Z BWR - A.N.T. International...4.3.4.2 RCP seals 4-51 4.3.4.3 RCS valve hard facings 4-51 4.3.4.4 Control rod drive mechanism (CRDM) 4-52 4.3.5 PWSCC of Alloy 600 MA 4-52 4.3.5.1 Laboratory

E F F E C T O F Z I N C I N B W R A N D P W R / V V E R O N A C T I V I T Y B U I L D - U P , I G S C C A N D F U E L P E R F O R M A N C E

© October 2010

Advanced Nuclear Technology International

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Sweden

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Effect of Zinc in BWR and PWR/VVER on Activity Build-up, IGSCC and Fuel Performance

Authors

Suat Odar Erlangen, Germany

Robert Cowan Livermore, California, USA

Jan Kysela Rez, Czech Republic

Reviewed by

Peter Ford Rexford, NY, USA

Peter Scott Noisy Le Roi, France

Friedrich Garzarolli Fürth, Germany


Disclaimer

The information presented in this report has been compiled and analysed by

Advanced Nuclear Technology International Europe AB (ANT International®)

and its subcontractors. ANT International has exercised due diligence in this work,

but does not warrant the accuracy or completeness of the information.

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Copyright © Advanced Nuclear Technology International Europe AB, ANT International, 2010.

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Contents

1 1-1 Introduction

2 2-1 Background information

2.1 2-1 BWRs shutdown dose rates (Robert Cowan)2.2 2-4 BWR plants start to inject zinc (Robert Cowan)2.3 2-4 PWR plants motivation to inject zinc (Suat Odar)

3 3-1 BWR plants (Robert Cowan)

3.1 3-1 The development of zinc technology for BWR applications3.1.1 3-1 Discovery of the zinc effect3.1.2 3-4 Confirmation of the zinc effect3.1.2.1 3-4 Effect of zinc ion on corrosion films3.1.2.2 3-5 Effect of zinc ion on Co incorporation60

3.1.2.3 3-9 Optimizing the ionic Zn concentration3.1.3 3-11 Theory/mechanism3.1.4 3-14 Plant qualification testing3.1.5 3-15 Plant Demonstrations3.1.5.1 3-16 Hope Creek demonstration3.1.5.2 3-19 Millstone I demonstration3.1.6 3-20 Isotopic Depleted Zinc3.2 3-24 Effect of Zinc on IGSCC3.3 3-30 Effectiveness of zinc injection in controlling shut down dose rates3.3.1 3-30 NWC3.3.2 3-33 HWC3.3.2.1 3-33 HWC basics3.3.2.2 3-35 HWC effect on shutdown dose rates3.3.3 3-41 NMCA and OLNC treated BWRs3.3.3.1 3-41 Noble metal basics3.3.3.2 3-45 Shut down dose rates and NMCA3.3.3.3 3-56 Shut down dose rates with OLNC3.3.3.4 3-58 Shut down dose rates with LTNC3.4 3-59 Zinc demand3.5 3-60 Fuel deposits with zinc injection3.6 3-66 Fuel performance with zinc injection3.6.1 3-66 Feedwater copper plants3.6.2 3-71 Enhanced spacer shadow corrosion (ESSC)3.6.3 3-72 NMCA and crud/oxide spalling3.6.4 3-76 Unexplained fuel failures3.7 3-77 Status of zinc injection worldwide3.7.1 3-77 GE Designed BWRs3.7.2 3-78 Siemens BWRs3.7.3 3-79 ASEA BWRs3.7.4 3-80 Toshiba/Hitachi BWRs3.8 3-82 Overall observations and conclusions


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4 4-1 PWR plants (Suat Odar)

4.1 4-2 Radiation build-up4.2 4-6 Mechanism of zinc technology4.3 4-11 Impact of zinc4.3.1 4-11 Oxide layers on out-of-core surfaces4.3.2 4-17 Coolant chemistry4.3.3 4-32 Radiation fields4.3.4 4-48 RCS material and design compatibility4.3.4.1 4-48 Construction materials4.3.4.2 4-51 RCP seals4.3.4.3 4-51 RCS valve hard facings4.3.4.4 4-52 Control rod drive mechanism (CRDM)4.3.5 4-52 PWSCC of Alloy 600 MA4.3.5.1 4-52 Laboratory data4.3.5.2 4-58 Field experience4.3.5.2.1 4-61 RPV head4.3.5.2.2 4-63 SG Tubing4.3.5.3 4-69 Summarizing discussions4.3.6 4-71 Fuel performance4.3.6.1 4-71 Background information4.3.6.2 4-75 Field experience: Influence of zinc on fuel deposits4.3.6.2.1 4-75 German PWR field experience4.3.6.2.2 4-78 US PWR field experience:4.3.6.3 4-81 Field experience: Influence of Zn on fuel clad corrosion4.3.6.3.1 4-81 USA PWR Experience:4.3.6.3.2 4-88 German PWR experience4.3.6.4 4-90 Field experience: Influence of Zn on AOA4.3.7

4-92 Chemical and volume control system ion exchange resins and mechanical filters

4.3.7.1 4-92 Effect of zinc on ion exchange resins4.3.7.2 4-94 Mechanical filters4.4 4-95 Experience with implementation of zinc chemistry and its basics4.4.1 4-95 Zinc compounds and solubility aspects4.4.2 4-98 Zinc injection concept4.4.2.1 4-98 Use of depleted vs natural zinc4.4.2.2 4-99 Injection locations and equipment4.4.2.3 4-102 Injection rate4.4.3 4-102 Zinc sampling and monitoring concept4.4.3.1 4-103 Sampling system for representative sampling4.4.3.1.1 4-103 General sampling system information4.4.3.1.2 4-104 Parameters impacting sampling representativeness4.4.3.2 4-108 Zinc monitoring program4.4.3.3 4-110 Analytical equipments4.4.4 4-111 Zinc surveillance programs4.4.4.1 4-111 Fuel surveillance program4.4.4.2 4-112 Radiation field surveillance program4.4.5 4-115 Control of zinc addition4.4.5.1 4-115 Control criteria4.4.5.2 4-118 Application of zinc chemistry during different plant operation modes4.4.5.2.1 4-118 Initial zinc injection at power operation4.4.5.2.2 4-120 Subsequent zinc cycles at power operation4.4.5.2.3 4-120 Plant start-up4.4.5.2.4 4-120 Plant shutdown4.5 4-121 Comparison of US and European experience4.5.1 4-122 Zinc absorption on RCS surfaces4.5.2 4-124 Coolant CP and radiocobalt concentrations4.5.3 4-125 Radiation fields


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IV(IV)

5 5-1 VVER plants (Jan Kysela)

5.1 5-1 Introduction5.2

5-1 Impact of zinc on material compatibility and oxide properties related to VVER

5.2.1 5-5 Experimental investigation in the loop5.2.1.1 5-5 EDX analysis of internal surfaces of the loop samples5.2.1.2 5-6 Linear EDX analysis of cross-sections5.2.1.3 5-7 Assessment of Bohunice and Paks SG tubes5.2.2 5-11 Results of experimental testing of zinc injection in VVER water chemistry5.3 5-16 Surface activity differences between VVER and PWR

6 6-1 Zinc influence on radwaste production

6.1 6-1 BWR plants (Robert Cowan)6.2 6-1 PWR plants (Suat Odar)6.2.1 6-2 Liquid waste effluents6.2.2 6-4 Spend resins and filters in CVCS6.2.3 6-5 Spend filter cartridges in CVCS6.2.4 6-8 Handling and storage of treated radwaste

7 7-1 Executive summary and recommendations

7.1 7-1 BWR plants (Robert Cowan)7.1.1 7-1 Summary7.1.2 7-2 BWR recommendations7.2 7-3 PWR plants (Suat Odar)7.2.1 7-3 Summary7.2.2 7-9 Recommendations7.3 7-11 VVER plants (Jan Kysela)

8 8-1 References

Nomenclature Unit conversion


1 Introduction This report describes the use of zinc injection technology in both Boiling Water Reactor (BWR) and Pressurized Water Reactor (PWR) plants worldwide. A review of the available laboratory work applicable to VVER1 plants is also included. The review covers the range from basic information to current knowledge and understanding of operational behaviour. The basis of this report is the published information including the proceedings of the recent International Conference on Water Chemistry of Nuclear Reactor Systems (NPC 2010) that took place at Quebec City on October 3-7, 2010. The personal opinions and interpretations of the authors are written in italic letters.

Zinc addition into the reactor coolant is now one of the most important considerations regarding coolant chemistry. The main objectives of zinc injection are: a) the control of radiation fields and b) the mitigation of Primary Water Stress Corrosion Cracking (PWSCC), in particular at PWR plants that used Alloy 600MA2 as construction material (SG tubes, Reactor Pressure Vessel (RPV) penetrations, nozzles and welds). This strategy of the zinc addition into reactor coolant started in the 1980s for BWR plants and 1990s in PWR plants. The basis for the “how and why” the plants started to inject zinc and their motivation for this strategy is explained briefly in Chapter 2.

