Coupled code FRAPTRAN - GENFLO for analysing fuel behaviour during PWR and BWR transients and accidents

Demands for reactor safety assessments and for the tools to make these are continuously growing. Apart from the general request for better accuracy and application of best-estimate type calculations, several concrete developments work to the same end. It may become necessary to consider new events, like Anticipated Transients Without a Scram (ATWS), in the basic licensing calculations. Also continuing is the quest for better fuel economy that is bringing around higher discharge burnups and increasingly complicated structural and neutronic solutions. Conventionally, the description of thermal hydraulics is kept simple in fuel transient behaviour codes. Attempts to combine proper models for the two have been made with varying success. Even in the best of the cases so far the result has been a gigantic assembly of codes the use of which has been limited by computing capacity at least until a few years ago. At VTT Energy in Finland, an effort was initiated to link a proven, fast running general thermal hydraulics module, developed in-house and known as GENFLO, with the U.S. Nuclear Regulatory Commission’s (USNRC) renovated version of the FRAPTRAN fuel transient code. Preliminary results of the work are now available. In the paper, the codes and the main principles of the coupling are described. The first fuel rod calculation cases performed with the coupled code include a large break Loss-of-Coolant Accident (LBLOCA) in a VVER-440 power plant and an instability incident in a Boiling Water Reactor (BWR). Separately calculated system behaviour analyses were available for the two scenarios. Low and high burnup figures for the fuel were considered. Some results of these calculations are given to demonstrate the performance of the coupled code.