Aim:  To develop and validate a waste repository acceptance characterisation system for use on site with very low, low and intermediate level radioactive waste. As the legacy waste on nuclear sites needs to be verified for alpha active isotopes and in particular for fissile materials, the system will include gamma scanning and active/passive neutron measurements (sensitive to fissile materials).

Work is progressing on the passive/active neutron measurement station and the segmented/ tomographic gamma scanning station of the waste characterisation system with the purpose to bring them into operation at the JRC Ispra site. The neutron measurement station is composed of a central drum cavity surrounded by a neutron detection system composed of ³He gas proportional counters.  The drum counter has 18 parallel electronics chains connected to the ³He detectors each providing a signal pulse train representing neutron detection events.  The detection system was taken into operation and key assay parameters such as the neutron detection efficiency and thermal neutron die-away time were determined (figure 4). The determination of the average neutron life-time from the exponential fit of the die-away time is essential for the calculation of the detection probability of subsequent neutrons from a fission event.

In the coming months, passive measurements with fissile material standards and preparations of the active neutron measurements (i.e. inducing fission in fissile isotopes of the waste item by means of an external neutron generator with subsequent detection of the prompt fission neutrons) will be undertaken.

Top: Relative count rate distribution in each detector channel. Each side of the drum cavity has three detector modules of which the centre module counts slightly higher than the two adjacent modules when exposed to a centred neutron source. The top and bottom modules (channels 13–18) have shorter ³He tubes and thus lower count rates. Bottom:  Exponential fit to the time distribution of fission neutrons (die-away time).