10.60. As a result of the boiling process, the axial power shape is much more of an important parameter than is the case for PWRs. Control rod sequencing during burnup also requires attention. Therefore, three­dimensional calculations are necessary, even at the preliminary core design level. In any fuel design method, a key requirement is to evaluate safety margins for a candidate reloaded core. For a BWR, power peaking effects require evaluation for a variety of operating conditions, including various control element insertion scenarios. By contrast, an initial PWR evaluation need be made only at the hot, full-power, control elements out, condition. In many cases the cold shutdown margin is a limiting criterion for a BWR. Therefore, a methodology that would accurately assess a design in terms of all of these requirements at reasonable computing costs is needed.

10.61. Most modeling methods have been developed in-house by re­actor or fuel vendors. For example, the General Electric Company, the major BWR vendor, uses a three-dimensional BWR nodal simulator code called PANACEA, which also describes two-phase flow behavior. This serves as the basis for an interactive method for reload and control element pattern design using a coupled mainframe and minicomputer [15].

10.62. Among other nodal codes useful for BWR reload core design, various versions of SIMULATE developed for the Electric Power Research Institute are noteworthy. In Sweden, an integrated system of codes for both static analysis and optimization was developed by the ASEA-ATOM group called CORE MASTER. This system is based on POLCA, a three­dimensional, two-group, nodal model which includes a thermal-hydraulic description [16].