The in-core power distribution is one of the most important parameters calculated in the design and operation of nuclear reactors because it is the basis for estimating other requisite parameters. For example, it is used to estimate the probability of the failure of fuel and cladding both during normal operation and accident situations. It is also used to estimate the amount of nuclear fuel to yield the required energy by a reactor in a specified period. The inaccuracy of this estimation may cause an unnecessary increase in the generation of high-level nuclear wastes, or an unexpected coast down during the operation of a reactor. Thus, the accuracy of the in-core power distribution calculation is very important from a safety and environmental point of view.
In conventional methods the in-core power distribution is calculated by an approximation (neutron diffusion theory) because available computing power has not been sufficient to use the rigorous method of solving the neutron transport equation. A margin for the error caused by this approximation is thus necessary in operating nuclear reactors, e.g., to secure the integrity of fuel elements. It is obvious that reducing the margin for such an error by improving the accuracy of the calculation will contribute to improve the safety of nuclear reactor operations.
The performance of the PC, on the other hand, has been improving and recently has become so impressive that there is a possibility of solving the rigorous neutron transport equations with a normal PC. Based on this background, a method (VNEM: Variational Nodal Expansion Method) of calculating the in-core power distribution by solving the rigorous neutron transport equations has been developed and verified by numerical benchmark problems as well as by comparing to real plant data. The implementation of the VNEM into a core simulator has been completed so that it may be easily connected to core monitoring systems like SCORPIO or the 3D Core Data Viewer