Experimental and Monte Carlo methods for burn up assessment of spent fuel elements

SCK-CEN is currently investigating spent fuel experimental methods and devices to assess their capability for burn up determination. This assessment is performed from an operator's point of view, meaning that certain information like initial enrichment of the fuel element, the irradiation history and material composition can be taken into account for the assessment. Monte Carlo based calculations were carried out for a better understanding of the parameters that influence the response to neutrons of a Fork detector. According to the results presented, the sensitivity of the neutron detector to the inner rows of pins can be increased removing the Cd sleeve and the boron in the water. However, these conditions result in an increased sensitive zone along the axial direction. Simulations oriented to the feasibility of measurements in dry conditions were also carried out. According to the results obtained measurements in dry conditions are feasible and would result in a decrease of efficiency of about 30 %. The axial importance function would be increased by about 35% in dry conditions. Therefore, a worse resolution for a fuel axial scanning is expected.