An analysis was performed within PHARE Project BG 01.10.01 in 2003-2004 that has discovered a vulnerability of LWR WWER-1000, V-320 design consisting in early (up to one hour) containment melt-through via ionization chambers channels during severe accident. Based on this analysis, a separate project was started to find out the possibilities to retain the melt in the ionization chamber’s channels and for prevention of early melt penetration and radioactive releases outside of the containment. An original engineering solution was developed, proposing ceramic plugs with a special design to be used for plugging the bottom of ionization chamber’s channels in the reactor pit of WWER-1000, B-320. This technical solution was substantiated trough analytical and experimental means. The analytical part consisted in using the NISA II FEM – DISPLAY III thermo-mechanical code for investigation of temperature and stress fields distributions within the plugs, when the corium falls over them.
Two experiments (hot and cold ones) were performed to prove the adequacy of the analyses for the proposed engineering solution, as well as the high temperature resistance and tightness of the ceramic plugs. This way the Design of WWER-1000, B-320 was enhanced against early containment failure during severe accident.
Finally, the solution was implemented on Units 5&6 of KNPP. All above works lasted as overall as of 4 years. The construction was patented.
Keywords: severe accident, early containment failure, ionization chambers channels, ceramic plugs.