Distribution of fission product residual decay heat in stratified core melt of LWR and its influence on sidewall heat flux

•FP decay heat in core melt of LWR under severe accident conditions is modeled.•The melt is simulated as a two-phase (oxide and metallic) stratified system.•We consider self-consistently FP thermochemistry, mass transfer and radioactivity.•Up to 15–25% of decay heat may be released in the metallic phase.•Change in decay heat distribution influences reactor pressure vessel failure time.

The model of spatial distribution of fission product (FP) activity and decay heat in core melt of light water reactor under severe accident conditions is proposed. It is based on thermodynamical consideration of “U–Zr–O–Fe–FP” system as a two-phase (oxide and metallic) stratified melt. The thermochemistry of FP phase distribution, heat-mass transfer and radioactive transformations of ∼100 isotopes of Cs, Mo, Ru, Ba, Sr, Zr, La, Ce, Eu, Nd, Nb and Sb are considered self-consistently. The calculations carried out with specially designed code FPMC (fission products–molten core) showed that up to 15–25% of decay heat may be released in the metallic phase of the melt. Estimation of this value is fundamentally important in simulations of heat and mass transfer in stratified melt, such as in-vessel melt retention and reactor lower head failure. For numerical assessment of the influence of decay heat spatial distribution in stratified molten pool on the lower head failure time the multivariate simulation of the later stage of large break loss-of-coolant accident in VVER-1000 reactor was carried out by SOCRAT and HEFEST codes coupled with FPMC.