ANALYSIS OF CORIUM CRITICALITY IN VVER-440 DURING SEVERE ACCIDENT
29th Symposium of AER on VVER Reactor Physics and Reactor Safety (2019, Energoland, Mochovce NPP, Slovakia)
spent fuel disposal and actinide transmutation
The paper studies the potential for a self-sustaining chain nuclear fission reaction during the progress of a severe accident at Ukrainian nuclear power plants with VVER-440 reactors. The Rivne NPP unit 2 core was modelled. Some models for criticality calculation at different stages of a severe accident in the VVER-440 pressure vessel were developed and corium multiplication properties were calculated.
The severe accident in the VVER-440 core was divided into eight major stages: intact reactor core, beginning of cladding damage (swelling), cladding melting and flowing down to the support grid, melting of structural materials, homogenization of the materials at the bottom of the reactor vessel, stratification of the corium at the bottom of the reactor vessel, spread of the corium from the reactor vessel. In view of criticality the supply of water (re-flooding) as a part of accident management is accounted. A compensatory measure such as addition of the boric acid solution to the cooling water was analyzed.
The results obtained in the study show whether the fuel rods remain intact at the beginning of a severe accident or whether criticality might appear. With further progression of the accident, the corium will be deeply subcritical if water cannot penetrate into its pores or voids. In the case of the formation of pores or voids in the melt with the ingress of water, criticality may occur.