Analysis of a severe beyond design basis accident for the EGP-6 reactor facility at Bilibino NPP. Release source term formation
An analysis has been performed for the dynamic phase progression in a severe beyond design basis accident caused by uncontrolled insertion of positive reactivity into the EGP-6 reactor in conditions of the emergency protection system failure. The initial event leads to the reactor power growing rapidly to 450% of the rated value, involving the fuel temperature growth and the growth in the coolant temperature and pressure. These factors result in the coolant boiling in some of the FAs, leading to a departure from nuclear boiling, the fuel damage in these SFAs and the escape of the steam-water fluid into the reactor stack. The stack pressure growth leads to the reactor shroud damage, the escape of the steam-water fluid into the space above the core and the subsequent steam-water fluid transport (into the reactor hall and the above-core space ventilation system and, further, into the environment). The dynamic processes were calculated using the RELAP5/Mod3.2 code. The stages of the fuel damage process were considered, and the dynamics in the number of and the extent of damage to fuel elements in the FAs was estimated. The estimated fission product release from the failed FAs was considered based on experimental data acquired at SCC IPPE for the Bilibino NPP FAs (tube-type steel-clad fuel elements, fuel composition: uranium dioxide grit in a magnesium matrix) for the severe beyond design basis accident conditions. The transport of fission products with water and steam was estimated based on experimental data for the fission product graphite stack transmission in conditions of a severe accident, also obtained at SCC IPPE. The emergency release into the atmosphere was estimated with regard for the fission product release dynamics and the use of two approaches to the release path formation (conservative or the most probable one). Good self-protection properties of the EGP-6 facility in conditions of a severe beyond design basis accident were noted.
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