Nuclear Engineering and Design 260 (2013) 104 120
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Nuclear Engineering and Design
j ourna l h om epa ge: www.elsev ier .com/ locate /nucengdes
esign of integrated passive safety system (IPSS) for ultimate passive safety ofuclear power plants
oon Heung Chang, Sang Ho Kim , Jae Young Choiepartment of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea
i g h l i g h t s
We newly propose the design concept of integrated passive safety system (IPSS).It has five safety functions for decay heat removal and severe accident mitigation.Simulations for IPSS show that core melt does not occur in accidents with SBO.IPSS can achieve the passive in-vessel retention and ex-vessel cooling strategy.The applicability of IPSS is high due to the installation outside the containment.
r t i c l e i n f o
rticle history:eceived 11 June 2012eceived in revised form 14 February 2013ccepted 12 March 2013
a b s t r a c t
The design concept of integrated passive safety system (IPSS) which can perform various passive safetyfunctions is proposed in this paper. It has the various functions of passive decay heat removal system,passive safety injection system, passive containment cooling system, passive in-vessel retention andcavity flooding system, and filtered venting system with containment pressure control. The objectives ofthis paper are to propose the conceptual design of an IPSS and to estimate the design characters of theIPSS with accident simulations using MARS code. Some functions of the IPSS are newly proposed and theother functions are reviewed with the integration of the functions. Consequently, all of the functions aremodified and integrated for simplicity of the design in preparation for beyond design based accidents(BDBAs) focused on a station black out (SBO). The simulation results with the IPSS show that the decayheat can be sufficiently removed in accidents that occur with a SBO. Also, the molten core can be retainedin a vessel via the passive in-vessel retention strategy of the IPSS. The actual application potential of theIPSS is high, as numerous strong design characters are evaluated. The installation of the IPSS into theoriginal design of a nuclear power plant requires minimal design change using the current penetrationsof the containment. The functions are integrated in one or two large tanks outside the containment.Furthermore, the operation time of the IPSS can be increased by refilling coolant from the containment
outside into integrated passive safety tanks (IPSTs). The coolant in the IPSTs is used for various functionsin accident scenarios. Also, potential problems for the realistic installation of the IPSS are proposed andthe solutions to these problems are schematically described. IPSS is the design for the passive safetyenhancement in preparation for a loss of AC power. Consequently, it is designed for the supplementationand enhancement of current nuclear power plants, not as a replacement. The specific optimization designfor each current or future reactor will be studied as further works.. IntroductionThe decay heat removal is one of the most important problemsn nuclear power plants. The integrity of the reactor core muste preserved by removing the decay heat after a shutdown. Also,
Corresponding author. Tel.: +82 42 350 3856; fax: +82 42 350 3810.E-mail address: email@example.com (S.H. Kim).
029-5493/$ see front matter 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.nucengdes.2013.03.018 2013 Elsevier B.V. All rights reserved.
