There is a need for a special type of nuclear reactor that can test fuel elements nondestructively to determine the total amounts and the distributions of both fissionable and parasitically absorbing materials in completed, unirradiated fuel elements. Such a reactor, called the Fuel Assay Reactor, is proposed. Theoretical considerations are presented to show the reactivity changes that result from the insertion of both fissionable and absorbing material into a central test hole. The choice of moderator depends upon the size of material to be tested. For testing small pellets of fissionable material, a light-water- moderated assembly is advantageous because of …
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There is a need for a special type of nuclear reactor that can test fuel elements nondestructively to determine the total amounts and the distributions of both fissionable and parasitically absorbing materials in completed, unirradiated fuel elements. Such a reactor, called the Fuel Assay Reactor, is proposed. Theoretical considerations are presented to show the reactivity changes that result from the insertion of both fissionable and absorbing material into a central test hole. The choice of moderator depends upon the size of material to be tested. For testing small pellets of fissionable material, a light-water- moderated assembly is advantageous because of its small critical mass; for large, completed fuel elements a graphite or heavy-watermoderated system is more suitable. An internal reflector gives increased sensitivity in the center and allows enough space surrounding the test element so that spectral changes in a nonfuel region may be produced in order to discriminate between fissionable and absorbing materials. The factors determining maximum sensitivity are discussed, as are several reactivity measurement methods suitable for testing fuel elements. The limits of accuracy are governed by the statistical variation of the neutron population within the reactor core. The theoretical limits of accuracy of all methods have the same order of magnitude, but the automatic-flux-level method appears to have advantages over the pile-period and oscillator methods for this purpose. (auth)
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