The individual channel monitoring (ICM) proposal to improve the safety performance of RBMK

Abstract

A comprehensive study has been completed in relation to the accident analysis of RBMK with key results documented in three companion papers in the present journal volume. The study also aimed at the comparison between selected accident analysis topics in Light Water Reactors (LWR) and RBMK. The relevance of the Fuel Channel Blockage (FCB) event within the area of accident analysis was confirmed. Owing to the probability of occurrence, also connected with the large number of channels, estimated in the order of 10 −2/reactor/year, the FCB is actually part of the Design Basis Accidents (DBA) for the RBMK. In case the FCB is part of the DBA, a noticeable difference occurs in results from safety evaluations between LWR and RBMK: in the latter case, a DBA causes a loss of integrity for the pressure barrier and, though to a limited extent, damaged fuel overpasses such a barrier. The FCB event also causes contamination in reactor cavity and in selected parts of the overall confinement, damage in various graphite blocks and, even excluding the MPTR risk owing to the findings from the second companion paper in this journal volume, mechanical loads on neighbouring fuel channels, graphite stacks and reactor tank that need extensive examination before reactor restart. Therefore, a proposal has been formulated for performing the feasibility analysis for the design of a system denominated ICM (Individual Channel Monitoring). The goal of the ICM is the early detection of the FCB event and the triggering of scram in such a way to prevent pressure tube damage and, definitely, over-passing of the pressure barrier by molten or damaged fuel during a DBA situation. The ICM is based upon the signals of pressure-drop (or flow-rate) and fluid temperature transducers installed in the bottom and the upper parts of the fuel channels, respectively. The performed study shows that steam superheating at fuel channel outlet occurs early after the blockage event and the related temperature signal can be used to cause scram. The availability of sophisticate computational tools including the detailed neutron kinetic model for each core channel, made possible the preliminary conclusion of the study.