Abstract: An accurate determination of whether the reactivity of individual spent fuel assemblies exceeds a threshold value is made while each assembly is being transferred from the reactor core to the fuel storage rack. The reactivity of each spent assembly is compared with that of a standard assembly by comparing the subcritical multiplication resulting from insertion of a neutron source into the assemblies. The measuring apparatus preferably exteriorly resembles a control element assembly wherein one control element finger containing the neutron source is yoked to another finger containing a neutron detector. The fingers are simultaneously inserted into a standard assembly having a known reactivity and subcritical multiplication, and the resulting flux signal is recorded. Thereafter spent assemblies are sequentially measured to assure that no assembly having a subcritical multiplication greater than that of the standard is ever placed into the storage rack.

Abstract: The upper portion of a nuclear reactor vessel supported in a concrete reactor cavity has a structure mounted below the top of the vessel between the outer vessel wall and the reactor cavity wall which contains hydrogenuous material which will attenuate radiation streaming upward between vessel and the reactor cavity wall while preventing pressure buildup during a loss of coolant accident.