Development of fission product chemistry database ECUME

J-STAGE Advance Publication date : 9 January, 2020Paper No.19-00537

© 2020 The Japan Society of Mechanical Engineers[DOI: 10.1299/mej.19-00537]

Vol.7, No.3, 2020Bulletin of the JSME

Mechanical Engineering Journal

1. Introduction

Improvement of source term under LWR severe accidents (SAs) is of crucial importance both for the continuous

enhancement of LWR safety (Morris, 2007; Haste, 2009; Klein-Heßling, 2014; Suehiro, 2015) and the decommissioning of Fukushima Daiichi Nuclear Power Station (1F) (Nuclear Damage Compensation and Decommissioning Facilitation Corporation, 2017). One main tool for the evaluation of source term has been and will be SA analysis codes such as MELCOR (Gauntt, 2001), MAAP (EPRI, 1994), SAMPSON (Ujita, 1999), THALES-2 (Kajimoto, 1991). The SA analysis codes can estimate the release of fission products (FPs) into environment and distribution of FPs in a reactor based on the models of thermal-hydraulics and FP behaviors at each region in a reactor, from reactor pressure vessel (RPV) to containment vessel (CV) and reactor building (RB) through reactor cooling system (RCS). Thus, the improvement of FP models is one of the key issues for the improved source term (IAEA, 2008). The FP models have been improved based on results of the researches on various FP behaviors (Soffer, 1995; Clément, 2007; Allelein, 2009). The main issues for the improvement are FP chemistry (Haste, 2009; Miwa 2015) and treatment of fine space resolution in a reactor.

As for the latter issue of fine space resolution, it is especially important for the decommissioning of 1F since a detailed and fine FP distribution is required (Nuclear Damage Compensation and Decommissioning Facilitation

Development of fission product chemistry database ECUME for the LWR severe accident

Shuhei MIWA*, Kunihisa NAKAJIMA*, Naoya MIYAHARA*, Shunichiro NISHIOKA*, Eriko SUZUKI*, Naoki HORIGUCHI*, Jiazhan LIU*, Faoulat MIRADJI*, Junpei IMOTO*, Afiqa MOHAMAD*,

Gaku TAKASE*, Hidetoshi KARASAWA* and Masahiko OSAKA*

Abstract We extended the first version of fission product (FP) chemistry database named ECUME (Effective Chemistry database of fission products Under Multiphase rEaction). The extended ECUME consists of three kinds of datasets: CRK (dataset for Chemical Reaction Kinetics), EM (Elemental Model set) and TD (ThermoDynamic dataset). The present ECUME is equipped with the CRK for the reaction of Cs-I-B-Mo-O-H system and Ru-N-O-H system in gas phase, the EM for the Cs chemical reaction with stainless steel (SS) (Cs chemisorption onto SS) and the TD for CsBO2 vapor species and solid Cs2Si4O9 and CsFeSiO4. A FP chemical reaction calculation in gas phase with the CRK of Cs-I-B-Mo-O-H system has shown the necessity of consideration of chemical reaction kinetics for more accurate estimation of Cs and I release amount into environment. The EM for Cs chemisorption has successfully achieved more accurate estimation of Cs distribution in a reactor by reproducing the effects of CsOH vapor concentration in gas phase and Si content in SS which were not considered by the existing model. The high quality vapor pressure data for CsBO2 vapor were evaluated based on the result of a high temperature mass spectrometry. Cesium species at high temperature can be estimated by the thermodynamic data with high reliability. Thermodynamic data for solid Cs2Si4O9 and CsFeSiO4 were successfully evaluated by the experiment and ab-initio based methodology, respectively. These results have shown the validity and importance of the ECUME application for the more accurate evaluation of FP chemistry during transportation in a reactor under a LWR severe accident.

