Spectral density measurements with a mock-up of the fast flux test facility reactor

Abstract

This paper describes two series of noise analysis measurements that were performed with a mock-up of the Fast Flux Test Facility Reactor (FFTF), which is a liquid-metal fast breeder reactor. The first measurements demonstrated the adequacy of in-core noise analysis measurements for reactivity determinations down to 34 dollars subcritical. In the first series, the ractivity of the mock-up was determined by breakfrequency noise analysis (BFNA) over a wide range of reactivities down to 34 dollars subcritical. The results from the BFNA with a pair of detectors in the core agreed with results from the modified source neutron multiplication measurements for reactivities down to 34 dollars subcritical. The purpose of the second series was to determine the feasibility of a new method of determining reactivity by using 252Cf. For these studies, the loading of the mock-up of the FFTF was changed to simulate the initial loading of the reactor. The auto-power spectral densities and the real and imaginary parts of cross-power spectral densities between each detector of a pair and a detector containing 252Cf, and between the pair of detectors were measured simultaneously as a function of frequency for reactivities from ∼0.8 to ∼75 dollars subcritical. The results of the second series demonstrated the feasibility of the method by verifying many of the predictions of the theory of this method which determines the reactivity in units of multiplication factors from measurements only for the subcritical state of interest. This determination does not depend on the detection efficiency or inherent source strength.