Masters Theses

Date of Award

8-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

John T. Mihalczo

Committee Members

Paul Stevens, Laurence Miller

Abstract

Fueling of the Advanced Neutron Source (ANS) reactor is unique in that the core is composed of two annular fuel elements with different diameters, each of which is supercritical when moderated and infinitely reflected by heavy water. Therefore, there is a need for careful design and detailed criticality safety analysis of each step of the fueling process to ensure that a criticality during fueling is not a major safety issue. Further, there is a need to measure the subcriticality associated with the loading of the fuel elements into the reactor during the critical experiments program. To maintain subcriticality of the fuel annuli, each fuel element will have material on the inner and outer radial surfaces to isolate the fissile material from the heavy water reflector as the fuel annuli are transferred into the reactor core. The ability of structural materials to decouple the annuli from the heavy water has been analyzed by using the KENO Va Monte Carlo code to determine whether they can be used as the isolation materials. With 25-mm-thick inner stainless steel isolators and outer aluminum isolators that are 127 mm thick for the upper fuel annuli and 57 mm thick for the lower fuel annuli in place, the effective neutron multiplication factors for the upper and lower fuel elements are 0.653 ± 0.002 and 0.769 ± 0.002 respectively. With only one of the isolators in place, the effective neutron multiplication factors for both the upper and lower fuel annuli are less than 0.95; therefore, if the isolators are independently supported, two mutually exclusive events must occur before a criticality event could occur. A Monte Carlo calculation of the 252Cf-source-driven noise analysis method has shown that it is feasible to measure the subcriticality of the fuel annulus/isolator assembly in the critical experiments program and, if appropriate, during the fueling of the ANS reactor. The calculated coherences have shown that a measurement of the ratio of spectral densities to a precision of 2% can be made in less than 11 minutes with the upper fuel annulus/isolator in the transfer shaft of the ANS reactor and with the source in the inner isolator and two 3He counters on each side of the fuel annulus in the outer isolator assembly. The calculated auto power spectral densities, the cross power spectral densities, the coherences, and the ratio of spectral densities can be compared directly to the data obtained from future measurements. The subcriticality can also be obtained, thus verifying the fueling scenario of the ANS reactor.

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