Operating nuclear fuel to higher discharge burnups reduces not only fuel cycle costs but also the volume of radioactive waste requiring disposal. In pressurized water reactors (PWRs), high local power densities are a prerequisite for achieving a high batch burnup.

The range of maximum power densities that can be exploited for in-core fuel management and operational flexibility is restricted by the limiting conditions for operation obtained from analyses of anticipated operational occurrences and hypothetical accidents.

Since utilities mainly use available margins for implementing advanced in-core fuel management strategies or for power uprating, a suitable parameter for making a rough comparison of the present thermal-hydraulic design status of different PWRs is the maximum local heat flux achieved during actual cycles under steady-state full-power conditions. A comparison between Siemens PWRs and the PWR designs of other vendors shows that the maximum local power densities during steady-state operation are usually higher in Siemens PWRs.

The main reasons why higher power densities are permissible can usually be attributed to different core surveillance concepts (instrumentation and control) in conjunction with different control assembly management schemes. Moreover, two representative studies conducted with a new methodology using the three-dimensional neutronics/thermal-hydraulics coupled code PANBOX for core transient analysis present additional margins. Especially in plants using the Siemens core surveillance concept, the new methodology yields significant additional margins for PWRs to be operated with even higher permissible local power densities.

The additional departure from nucleate boiling ratio (DNBR) margin gained in the representative studies was 0.38. However, utilization of this additional margin is accompanied by larger void fractions within the upper section of the hot channel during normal operation. Therefore, increasing steady-state maximum power densities has to be done gradually while collecting and evaluating operating experience each time. Depending on the specific circumstances at a plant, the gained margin can be utilized not only for more economical core loading patterns (improved low-leakage loading and/or elimination of burnable absorbers) or power uprating but also, in Siemens PWRs, to eliminate having to readjust the DNBR limitation circuit for one or more cycles.

Although the concept presented here is specifically tailored to Siemens PWRs, it is obvious that the application of a three-dimensional neutronics/thermal-hydraulics coupled code could also provide significant benefits for non-Siemens PWRs as well.