The beneficial effect of zinc addition on radiation field reduction was first discovered in BWR plants in the early 1980s. This finding was followed by detailed qualification work to confirm structural material and fuel compatibility of zinc addition for BWR plants including the conduction of plant demonstration tests. This history is explained in detail in Chapter 3 along with information regarding the positive effect of zinc on Stress Corrosion Cracking (SCC) in BWR plants. The effect on zinc addition on radiation field control under different BWR chemistry conditions such as “Normal Water Chemistry” (NWC), “Hydrogen Water Chemistry” (HWC), “Nobel Metal Chemical Addition” (NMCA) and “On-Line NobleChem” (OLNC) is explained in Section 3.3. Zinc demand under different BWR chemistry conditions is explained in Section 3.4. Sections 3.5 and 3.6 summarize the information related to zinc influence on fuel deposits and fuel performance. At the present, zinc injection is applied worldwide in BWR plants of different vendors and design, and is detailed in Section 3.7. Finally, the overall observations related to zinc injection within the BWR world is summarized and concluded in Section 3.8.

In the case of PWR plants, based on good results of zinc addition gained in BWR plants, many plants started to think also to inject zinc in their reactor coolant to control the radiation fields and the PWR plants with Alloy 600MA also as remedy to mitigate PWSCC. For this purpose, detailed investigations were performed to qualify the zinc addition for PWR units in particular, to confirm its effectiveness for dose rate reduction, its compatibility with PWR construction materials and with fuel performance. The results of these extensive investigations and the field experience gained worldwide by applying the zinc injection at PWR units are summarized in Chapter 4. Chapter 4 starts in Section 4.1 with the information how oxide films on the out-of-core surfaces of Reactor Coolant System (RCS) is formed and how the radiation fields are built-up in the PWR units. The purpose of giving this information is to provide better understanding of the mechanism how zinc addition works, which is explained in Section 4.2. The investigations performed to qualify the zinc addition for PWR units confirmed that zinc injection has some impacts on RCSs and on fuel. These were explained in Section 4.3: Oxide films on out-of-core surfaces in Section 4.3.1; Coolant chemistry in Section 4.3.2; Out-of-core dose rates in Section 4.3.3; RCS material compatibility in Section 4.3.4; Influence on PWSCC in Section 4.3.5; Fuel performance in Section 4.3.6; and finally Influence on Chemical and Volume Control System (CVCS) in Section 4.3.7.

1 Voda Voda Energo Reactor (Russian acronym for the Russian type of PWR) 2 Mill Annealed


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The worldwide experience gained in the field by applying zinc addition is summarized in Section 4.4. The information included in this Section is important and valuable not only for those PWR utilities that intend to introduce zinc addition in their units, but also for the plants that already apply zinc addition to optimize their zinc addition program. This information include: Solubility aspects of zinc compounds in Section 4.4.1; Implementation of zinc injection concept in Section 4.4.2; Sampling and monitoring concept of zinc injection in Section 4.4.3; Zinc surveillance programs for fuel performance and dose rate reduction in Section 4.4.4; Control of zinc addition criteria in different plant operation modes in Section 4.4.5. Finally, the results of zinc addition in US and European PWR units are compared in Section 4.5.

Zinc addition is not yet applied in VVER plants. However, lots of preparatory investigations such as laboratory tests were performed. The results of these investigations might be also useful for the PWR units. They are explained in Chapter 4.

There is a common concern that zinc injection might have negative impact on plant radwaste production due to release of radiocobalt into reactor coolant by zinc injection and also due to production of 65Zn, particularly if natural zinc is used. The confirmation that zinc addition doesn’t have any negative effect on radwaste production is explained by the results of field experience gained in zinc addition BWR and PWR plants in Chapter 6.

Finally, Chapter 7 summarizes the information related to zinc injection in BWR and PWR plants and gives valuable recommendations regarding implementation of zinc addition.


2 Background information In this Section, the technical basis for both BWR and PWR plants to inject zinc is briefly described.

2.1 BWRs shutdown dose rates (Robert Cowan) By the early 1980s, it became apparent that the shut down dose rates of GE designed BWRs was variable and not easily controlled. Some reactors were showing very acceptable behaviour but others were showing annual shut down dose rate increases of up to 150 mR/hr, as detailed in Figure 2-1.

Figure 2-1: Radiation field buildup on GE designed BWR recirculation system piping [Lin et al3, 1980].

The primary source of radiation fields on out-of-core surfaces in BWRs is 60Co, which in mature plants usually accounts for 80 to 90% of the total dose rate. The 60Co originates when 59Co, the naturally occurring form of elemental cobalt, is activated by an n,p reaction in the reactor core. The 60Co isotope has a relatively long half-life of 5.27 years and the energies of the two principal 60Co gamma rays are 1.17 and 1.33 MeV. This combination makes its effect on shutdown dose rates very potent. The naturally occurring 59Co is present as an impurity in the nickel in structural alloys used in BWRs (e.g., stainless steels, carbon steels and nickel alloys), and is the main constituent of wear-resistant alloys (e.g., Stellite™), used as hard facing in valve trim and other applications requiring outstanding wear resistance. Another gamma emitting cobalt isotope, 58Co, is formed by the activation of nickel from stainless steel and nickel-based alloys. 58Co has a shorter half-life and is not as significant a contributor to radiation fields as is 60Co. Other fuel deposit activated transition metals such as 54Mn, 59Fe, 51Cr and 65Zn contribute to the remainder of the dose rate.

3 et alia


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For elements such as 59Co to become activated and contribute to shutdown dose rates, they must spend significant residence time in the reactor core. The core residence time can occur because (1) components in the core activate and corrode, such as cobalt alloy control blade pins and rollers or (2) the element deposits on the fuel rod as crud, gets activated and then releases to the coolant to deposit in an out of core location. This process is shown schematically in Figure 2-2 for 59Co/60Co.

Figure 2-2: Schematic diagram of 60Co Isotope creation, transport and deposition in a BWR [Cowan & Kiss, 1993].

The radiation fields are commonly measured in a BWR at the straight vertical section of pipes are considered to be more representative for the purposes of radiation buildup trending and comparison with other plants. These measurements represent primarily the incorporation of soluble 60Co into the corrosion film on the piping surfaces and tend to be a good predictor of general drywell dose rates. The deposition of particulate oxides that contain 60Co and other activated species can also contribute significantly to out-of-core radiation levels in BWRs, especially in hot spots. The particulate oxides, which vary in size, originate primarily from corrosion of the steam/condensate system and are introduced via the feedwater.


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Cobalt source removal is clearly important if radiation fields are to be minimized, but experience has shown that effective cobalt removal at an operating plant is difficult to do in a cost effective manner because there are so many cobalt sources and removal of no one source makes a significant impact on total cobalt input to the reactor. New plants have the advantage of "engineering in" low cobalt alloys, the use of alternate valve trim, low cobalt specifications for structural materials, etc. An additional approach is to limit the amount of mass entering the reactor via the feedwater that can deposit on the fuel surface to minimize the amount of species that become activated. The combination of both of these strategies has worked well in the new BWRs constructed in Japan [Ishigure, 2002]. Similar new plant results were reported in Scandinavia and Germany.

In the 1980s, the only viable approach to shutdown dose rate reduction in operating plants was recirculation system decontamination. This solution was expensive and used critical path time at an outage, so was only used sparingly. In addition, the effects of repeated chemical treatment of systems prone to Inter Granular Stress Corrosion Cracking (IGSCC) was worrisome. Fortunately, feedwater Zn injection was developed at about this time as a new approach to controlling shutdown dose rates. Zn in the reactor water at a level of 5 to 10 ppb operates in two ways. First it promotes the formation of a fuel deposit composition that 'traps' the 60Co into its matrix and lowers the amount of soluble 60Co in the reactor water. This in turn lowers the 60Co available to incorporate into the corrosion film oxides of the out of core surfaces that cause shut down dose rates surfaces. Secondly, the Zn is incorporated into these corrosion films into sites that could be occupied by 60Co, as they share the same preferred locations. The net result is that feedwater Zn injection can reduce the shutdown dose rates from 10% to 90%, depending on host of factors, which are addressed in detail in Chapter 3.

Adapting Zn injection to the BWR has not been a straightforward effort, as the environment in the BWR has been a "moving target" in many case, as shown in Figure 2-2. The implementation of HWC, zinc addition and noble metal chemical addition has been broadly seen across the GE BWR fleet with all but for plants worldwide injecting zinc for radiation field control and all of the fleet injecting hydrogen for IGSCC control. Likewise with parts of the Scandinavian and Japanese BWR fleets. A large majority of the GE fleet have also implemented the noble metal chemical addition technology.

Early investigations also showed that small amounts of Zn in the water also appeared to positively affect IGSCC in both oxidizing and reducing reactor conditions. This later observation lead to investigation and evaluations by PWR researchers into adapting Zn injection to PWR units (see Section 2.3).

Figure 2-3: BWR chemistry changes through the last four decades [Garcia & Wood, 2008].


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3 BWR plants (Robert Cowan)

3.1 The development of zinc technology for BWR applications

3.1.1 Discovery of the zinc effect

A joint program by the EPRI and General Electric (GE) was conducted in the late 1970s and early 1980s to identify and understand the variables affecting occupational exposure in Boiling Water Reactors (BWR) with an emphasis on reducing shut down radiation fields. This effort was called the BWR Radiation Assessment and Control (BRAC) program. The conclusion of this program in the final BRAC report [Anstine, 1983] can be paraphrased as follows:

“BWR radiation buildup is a complex combination of mechanisms which are not totally understood at this time. Many recommendations, such as minimum crud input, minimum reactor water conductivity, and surface polishing of stainless steel, will contribute to reduced radiation buildup but do not ensure it and may not be mandatory to achieve it. It is hypothesized that an unidentified key parameter existed that might be controlling the buildup process and further study is warranted.”

