Validating Criticality Calculations for Spent Fuel with 252Cf-source-driven Noise Measurements

The 252Cf-source-driven noise analysis method can be used for measuring the subcritical neutron multiplication factor k of arrays of spent light water reactor (LWR) fuel. This type of measurement provides a parameter that is directly related to the criticality state of arrays of LWR fuel. Measurements of this parameter can verify the criticality safety margins of spent LWR fuel configurations and thus could be a means of obtaining the information to justify burnup credit for spent LWR transportation/storage casks. The practicality of a measurement depends on the ability to install the hardware required to perform the measurement. Source chambers containing the 252Cf at the required source intensity for this application have been constructed and have operated successfully for —10 years and can be fabricated to fit into control rod guide tubes of PWR fuel elements. Fission counters especially developed for spent-fuel measurements are available that would allow measurements of a special 3 x 3 spent-fuel array (k = 0.90) and a typical burnup credit rail cask with spent fuel in unborated water (k = 0.92). Adding a moderator around these fission counters would allow measurements with the typical burnup credit rail cask with borated water (k = 0.76) and the special 3 x 3 array with borated water (k = 0.75). The recent work of Ficaro on modifying the KENO Va code to calculate by the Monte Carlo method the time sequences of pulses at two detectors near a fissile assembly from the fission chain multiplication process, initiated by a \"2Cf source in the assembly allows a direct computer calculation of the noise analysis data from this measurement method. Calculations for a special 3 x 3 array of spent fuel in unborated water show that these measurements can provide data for a more sensitive test of calculational capability than comparisons between calculated and measured neutron multiplication factors. The sensitivity to changes in fission product cross sections were a factor of 10 higher for calculated noise analysis ratios of spectral densities and a factor of —40 higher for coherences than for neutron multiplication factor comparisons. Thus, a measurement by the 252Cf-source-driven noise analysis method for this application not only can measure the subcriticality but also can be used to provide data for better validation of calculational methods and cross sections than by using comparisons of calculated and measured neutron multiplication factors.