Review of Subcritical Source-Driven Noise Analysis Measurements

Subcritical source-driven noise measurements are simultaneous Rossi-a and randomly
pulsed neutron measurements that provide measured quantities that can be related to the
subcritical neutron multiplication factor. In fact, subcritical source-driven noise
measurements should be performed iii lieu of Rossi-a rneasurements because of the
additional information that is obtained from noise measurements such as the spectral ratio
and the coherencc functions. The basic understanding of source-driven noisc analysis
measurements can be developed from a point reactor kinetics model to dcinoiistrate how
the measured quantities relate to the subcritical neutron multiplication factor. More
elaborate models can also be developed using a generalized stochastic model. These
measurements can be simulated iising Monte Carlo codes to determine thc subcritical
neutron multiplication factor or to determine the sensitivity of calculations to nuclear
cross section data. The interpretation of the measurement using a Monte Carlo method is
based on a perturbation model for the relationship between the spectral ratio and the
subcritical neutron niultiplication factor.
'The subcritical source-driven noise measurement has advantages over other subcritical
measurement methods in that reference measurements at delayed critical arc riot required
for interpreting the measurements. Therefore, benchmark or in-situ subcritical
measurements can be perfornied outside a critical experiment facility. Furthermore, a
certain ratio of frequency spectra has becn shown to be independent of detection
efficiency thereby making the measurement more robust and unaffected by drifts or
changes in instrumentation during the measurement. Criteria have been dcfined for
application of this measurement method for benchmarks and in-situ subcritical
measurements. An extension of the source-drivcn subcritical noise measurcment has also
been discussed that eliminates the few technical challenges for in-situ applications