Loading Pattern Optimization for Standard and End-Of-Life Cycles of Ringhals PWR's with ROSA

The core-loading pattern is decisive for fuel cycle economics and safety parameters. ROSA, NRG's manual and automatic loading pattern optimization code system for PWR's, has proven to be a valuable tool for reactor operators to improve their fuel economics for over two decades. ROSA uses simulated annealing as optimization technique, in combination with an extremely fast 3-D neutronics code for loading pattern calculations. This paper outlines two recent applications of the ROSA code system for different Ringhals PWR units. The purpose of the work was twofold: first to demonstrate the value of the ROSA tool by a blind 5 cycle core design benchmark against utility core design methods for Ringhals-4. A predefined discharge burnup increase with respect to 5 reference cycle designs had to be demonstrated. A complicating factor was the use of shielding assemblies, causing reduced core design flexibility. The second part of the work was an End-Of-Life study for Ringhals-2 aiming at máximum energy production during the last two cycles. Reference designs for comparison were available. ROSA models for quarter and full core were setup for both Ringhals units and successfully validated against license code results for natural cycle length (boron curve), peaking, EOC assembly and rod burnups, MTC, and SDM. In order to compensate for (small) peaking differences between ROSA and license codes an easy to use peak-matching method was implemented in ROSA. This helped a lot to generate optimized solutions that also worked in the license code after one or a few iterations. For Ringhals-4 the average discharge burnup of 212 assemblies (5 cycles) could be increased by 0.6 MWd/kg, whereas the target was 0.24 MWd/kg. For Ringhals-2 an extra energy production of 10 EFPD could be demonstrated during the same number of calendar days and with the same fuel inventory. Both studies were confirmed by license code calculations.