EPRI and GE decided to co-sponsor a small follow-on program to review the data collected by BRAC with an emphasis on the processes occurring on the stainless steel piping surface [Shaw et al, 1984]. Marble was the GE investigator assigned to the program and, based on an observation that plants with soluble Zn ion in the reactor water tended to have significantly lower shutdown dose rates, he developed the following working hypotheses [Marble & Wood, 1985]:

“The zinc cations will incorporate into and modify the normal magnetite crystal defect structure so that a more protective film is formed and corrosion is significantly inhibited.”

Figure 3-1: BWR radiation behaviour for generic plants when sorted by condenser metals and condensate type [Marble & Wood, 1985].


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Marble was able to categorize the plant shut down dose behaviour based on the type of condensate treatment system, as shown in Figure 3-1. The brass condenser/filter demineralizer (Powdex) treatment plants were known to have μg/kg5 concentrations of both Zn and Cu in the reactor water and Marble felt the Zn ions were incorporating into the oxide film. (These plants are referred to in subsequent discussion as 'natural zinc' plants.) It had previously been established [Lin et al, 1980] that the majority of the shutdown radiation field in the BWR originates from soluble 60Co in the reactor water that is incorporated into the stainless steel corrosion film. Experiments at GE [Niedrach, 1980] showed that under BWR conditions, the incorporation of soluble cobalt into the corrosion film of stainless steel was proportional to the soluble cobalt concentration of the water, all other things being equal (Figure 3-2). However, Marble felt that with zinc present in the water, "all other things were not equal". He plotted the available corrosion film composition data vs. plant shut down dose rate and developed the relationship shown in Figure 3-3.

Clearly, the Zn in the reactor water was incorporating into the corrosion film, and there seemed to be a favourable correlation with zinc and lower dose rates. The concentration of zinc in the Hatch 2 BWR reactor water was typically in the range of 5 to 10 μg/kg and the corrosion film zinc concentration was 20%, a factor of 107 enrichment. The reactor water copper concentration was typically in the range of 10 to 20 μg/kg or higher and the measured copper in the Hatch 2 corrosion film oxide was 4.5%. Other plant parameters also indicated the role of zinc, including fuel deposit data and variations of zinc concentration and 60Co along coupon trains in the Hatch 2 unit [Marble, 1983]. The other obvious shut down dose rate correlation to make would have been with reactor water 60Co. However, at that time it was not a standard plant practice to make reactor water 60Co determinations and for those plants that did, it was even rarer to find a separation of soluble and insoluble 60Co. However, even a cursory inspection of the available data showed that 60Co concentrations alone couldn't explain the wide variations in observed dose rates.

5 μg/kg is used throughout this work instead of 'μg/kg '


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Figure 3-2 Incorporation ratio of cobalt in stainless steel corrosion film as a function of water cobalt concentration in pure, oxygenated water at 285 °C [Niedrach, 1980].

Figure 3-3: BWR radiation buildup vs. measured corrosion film Zn concentrations [Marble & Wood, 1985].


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3.3 Effectiveness of zinc injection in controlling shut down dose rates

3.3.1 NWC

Zinc addition was originally practiced with natural zinc oxide in plants operating on NWC. But as discussed in 3.1.6, the production of 65Zn from natural zinc can offset many of the advantages of Zn in inhibiting dose rates. The use of DZO quickly became the standard method of implementing Zn injection, and those BWRs that started with natural Zn switched to DZO. The 60Co and 65Zn history in such a plant is shown in Figure 3-28 [Garcia & Cowan, 1998]. The 60Co reduction phenomenon, observed at the first use of Zn injection in an operating plant, proved to be very reproducible in subsequent Zn injections as shown in Figure 3-29. During the time frame of the adoption of DZO addition many BWRs were either decontaminating and repairing their recirculation system piping or decontaminating and replacing the piping with Type 316NG stainless steel. It was found that Zn injection was much more effective after a decontamination or on new piping surfaces, as shown in Figure 3-30.

Figure 3-28: Piping activity impact of natural zinc and DZO injection [Garcia & Cowan, 1998].

Figure 3-29: Effect of Zn addition on the 60Co content of reactor water under NWC conditions [Garcia & Cowan, 1998].


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Figure 3-30: Comparison of BRAC piping dose rates on existing recirculation piping vs. decontaminated or replaced piping [Garcia & Cowan, 1998].

A summary of the BRAC dose rates in NWC before and after Zn injection is shown in Table 3-9. The reduction in BRAC dose rate with NWC and DZO injection was real and consistent, but the steady state factors of reduction of these rates were only on the order of a factor of 1.5 to 5, excluding plants that augmented 'natural zinc' with DZO (Peach Bottom 2 and Columbia). Looking at the reduction in 60Co (a factor of 2 to 5) in reactor water (Figure 3-29) and the results of the laboratory testing (a factor of 3 times decrease in 60Co with Zn addition) in Figure 3-6, a much larger reduction in BRAC rates would have been expected under NWC condition than was observed. (See Section 3.1.5.2 for an explanation of why the reactor water 60Co drops with Zn injection).

Lin [Lin, 2004] has suggested that the observed reduction in BRAC dose rates is due only to the reduction of reactor water soluble 60Co seen at NWC Zn injection plants. He contends that the environment in his test loops simulating NWC contained only 200 μg/kg O2 and did not represent real BWR NWC conditions. Since this testing [Lin et al, 1986] and [Lin et al, 1988b], it has been discovered that a more realistic NWC environment consists of approximately 300 μg/kg H2O2 + 300 μg/kg O2 [Rooth et al, 1989]. Tests with 200 μg/kg H2O2 vs. 200 μg/kg O2 [Kim, 1999] showthat there is a marked difference in the composition and structure of the oxide films formed in these two environments, as shown in

tions.

Table 3-10. (Similar results are reported by [Uchida et al, 1987]). Note that with 200 μg/kg H2O2 the inner continuous layer is composed of NiFe2O4. Thesource of the Ni must be the alloy rather than the water phase, as no Ni was added to the water. With this formation mechanism, Zn in the water at a concentration of 10 μg/kg is presumably not effective in changing the composition of the outer layer or affecting cobalt incorporation rates. Tests with pure NiFe3O4 [Permér & Österlundh, 1996] showed that with 20 μg/kg Zn and 0.1 μg/kg Co under NWC ECP values of 125 mV(SHE), uptake of Co from the water into pure NiFe2O4 wafers was not significantly inhibited by the presence of Zn. Conversely, wafers of pure FeCr2O4 did have a significant uptake of Zn. For the case of the laboratory test in 200 μg/kg O2

in Figure 3-6, the inner layer is a FeCr2O4 structure that is easily substituted with transition metals from the water, which explains the effectiveness of Zn addition under these test condiUnfortunately, there are no loop data under more realistic NWC test conditions using 200 μg/kg H2O2 to study the interaction of Zn and 60Co uptake to quantify the ineffectiveness of Zn under these conditions.


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Table 3-9: BRAC values for NWC plants before and after Zn injection [Cowan, 2010b].

Plant Before Zn After Zn BF2 425 315

Fermi 162 112

Monticello 600 300

Columbia 400 318

FitzPatrick 168 113

KKL 400 150

KKM 346 300

Laguna Verde 1 336 140

Laguna Verde 2 275 60

LaSalle1 460 181

Limerick1 133 93

Nine Mile Point 2 245 178

Peach Bottom 2 110 110

Table 3-10: Oxide Structure Formed on Type 304 SS After Free Immersion for 2 Weeks in 288 °C Water Containing 200 µg/kg O2, 200 µg/kg H2O2, or 150 µg/kg H2 [Kim, 1999].

Water chemistry Outer oxide layer, large particle

Outer oxide layer, small particle Inner thin oxide layer

200 ppb H2O2 γ-Fe2O3 γ-Fe2O3 NiFe2O4

200 ppb O2 α-Fe2O3 α-Fe2O4 FeCr2O4

150 ppb H2 Fe3O4 FeCr2O4 FeCr2O4

A recent paper from Japan [Kondou et al, 2009] sheds some interesting light on the effect of Zn under NWC conditions. The Hamaoka-3 Unit uses ultra-low feedwater iron (≤ 0.1 μg/kg) which under NWC conditions results in fairly high soluble Ni in the reactor water (1-3 μg/kg). This strategy, in which soluble Ni acts like Zn in lowering 60Co uptake (see Section 3.1.2.2), is used at most Japanese BWRs. After a decon at the end of the Cycle11, the shut down dose rate built up as expected for an ultra low feedwater iron BWR, see the projected line in Figure 3-31. At the Beginning of Cycle (BOC) 13 Hamaoka-3 began feedwater Zn addition to establish 3 μg/kg Zn in the reactor water and saw a significant reduction of the shutdown dose rate after Cycle 13 and an even larger reduction at the End of Cycle (EOC) 14. [Kondou et al, 2009] claim that the Ni ion in the reactor water is effective in decreasing the 60Co deposition in the outer layer of the stainless steel oxide, while Zn injection reduces the 60Co incorporation into the inner oxide. (In Cycle 15, feedwater hydrogen injection began, see next section).

In summary, under NWC conditions, Zn addition to BWRs is effective in reducing the concentration of 60Co in the reactor water by changing the composition and structure of the fuel deposit (Section 3.1.5.2) and the majority of the observed dose reduction in high feedwater iron plants appears to be due to this effect. In lower feedwater iron plants feedwater iron (≤ 0.1 μg/kg) Zn seems to be more effective at inhibiting60Co uptake into the corrosion film.


