Isotropic Analog—a Method for Penetration Dose Rate Analysis

Penetration shielding design for primary and secondary shield walls in a nuclear power plant proceeds in several iterative cycles. These cycles are needed to refine the conceptual designs for numerous, often conflicting, requirements. These requirements include the following: worker occupancy, in-service inspections, ventilation, pressure and temperature transient controls, equipment qualification, etc. Because the determination of neutron and gamma radiation levels in the containment building of a nuclear power plant requires a three-dimensional calculation, which is both very complicated and expensive, simplified but conservative procedures are needed to provide that input for various other analyses. Once an optimized design is developed, it can be confirmed by either a full three-dimensional analysis or acceptable combinations of discrete ordinates and Monte Carlo methods. The isotropic analog method and its enhancement are presented to provide such an alternative. Included are the methodology, its justification, confirmation, limitations, and suggestions for additional development. This method has already been used for the shielding design of two nuclear power plants and shown to be conservative by a factor of between 2 and 5.