the integrity of the containment has to be protected in the eventof BDBAs to prevent a large release of radioactive materials. Inaddition, all accidents must be appropriately managed by meansof correct interpretations and prompt actions by operators. Fromthree points of views, three important lessons are derived for thesafety of NPPs from the Fukushima accidents.First, the decay heat must be removed from core even if there isno AC power. The original cause of the Fukushima accident was theoccurrence of a station black-out (SBO). The decay heat could beremoved by passive safety systems during the initial stages of the
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ADS automatic depressurization systemADV atmospheric dump valveAFWS auxiliary feedwater systemAFWST auxiliary feedwater storage tankBAMP boric acid makeup pumpBDBA beyond design based accidentBWR boiling water reactorCARR center for advanced reactor researchCFS cavity flooding systemCHF critical heat fluxCLI cold leg injectionCMT core makeup tankDBA design based accidentDCH direct containment heatingDVI direct vessel injectionEC emergency condenserECCS emergency core cooling systemEDG emergency diesel generatorERVC external reactor vessel coolingFVS filtered venting systemHPCI high pressure coolant injection systemHPME high pressure melt ejectionHPSIP high pressure safety injection pumpHX heat exchangerIC isolation condenserIPSS integrated passive safety systemIPST integrated passive safety tankIRWST in-containment refueling water storage tankIVR in-vessel retentionLBLOCA large break loss of coolant accidentLOCA loss of coolant accidentLOCV loss of condenser vacuumLPSIP low pressure safety injection pumpMCCI molten corium concrete interactionMDP motor driven auxiliary feedwater pumpMFIV main feedwater isolation valveMSIV main steam isolation valveMSSV main steam safety valveMFL main feedwater lineMFLB main feedwater line breakNPP nuclear power plantRCP reactor coolant pumpRCS reactor coolant systemRV reactor vesselPAFS passive auxiliary feedwater systemPCCS passive containment cooling systemPCCT passive condensate cooling tankPCCWST passive containment cooling water storage tankPCT peak cladding temperaturePDHR passive decay heat removalPRHR passive residual heat removalPSIS passive safety injection systemPWR pressurized water reactorSBLOCA small break loss of coolant accidentSBO station black outSCP shutdown cooling pumpSG steam generatorSGTR steam generator tube ruptureSI safety injection
SIP safety injection pumpSIT safety injection tankTDP turbine driven auxiliary feedwater pumpnd Design 260 (2013) 104 120 105
accident. On the other hands, many currently operating reactorsare dependent on the restoration of AC power in preparation for aSBO. However, in the Fukushima accident, it took about nine days torestore AC power due to the conditions around the site (Hatamuraand Yotaro, 2011). As shown in the accidents, it seems that the ACpower restoration from the off-site is not easy to mitigate accidents.Also, passive systems are needed to remove decay heat for morethan a few days. Therefore, the enhancement of passive decay heatremoval systems for long time mitigation on the current operatingand future reactors is necessary in some way or other.
For the second lesson, severe accident mitigations are neededin severe accidents. All the NPPs are designed to cope with severeaccidents. However, they have to be enhanced in preparation witha SBO. One of the methods is supplying the water coolant to themolten corium. It is important to decrease the pressure of contain-ment and confine radioactive materials in the water. There was nospecific passive safety system to retain the molten core in the reac-tor vessel in the Fukushima accident. Also, it is estimated that therelease of radioactive materials was quite low at 10 Sv/h owingto the effect of water scrubbing in Unit 1 and 3. This figure can becompared to the release of 100010,000 Sv/h after the explosionof Unit 2 without confinement by water (TEPCO, 2011). These find-ings show that injecting water into the core, even in the event ofsevere accidents, is feasible in water reactors to cool the coriumand for retaining purposes in the form of a passive method duringa SBO.
Thirdly, the Fukushima accident implies the necessity of theaddition of many passive safety systems, but simplicity and main-tenance of passive safety systems must be also considered to copewith severe accidents. During the Fukushima accident, none of theactive safety systems could be operated due to the SBO caused bythe tsunami. Also, even if there were some passive safety systems,they could not be operated feasibly in the event of human errorsand the system failures due to the lack of accident interpretationmeasures. In the case of unit 1, although the isolation condensers(ICs) had malfunctioned, personnel were under the impression theywere normally operating. In addition, after they knew of the mal-function of the ICs, they could not be repaired due to the lackof accessibility to the inside of the containment. In the case ofunit 3, shift operators became concerned about insufficient waterinjection by the high pressure coolant injection system (HPCI) andswitched off the HPCI manually at 2:42 a.m. on March 13, 2011 inspite of the absence of preparation measures to realize an alter-native water injection source (Hatamura and Yotaro, 2011). Whilethe nuclear power plants at Fukushima were BWRs, the accidentsequence shows the importance of simplicity and maintenance forthe proper and prompt operation of passive safety systems in alltypes of nuclear power plants.
These three lessons about safety systems have to be reflected inthe design of reactors. On the other hand, most current PWRs aredesigned with numerous active safety systems and a few passivesafety systems as well compared to BWRs. There have been manyresearches about passive safety systems for PWRs.
