 It is beryllium-reflected with a very low critical mass but provides neutron fluxes higher than available from a small particle accelerator or other radioactive sources.The SLOWPOKE-2 originally used 93% highly enriched uranium in the form of 28% uranium-aluminium alloy with aluminium cladding, and then in 1985 a new low enriched uranium design (~19.9 % enriched) was commissioned using ceramic UO2 fuel.The core is an assembly of about 200-300 fuel pins, only 22 cm diameter and 23 cm high, surrounded by a fixed beryllium annulus and a bottom beryllium slab.