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3.5 Fuel deposits with zinc injection BWR fuel deposits usually consists of an outer loose layer that can be removed with a brush and a second adherent inner layer that can only be removed by a grinding stone or some other robust method. With no additions of Zn or H2 to the feedwater of a BWR (and the absence of a copper/zinc condenser), the fuel deposits are composed primarily of iron oxide product that comes in via the feedwater (in low feedwater iron input plants, < 0.5 μg/kg Fe, other CPs can play a more important role.) These plants usually have a loose outer layer comprised of α-Fe2O3 and an inner layer of the same compound plus traces of the mixed metal spinel-like NiFe2O4 type. The spinel phase forms as the soluble Ni (and other transition metals such as Co) concentrate at the bottom of the wick-boiling channels (micro boiling) and reacts with the iron oxide. For plants with a natural source of Zn and Cu (brass alloy condenser tubes coupled with a Filter Demineralizer (F/D) condensate treatment system), ZnFe2O4 and CuO are found as well. A summary of the crystal structures and distributions is shown in Table 3-17.

Table 3-17: Crystal structures in Oxide Deposits on fuel cladding surfaces [EPRI, 2004b].

Plant type Major component (>50%)

Minor component (1-50%)

Trace component (< 1%)

DB, no Zn Brush α-Fe2O3 NiFe2O4

Scrape α-Fe2O4

DB, with Zn

Brush α-Fe2O3 ZnFe2O3

Scrape α-Fe2O4 ZnFe2O4 NiFe2O4

F/D, no Zn

Brush α-Fe2O3 NiFe2O4

Scrape α-Fe2O4 NiFe2O5

F/D, with Zn

Brush α-Fe2O3 ZnFe2O4 NiFe2O4

Scrape ZnFe2O4 NiFe2O4

A comparison of the average fuel deposit composition from BWRs utilizing a) admiralty brass condenser + F/D b) admiralty brass condenser + DB demineralizer and c) stainless steel condenser + F/D is shown in Table 3-18. These data are for third cycle, high duty, sound rods that were exposed under NWC conditions without zinc injection during the 1980s. Note that the "loose" fraction is somewhat enriched in copper but the adherent deposit is especially enriched in both copper and zinc. (These data are rod average values and the copper and zinc levels can be significantly higher at mid core elevations). Structural analysis of adherent deposits showed it to be primarily a mix of Fe2O3 (55 to 75%) and a zinc/copper rich spinel structure (25 to 45%) [Levin & Garcia, 1995]. The feedwater iron input for these reactors at the time these data were generated were typically 2-8 μg/kg for the DB plants and 0.5 to 2.0 μg/kg for the F/D plants.


3-60(3-83)=


Table 3-18: Summary of fuel deposits sorted by condenser type and condenser tube material for BWRs with no deliberate Zn additions [Cowan & Garcia, 2010].

Condenser and treatment type Deposit loading (Micrograms per

square centimetre)

Percent composition

Fe Cu Zn Ni Cr Mn Co Loose deposit (removed with brush)

Admiralty brass + filter/demin 948 85.6 6.8 4.9 2.2 0.8 2.2 0.2 Admiralty brass + DB 7317 95.7 0.8 0.7 2.9 1.7 0.8 0.3

Stainless + filter/demin 1497 93.9 0.1 0.2 2.6 1.2 1.6 0.2

Adherent deposit (removed with grinding stone)

Admiralty brass + filter/demin 802 69.3 16.2 10.2 3.7 1.4 3.3 0.4

Admiralty brass + DB 2647 90.3 0.5 0.2 2.9 5.2 0.8 0.1 Stainless + filter/demin 557 85.1 0.6 1.5 5.2 2.5 4.4 0.7

The first deliberate injection of Zn to the feedwater was at the Hope Creek BWR in 1987, see Section 3.1.5.1. During the first three cycles of operation, Hope Creek operated with high feedwater iron input of approximately 8 μg/kg and an average reactor water Zn concentration of only about 2 μg/kg, as the high iron input was utilizing the Zn at a very high rate. A comprehensive fuel surveillance program was initiated that included visual examination, fuel deposit sampling (crud scraping), and lift-off eddy current measurements to characterize fuel corrosion performance with zinc injection. Fuel performance was characterized over a fuel bundle lifetime of three cycles. Prior to taking any eddy current lift-off measurements to determine the ZrO2 thickness, the fuel rods were cleaned with a stainless steel brush to remove any deposited crud that could affect the measurements. After brushing, it was evident that the stainless steel brushing did not remove all of the deposited crud and a roughened adherent layer remained. The eddy current lift-off device recorded extremely high apparent fuel clad oxide thickness measurements that was only resolved by a combination of fuel deposit sampling measurements and eddy current measurements in areas where all fuel deposits had been removed by a grinding stone, followed up by confirmatory hot cell examinations [Levin & Garcia, 1995] and [Davies et al, 1996]. It was concluded that the fuel deposit contained a high concentration of ferromagnetic spinel phase that was biasing the eddy current clad corrosion oxide thickness measurements. This biasing of eddy current data by Zn injection inducing ferromagnetic spinel phase in the fuel deposit was confirmed by [Gavillet et al, 1996] and they also confirmed that the phase was ferromagnetic14.

The analysis of the fuel deposits at Hope Creek showed Zn levels in the range of 2 to 10% Zn for the brushed off layer and 29 to 33% in the adherent layer. This is very close to the 2:1 ratio of 100% ZnFe2O4 in the adherent layer. Metallographic analysis of the cladding and deposit confirmed that the deposit thickness was on the order of 1.2 to 9.7 μm and the zirconium oxide thickness was 8to 19 μm in thickness. These values fell well within the GE fuel data base at that time as shown in Figure 3-56. The eddy current method was subsequently improved to correct for the ferromagnetic effect of the Zn spinel.

996].

14 ZnFe2O4 is normally antiparamagnetic, but turns ferromagnetic when doped with a small amount of nickel [Gavillet et al, 1


3-61(3-83)=


4 PWR plants (Suat Odar) At the beginning of 1990’s PWR vendors and utilities started to become interested in zinc injection because of the reported beneficial effect on dose reduction and corrosion inhibition from BWR experience (see Chapter 3). Therefore, comprehensive research work started mainly in two countries, in US and Germany, to investigate the impact of zinc addition on material compatibility, fuel performance and operational conditions in PWR plants. In US the main interest was to mitigate PWSCC of Alloy 600 Meal Annealed (MA), a material that is used for SG tubing material and widely in many penetrations of the RCS. In collaboration with Westinghouse Owner’s Group (WOG) and EPRI, Westinghouse initiated laboratory studies to examine the impact of zinc on PWSCC in the PWR primary water environment [Esposito et al, 1991]. In contrast to the US, PWSCC was not an issue for PWR plants in Germany due to use of Alloy 800NG as SG tubing material and avoidance of the use of Alloy 600MA in the RCS. The main interest for the German PWR utilities was radiation field reduction in their old PWR plants, which had rather high dose rates due to use of cobalt containing hard facing-materials (stellites) in the RPV internals. Siemens performed a research program in collaboration with VGB and German PWR utilities with the aim of studying the impact of zinc addition solely for dose reduction, [Stellwag & Staudt, 1995].

After completion of the investigation programs and performing plant specific qualification work, zinc injection was introduced in the US PWR plants. The Farley-2 plant was selected as the first candidate plant for zinc injection, as it had a sufficient number of cracked SG tubes to allow for a good statistical basis for monitoring of crack development. Zinc injection started in 1994 at Farley-2 plant, and followed by Diablo Canyon-1 and 2 in 1997-1998.

In Germany Biblis-B was the first PWR plant that introduced zinc injection in 1996, which was followed by Biblis-A and Obrigheim plants in the beginning of 1998. Due to convincing good field results from these leadplants that confirmed the beneficial effect of zinc chemistry, many PWR plants started to inject zinc in their reactor coolant. As of 2010, there are now almost 70 PWR plants that are injecting zinc (see Figure 4-1), [Fruzzetti et al, 2010].

Figure 4-1: Number of PWR units injecting zinc worldwide [Fruzzetti et al, 2010].

The following sections explain the mechanism by which zinc chemistry works and summarize the laboratory and field results gained within the last fifteen years with respect to radiation field reduction and mitigating the PWSCC of Alloy 600MA.


