A new hybrid fast-slow ADS for research and applications

We report on the studies of an irradiation facility based on an accelerator-driven subcritical nuclear research reactor, which can simultaneously provide a fast flux in the core and a thermal flux in the reflector, that we will call a hybrid fast-slow ADS. The conceptual design presented here, inspired by [1], starts from a 432 kW (keff=0.967) ADS composed by 110 solid lead fuel assemblies each with size 9.7 x 9.7 x 87 cm3, filled with 81 MOX pins (16.5% Pu+Am) of 0.357 cm radius and surrounded by a 0.068 cm thick AISI steel cladding. Source neutrons are produced by a 70 MeV 1 mA proton beam impinging on a beryllium target (~ 8 x 1014 n/sec) [2].The core is cooled by helium flowing in very thin pipes, 0.25 cm in diameter and is surrounded by a 80 cm lead reflector. Core and reflector are contained within a 2 cm steel vessel. The hybrid version (keff 0.972= P=527 kW) is instead composed by 59 fuel assemblies, each hosting 81 MOX pins (22% Pu+Am) where: - The lead reflector has been replaced by three concentric layers, the first of 35 cm lead, followed by 50 cm graphite and finally 10 cm lead. - In the cooling system water flows in wider pipes, 0.5 cm in diameter, which allows to increase keff while maintaining the fast character of the spectrum. We simulated the neutron flux in three core positions (internal, intermediate and external) and in two graphite reflector positions (internal, intermediate), finding that the flux is still mostly fast in the core, while it exhibits a strong thermal component in the reflector, as shown in the following table. This work is partially supported by the 7th Framework Programmes of the European Commission (Euratom) through the CHANDA contract FP7-Fission-2013-605203.