Keywords : Fission product, Chemistry, Database, Cesium, Chemisorption, Thermodynamic

* Nuclear Science and Engineering Center, Japan Atomic Energy Agency 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan

E-mail: [email protected]

Received: 28 October 2019; Revised: 11 December 2019; Accepted: 25 December 2019

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2© 2020 The Japan Society of Mechanical Engineers

Miwa, Nakajima, Miyahara, Nishioka, Suzuki, Horiguchi, Liu, Miradji, Imoto, Mohamad, Takase, Karasawa and Osaka,Mechanical Engineering Journal, Vol.7, No.3 (2020)

[DOI: 10.1299/mej.19-00537]

Corporation, 2017). As the present SA analysis code treats coarse space mesh with homogenized geometry of gas phase region and structure (Gauntt, 2001; EPRI, 1994; Ujita, 1999; Kajimoto, 1991), localization and enrichment of the FPs into a specific geometrical structure are hardly evaluated. Therefore, the localization and enrichment of FPs should be treated as a risk. An advanced geometrical treatment will be required for the evaluation of localization and enrichment of FPs in narrow space like as a thin gap between structures.

Regarding the FP chemistry issue, it has been recognized as the most effective and promising way for the overall improvement of source term (Haste, 2009; Miwa 2015). This is because the FP chemistry significantly affects all the FP release and transport behaviors. Therefore, various researches on FP chemistry aiming at the improvement of models have long been conducted since the TMI-2 SA, and are still in progress (IRSN and CEA, 2007; Osaka, 2017). The key point for the FP chemistry research is to prepare comprehensive but rational FP chemistry database that cover all important chemical features of the possible LWR SA sequences. The FP chemical models could be constructed for covering a representative chemical condition at SAs based on such a comprehensive and rational FP chemistry database. However, such database have hardly been available so far.

In response to the FP chemistry issues mentioned above, we have been conducting a fundamental study on FP chemistry in order to construct a comprehensive and rational FP chemistry database (Osaka, 2016; Osaka, 2017). The role of database in the framework of our FP chemistry study is shown in Figure 1. Our FP chemistry database was named as ECUME (Effective Chemistry database of fission products Under Multiphase rEaction) (Miyahara, 2018). The outputs based on the ECUME are not only FP chemical models but also basic knowledge and data obtained through the chemical analyses by using a 3D-CFD analysis code with fine space resolution. These outputs will be reflected to the improved source term for both the enhancement of LWR safety after the 1F SA and the 1F decommissioning work. This study covers important chemistry-related phenomena revealed after the 1F SA. Namely, boron (B) chemical effect (Miwa, 2016; Nakajima, 2017), chemical interaction between FP vapor/aerosol and structural material (Di Lemma, 2016; Di Lemma, 2017), solubility of FP deposit compounds for the leaching behavior and so on. The aerosol behavior is also included because it is also affected by the chemical properties such as condensation and hygroscopic properties.

In this paper, the extended ECUME from the first version (Miyahara, 2018) is presented, together with several examples of effects of ECUME application on the evaluation of FP chemistry. The ECUME could provide useful datasets and models to the commercially/freely available calculation tools such as SA analysis codes for more accurate estimation of the FP chemistry.

Fig. 1 The role of ECUME in the framework of the FP chemistry study

2. FP chemistry database ECUME

The extended ECUME consists of three kinds of datasets: CRK (dataset for Chemical Reaction Kinetics), EM

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(Elemental Model set) and TD (ThermoDynamic dataset). Table 1 shows the contents of extended ECUME. The focal phenomena and behaviors are chosen on the basis of importance and priority in terms of both the enhancement of LWR safety after the 1F SA and the 1F decommissioning work.

