12.130. The modeling of various aspects of reactor system behavior is the objective of a large number of safety-related computer codes. Also, by assembling specialized codes into related modules, it is common practice to provide a description of system behavior following a spectrum of dis­turbances and accidents of varying degrees of seriousness. Since modeling methods are continually being updated and many are proprietary, our primary purpose here is to provide orientation and describe some typical approaches.

12.131. Modeling methods may be classified by their intended appli­cation.

1. Licensing requirements. Licensing codes, which are primarily thermal — hydraulics in nature, confirm the protection provided by LWR emergency core cooling systems.

2. Modeling as an integral part of design procedures. Design codes provide “best estimate” or “realistic” predictions. They, too, normally model thermal — hydraulics behavior, although some deal with containment response.

3. Severe accident analysis. Analysis complements risk assessment and source term development activities.

Many codes, particularly those in the second category, have been de­veloped by reactor vendors and are proprietary. Others have been devel­oped with government or utility support. Some codes serve more than one application. Each of these general modeling categories are considered in the following sections.