DESIGN OF ACTIVE-NEUTRON FUEL-ROD SCANNER

An active-neutron fuel rad scanner has been designed for the assay of fissile materials in mixed oxide fuel rods. A Cf-252 source is located at the center of the scanner very near the through hole for the fuel rods. Spontaneous fission neutrons from the californium are moderated and induce fissions within the passing fuel rod. The rod continues pas t a combined gamma-ray and neutron shield where delayed gamma rays abo ve I MeV are detected. We used the Monte Carlo code MCNP to design the scanner and review optimum materials and geometries. An inhomogeneous beryllium, graphite, and polyethylene moderator has been designed tha t uses source neutrons much more efficiently than assay systems using polyethylene moderators. Layers of berated polyethylene and tungsten a re used to shield the detectors. Large NaI(Tl) detectors were selected to measure the delayed gamma rays. The enrichment zones of a thermal reactor fuel pin could be measured to within 1% counting statistics fo r practical rod speeds. Applications of the rod scanner include accoun tability of fissile material for safeguards applications, quality cont rol of the fissile content in a fuel rod, and the verification of reac tivity potential for mixed oxide fuels.