Analysis of the ZED-2 reactor for the benchmarking of newly measured Gadolinium capture cross-sections

In nuclear technology, a detailed knowledge of nuclear data is of primary importance to optimize the design characteristics of nuclear facilities and to enhance related safety margins. In particular, Gen-II and Gen-III nuclear power reactors take advantage of neutron absorbing materials – called burnable poisons – that are mixed with the fuel in the pins of certain Fuel Assemblies (FA) in the core and are consumed during the cycle. This is a common practice that allows to pack more fissile material into each FA at the beginning of the cycle. This increases the amount of energy extracted from it over the whole cycle with no safety issues, since the poison compensates the extra-reactivity. There is a consequent economic gain for the utility in the availability factor and plant capability. The most important burnable poison is gadolinium, through its odd isotopes 157Gd and 155Gd which have a high capture cross-section. An improvement of the current nuclear data for these gadolinium isotopes has a feedback on nuclear plant management and safety. For that reason, a joint ENEA and INFN research team has carried out new measurements for these nuclides at the n_TOF facility at CERN. In the present report, the new evaluated nuclear data are considered in comparison to standard ENDF/B-VII.1. The ZED-2 nuclear research reactor (at Chalk River Laboratory – CRL, Canada) is considered as a possible case study for such evaluation, since a reference experiment with gadolinium diluted in the moderator has been performed there. Some simulations have been made with different quantities of gadolinium poison in the reactor moderator, employing the MCNP6.1 Monte Carlo transport code. The neutron spectrum in the moderator is calculated as well as k effective and its sensitivity to the microscopic neutron capture cross-section of 157Gd and 155Gd. The results highlight the relevance of the system since it presents a thermal neutron spectrum and it is sensitive to the data of interest. Comparison with results obtained at the ZED-2 facility by the CRL research team is not straightforward due to employment of different nuclear data sets and simulation parameters. It is shown that model calibration cannot cover and compensate all these aspects. More details are required and a sensitivity calculation concerning other parameters (temperature of fuel, moderator and thermal scattering data) is needed. Possible other benchmark cases should be investigated as well.