Verification of Severe Accident Management strategies for VVER 1000 (V320) reactor

Severe accident analysis of a reactor is an important aspect in evaluation of source term. This in turn helps in emergency planning and Severe Accident Management (SAM). The use of the severe accident management guideline (SAMG) is required for accident situation which is not handled adequately through the use of Emergency Operating Procedures (EOP), thus leading to a partial or a total core melt. Actions recommended in the SAMG aim at limiting the risk of radiologically significant radioactive releases in the short- and mid-term (a few hours to a few days). Initiation of SAMG for VVER-1000 is considered at two core exit temperatures viz. 650°C as a desirable entry temperature and 980°C as a backup action. Analyses have been carried out for VVER-1000 (V320) for verification of some of the strategies namely water injection in primary and secondary circuit. These strategies are analysed for a high and low pressure Primary Circuit transients. Station Blackout (SBO) is one such high pressure transient for which core heat can be removed by natural circulation of the primary circuit inventory by maintaining the secondary side inventory. This strategy has been verified where the feed water injection to secondary side of SG is considered from external power sources (e.g, mobile DG sets) as suggested in SAM guidelines. The second transient analysed for verification of the core flooding during Large Loss of Coolant Accident (LOCA) along with SBO, a low pressure event. The injection to secondary circuit is initiated with the available safety pumps and mobile DG sets as soon as feed pumps trip. The analysis shows that SG flooding is not adequate to arrest the degradation of the core. In the second strategy for LOCA transient, the injection to primary circuit has been initiated at 650°C core exit temperature. The analyses show that core flooding is not adequate to arrest the degradation of the core for the large LOCA where as for small break LOCA the inj- ctions through available safety systems are adequate. The assessments are carried out with integral severe accident computer code ASTEC V1.3.