For a safe, economic and ecological final disposal of spent nuclear fuel (SNF), the fuel has to be characterised for decay heat, neutron and gamma-ray emission rate and reactivity. The main challenge of final depositories is to predict these characteristics in timescales from post-irradiation to millennia. This requires a full confidence in the predictive power of the theoretical models and simulation tools, including the nuclear data. In addition, the predictions strongly rely on the correctness of the declared history of the assembly.
Within this framework we aim at understanding how the neutron emission of a spent fuel pellet, originating from an assembly with well-known original characteristics and history, can be used to validate part of the nuclear data and to verify the declared history of a SNF. Therefore, the development of a dedicated neutron detection device and signal processing algorithms are needed and being developed within a SCK•CEN – JRC collaboration.
This project relates to the development of the signal processing algorithms and codes to process the signals originating from a neutron detection device that is optimised to measure the neutron output of a spent fuel pellet. It involves special algorithms to differentiate between neutrons produced by spontaneous fission and neutrons produced by the (α,n) reaction in the SNF. In addition it includes an algorithm to discriminate between neutrons produced in the SNF and background contributions.
The neutron detector system consists of 42 3He proportional counters embedded in high-density polyethylene. The TTL output signals of all detectors are logically OR-ed to a single pulse train, which is digitised by means of a fast digital data acquisition device. The digitizer DT5730B from CAEN will also be used in this work. Through the application of time-interval analysis routines (based on e.g. Rossi-alpha and Feynman-alpha distribution), it is possible to calculate the spontaneous fission rate and the (α,n) reaction rate in the SNF. However, muons created in the atmosphere by the interaction of highly energetic particles from space can cause spallation in (heavy) materials close to or inside the neutron detector system, causing an unwanted background of time-correlated neutron detections. To reduce this background, a veto-system will temporarily disable the signal from the neutron detector system when a muon is detected simultaneously by two plastic scintillators, placed on top of the neutron detector system.