28th Symposium of AER on VVER Reactor Physics and Reactor Safety (2018, Olomouc, Czechia)
Reactor dynamics and safety analysis
Ievgen Bilodid (SSTC NRS)
The paper studies the potential for a self-sustaining chain nuclear fission reaction during the propagation of a severe accident at Ukrainian nuclear power plants with VVER-1000 reactors. The beginning of the third fuel cycle of the Х2 core loaded with TVSA fuel was modelled. Some models for criticality calculation at different stages of a severe accident in the VVER-1000 pressure vessel were developed and corium multiplication properties were calculated.
The severe accident in the VVER-1000 core was divided into seven 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. A compensatory measure such as addition of the boric acid solution to the cooling water was analyzed.
The results obtained in the study show, if fuel rods are maintained intact at the beginning of a severe accident, criticality might appear even if the emergency protection rods is triggered. With further propagation 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, a criticality may occur.