2.1 Chemical system

The cesium (Cs) and iodine (I) are most important FPs because large amounts of Cs and I are released into

environment at a SA, which significantly affects the public exposure. In particular, Cs is important for the evaluation of dose distribution in 1F (Nuclear Damage Compensation and Decommissioning Facilitation Corporation, 2017). Thus, the main datasets and models are for the chemical system of Cs and I. Since the BWR control and FP materials, boron (B) and molybdenum (Mo) respectively, have high chemical affinities and thus affects the chemistry of Cs and I, they are also included (Miwa, 2015; Miwa, 2016). In addition, as Cs can form compounds with iron (Fe) and silicon (Si) in the chemical reaction of Cs with stainless steel (SS) (hereafter, Cs chemisorption onto SS) (Di Lemma, 2016; Di Lemma, 2017), they are also considered.

Besides, since ruthenium (Ru) has potentially high radiological impacts, it was also included in the ECUME. This is because the Ru release can significantly increase in the air-ingress scenario of PWR-SA and reprocessing plant accident (Powers, 1994). In addition, significant amount of strontium (Sr) was released to outside of the 1F RB through the aqueous phase (Koma, 2017). There is possibility that Sr release increases in a likely chemical condition of 1F, such as co-existence of chlorine (Cl) by sea water injection (Kurata, 2013).

Consequently, chemical systems of Cs-I-Mo-B-Fe-Si-O-H, Ru-O-N-H and Sr-Cl-O are treated in the ECUME.

2.2 Phenomena and behaviors

The phenomena and behaviors covered in the ECUME are not only chemical reaction in gas phase but also that in

solid-gas phase. The main targets for solid-gas phase reaction are chemical interactions of B with deposits of Cs and I, and Cs chemisorption onto SS and re-vaporization. The chemical interaction of B with deposits of Cs and I can cause a re-vaporization of gaseous iodine and Cs vapor species (hereafter, B-driven Cs and I re-vaporization) (Sato, 2015). This interaction should be important in terms of a late phase environmental release. The Cs chemisorption and re-vaporization should affect Cs chemical speciation during transportation and therefore Cs distribution in RPV of 1F

Table. 1 Contents of FP chemistry database ECUME

Underline; Equipped in present version of ECUME, TBD: to be determined Phenomena / behaviors

(chemical system)

ECUME-CRK: Chemical Reaction Kinetics data

ECUME-EM: Elemental Model

ECUME-TD: Thermo-Dynamic data

Reaction Model Species

Chemical reaction in gas phase

Changes in speciation (Cs-I-Mo-B-O-H system)

More than 200 elementary reactions ―

CsBO2, Cs2B4O7(s)、CsB3O5(s)

Changes in speciation (Ru-N-O-H system)

More than 100 elementary reactions

RuO(g), RuO2(g), RuO3(g), RuO4(g)

Chemical reaction for solid-gas phase

Cs and I re-vaporization by B vapor species (Cs-I-B-O-H system)

CsI(s) + HBO2(g,s) → CsBO2(s) + HI(g)

nth order reaction model with reaction kinetics constant composed of Arrhenius equation, or Mass transfer model with partial pressure of gas species

HBO2

Cs chemisorption on to SS and re-vaporization (Cs-Fe-Si-O-H system)

CsOH(g) + (Fe, Si) → Cs-Si-Fe-O compound → Cs compound (g)

Cs2Si2O5(s,l), Cs2Si4O9(s,l), CsFeSiO4(s), CsFeSi2O6(s)

Sr chemisorption (Sr-Cl-O system) ― TBD (same as Cs

chemisorption) ―

Aerosol behavior

Detailed Aerosol deposition behavior (Cs-I-O-H system)

―

TBD (Aerosol deposition model on the crack and so on)

― Aerosol removal by droplets (filtered vent system, etc) (Cs-I-O-H system)

Aerosol removal model based on that proposed by M. Ali (Ali, 2013)

Resuspension of aerosol (Cs-I-O-H system)

The model using critical friction velocity

Aqueous phase behavior

Leaching of Cs compounds (Cs-I-Mo-B-Fe-Si-O system)

Cs compound (s) → Cs or Cs complex ions

Nernst-Noyes-Whitney model

CsI(s), Cs2MoO4(s), Cs-B-O compound (s), Cs-Fe-Si-O compound (s)

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(Di Lemma, 2016; Di Lemma, 2017). This is important point for the decommissioning work and debris removal work of 1F.

