Integral experiments at Sandia with molybdenum sleeves for testing 95Mo cross sections in thermal energy spectrum

International audienceMolybdenum plays an important role in nuclear criticality-safety as it is a fission product in reprocessing plants under the shape of highly concentrated UPuMoZr deposits but also in research reactors, naval and space reactors. 95Mo, 96Mo and 97Mo are the main isotopes of interest in terms of abundance; 95Mo has the largest capture cross section. Few integral experiments sensitive to the cross sections of molybdenum are available in the literature. The French MIRTE experiments [1] consisting of a 10 mm molybdenum screen between four lattices of Valduc U(4.738 %)O2 rods, are sensitive to the capture cross section of 95Mo mainly in thermal energy range. However, due to the limitation in the number of available rods, the French MIRTE experiments are not sensitive enough to molybdenum, mainly for the first resonance of 95Mo around 45 eV. Unfortunately, high sensitivity in this range is important to improve accuracy of criticality computations of some French reprocessing plants application cases.As a result, to address the needs of criticality-safety, an IER sheet was submitted in 2015 and approved by NCSP program management team. A preliminary design report (CED-1) was issued in 2018 and a final design report (CED-2) at the end of 2020. This latter design proposes to test natural 0.762-mm thick molybdenum sleeves (external diameter = 12.7 mm) around 397 7uPCX rods (UO2 rods enriched at 6.90%), in a test zone that is surrounded by un-sleeved 7uPCX rods. The UO2 rods were set according with a hexagonal 1.55-cm pitch. The total number of rods was 877. The approach to criticality is reached through the addition of rods, the rods being totally immersed in water. Reference experiments without molybdenum are planned to have feedback on cross sections and reproducibility experiments are proposed for confirming the estimate of uncertainties. The material supply study analyzing the costs was done beginning of 2021 and the sleeves were ordered in April 2021. They have been sent to SANDIA national laboratories last September. Experiments are assumed to be performed by the end of 2021 or at the beginning of 2022. The final paper will address the final design, an estimation of experimental uncertainties and will give the calculated keff values with various codes and nuclear data libraries, as well as sensitivities of keff to the main reactions. These experiments will also help improving molybdenum cross sections in thermal energy range as they will complement the work performed on differential measurements (transmission, capture) from JPARC, n-TOF, GELINA and RPI on natural and enriched molybdenum samples and the assessment work of IRSN with the SAMMY and CONRAD codes aiming at producing new resonance parameters and covariance data.[1] International Handbook of Evaluated Criticality Safety Benchmark Experiments, NEA/NSC/DOC(95)/03, OECD NEA, Paris, France – LEU-COMP-THERM-106 – “4.738-Wt.%-Enriched-Uranium-Dioxide-Fuel-Rod Arrays In Water Reflected Or Separated By Various Structural Materials (Sodium Chloride, Rhodium, PVC, Molybdenum, Chromium, Manganese)”, N. LECLAIRE, F.X. LE DAUPHIN, I. DUHAMEL, J.BESS