Instrumentation assembly for nuclear reactor

Claim 1

A neutron detector assembly for use in a fluid moderated thermal neutron reactor core wherein said core includes a plurality of replaceable fuel assemblies of substantially square cross section shape and wherein said fuel assemblies are positioned in spaced relation to provide gaps for fluid moderator therebetween, comprising: an elongated solid moderator-displacing member adapted for positioning in said core between the corners of an adjacent four of said spaced fuel assemblies, said member extending through said core for a length at least equal to the active height of said core, said member having transverse cross section dimensions equal to the diagonal distance between the corners of said four fuel assemblies less a tolerance required for insertion, removal and thermal expansion of said fuel assemblies whereby the moderator in the space between said corners of said adjacent four fuel assemblies is substantially completely displaced to thereby depress and flatten the neutron flux therebetween, said moderator-displacing member being formed with at least one longitudinal passage for receiving a neutron detector for monitoring the neutron flux in said space over the power range of operation of said core, said moderator-displacing member being formed of a metal selected such that said transverse cross section dimensions thereof are less than one-fourth of the reciprocal of the slowing-down power of said metal for fast neutrons and less than one-half of the reciprocal of the macroscopic absorption cross of said metal for thermal neutrons.