The FP aerosol behavior is also included in the ECUME. The aerosol behavior in RB, specifically interaction of aerosol and gas with water droplet, and resuspension behavior, were considered to be important for the decrease of uncertainty in evaluation of source term. Models for these behaviors in the present SA analysis code need to be improved.

The long-term FP migration behavior through the aqueous phase becomes important for the 1F decommissioning. The migration of dissolved FPs from fuel debris or deposit compounds through the aqueous phase may cause the secondary contamination of structure materials such as concrete and SS for many years after the SA. Such migration and contamination would affect the debris removal and waste disposal works. 2.3 Dataset and model

Each dataset and model in ECUME is designed to be able to be implemented to the present SA analysis code

directly or easily. A brief description of each CRK, EM and TD in ECUME is given below.

2.3.1 Dataset for chemical reaction kinetics (CRK) The CRK consists of main chemical reactions and their effective chemical reaction kinetic constants. As described

in the sub-section 2.1, the reaction mode will be not only the gas phase reaction of Cs-I-B-Mo-O-H system but also the gas-solid phase reaction of B-driven Cs and I re-vaporization and the Cs chemisorption onto SS. The Ru-N-O-H system in the gas phase reaction was also incorporated to the ECUME. The CRK can be applied to the mechanistic codes such as SOPHAEROS module in ASTEC code (Cantrel, 2014), which can evaluate the FP chemistry with the reaction kinetics.

The CRK in gas phase was based on either the literature data or, if the literature data were not available, calculation results using a combinational method of ab-initio calculation with statistical physics (Miyahara, 2018). By comparing the species calculated by using the CRK for a time long enough to regard it as the chemical equilibrium condition to those obtained by chemical equilibrium calculations, it was confirmed that there was no missing of major reaction paths in the dataset (Miyahara, 2018). The CRK in gas-solid phase will be obtained by separate-effect experiments reproducing FP behavior with various chemical conditions as parameters, as it is difficult to obtain the chemical reaction kinetic constants in the same way as those in gas phase. In near future, the CRK will consist of effective chemical reaction kinetic constants for chemical reactions including not only gas phase but also solid-gas phase for several representative SA sequences.

2.3.2 Elemental model set (EM)

The EM consists of improved models from those incorporated in the present SA analysis codes. Thus, most of EM

can be directly incorporated into the present SA analysis codes. The following models are being improved based on fundamental experiments: the B-driven Cs and I re-vaporization, the Cs chemisorption onto SS and re-vaporization from chemisorbed compounds, the aerosol behavior and solubility of FP compounds.

The EM is established based on separate-effect experiments reproducing FP behaviors with various chemical and thermo-hydraulic conditions as parameters. The model coefficients are determined by fitting the model equations, which are determined by considering major factors for the FP behaviors, to the experimental results.

2.3.3 Thermo-dynamic dataset (TD)

The TD consists of thermodynamic data, namely, heat capacity, Cp°, entropy, S°, Gibbs energy

function,-{G°-H°(298.15 K)}/T, enthalpy increment, H°-H°(298.15 K), enthalpy of formation, ∆fH°, and Gibbs energy of formation, ∆fG° of compounds. The TD will basically be utilized for the improvement of the CRK and EM. The thermodynamic data of new compounds observed in the experiments will be derived from the experiments such as high temperature Knudsen Effusion Mass-Spectrometry (KEMS) and will be estimated by an empirical method, or an

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ab-initio calculation if experimental data are not available. Recommended values and uncertainties of thermodynamic data in TD were determined from the second- and

third-law analyses (Chase, 1998) if the Gibbs energy functions for the reactants and the products were available. Uncertainty of the thermodynamic data was evaluated from the difference between the second- and the third-law values of the reaction enthalpy, and the thermodynamic dataset having the smallest difference was selected as the recommended one.

