The software team at NRG works to improve our ROSA code every year. Our team:

Vattenfall’s Ringhals - 4 is a 3292 MWth, 157 assembly core nuclear power plant. It uses 17x17 fuel assemblies, in combination with twelve so-called shielding assemblies for reactor vessel shielding. These assemblies contain little to no uranium and produce almost no power. Because of the high power density of the core, a very balanced loading pattern for the rest of the assemblies is required. Moreover, multiple assemblies are close to thermal safety margins. Vattenfall set a goal for ROSA to increase the assembly discharge burnup by 0.25 MWd/kgU. The ROSA solution was demonstrated on five consecutive cycles, in blind competition with Vattenfall’s own core design team using other software. Vattenfall provided NRG with data for the reactor model and the fuel history.

During the demonstration a new feature for ROSA was  developed in cooperation with Vattenfall, enabling further fine tuning of ROSA’s core loading  predictions with those of the license code, thus reaching a very high accuracy between the two codes.

Similar to other nuclear power plants, the way you load your core is important. At the end of every cycle, spent fuel is taken out and new fuel is put in. The configuration of the fuel assemblies determines the power distribution in the reactor core. After the model was set up, NRG optimized the loading patterns for the five cycles. The performance of the optimized loading patterns, in particular the safety margins and the assembly discharge burnup, were subsequently reproduced with Vattenfall’s license code. ROSA’s optimized core loading for Ringhals 4 outperformed Vattenfall’s reference case with a higher than required increase of the assembly discharge burnup of 0.6 MWd/kgU per reactor core assembly. This relates to fuel cost savings of about 500,000 Euro per cycle.