Status on performance study of mixed nitride fuel pins of BREST reactor type

•Experimental research of out-of-pile properties of the mixed nitride.•Reactor tests.•Post-irradiation examinations.•Development of methods, codes and criteria justifying fuel pins performance.•Improvement of fuel pins with (U, Pu) N. Within the framework of the “PRORYV” project the “The comprehensive program for the calculation and experimental justification of fuel pins with mixed uranium-plutonium nitride fuel of BN-1200 and BREST reactors for the period up to 2020” has been developed Tpoянoв et al., 2015 [1]. The purpose of the program is to judge fuel pins performance, to ensure stability and required quality of the experimental technology for manufacturing of mixed nitride, fuel pins and experimental fuel assemblies (EFA) for testing in BOR-60 and BN-600 reactors, to optimize the design and technology of fuel pins with mixed nitride for the developed BN-1200 and BREST-OD-300 reactors based on the results of reactor testing. Currently, the program aims at justifying the starting operation stages of BREST-OD-300 and BN-1200 reactors. The following results were obtained to judge the fuel performance at the starting operation stage of BREST-OD-300 reactor (the maximum mixed nitride burn-up of 6 at%, the maximum damage dose of 89 dpa): 1. A set of out-of-pile investigations has been performed in order to obtain the necessary data on the properties of fuel and cladding materials, which are required for the development of calculation models of mixed nitride behavior under irradiation and design justification of their performance, including in design accidents. 2. To March 2021 the irradiation of eight BN-600 EFAs with EP823-Sh cladding was finished. In EFA-11 the maximum burn-up 9.0 at% and the maximum damage dose 107.6 dpa have been reached. PIE of fuel pins of two OU irradiated in BOR-60 and four EFAs irradiated in BN-600 with maximum burn-up of 5 at% and damage dose of 74.7 dpa are completed. Experimental data required for verification of calculation results of temperature and stress-strain state of mixed nitride pins with EP823-Sh steel cladding, as well as for evaluation of fuel pin performance and establishment of allowable lifetime of the BREST-type fuel pins with mixed nitride have been obtained. 3. The KORAT, DRAKON and BERKUT-U codes were modified and verified. Certification of the DRAKON code has been completed and the BERKUT-U code has been submitted for certification. Obtaining the permission to extend the irradiation of EFA beyond the paramete