Assessment of Steady State Uncertainties Impact on RIA Modelling

RIA scenarios, as design basis accidents, are studied to set reactor operational safety limits aimed to ensure the long-term core coolability. This is of particular interest nowadays, as nuclear fuel is driven to higher burnups under more demanding operation conditions. The present understanding of fuel behaviour under such events is encapsulated in computer codes, which predictability is extended and validated through separate effect and integral experiments data. These computational tools have become a key element to assess safety margins and, hence, their accuracy is of utmost interest in nuclear safety studies. Given the complexity of working phenomena under RIA conditions, neither existing thermo-mechanical models nor rod state characterization at the moment of power pulse reflect exactly reality. So, it is essential to know how these deviations from reality influence code accuracy. This study illustrates how uncertainties related to pre-transient rod characterization affect the estimates of nuclear fuel behaviour during the transient. To do so, the RIA scenario chosen as a basis for the study has been that of the CIP0-1 test of the CABRI program, and the codes involved in simulation have been FRAPCON-3 (steady state irradiation) and FRAPTRAN1.4 (transient). The rod characterization uncertainties have been estimated by analysing the propagation of uncertainties in modelling steady state irradiation. Then, those RIA initial uncertainties are fed into the transient analysis, estimates bands are discussed and input variables are ranked according to their effect on the RIA modelling. In addition to be an example of uncertainties assessment, this study intends to set a methodology that allows getting more reliable insights when modelling RIA scenarios. This work is framed within the CSN-CIEMAT agreement on ''Thermo-Mechanical Behaviour of the Nuclear Fuel at High Burnup''. (author)