As a representative passive pressurized water-cooled reactor,AP1000 developed by Westinghouse incorporates many passivesafety systems (Schulz, 2006). The main reactor coolant system(RCS) is identical to that of 2/4 loop PWRs, meaning two steam gen-erators and four reactor coolant pumps (RCP). The main passivesafety system consists of passive emergency core cooling system(ECCS) and passive residual heat removal system (PRHR), as shownin Fig. 1. Passive ECCS is the mitigation system for a loss of coolantaccident (LOCA). When a LOCA occurs, core makeup tanks (CMTs)
can supply coolant into the core in the reactor vessel at a high pres-sure injection because they are connected to the cold lines of theRCS during operation of the reactor. After the injection from theCMTs, the coolant in the accumulators is injected into the core as
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steam line, are closed for isolation of the turbines and condensers.At the same time, valves on the line connected to the HXs in passivecondensate cooling tank (PCCT) are opened. The steam from thesteam generator goes to the condensation HX through the steamFig. 1. AP1000 RCS and passive
mid-level pressure injection. After depressurization by either theutomatic depressurization system (ADS) or the break of the LOCA,he refueling water in in-containment refueling water storage tankIRWST) can be injected into the reactor vessel by gravity, as IRWSTs installed at an elevation higher than that of the reactor vessel. Theater in the IRWST is originally used as supply water for refueling.lso, the sump has the function of gathering the water spilled in theontainment. As a main part of the preparation for non-LOCA, theres PRHR in the RCS. A loop is installed from the hot leg to the steamenerator. In the loop, a PRHR heat exchanger (HX) is installed toemove the decay heat. Finally, the decay heat is transferred fromhe core to the water in the IRWST through the PRHR HXs. It cane operated even at a high pressure about 15.0 MPa for shutdownooling.
As the AP1000 is used in steel containment, direct containmentooling is possible by water and natural air, shown in Fig. 2. Passiveontainment cooling system (PCCS) in the AP1000 mainly consistsf two systems. The first spills out the water from passive contain-ent cooling water storage tank (PCCWST) which is installed at theighest elevation of the containment. Heat from the containments removed by the evaporation of the water on the surface of theteel containment. The second method utilizes the flow path forhe intake and discharge of outside air. Heat from the containments transferred to the outside air along the designed flow path. Ashe PCCS from the containment outside can form a high heat sink,nternal condensation and natural recirculation in the containmentnside is possible with a lower heat source. The AP1000 achievesigh passive safety due to its use of various passive safety systems.owever, it is difficult to apply the concepts of the proposed passiveafety systems to other reactors to enhance the safety due to theses of original designs such as the IRWST and steel containment.The center for advanced reactor research (CARR) in Korea pro-
osed a new concept of a passive PWR termed the CP-1300 (Changt al., 1996). The CP-1300 is a CARR passive PWR-1300 MWe. CARR
roposed the concept for decay heat removal by natural circula-ion through the steam generator on the secondary side. It has beenpplied in researches on passive auxiliary feedwater system (PAFS)Bae et al., 2012). The PAFS was developed for use in the designcooling system (Schulz, 2006).
of Advanced Power Reactor Plus (APR+) in Korea. Fig. 3 shows theconcept of the PAFS. For decay heat removal, the main feedwa-ter isolation valves (MFIVs) and the main steam isolation valves(MSIVs), which are installed on the main feedwater line and mainFig. 2. Passive containment cooling system in the AP1000 (Schulz, 2006).
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Fig. 3. Concept of the PAFS in the APR+ (Cho et al., 2012).
upply line. It is condensed in the HX by heat transfer into theCCT. The condensed water comes back into the steam generatorhrough the return water line. This process forms the natural circu-ation by the steam generator as a lower heat source with the HXsn the PAFS forming a higher heat sink. In the design of the PAFS, theesign parameters of the HXs and pipes crucially affect the coolingapacity for decay heat removal. Some researchers have workedn designing a condensation HX for the PAFS and evaluating theooling performance using the thermal hydraulic system analysisode MARS (Multi-dimensional Analysis for Reactor Safety) (Baet al., 2012). It wa...