On the combination of delayed neutron and delayed gamma techniques for fission rate measurement in nuclear fuel

Novel techniques to measure newly induced fissions in spent fuel after re-irradiation at low power have been developed and tested at the Proteus zero-power research reactor. The two techniques are based on the detection of high energy gamma-rays emitted by short-lived fission products and delayed neutrons. The two techniques relate the measured signals to the total fission rate, the isotopic composition of the fuel, and nuclear data. They can be combined to derive better estimates on each of these parameters. This has potential for improvement in many areas. Spent fuel characterisation and safeguard applications can benefit from these techniques for non-destructive assay of plutonium content. Another application of choice is the reduction of uncertainties on nuclear data. As a first application of the combination of the delayed neutron and gamma measurement techniques, this paper shows how to reduce the uncertainties on the relative abundances of the longest delayed neutron group for thermal fissions in ²³⁵U, ²³⁹Pu and fast fissions in ²³⁸U. The proposed experiments are easily achievable in zero-power research reactors using fresh UO₂ and MOX fuel and do not require fast extraction systems. The relative uncertainties (1σ) on the relative abundances are expected to be reduced from 13% to 4%, 16% to 5%, and 38% to 12% for ²³⁵U, ²³⁸U and ²³⁹Pu, respectively.