2.4 Validation of ECUME

The ECUME will be validated by the analysis of FP release and transport experiments using the following facility

and analysis tool (Miyahara, 2017; Osaka, 2017). The experiments have been conducted by using the experimental setup reproducing the FP release and transport called TeRRa (Test bench for FP Release and tRansport). The TeRRa covers temperature range representing those from core to PCV conditions during a SA. The airborne aerosols can be extracted at each 100 K point of the downstream region representing RCS. Thus, the detailed and consecutive data on the FP chemistry during transportation can be obtained by the TeRRa experiments. The analysis on the TeRRa results will be conducted by using a 3D-CFD analysis tool called CHASER (CFD analysis tool for FP cHemistry And aeroSol bEhavioR) developed in JAEA. The CHASER incorporates the detailed models for 3-D fluid dynamics, chemical reaction kinetics, aerosol movement tracking, and some additional models to calculate the FP chemical reactions during release and transport. Such setup and analysis tool having a wide application range and fine space resolution are suitable for the validation of the comprehensive and rational chemistry database ECUME.

3. Effects of ECUME application on FP Chemistry

The present ECUME is equipped with the CRK for the reaction in Cs-I-B-Mo-O-H system and Ru-N-O-H system

in gas phase, the EM for Cs chemisorption onto SS, and the TD for CsBO2 vapor and solid Cs2Si4O9 and CsFeSiO4, as shown in the underlined parts of Table 1. This section describes the effects of ECUME application on FP chemistry.

3.1 Chemical speciation in Cs-I-B-Mo-O-H system in gas phase using CRK

The present CRK is for the reaction of Cs-I-B-Mo-O-H system and Ru-N-O-H system in gas phase. In order to

estimate the effects of the CRK on the FP chemistry, the Cs vapor species of Cs-I-B-Mo-O-H system during transportation in gas phase were calculated by using the CRK. The results were compared with those calculated by chemical equilibrium calculation which is applied to the present SA analysis codes. The chemical reaction calculation in a simple stream line with a linear temperature gradient was carried out using the chemical kinetics simulation software CHEMKIN-PRO 15151 (ANSYS, 2016) with the CRK. Table 2 shows the calculation conditions based on the data for 1F (Nishihara, 2012; Hoshi, 2012). The steady-state flow condition was assumed. FP compounds, namely CsI, I2, CsOH, B2O3 and MoO3 were assumed to flow from the high temperature side together with the carrier gas species of H2O into the lower temperature region of a reactor through RCS in 0.5 s. The chemical equilibrium calculations were carried out by using Thermo-Calc software (Sundman, 1985) with SGTE substance thermodynamic database (Dinsdale, 1991) under the same conditions as shown in Table 2.

Figure 2 shows the calculated mole fractions of the representative Cs and I vapor species at each temperature. The Cs and I vapor speciation calculated with the CRK agreed well with the equilibrium calculations above 1700 K. On the other hand, the discrepancies with the equilibrium calculations for CsBO2, Cs2MoO4 and I vapors appeared in lower temperature regions of below 1700 K. This result shows that the Cs and I vapor species were transported toward the lower temperature region before reaching to the chemical equilibrium. In this condition, it should be noted that the present SA analysis codes with chemical equilibrium calculation can underestimate the release amount of gaseous iodine (monomolecular of I) into environment by about one thousandth. Thus, although the CRK needs further improvement for various chemical conditions, the CRK could provide more accurate chemistry in an SA condition.

In addition, the CRK provides new finding. The temperature where the equilibrium assumption breaks for Cs-I-O-H system was estimated to be about 1,100 K (Cantrel, 2013). The result shows that the break temperature for Cs-I-B-Mo-O-H system obtained was higher than that for Cs-I-O-H system (Cantrel, 2013) due to the formation of

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CsBO2 and Cs2MoO4 vapors. Although the CRK can directly contribute to the improvement of the mechanistic code, such finding as the break temperature should become a useful information also for the improvement of SA analysis codes with no consideration of the reaction kinetics.

