Neutron Measurements outside Containers of Spent fuel for Safeguards

Measurements are routinely carried out on individual spent fuel assemblies, but when they are packed for storage into large containers they become difficult to measure. Up until now, the safeguards approach has relied on maintaining continuity of knowledge by the use of appropriate containment and surveillance. This paper presents the implementation of a neutron measurement device for verification measurements on such large storage containers. The method establishes the internal total neutron source strength of the container from individual measurements on each assembly using a Fork detector. (These measurements are routinely carried out by Euratom). The \"calibration\" factor which related the internal source strength into a surface neutron flux (and hence counting rate) was determined by using spent fuel containers of identical design that were under C/S. The detector was made form two standard neutron monitors separated by polyethylene, which allowed the front (signal) flux to be distinguished from the rear (background) flux. This was necessary because the containers are stored close together. Radial measurements were made on three spent fuel containers. Uncertainties (overall 5%) were assessed as follows neutron emission from Fork measurements 2-3% reproducibility ( positioning and counting statistics) 0.5% background subtraction 3% effect of distribution of assemblies in the container 2% Routine Neutron flux measurements on all spent fuel containers would reduce the uncertainties on reverifications for Safeguards purposes Vertical neutron flux profiles were also measured.