Possibility of misleading readings of water level in VVER steam generator during severe accidents with account for the Fukushima lessons

Abstract

The severe accident at Fukushima Daiichi in 2011 has emphasized the importance of keeping instrumentation reliable in the course of a severe accident, for successful diagnosis of current state of the reactor and adequate decision making. After the electrical current was partially restored several hours into the accident at Unit 1, the readings of the water level in the core have been for a long time misleading. Given the conditions at the Unit, they were clearly contradicting the analytical estimates of coolability and degradation level of the reactor core. In the post-accident investigations this contradiction was explained by TEPCO by the effect of water evaporation in the reference leg due to heat up of the drywell atmosphere around. The water level instrumentation in different vessels of the VVER NPPs like steam generator, pressurizer is very similar to that at Fukushima BWR. Although the VVER and BWR designs are different in many aspects, it is important to verify if the lessons learnt from the Fukushima accident are applicable in case of severe accidents at VVER.This paper presents the results of numerical study for the possibility of misleading readings of water level in VVER steam generator during hypothetical severe accidents. These results demonstrate the importance of learning the lessons from the accidents that have happened at structurally similar NPPs, and their consideration in safety analyses of NPP units in operation and under design process. Accumulation of lessons learnt from previous operation and emergency experience is a very important advantage of nuclear fission technology that allows enhancing the safety of new designs of nuclear reactors.