4-1(4-127)=


4.1 Radiation build-up The major construction materials used in PWR RCS materials are high alloyed CrNi-Steels, nickel based alloys, and in small quantities cobalt based alloys. At ambient temperatures all these construction materials have very low corrosion rates because of the formation of the passive oxide layers that protect them. The thickness of these low temperature passive layers is in the range of several nanometres and their chemical composition is chromium rich (> 30% Cr and usually free of nickel), either Cr2O3 or a mixture of Cr2O3 and hydrated chromium hydroxide [Kaesche, 1990], [Brüesch et al, 1984] and [Sato & Okamoto, 1981]. With increasing temperature above 150 °C the general corrosion rate of these metals increase [Winkler & Lehmann, 1985] because the passive oxide layers become thicker and more porous but still protective. The composition and thickness of these protective layers depends on different factors, like the chemical composition of the base material, environmental conditions like the ECP, pH and temperature of the medium to which they are exposed. The thickness of these protective oxide layers is in the range of several hundred nanometres. Oxide layers built under the PWR primary coolant operating conditions, i.e. reducing at 300-330 °C, have duplex structure (see Figure 4-2) [Lister et al, 1987]. They have chromium-rich inner layer and an iron and nickel-rich outer layer. The existence of duplex oxide layers is a result of different diffusion rate of the cationic substituents of the alloy or steel through the inner protective oxide layer. The inner oxide layer grows at the interface oxide and metal as the water penetrates through the pores and cracks in the oxide layer to the metal surface, where it reacts with the metal ions to build oxides. This inner oxide layer grows mainly by anion mass transport towards the oxide metal intersection although there is clearly iron and nickel cation transport in the reverse direction. The driving force for metal cation diffusion is the metal ion concentration difference between the metal surface and the coolant. Due to different diffusion coefficients of different metal ions their diffusion rates are also different. Iron and nickel have much higher diffusion rates than chromium. Therefore, the inner part of the protective oxide layer becomes chromium-rich, whereas the outer part is iron and nickel-rich. The iron and nickel-rich outer oxide layers are also more soluble in hydrogenated PWR primary water whereas chromium is for all practical purposes completely insoluble (see Figure 4-3) [Ahlberg & Rebensdorff, 1992]. Therefore the growth of outer oxide layers is controlled by dissolution and precipitation of iron/nickel oxides. For example, if the outer layer is exposed to iron and nickel-free coolant in PWR primary water, only inner oxide layers and no outer layer are observed in the laboratory work due to dissolution of the outer layer [Lister et al, 1987]. On the other side, in iron-saturated coolant, large crystals of iron/nickel spinell oxide usually exist in this outer oxide layers that are produced from coolant by precipitation reactions. However, due to the higher solubility of iron/nickel oxides in hydrogenated PWR coolant, the oxide layers produced under reducing conditions of PWR reactor coolant are usually thinner than that produced under oxidizing BWR conditions.


4-2(4-127)=


Figure 4-2: Structure of the protective oxide layers on stainless steel under PWR operating conditions [Lister et al, 1987].

Figure 4-3: Iron solubility as a function of the potential at neutral pH, at varying temperatures, from thermodynamic calculations [Ahlberg & Rebensdorff, 1992].


4-3(4-127)=


As already mentioned, duplex oxide layers are normally built under PWR coolant conditions. Both oxide layers have spinel structure (see Section 4.2), the inner layer being a regular spinel and the outer layer an inverse spinel; the two forms having only limited (<30%) solubility in each other. The chemical composition of these oxide spinels depends on the chemical composition of materials exposed to the reactor coolant. In a typical PWR, the wetted RCS surface area consists of approximately 25% fuel cladding (zirconium alloy; Zircaloy-4, Zirlo, or M5), 65% SG tubing (either nickel base alloy like Alloy 600MA, Alloy 600TT20, Alloy 690TT or iron base alloy, Alloy 800NG) and about 10% stainless steel. The chemical composition of these materials is shown in Figure 4-4. Accordingly, the following protective oxides are produced on these construction materials.

In the core region, a film consisting principally of the zirconium dioxide (ZrO2) forms from the zirconium alloy on the fuel assembly cladding, guide tubes and fuel grids. In addition to ZrO2 CP deposits (nickel ferrites, metallic nickel and nickel oxides) are also found on fuel rods, which are precipitated from dissolved cations transported by coolant (see below).

On stainless steel and SG tubing, the film builds duplex oxide layers with chemical composition of NixFe1-xCr2O4 (nickel-iron chromites) for the inner layers and NixFe1-xFe2O4 (nickel ferrites) for the outer layers. If, a nickel base alloy is used for the SG tubing, in that case the outer oxide layers can also contain metallic nickel on out-of-core surfaces. On fuel assembly rods, metallic nickel exists in the lower part of the core, whereas in the upper part, where boiling is expected, nickel exists usually as nickel oxide deposit.

Much less, but consistent information is available on the oxide layers formed on cobalt based alloys (stellites). According to Lister, cobalt chromite CoCr2O4 was identified after long-term exposure as a major phase, but strongly depleted cobalt, relative to the base metal [Hocking & Lister, 1998].

The solubility of these spinel oxide films is very low and they protect the materials very well. Accordingly, the associated metal ion release is extremely low and their concentrations in the reactor coolant are in trace levels (see Figure 4-5). Even so, at the liquid-oxide interface some metal oxide does dissolve and are transported by coolant flow to the core area, where they precipitate by boiling and/or due to their decreased solubility by increasing temperature (i.e. retrograde solubility). The deposited CPs are activated in the core area by the neutron flux and form radiocobalts 58Co and 60Co, according to the transmutation reactions.

58Ni (n,p)58Co, and 59Co (n,γ) 60Co.

Figure 4-4: Chemical composition of materials used in PWR plants [Garbett, 2006].

20 Thermal Treated


4-4(4-127)=


4.4 Experience with implementation of zinc chemistry and its basics

In this section basic information is given that is important for the implementation of zinc injection in the PWR units, such as solubility of the zinc compounds under PWR operation conditions, injection, sampling and monitoring of zinc, control criteria of zinc addition, and fuel surveillance requirements.

4.4.1 Zinc compounds and solubility aspects

Zinc is injected into reactor coolant of PWR plants to reduce the radiation fields and/or to mitigate PWSCC (see Sections 4.3.3 and 4.3.5). The main concern for the zinc injection was all the time to be sure that fuel integrity and performance is not affected by possible zinc precipitation on the fuel rod surfaces and/or in the fuel deposits in particular, in the region of SNB. Therefore, zinc solubility behaviour under PWR coolant conditions with varying temperatures, pH values was one of the important subjects for the PWR industry investigations. The previous data regarding the solubility of the zinc compound were established by temperature extrapolations of ZnO solubility data, which were known for temperatures at ≤ 200 °C, to 300 °C. These extrapolations resulted in a conservative solubility limit of ≤ 40 μg/kg, [Lindsay, 1998]. Another predictions of 80 to 110 μg/kg for zinc oxide solubility was also used by PWR industry, which were based on some conventional, limited data (in aqueous phosphate solutions) gained from previous studies [Ziemniak et al, 1992]. Finally, at the beginning of 2000s, the Oak Ridge national Laboratories (ORNL) had performed experimental investigations to measure the solubility of zinc compounds under PWR coolant conditions at different temperatures and pH values. The results of their work that was sponsored by EPRI/DEO can be summarized as follows [Bénézeth et al, 1999] and [Palmer et al, 2004]:

• The solubility of zinc at the pH values of PWR operating conditions is between 100 and 200 μg/kg, depending on temperature between 150 and 350 °C (see Figure 4-103 for 200 and 300 °C as example).

• In the steam phase that is relevant for boiling on fuel rod surfaces, the solubility is about 1 μg/kg as expected (see Figure 4-104).

Figure 4-103: Solubility of zinc at 200 and 300 °C as a function of pHT values [Palmer et al, 2004].


4-95(4-127)=


Figure 4-104: Solubility of zinc at 350 °C as a function of pHT values [Palmer et al, 2004].

In their work they have also calculated from the solubility constants, the distribution of aqueous zinc species as a function of pH and temperature as shown in Figure 4-105. As can be seen, Zn(OH)+ and Zn(OH)2 species are dominating and not the Zn2+ in the near neutral pH to mildly basic region relevant for PWR coolant conditions. As temperature increases, Zn(OH)2 and

get more dominant. 3Zn(OH)−


4-96(4-127)=


Figure 4-105: Distribution of aqueous zinc species as a function of pH at different temperatures [Palmer et al, 2004].


4-97(4-127)=


5 VVER plants (Jan Kysela)

5.1 Introduction VVER reactors operated in Eastern Europe, some Asian countries and the Russian Federation differ from PWR in their primary circuit technology. This is given by SG design (horizontal tubing as opposed to vertical in PWR reactors), different materials (SGs are made of stainless steel 18 % cr-10 % Ni, Ti, and fuel is clad with Zr/Nb alloys) and water chemistry, which in a VVER uses an alkaline agent (KOH) and ammonia, whose decomposition results in radiolytic hydrogen.

The radiation situation in VVER units is among the lowest in the world. This due to good corrosion resistance of SG stainless steel tubing, reduced cobalt content in all primary circuit materials, and avoidance of the use of highly resistant cobalt alloys (stellites) in valve seats. VVER reactors are operated at lower parameters of temperature than PWR reactors, which can play a role in the transport of activity in the primary circuit.

Despite the above, it is in the interest of operators to use zinc to reduce activity transport. Western PWR and BWR reactors widely utilize zinc injection, with excellent results. The goal of operators is to maintain low levels of Operational Radiation Exposure while also extending power station life, and at the same time resolve the situation in some VVER units where staff radiation dosage levels continue to be high.

The first part of this Section deals with the corrosion behaviour of stainless steels in the presence of zinc in coolant. Zinc also has a beneficial effect on transport activities in the primary circuit by replacing cobalt in the oxide layer coating equipment.

The second part of this Section assesses the results of an experimental programme at NRI Rez (Czech Republic) whose goal was to compare water chemistries with ammonia, hydrogen, and zinc injection. It discusses the results of crud behaviour in the coolant and active crud deposition on surfaces.

5.2 Impact of zinc on material compatibility and oxide properties related to VVER

The utilization of zinc injection must be preceded by an analysis of possible effects on circuit materials and chemistries. We focused on data regarding the influence of zinc on corrosion behaviour of the primary material of the VVER circuit, stainless steel.

This section summarizes some studies of the influence of zinc on the corrosion resistance of stainless steel and its behaviour in corrosion layers.

