Abstract-

Abstract-CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute (IRSN) and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under RIA conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber ( 3 He) situated in transient rods inside the reactor core. The shapes of power transients depend on the total amount of reactivity injected and on the injection speed. The injected reactivity can be calculated by conversion of the 3 He gas density into units of reactivity. So, it is of upmost importance to properly master gas density evolution in transient rods during a power transient. The 3 He depressurization was studied by CFD calculations and completed with measurements using pressure transducers. The CFD calculations show that the density evolution is slower than the pressure drop. Surrogate models were built based on CFD calculations and validated against preliminary tests in the CABRI transient system. Studies also show that it is harder to predict the depressurization during the power transients because of neutron/ 3 He capture reactions that induce a gas heating. This phenomenon can be studied by a multiphysics approach based on reaction rate calculation thanks to Monte Carlo code and study the resulting heating effect with the validated CFD simulation.

CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute (IRSN) and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under RIA (Reactivity Initiated Accident) conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber ( 3 He) situated in the so-called "transient rods" inside the reactor core. The CABRI reactivity injection system allows us to generate structured transients based on specific sequences of depressurization. For such transients, the time difference between the openings of two valves of the reactivity injection system has an important impact on the power pulse shape. A kinetic point code, SPARTE, was developed in order to replace the older DULCINEE code dedicated to the modeling and prediction of CABRI power transients. The SPARTE code includes new models of 3 He depressurization based on CFD calculations, variable Doppler coefficient based on Monte Carlo calculations and variable axial neutron flux profile. The density and Doppler models have a large impact on power transient prediction. For low initial pressure transients, the major uncertainty comes from the reactivity injected by the 3 He depressurization. For high initial pressure transients, the 3 He heating during the power pulse ("TOP effect") is responsible of an additional injection of reactivity that needs to be modeled precisely.

The CABRI International Program (CIP) tests irradiated UO2 or MOX fuels submitted to Reactivity Initiated Accidents (RIA) representative power pulses in prototypical PWR thermal-hydraulic conditions. CIP is managed by The Institut de Radioprotection et de Sûreté Nucléaire (IRSN) within a OECD/NEA framework. Experiments are conducted in the CABRI reactor operated by CEA. For CIP, CABRI benefits from a new pressurized water loop. An important refurbishment program enhanced the facility safety and upgraded the experimental equipment such as the nondestructive examination bench IRIS and the Hodoscope on-line fuel motion monitoring system. Specific test devices with appropriate innovative instrumentation follow the test rod behavior during the transient.The successful first test in the pressurized water loop demonstrated the CABRI capability to perform fully instrumented RIA tests in PWR conditions and to provide highly valuable results for RIA phenomena modelling (boiling crisis, post-failure events), for code validation, and for assessing PWR safety criteria. Highlights• The CABRI international Program (CIP) tests industrial irradiated fuel rods in a circulating test loop ensuring prototypical PWR conditions.• The CABRI reactor provides experimental conditions and power pulses representative of PWR RIA transients.• A complete instrumentation is available: online fuel motion monitoring system (Hodoscope), instrumented test device, pre-test and post-test non-destructive examination (IRIS).• Results will help modelling, code qualification and validation, and provide a new technical basis for PWR fuel safety criteria assessment for RIA conditions.

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