3.2 Improvement of Cs chemisorption model in EM

The present EM includes the Cs chemisorption model. The existing Cs chemisorption model (Bowsher, 1990) was

constructed based on limited experimental data and has a simple treatment of chemistry. Therefore, as experiments reproducing Cs chemisorption onto SS type-304 (SS304), reaction tests of CsOH vapor with SS304 have been conducted to investigate the effects of such chemical factors on the Cs chemisorption behavior as atmosphere, minor elements in SS, CsOH vapor concentration (Nishioka, 2019). The conditions and parameters are shown in Table 3. The surface reaction rate constant, vd, that is used in the existing Cs chemisorption models (Elrick, 1984; Bowsher, 1990) was obtained by fitting the equation to the test results. The vd can be calculated by using Eq. (1).

𝑁𝑁 = vd𝐶𝐶g (1)

where N is the Cs chemisorption amount per unit time unit area (μg Cs/cm2/s) and Cg is the concentration of CsOH vapor in the gas phase (μg/cm3). From the analysis of test results, it was found that the vd was influenced by not only temperature, as already known (Elrick, 1984; Bowsher, 1990), but also the concentration of CsOH vapor in the gas phase and Si content in SS304 (Di Lemma, 2016; Nishioka, 2019).

A Cs chemisorption model was improved by using available experimental data based on the penetration theory for gas-liquid mass transfer with chemical reaction (Matsuyama, 1950; Fujita, 1954). The finally obtained model equation, in which vd was expressed as a function of temperature, T (K), the concentration of CsOH vapor in the gas phase, Cg (µg/cm3) and Si content in SS304, CB, (wt.%) was determined as Eq. (2) (Nakajima, 2019).

vd = 7.027�CBCg−0.5225exp �− 6552

T� (2)

In order to estimate the effects of the EM on the FP chemistry, the uncertainties of existing model incorporated to

the present SA analysis code were estimated. Figure 3 shows dependence of relative Cs chemisorption amount calculated by the existing model to that by improved model on CsOH concentration in gas phase (Bowsher, 1990; Nakajima, 2019). The result indicates that there is possibility that the existing model under/overestimates Cs chemisorption amount onto SS up to ten times according to CsOH concentration in gas phase. Thus, it can be said that

Fig. 2 Calculated mole fractions of the representative Cs and I

vapor species (solid line: CRK, marker: chemical equilibrium calculation).

Table. 2 Conditions of the chemical reaction calculation for Cs-I-B-Mo-O-H system Conditions Flow model Tube geometry Inlet flow rate Temperature Pressure Inlet amount

H2O CsI I2 CsOH B2O3 MoO3

Steady-state plug flow model Diameter: 720 mm Length: 10 m 5.3×105 NL/min 2500 – 700 K (linear gradient) 0.4 MPa 2.9×105 mol 8.8×101 mol 2.5×100 mol 1.0×103 mol

5.0×103 mol

1.8×102 mol

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the Cs distribution in various chemical condition in 1F accident can be estimated with higher reliability by implementing improved model to the present SA analysis code.

3.3 Thermodynamic data of Cs compounds in TD

The present TD consists of the thermodynamic data for cesium metaborate vapor, CsBO2(g), solid cesium silicate,

Cs2Si4O9(s), and iron-containing cesium silicate, CsFeSiO4(s). Previous Cs-chemisorption tests indicated that these cesium silicates were suggested or identified as major compounds (Elrick, 1984; Di Lemma, 2016; Di Lemma, 2017).

