Review of Safety Case Documents SAR, Chapter 15, SB LOCA

1. ReviewofSafetyCaseDocumentsSAR, Chapter 15, SB LOCA SafetyAssessment EssentialKnowledgeWorkshop IAEA Safety Assessment Education and Training (SAET) Programme Presented by: Marián Krištof, IAEA

2. Session Outline • Overview of LOCA events • Safety aspects of SB LOCA and phenomena • Experimental programs on SB LOCA • SAR analysis of SB LOCA

3. LOCA • Lossof integrity of the primary circuit or itsassociated pipes and devices • Allsmall lines that penetrate the primary boundary including relief and safety valves, charging and letdown lines, drain lines, instrumentation lines, etc. • The direct cause of such an accident is a materialdefect, material fatigue, an external impact (internal missiles or heavy loads) ora device failure during the operation of the plant • Can bean actual break, as well as a stuck – open valve

4. LOCA features • Primarysystempressure drop • Increaseofthepressure and temperature in thecontainment • Reductionofthecoolantinventory in theprimarysystems • Posibilityofboilingcrises • Increaseofthefuel and claddingtemperature

5. Typesof LOCA • LB-LOCA • Fullor partial rupture of the maincirculation line, typically with break areas higher than 25% of the cross-sectionof the main circulation line • Ruptures of the major pipes connected to theprimary circuit, such as the pressurizer surge line or the accumulator dischargelines, can also be considered as LB-LOCAs • SB-LOCA • Smallerin size in comparison with LBLOCAs,which cannot, however, be compensated for by the make-up systemand thus require activation of the ECCS

6. Typesof LOCA • PRISE • Lossof primary coolant into the secondary circuit, for example due to therupture of the SG tubes • Interfacing LOCA • Breach in a system that interfaces with the reactor coolant system (RCS) and could cause a loss of coolant accident, if the breach is not isolated from the RCS

7. Safetyaspectsof LOCA • The high velocity stream of the escaping primary coolant generates jetforces and reaction forces (leading to pipe whip) that endanger other systemsclose to the ruptured pipe and containment internals. Similarly,mechanical damage can be caused by the MCP rotor overspeedinducedby a very high primary coolant flow directed to the break • Pressure wave propagation in the primary circuit at the very initial stageof the accident leads to pressure differences across the reactor internalswith large forces acting on the internals • Loss of coolant resulting in core dry-out leads to loss of coolability of thecore in spite of reactor shutdown; fuel rods are heated, claddingmechanical properties are degraded and the integrity of the cladding canbe lost due to internal fission gas overpressure or thermally inducedstresses • Cladding ballooning and geometrical distortions of the fuel assembliesmay endanger the long term coolability of the reactor core • At high temperatures, the cladding material reacts with the steam in anexothermic reaction, with hydrogen as a by-product. This reaction representsan additional heat source for the cladding and can cause furtherdegradation of the cladding material due to oxidation, and the potentialfor hydrogen burning or explosions inside the containment • High energy coolant outflow into the containment leads to pressurizationofthecontainment • Containment pressurization together with high radioactivity in the containmentatmosphere (due to fuel damage) leads to leakages into theenvironment with potential radiological consequences

8. SB LOCA • The major concern during SB LOCA is the loss of primary coolant inventory and the ability of the protection systems to detect this in time and restore the water inventory • Usually such a break is not sufficient to depressurize the system fast. For this reason, initiation of ECC injection is not possible until late in the accident sequence. • SB LOCA phenomena are usually quite complex and the magnitude and timing of these phenomena vary as a function of • break size, • break location, • plant design parameters and • emergency procedures (automatic or manual). • The long time scales in SBLOCA accidents mean that operator interventions can be important factor in aggravating or ameliorating the accidents sequences

9. SB LOCA – various break sizes

10. SB LOCA – sequenceofevents (LSTF 5% HL break)

11. SB LOCA phenomena - CCVM for Small and Intermediate Breaks • Pool information in UP / CCFL • Core wide void and flow distribution • Heat transfer in covered core • Heat transfer in partially uncovered core • Heat transfer in SG primary side • Heat transfer in SG secondary side • Pressuriser thermal hydraulics • Surge line thermal hydraulics • 1-phase and 2-phase pump behavior • Structural heat and heat losses • Non-condensable gas effect • Boron mixing and transport • Natural circulation in 1-phase, primary side • Natural circulation in 2-phase, primary side • Reflux condenser mode and CCFL • Asymmetric loop behavior • Break flow • Phase separation without level formation • Mixture level and entrainment in SG secondary side • Mixture level and entrainment in the core • Stratification in horizontal pipes • Phase separation in T-junctions and effect on leak flow • ECC mixing and condensation • Loop seal clearing

12. SB LOCA experimental program

13. SB LOCA experimental program

14. SB LOCA experimental program

15. SB LOCA analysis • Categorization • Acceptance criteria • Computer code • Analysis method • BIC • Availability of systems and components • Operator actions

16. Acceptance criteria • 10 CFR 50.46 • Maximum zircaloy cladding temperature • Maximum oxidation of cladding • Maximum amount of hydrogen generated by chemical reaction of the zircaloycladding with water and/or steam • Coolable core geometry • Long term cooling

17. BIC • Maximize the core heat-up • Maximum stored energy in fuel • All sources of generated and stored energy (e.g. metallic structures to account for the accumulated heat) • Minimum heat transfer to the secondary side • Sensitivity: heat transfer through fuel gap • Minimum coolant inventory (low PRZ level) • Low characteristics (low efficiency, low capacity, long delays) for reactor scram, ECCS • Sensitivity: pressure in ACCU • MCP characteristics (turbine regime data are needed)

18. BIC • Feedback coefficients • Moderator density – weak (until rector scram density decreases -> lower power decrease) • Fuel temperature – strong (until rector scram fuel temperature decreases -> increase of power) • Boron density – weak (during transient density increases -> lower power decrease)

19. Availability of systems and components • Single failure on active ECCS train • LOOP is usually assumed • Reduced number of accumulators • Most effective control rod stuck

20. Relevant documents • IAEA SRS-23: Accident analysis for NPP • IAEA SRS-30: Accident analysis for NPP with PWR • US NRC NUREG-0800 • US NRC RG 1.70, IAEA GS-G-4.1 • OECD/NEA CCVM matrices