Ke. Holbert et N. Venkatesh, VALID RANGES FOR USING THE CROSS-POWER SPECTRAL DENSITY PHASE-ANGLE FOR MODERATOR TEMPERATURE-COEFFICIENT SIGN DETERMINATION, Nuclear science and engineering, 119(3), 1995, pp. 203-211
The value of the moderator temperature coefficient (MTC) of reactivity
is contained in correlations between fluctuations of the neutron flux
and core-exit coolant temperature. The absolute magnitude of the MTC
is obtained from noise analysis by using the root-mean-square method a
nd the frequency response function technique. Both approaches are used
in conjunction with the phase angle method, which determines the MTC
sign, to obtain complete information about the MTC. Analytical express
ions that are derived show that a limitation exists on the range of MT
C values for which the cross-power spectral density phase angle can be
used to establish the MTC sign. This research shows that small positi
ve values of the MTC (an unstable condition) can result in a -180-deg
phase angle shift, contrary to earlier studies that indicated a stable
reactor. The range of sign determinate MTC values is dependent on the
driving noise source. Simulated noise data are generated for differen
t MTC values and analyzed to verify the theoretical work. A comparison
of the indeterminate regions to allowable MTC values for an operating
pressurized water reactor is also presented.