Abstract
The need for a methodology to assess the degree of core damage was part of the U.S. Nuclear Regulatory Commission's change in licensing and operation requirements after the Three Mile Island accident. Core damage assessment is used by the emergency response organization to determine the appropriate Emergency Action Levels (EALs), to select the source term for estimation the offsite dose, to predict the radiation protection actions that should be considered for long term recovery activities. In the Core Damage Assessment Guidance (CDAG) documented in WCAP-14696 report, Westinghouse Owners Group suggested that the containment radiation level and hydrogen concentration in the containment together with some other existing instrumentation, e.g. core exit thermocouples, reactor water level, can be used to determine the degree of core damage. It is well known that the amount radionuclides released from the fuel, retained in the reactor coolant system (RCS), and its distribution among containment compartments depend on the accident sequence. The amount of hydrogen generation and the containment hydrogen concentration are also sequences dependent. In the present analysis, a systematic MAAP calculation is performed to investigate the impact of accident scenarios on the amount of radionuclides and hydrogen concentration in the containment. The purpose of the study is to support the developed of the plant specific CADG. Eleven sequences are analyzed in the present study. The sequences analyzed included loss coolant accidents (several breach sizes and locations) and station blackout sequence. Various mitigation actions are considered in the calculations to study the impact of these actions on the containment radiation level and hydrogen concentration. The major results discussed in the paper include: fractional release of radionuclides from the fuel pellets, retention factor of radionuclides within RCS, oxidation fraction of cladding, retention factor of hydrogen within RCS, possibility of hydrogen combustion.