Integral Codes for Severe Accident Analyses

Publisher Summary This chapter deals with the computer codes for simulation of severe accidents (SA). Three classes of SA codes can be defined, depending on their scope of application—integral codes, detailed codes, and dedicated codes. This chapter focuses on integral codes. Integral codes (also called engineering-level codes) simulate the overall nuclear power plant (NPP) response, that is, the response of the reactor coolant system, the containment, and, most important, the source term to the environment, using “integrated” models for a self-consistent analysis of the accident. They include a combination of phenomenological and parametric models for the simulation of the relevant phenomena. They must be (relatively) fast-running to enable a sufficient number of simulations of different scenarios to be performed, accompanied by uncertainty studies. These codes are not designed to perform best-estimate simulations, but rather to allow the user to bound important processes or phenomena, often using user-defined parameters. They are generally used to support Probabilistic Safety Assessment level 2 (PSA2) analyses, for a good estimate of risks for SA scenarios, and for the development and validation of accident management programs. The principal internationally used codes today are MAAP (developed by Fauske & Associates Inc., USA), MELCOR (developed by Sandia National Laboratories, USA, under USNRC sponsorship), and ASTEC (jointly developed by IRSN, France, and the GRS, Germany).