15 Severe accident prevention and mitigation in pressurized heavy water reactors

A summary of the unique design features of the pressurized heavy water reactors (PHWRs), also known as CANada Deuterium Uranium (CANDU) reactors in Canada and some other operating nations of PHWRs, which enable it to support accident management is described. The strategies and success paths in severe accident prevention and mitigation are provided to improve understanding. Severe accidents in PHWRs/CANDUs consist of a wide range of interacting phenomena that can be grouped into in-vessel and ex-vessel phases. Some of the interacting phenomena that will be discussed are degradation of the fuel channels and calandria tubes, the oxidation of fuel rod cladding, corium (which is melted fuel and the associated core debris) melt formation, relocation of corium material to the bottom of the calandria vessel, melt pool behavior, and calandria vessel failure. The PHWR/CANDU design and its large inherent copious heat sinks can enable an event to be mitigated at any stage in the accident sequence as outlined above and increases its degree of success in preventing a PHWR/CANDU reactor accident from progressing to a severe accident. The consequences of severe core degradation if accidents cannot be prevented and/or terminated, e.g., molten fuel–concrete interactions, production of significant hydrogen and other combustible gases that could challenge containment integrity will also be mentioned. The behavior and release of fission products from the fuel and its transport in containment and aerosol behavior will also be briefly discussed. Description of severe accident mitigation strategies that can be used to terminate and/or prevent an accident from progressing to severe core degradation (a low probable event) will also be discussed. A brief comparison to light water reactors will also be provided.