The oxidation of AISI 316L(NG) stainless steel in simulated PWR coolant with or without addition of 1ppm Zn at 280 °C for up to 96 h has been characterized in situ by Electrochemical Impedance Spectroscopy (EIS), both at the corrosion potential and under anodic polarization up to 0.5 V vs. the Reversible Hydrogen Electrode (RHE) [Betova et al, 2009]. Additional tests were performed in simulated PWR coolant with the addition of 0.01M Na2B4O7 to exclude the effect of pH excursions probably due to Zn hydrolysis reactions. The thickness and in-depth composition of the oxide films formed at open circuit and at 0.5 V vs. RHE in the investigated electrolytes have been estimated from XPS depth profiles. The kinetic and transport parameters characterizing the oxide layer growth have been estimated using a calculation procedure bathe mixed conduction model for oxide films. Successful simulations of both the EIS and XPS data have been obtained. The parameter estimates are discussed in terms of the effect of Zn on the oxide layers on stainless steel in PW

sed on

R conditions.

The ultimate goal of this work was to develop a deterministic model for activity incorporation in coolant circuits of LWRs and to increase the theoretical understanding of the oxide film build-up and breakdown, controlling the localized corrosion phenomena of structural materials in such reactors.


5-1(5-20)=


The predictions of the MCM (mixed-conduction model for oxide films) are for the first time fitted to both in situ EIS and ex situ XPS data for oxides on AISI 316L(NG) formed in simulated PWR water at 280 °C with or without Zn addition.

The electrolyte was simulated PWR coolant (1200 ppm B as H3BO3, 2.2 ppm Li as LiOH) with or without the addition of 1 ppm Zn as Zn(NO3)2 and/or 0.01M Na2B4O7 which was added tobuffer the electrolyte and prevent any pH excursions due to hydrolysis reactions of soluble Zn.

Figure 5-1 presents the XPS depth profiles for the films formed in this type of solution at open circuit with and without Zn injection.

In the presence of tetraborate, the thicknesses of the films in both Zn-free and Zn-containing electrolytes are more than two times larger than those formed in solutions that did not contain tetraborate. This effect could be related to the decrease of the solubility of Fe with increasing pH. Such an explanation is corroborated by the fact that outer layers (i.e. layers that are formed by dissolution–precipitation mechanism) are observed in both Zn-free and Zn-containing electrolytes and their thickness is larger than that of the inner layer.

A notable effect of Zn injection is that the Ni content of the inner layer formed as a result seems to be larger than that of Cr, which is opposite to the inner layer formed in the absence of Zn. Thus Zn incorporation seems to play an important role in there structuring of the inner layer also in tetraborate-containing PWR water.

Figure 5-1: XPS depth profiles of the oxides formed on AISI 316L(NG) in simulated PWR water+0.01M Na2B4O7 without (a) and with 1 ppm Zn (b) at 280 °C at open circuit.


5-2(5-20)=


Profiles of the atomic concentration of oxygen, as well as Ni, Cr, Fe and Zn contents normalized to the total metallic content of the film are presented on Figure 5-1.

The experimental and calculational results obtained by [Betova et al, 2009] have clearly indicated the significant effect of Zn injection in simulated high-temperature PWR water on the initial stages of the growth and restructuring of the oxide film on AISI 316L(NG). In unbuffered PWR water, a drastic decrease of the thickness of the oxide is observed after Zn injection. This is demonstrated to be due to both the change in the pH of the electrolyte as a reset of Zn hydrolysis reactions and the Zn adsorption on/incorporation in the oxide film, which leads to significant decrease of the rates of ion transport in the inner layer and the interfacial reactions at the inner layer/electrolyte interface. On the other hand, the effect of Zn on film growth and restructuring in buffered PWR water is much less dramatic, but still significant. This effect is demonstrated to be due to Zn incorporation in the inner layer.

Summarizing, Zn injection has a significant effect on the oxides formed for 72 h in both unbuffered and buffered PWR water, altering both the thickness and the in-depth composition of the oxide and its electrical and electrochemical properties as reflected by the impedance spectroscopic results.

Several studies have been carried out by [Laitinen et al, 1998], to evaluate the effects of different blocking agents to reduce the incorporation of radioactive species into oxide films growing in typical PWR environments. The results have shown that dissolved zinc in the range of 10 to 40 ppb reduces the pickup of Co-60 by a factor of 8 to 10. The aim of the current work was to clarify the long-term beneficial effects of zinc addition as well as to find out the possible changes in activity pickup behaviour when the zinc addition is interrupted.

The oxides formed on stainless steel samples exposed to simulated primary PWR coolant during three subsequent zinc injection periods were thin and dense, and the oxide thicknesses remained almost constant (see Figure 5-2). After the zinc dosing periods, the oxide films were found to be enriched with Cr when compared with the base metal composition, and incorporation of Zn was clearly detected (see Figure 5-3).

Incorporation of Co-60 into the oxide films was found to be low during the zinc injection periods. The elemental depth profiles in the oxide films were found to remain unchanged during the subsequent injection of Zn.

The activity incorporation into the stainless steel oxides started to increase as soon as zinc dosing to the coolant was stopped (Figure 5-2). However, the Co-60 concentration was lowest on the samples, which were first oxidized in Zn containing primary coolant. This could indicate that zinc in the oxide continued to block the adsorption sites for Co-60 incorporation even though Zn concentration in the coolant was lower than 1 ppb. After the zinc injection period the thicknesses of all oxide films increased, which partly explains also the increase in total activity incorporation.


5-3(5-20)=


Figure 5-2: Oxide thicknesses and normalised Co-60 pickup rates in the new SS samples during exposures to different test phases [Laitinen et al, 1998].

Figure 5-3: Profiles of alloying elements in the oxide on the new SS 304 sample after 3 zinc addition periods (over 300 days [Laitinen et al, 1998].


5-4(5-20)=


Final evaluation of the water chemistries (including zinc) tested at the reactor water loop was based on CPs measurements (soluble and insoluble) and activity measurement in the coolant and surface measurements on SG tube model.

The best results showed II. Experiment (Hydrogen) followed by III. Experiment (Elevated Ammonia) and IV. Experiment (Zinc) and I. Experiment (Standard Ammonia) at the end.

5.3 Surface activity differences between VVER and PWR

Russian VVERs differ significantly from Western designed PWRs. Most have only stainless steels and zirconium alloys in contact with the primary coolant and use a different primary water chemistry.

To review radiation situation in VVER plants, reviewed by [Garbett, 2002], comparing with situation in PWR is important to discuss what could be benefit for VVER operators to use zinc injection

Currently, there are two major types in operation, the 6-loop designs rated at 440 MWe (1375 MWth) and the later 4-loop designs rated at 1000 MWe (3000 MWth). Higher power VVER-1500 units are under development. VVERs differ from PWRs in that the only alloys used in the primary circuits are stainless steel alloys and Zr-1 %Nb fuel clad. The primary coolant chemistry is also different. Here, the principle differences are the addition of potassium hydroxide to create alkaline conditions, instead of lithium hydroxide, and the addition of ammonia to generate hydrogen by radiolysis, instead of adding dissolved hydrogen gas.

VVER stations have excellent CP behaviour, which produces low primary circuit radiation fields, low Co 58 and Co 60 surface activities, low circulating nickel and cobalt concentrations and generally low ORE. These are most probably linked to the absence of nickel alloys and cobalt containing hard facing alloys in the primary circuit.

It is difficult to make a true comparison with PWR channel head radiation fields. Because of the different geometry - the only is to compare it with SG collector radiation at VVER. On the Table 5-3 to Table 5-5 there are surface activities deposited at VVER and PWR units.

Compared with VVERs, fewer measurements of surface activities have been reported for PWRs. Table 5-5 gives the data available, expressed in the same units that are normally used for VVERs for comparison. If these data are compared with those in Table 5-3 and Table 5-4, it is clear that although surface activities vary widely, activities on Westinghouse PWR stainless steel surfaces are significantly higher than in VVERs, including VVER-1000 stations that operate at a similar primary coolant temperature. This is particularly the case for 58Co and 60Co, which are typically one to two orders of magnitude higher on PWR surface activity that results. The dominant role played by 60Co in PWRs is, absent.


5-16(5-20)=


Table 5-3: Surface activities deposited VVER-440 V213 units at Dukovany (up to about 1993) and Bohunice 1 (after 20 years) (4-16 and 4-49).

Nuclide

Dukovany SG hot collector

Dukovany SG cold collector

Bohunice 1

Hot leg Cold leg

kBq cm-2 % kBq cm-2 % kBq cm-2 % 58Co 5-52 45-78 3-36 20-66 19.7 9.2 60Co 1-33 4-18 1-16 4-25 35.7 27.3 54Mn 0.5-14 10-24 1.5-14 5-35 6.8 3.3 59Fe 0.3-2.2 2.2-5.6 0.1-2.2 1.2-12 0.9 0.5 51Cr 0.1-0.3 0.4-2 0.1-0.3 0.3-0.6 2.3 0.3

110mAg 0.01-0.7 0-4 0.5-24 1.4-33 4.9 5.1 124Sb 0.03-2.2 0.1-9.4 0.1-25 0.1-58 1.3 0.8 95Zr - - - - 1.3 0.3 95Nb - - - - 2.8 -

37 kBq cm-2 = 1 µCi cm-2

Table 5-4: Surface activities at the Russian VVER-440 V230 unit Kola from 1978 to 1980 and at two VVER-1000 units (4-58 and 4-59).

