Progress of IVMR Investigations in WWER1000 Reactor Type with ASTECv2.1.1.3 Computer Code – R. Gencheva, A. Stefanova, P. Groudev

It has been investigated the In-Vessel Melt Retention (IVMR) strategy with external vessel water cooling after the
initiation of Large Break LOCA (2  850 mm) simultaneously with full station blackout (SBO) at WWER-1000 reactor. The
reference nuclear power plant for this analysis is Units 5 and 6 of the Kozloduy NPP.
Two calculations have been done with ASTECv2.1.1.3 computer code:
In the first calculation ICARE and CESAR modules of ASTECv2.1.1.3 computer code have been used in a coupled mode. The
calculation starts at 4910 s. after the initiation of LBLOCA and SBO. At this moment a large amount of molten corium is
poured in the lower vessel bottom head. In this calculation just the vessel bottom head was modelled without water and
without internals. The external water cooling is initiated also at 4910 s.
The second calculation is integral plant simulation of LBLOCA simultaneously with SBO. The modules ’CESAR’, ’ICARE’,
’SOPHAEROS’ and ’CPA’ from ASTECv2.1.1.3 computer code have been used in the calculation. LBLOCAand SBO are initiated
at 0 s. The external water cooling is initiated at 1025 s. This is the time of the first material and fission products (FP) slump
in the lower head vessel.
The investigations discussed the maximal values of the internal and the external heat fluxes inside and outside the vessel
bottom head for the different segments and other major parameters.

Keywords: in-vessel melt retention, severe accident, WWER-1000, ASTEC v2.1.1.3 computer code.