Severe Accident Prevention and Mitigation in Pressurized Heavy Water Reactors

Abstract of book chapter in Pressurized Heavy Water Reactors, Vol. 7, in the JSME Series in Thermal and Nuclear Power Generation, Elsevier, 2021, edited by J. Riznic

Author(s): Samuel Gyepi-Garbrah and Thambiayah Nitheanandan
Canadian Nuclear Safety Commission


This document summarizes the unique design features of pressurized heavy water reactors (PHWRs), which enable them to support accident management. PHWRs are also known as Canada Deuterium Uranium (CANDU) reactors in Canada and among some operating nations of PHWRs. The strategies and success paths in severe accident prevention and mitigation are presented to improve understanding. Severe accidents in PHWRs / CANDU reactors consist of a wide range of interacting phenomena that can be grouped into in-vessel and ex-vessel phases. Some of the interacting phenomena discussed include degradation of the fuel channels and calandria tubes, oxidation of fuel rod cladding, corium (melted fuel and associated core debris) melt formation, relocation of corium material to the bottom of the calandria vessel, melt pool behaviour and calandria vessel failure. The PHWR/CANDU design and its large inherent heat sinks allow for event mitigation at any stage in the accident sequence as outlined above and increase the degree of success in preventing a PHWR / CANDU reactor accident from progressing to a severe accident. This document will also look at the consequences of severe core degradation accidents that are not prevented and/or terminated, such as molten fuel–concrete interactions and the production of significant quantities of combustible gases. Aerosol behaviour and the behaviour and release of fission products from the fuel and its transport in containment will also be briefly discussed. Severe accident mitigation strategies that can be used to terminate an accident and/or prevent it from progressing to severe core degradation (a low probable event) will be described. A brief comparison to light water reactors will also be made.

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