Unit Position Cycle Surface activity (kBq cm-2) 51Cr 58Co 54Mn 60Co 59Fe

Kola 2

SG hot collector 4 24.8 48.1 34.8 13.0 8.5 SG hot collector 5 14.8 37.0 13.0 11.8 10.7 SG hot collector 6 7.1 33.7 9.6 10.7 4.4 SG cold collector 4 8.9 13.3 10.0 3.4 1.7 SG cold collector 5 5.9 28.5 15.9 6.7 3.6 SG cold collector 6 1.4 12.6 10.0 5.2 1.3

MCP 4 8.5 16.7 22.2 8.5 0.2 MCP 5 5.2 38.5 20.7 10.7 10.7 MCP 6 15.9 70.3 47.4 23.3 23.3

Zaporozhe

SG2 hot collector - - 48.7 11.2 18.2 11.2 SG2 cold collector - - 38.5 11.7 14.9 4.2 SG3 hot collector - - 36.6 8.0 13.1 2.2 SG3 cold collector - - 27.4 5.7 10.7 2.0

MCP2 - 63.6 62.2 9.1 33.7 44.6

Khmelnitsky SG3 hot collector - 15.2 21.1 7.8 4.4 25.9 SG3 cold collector - 12.2 59.2 29.2 11.8 9.6

MCP - 19.6 6.4 2.6 4.6 1.6 37 kBq cm-2 = 1 µCi cm-2


5-17(5-20)=



5-18(5-20)=

Table 5-5: Surface activities measured by gamma-spectroscopy on stainless steel surfaces in PWRs (4-67 to 4-78).

Station Cycle Position Surface activity (kBq cm-2)

58Co 60Co 51Cr 54Mn 59Fe

EdF PWR 3 SG channel head hot side 700 370 - 59 11

SG channel head cold side 1184 630 - 52 11

Fessenheim 4 SG channel head hot side 890 550 - 74 22

SG channel head cold side 1260 700 - 100 44

EdF PWRs 1 to 9 Hot legs of various stations 200 to

2600 50 to1100 - -

Cold legs of various stations 400 to 3000

50 to1900 - - -

R E Ginna 4 Hot side SG manway insert 592 537 11 52 11

Cold side SG manway insert 374 392 26 30 7.4

Turkey point 3 2

Hot side SG manway inserts 1214 374 4.4 52 15

Cold side SG manway inserts 866 311 19 37 7.4

Crossover leg during shutdown 274 153 - 8 6

Indian point 2 8 Hot leg 237 659 - - -

Cold leg 444 1613 - - -

Trojan

3 (2.42 efpy) Average crossover legs

4477 3441 - 278 -

4 (2.98 efpy) 3478 2331 - 133 -

2 (1.56 efpy) Average SG hot and cold side manway inserts 4600 814 - 226 -

Beaver Valley 1

2 (1.69 efpy) Average crossover legs 1591 1103 - 155 -

~1 (0.77- 1.02 efpy)

Average SG hot and cold side manway inserts 463 1462 - 122 -

Salem 2 1 Crossover leg ~3885 ~370 - - -

Sequoyah 2 1

Crossover legs 6001 807 - - -

2 (1.83 efpy) 5883 1110 - - - 37 kBq cm-2 = 1 µCi cm-2. Although 51Cr will be present on all surfaces, it cannot easily measured in-situ and is only normally detected on SG manways after they have been removed. (a) Measured using self-shielded non-collimated detector. (b) 51Cr also present at 25 kBq cm-2.

In Table 5-6 CP concentrations for different reactors types are shown.


6 Zinc influence on radwaste production

6.1 BWR plants (Robert Cowan) During the initial evaluation of Zn injection in BWRs, a review of NRC records showed that 'natural zinc' plants were shipping significantly less curries of radwaste, and in particular, 4 to 5 times less 60Co curies [Marble, 1986]. The 65Zn isotope was present in the radwaste (from 6 to 30% of the curie content), but trading it for 60Co curies was concluded to be a net positive because of its shorter half life (276 days vs. 5.27 years for 60Co) and because of the lower energy of 65Zn radiation. From a shutdown radiation point of view, even if the only effect of zinc injection was trading 60Co in the corrosion films for equal amounts of 65Zn, the effect would be reduced dose rates. Release of 65Zn during shutdowns was not reported as an issue at 'natural zinc' plants and radwaste from water cleanup during outages indicated nothing unusual [Marble, 1989].

Actual experience with Zn injection showed that it reduced the level of 60Co in the reactor water because the 60Co was more tightly bound to the fuel deposits. Thus, the 60Co curies in radiation waste associated with water treatment decreased significantly, only partially offset by an increase in 65Zn curies. With the introduction of DZO as the Zn source, even this source of curies was eliminated.

Any effect of Zn is also 'confounded' because of the introduction of mHWC, NMCA and OLNC soon after Zn implementation at many BWRs.

The one possible negative effect that Zn injection could have on radwaste shipments is the increased release of 60Co and other radioisotopes during plant shutdown, as shown in Figure 3-50. The increased level of clean-up flow and resin usage to achieve desired re-fuel outage levels of isotopes in the cavity water should produce more curies (and possibly more volume) of waste. However, no quantification of this effect was found.

A search of the literature and a significant number of personal enquires of those technically active in the field by the author have revealed no additional sources of information on this subject. If Zn injection was causing any issues in the radwaste area, it would certainly have surfaced in the technical community. The authors hunch is that the use of Zn injection had a small positive impact on overall radwaste issues but the effect has been ignored.

6.2 PWR plants (Suat Odar) Zinc injection at PWR units results in significant increase of radiocobalt and CP concentrations in the reactor coolant. Therefore zinc addition has potential to affect adversely the liquid and/or solid radwaste production in the zinc addition PWR plants. However, no problems or difficulties have been reported neither by US nor by European PWR units with the performance of CVCS demineralizers, handling or storage of resins, or performance of other waste treatment systems that could be attributed to zinc addition. Potential areas of waste generation, where zinc addition might have impact are the following:

• Liquid waste effluents.

• Spend resins and filters in the CVCS.

• Handling and storage of treated radwaste.

• In the following possible impact of zinc addition in these areas is discussed.


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6.2.1 Liquid waste effluents

Since the addition of zinc constitutes a net put of water into reactor coolant, there is a comparable removal of coolant to compensate for the added zinc solution. However, the amount of zinc solution added is negligible to be considered compared to the amount of coolant that has to be removed for boron dilution to compensate the fuel burn-up.

The amount of coolant removed for dilution needs to be considered with respect to influence of zinc on liquid waste effluents. If plant has Boron Recycling System (BRS), this dilution coolant is directed to BRS where it is recycled. In this case, zinc addition doesn’t impact the liquid waste effluent of the plant.

If the plant has no BRS, the dilution coolant is usually discharged via a waste hold-up tank. In such cases, am impact of zinc addition on liquid waste effluent may occur depending of the zinc injection rate. For example, for the zinc injection demonstration program at Farley-2 during fuel cycle 10 an increase in liquid waste effluent of about 20% was reported, [EPRI, 2003]. During that time the zinc injection rate was relatively high with 15-20 gph (57-75 l/h). If injection rates were much lower, the additional waste processing load for discharges due to zinc injection would be much less, as confirmed by field experience. For example, Callaway plant, that had injection rate of about 0.016 gph (1 ml/min), has reported that the additional liquid waste effluent processing due to zinc injection at their plant was not discernible.

EdF has compared the analyses of the liquid waste effluents before and after starting the zinc injection at Bugey-2 and 4 (see Figure 6-1). Based on the results of their surveillance they have confirmed that the increase in radiocobalt concentrations in the reactor coolant during power operation and plant shutdown does not necessarily cause an increase in the plant liquid waste effluents if the correct and efficient operation of the plant purification systems is ensured, [Tigeras et al, 2008b].


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Figure 6-1: 58Co and 60Co contribution to liquid waste effluent at EdF Bugey plant [Tigeras et al, 2008b].

The more recent information published by EdF confirms that three cycles of zinc addition at Bugey plant didn’t cause any significant changes even in the detailed isotopic distribution in the liquid waste effluent [Tigeras et al, 2010], (see Figure 6-2).

Based on this field experience, it can be concluded that the liquid waste effluent releases remain manageable with the addition of zinc into reactor coolant. It should further be mentioned that other plant operation events such as shutdown, plant trip, fuel defects, etc could impact the reactor coolant activity and pollution that exceed the effect of zinc injection by several orders of magnitude.


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7 Executive summary and recommendations

7.1 BWR plants (Robert Cowan)

7.1.1 Summary

The effect of Zn on shutdown dose rates was discovered because of the beneficial effect it had on shut down dose rate of plants with Cu/Zn alloy condensers. Further investigations showed that at very low concentrations (5 to 10 μg/kg) in water at reactor temperatures, it had a very pronounced effect on the structure and composition of corrosion films and fuel deposits. Under the oxidizing conditions of normal BWR water chemistry, soluble Zn in the water promotes the formation of Zn containing spinels with a range in composition from MFe2O4 to ZnFe2O4, whereM is composed of various percentages of the transition metals Fe, Zn, Co, Ni, etc. By substituting for Co in the spinels formed as an integral part of stainless steel corrosion films, Zn reduces the concentration of

m

O4.

60Co found in the spinel structure (both regular and inverse) of these films, such as Fe3O4, γ-Fe2O3 and NiFe2O4. Under laboratory BWR conditions, 5-10 μg/kg Zn in the waterresults in a much thinner, more protective film. Laboratory tests also showed that 5-10 μg/kg zinc was very effective in reducing the amount of incorporation of 60Co into the corrosion films of stainless steels and even more effective under the reducing conditions of HWC when the ECP was near or at the H2 redox potential.