Thermodynamic data for CsBO2(g) was evaluated based on the vapor pressure measurement results (Nakajima, 2016; Nakajima, 2017). According to the compilation on thermodynamic data for reactor materials and compounds of fission product elements (Cordfunke, 1990), uncertainties of the thermodynamic data have been marked as “A”, “B”, “C” and “D”, in which “A” means a good quality (generally better than 1% of uncertainty), “B” medium quality (1 - 5% of uncertainty), “C” poor (5 - 10% of uncertainty) and “D” is mainly based on estimates. The thermodynamic data of CsBO2(g) is judged to be “C” (poor quality). However, regarding the Gibbs energy function of CsBO2(g), there was only about a 0.1% difference between the values given by the compilation (Cordfunke, 1990) and derived from the latest experimental data on molecular constants of CsBO2 (Ezhov, 2000), in spite of a maximum discrepancy of approximately 20% among all the molecular constants (Nakajima, 2017). Therefore, based on the above mentioned fact, uncertainty of the Gibbs energy function of CsBO2(g) should be marked as “A”. On the other hand, there were differences greater than 5 % between the second law- and the third law-values of sublimation enthalpy of CsBO2 at 298.15 K before our study. Thus, equilibrium vapor pressure measurement of CsBO2 was carried out by using a KEMS to obtain a reliable thermodynamic data of CsBO2(g). As the result, difference between the second law- and the third law-values of sublimation enthalpy of CsBO2 was reduced up to 0.6 % (Nakajima, 2016) and uncertainty of the thermodynamic data of CsBO2(g) can be marked as “A”.

Thermodynamic data for Cs2Si4O9(s) had been evaluated based on the experimental H°-H°(298.15 K) and ∆fH°(298.15 K) and the estimated S°(298.15 K) (Ball, 1992). Thus, low temperature heat capacity measurement of Cs2Si4O9(s) was carried out by a thermal relaxation method in the temperature range of 1.9 - 302 K to obtain the experimental S°(298.15 K) (Suzuki, 2018). The Cp°(298.15 K) and the S°(298.15 K) were determined to be 249.4 ± 1.1 J K-1 mol-1 and 322.1 ± 1.3 J K-1 mol-1, respectively. The Cp°(298.15 K) given by Suzuki was in good agreement with that obtained at 298.15 K by extrapolation of the high temperature Cp° derived from the experimental H°-H°(298.15 K) (Ball, 1992). Thus, thermodynamic data for Cs2Si4O9(s) was reevaluated by using the experimental S°(298.15 K).

There is no experimental thermodynamic data available for CsFeSiO4(s). Then, a density-functional theory (DFT) calculation with the Vienna Ab initio Simulation Package was performed checking the applicability of estimation of

Fig. 3 Dependence of relative Cs chemisorption amount

calculated by the existing model to that by improved model on CsOH concentration in gas phase ( Bowsher, 1990; Nakajima, 2019)

Table. 3 Conditions of Cs-chemisorption tests

Conditions Temperature 873-1273 K

Atmosphere Ar-5%H2 Ar-5%H2-5%H2O Ar-4%H2-20%H2O

CsOH concentration in gas phase 0.2 - 20 μg/cm3 Deposition time 180 - 360 min Si content in SS304 specimen 0.1 - 4.9 wt.%

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thermodynamic properties of Cs silicates (Miradji, 2019; Miradji, 2020). As the result, our computational methodology provided excellent agreement with literature data of ∆fH°(298.15 K) for Cs2Si2O5(s) and Cs2Si4O9(s) within 4% error and the experimental Cp° for Cs2Si4O9(s) within 5% error. The thermodynamic data for CsFeSiO4(s) was successfully estimated by using the established DFT calculation.

In order to estimate the effects of the TD on the FP chemistry, the changes in fractions of Cs species were estimated by the uncertainty of thermodynamic data for CsBO2. Table 4 lists the calculation conditions which were set based on the data for 1F (Nishihara, 2012). The fractions of Cs species were calculated by using Thermo-Calc software (Sundman, 1985) with SGTE substance thermodynamic database (Dinsdale, 1991). Figure 4 shows the widths of prediction errors for fractions of Cs species calculated with thermodynamic data of CsBO2 for “A” (1 % of uncertainty) and “C” (5 % of uncertainty). Although the fractions of Cs2MoO4(l) and CsBO2(g) can be estimated due to the small width for the case of 1 % of uncertainty, they cannot be estimated for the case of 5 % of uncertainty. From the results, it can be said that the fraction of Cs species at high temperature can be estimated with higher reliability by implementing improved TD to the present SA analysis code.

4. Conclusions

We extended the first version of FP chemistry database ECUME. It consists of three kinds of datasets: CRK

(dataset for Chemical Reaction Kinetics), EM (Elemental Model set) and TD (ThermoDynamic dataset). The present ECUME is equipped with the CRK for the reaction of Cs-I-B-Mo-O-H system and Ru-N-O-H system in gas phase, the EM for Cs chemisorption onto SS and the TD for CsBO2 vapor and solid Cs2Si4O9 and CsFeSiO4. The ECUME is the first database for Cs-I-B-Mo-Fe-Si-O-H system in the world applicable for the evaluation of FP chemical reaction kinetics.

The CRK constants for Cs-I-B-Mo-O-H system and Ru-N-O-H system have been prepared based on an ab-initio calculation and literature survey. A FP chemical reaction calculation of Cs-I-B-Mo-O-H system with the CRK for a simple flow condition has revealed the necessity of consideration of chemical reaction kinetics for the evaluation of the FP chemistry in the lower temperature region.

The EM for Cs chemisorption has successfully reproduced the effects of CsOH vapor concentration in gas phase and Si content in SS on the Cs chemisorption behavior, which were not able to be considered by the existing model. There is possibility that the existing model under/overestimates Cs chemisorption amount in 1F according to CsOH concentration in gas phase.

Fig. 4 Widths of prediction errors for fractions of Cs

species calculated by chemical equilibrium calculation with thermodynamic data of CsBO2 for “A” (1 % of uncertainty) and “C” (5 % of uncertainty).

Table. 4 Conditions of the chemical equilibrium calculation Conditions Temperature Pressure Inlet amount

Cs I B Mo O H

1300 – 1500 K 7.6 MPa 1.1×103 mol 9.3×102 mol 4.3×104 mol 1.8×103 mol

2.9×105 mol

5.8×105 mol

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The high quality vapor pressure data for CsBO2 vapor were obtained experimentally. The recommended thermodynamic data for the equilibrium vapor pressures of CsBO2 vapor were determined based on the evaluation of experimental data and its uncertainty. The fraction of Cs species can be estimated by the thermodynamic data of CsBO2 with higher reliability. Thermodynamic data for Cs2Si4O9(s) and CsFeSiO4(s) were successfully evaluated by the experiment and ab-initio based methodology developed for the present purpose, respectively.

The present ECUME was successfully constructed for the more accurate evaluation of FP behavior under a LWR SA. The effects of B vapor and Fe, Si in SS on Cs behavior can be evaluated through the improvement of present SA analysis code using the ECUME, which should lead to the more accurate evaluation of Cs distribution in 1F. Thus, the ECUME can provide the fundamental solution for the FP chemistry issues towards the enhancement of LWR safety after the 1F SA and the 1F decommissioning work.

Acknowledgements

The authors express their sincere gratitude to Dr. Tokio Fukahori, Mr. Tomohiro Kawanishi and Dr. Hiroyuki

Yoshida, Dr. Ikuo Ioka, Dr. Chikashi Suzuki, Dr. Akihide Hidaka for their useful discussions and encouragement for the construction of ECUME, Mr. Junichiro Takada, Mr. Kunihiro Sumiya, Mr. Koei Ebata, Mr. Hiroshi Terakado, Mr. Yukihiro Hagiya, Mr. Kenta Sato for their technical support on the experiment.

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Development of fission product chemistry database ECUME