Testing in actual BWRs showed that Zn injection was effective in lowering both the reactor water 60Co and the BRAC shut down doe rates. Adoption of Zn injection was slow because the use of natural Zn added significant 65Zn to the reactor water during shutdown/refuelling operations and it also contributed to the BRAC does rates, especially in the higher feedwater iron plants. This drawback to Zn injection was solved technically by the introduction of 64Zn isotope depleted ZnO (DZO) as a Zn source, but adoption of DZO was slow because of its relatively high cost to benefit ratio.

With the advent of HWC, the injection of Zn was found to offset the very high shutdown dose rates that accompanied HWC, and the price of DZO came down enough to make its use very cost beneficial. The use of higher levels of feedwater hydrogen injection or alternatively the use of noble metal technologies such as NMCA and OLNC made the use of Zn addition almost mandatory to control shutdown dose rates. Today, all of the BWRs worldwide that employ HWC to protect their internals (which lowers the ECP of the recirculation piping to the H2 redox potential) inject Zn (as DZO), except for the 4 BWRs in Taiwan. This practice has resulted in significantly lowered BRAC shutdown dose rates and the added 2-4 times reduction in IGSCC CGRs that 5-10 μg/kg Zn affords is a bonus for these BWRs.

Zn addition affects the structure of the fuel deposits as well as the structure of the corrosion films. In a BWR with NWC and significant feedwater iron, the fuel deposit is composed primarily of hematite (α-Fe2O3). Co is adsorbed to the surface of the hematite and is the source of 60Co. The 60Co concentration in the reactor water is set by the steady state equilibrium between the water and the 60Co adsorbed on the hematite. When Zn addition is initiated in such a case, there is a significant reduction in reactor water 60Co. This is due to the formation of ZnFe2O4 deposits, which incorporate the Co (and thus 60Co) into its spinel crystal structure. With enough Zn addition, there will be enough ZnFe2O4 present in the deposit (and much less α-Fe2O3) so that the steady state 60Co in the reactor water is now controlled by the steady state equilibriuconcentration between Co in the water and the Co dissolved in the ZnFe2O4 spinel. Theoretically, the reactor water 60Co would decrease with Zn injection until the fuel deposit was 100% spinel. This situation would occur when the stoichiometric ratio of feedwater Fe to other feedwater transition metals (Zn + Ni) was equal to 2. Under a stoichiometric ratio of feedwater Fe/(Zn + Ni) < 2, a different effect can occur, as all of the deposit is already in the MFe2O4 form. In this case, Zn injection would result in a small to moderate increase in reactor water 60Co, as the resulting mixed deposit of ZnFe2O4, NiFe2O4 and NiO will have a higher percentage of ZnFe2O4, which has a similar or greater 60Co equilibrium with the reactor water than NiFe2


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Under mHWC conditions of hydrogen + NMCA (or OLNC), the reducing environment on the fuel rod where most of the crud mass is deposited (lower third of the rod) results in a complete conversion of the deposit to a spinel form. In a plant with a stoichiometric ratio of feedwater Fe/(Zn + Ni) > 2, the deposit is composed of ZnFe2O4 + Fe3O4. In a plant with a stoichiometricratio of feedwater Fe/(Zn + Ni) < 2, the deposit is ZnFe

2O4, NiFe2O4 and NiO. The net result in

the iron rich case is a significant increase in 60Co in the reactor water as all of the pre-HWC α-Fe2O3 is converted to a spinel form.

The nature of the fuel deposit formed under mHWC conditions or hydrogen + NMCA (or OLNC) is quite adherent and dense layer with a possibility of spalling if the thickness is too great. Fuel specialists worry that this condition might cause local steam blanketing and overheating as the deposit "cracks away" but is held in place by the loose crud layer. This condition could cause increased and possibly 'run away' corrosion, leading to local cladding failure. There have been no reported fuel failures by this cause (and hot cell exams show no increased corrosion under the spalling areas examined to date), but prudence has suggested limiting the feedwater Zn injection to under 0.4 μg/kg average per cycle to eliminate any chance of crud spalling in NMCA and OLNC plants. mHWC plants are limited to 0.6 μg/kg cycle averages.

In conclusion, Zn injection using DZO is a practical way to deal with undesirable shut down dose rates and should be considered mandatory for those plant using mHWC or hydrogen + (NMCA or OLNC) to protect reactor internals.

7.1.2 BWR recommendations

1) Zn injection using DZO for shut down dose rate control should be considered mandatory for those plant using mHWC or hydrogen + (NMCA or OLNC) to protect reactor internals.

2) BWRs should implement Zn injection well before implementing HWC or hydrogen + (NMCA or OLNC). A full fuel cycle of Zn operation is optimum but a 6 months period might be satisfactory if the reactor water soluble Zn concentration reaches a steady state level of 5 to 10 μg/kg.

3) Potential fuel corrosion concerns suggest limiting the feedwater Zn injection to under a fuel cycle average of 0.4 μg/kg to eliminate any chance of crud spalling in NMCA and OLNC plants. The available data also suggests that operating one quarter a cycle at feedwater Zn injection rate of 0.5 μg/kg is benign if the entire cycle average is < 0.4 μg/kg. mHWC plants should limit their cycle average feedwater Zn to < 0.6 μg/kg.

4) The Co-60(s)/Zn(s) ratio should be kept below 2.0 x 10-5 μCi/ml per μg/kg during the first fuel cycle after mHWC or hydrogen + (NMCA or OLNC). The most critical time period to try to attain the ratio as near this value is during the first several months after the ECP on the piping surfaces are driven to the -500 mV(SHE) range, as this is when the major portion of the oxide restructuring occurs.

5) For long term operation, the reactor water Zn should be kept in a steady state concentration range of 5 to 10 μg/kg.

6) Only Zn depleted in the 64Zn isotope should be used for BWR shutdown dose rate control. The ZnO oxide form of 64Zn isotope depleted Zn, DZO, is currently used by all BWR operators.

7) The usage of Zn depleted in the 64Zn isotope can be limited by establishing the feedwater iron input at the lowest level practical.

8) There is significant evidence that soluble Zn in the 5 to 10 ppb range can reduce the CGR of stainless steels and nickel base alloys. When making crack growth projections using data generated in Zn free water, this effect should add extra margin to the projections. Work is underway to further quantify this effect.


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9) The best source of engineering details for implementing, monitoring and evaluating Zn injection for BWRs can be found in the EPRI Water Chemistry Guidelines [BWRVIP-190, 2008].

10) EPRI has formed a “BWR Zn Owners Group” and membership should provide an excellent forum to remain current on all of the topics discussed in this Chapter.

11) There are additional options available for reducing the 60Co concentration in reactor water such as cobalt source term reduction, optimized reactor water cleanup operation, the potential for BWR fuel cleaning, and others. These techniques should be explored to be used in conjunction with Zn injection to further minimize shutdown dose rates.

7.2 PWR plants (Suat Odar)

7.2.1 Summary

In this Section summary of the Chapter 4 of this report is given that comprises the most relevant information:

Zinc is injected in PWR plants since sixteen years with the objective to reduce the dose rates and/or to mitigate PWSCC of Alloy 600MA. Whereas zinc is injected at target concentrations of about 5 μg/kg for reduction of dose rates, the selected target concentration is in the range of 15–40 μg/kg for mitigation of PWSCC. In both cases, zinc injected into reactor coolant incorporates into oxide films on the RCS surfaces and improves their protective behaviour. This improvement of oxide films leads to reduction of radiation fields and can inhibit the occurrence of PWSCC of Alloy 600MA. The mechanism how the zinc injection works and influence positively the dose rate evolution, material compatibility of structural materials, PWSCC mitigation of alloy 600MA and fuel performance can be summarized as follows:

Dose rate reduction:

The worldwide most accepted model that explains how zinc injection into reactor coolant can fulfil the above mentioned objectives is the “Site Preference Energy” model. According to this model, zinc has high incorporation affinity for the tetrahedral sites of the spinel oxide structure. Its site preference energy is the highest among all other cations that are relevant for the PWR reactor coolant. Hence, zinc can displace these cations that occupy tetrahedral spinel sites, such as iron, nickel from the oxide films. Whereas zinc has the highest site preference energy for tetrahedral sites, chromium has the highest site preference energy for the octahedral sites. Therefore, zinc is incorporated preferentially into inner oxide layers, which mainly consists of chromites, and builds very stable oxide films that improves the protective behaviour of the passive layers. This effect of the zinc addition results in reduced metal release and corrosion rates of the RCS structural materials, which leads to formation of thinner oxide layers due to reduction of source term. In addition, once the tetrahedral sites are occupied by zinc, this incorporated zinc inhibits the further incorporation of fresh radiocobalt into oxide layers; because its site preference energy for tetrahedral sites is higher than that of cobalt. These two behaviours of zinc, displacement of radiocobalt from the oxide layers and inhibition of fresh radiocobalt into oxide layers, are the mechanisms how zinc addition results in reduction of radiation fields.


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Documents

Z BWR - A.N.T. International...4.3.4.2 RCP seals 4-51 4.3.4.3 RCS valve hard facings 4-51 4.3.4.4 Control rod drive mechanism (CRDM) 4-52 4.3.5 PWSCC of Alloy 600 MA 4-52 4.3.5.1 Laboratory