Fuel Assembly Self Shielding of Interrogation Neutrons in a Lead Slowing-Down Spectrometer

Recent attention to nuclear safeguards has stepped up the need for additional non proliferation safety measures. One of these safeguards is the tracking of 239Pu and other fissile materials in spent nuclear fuel. Noninvasive methods are being investigated, including neutron interrogation. RPI has modeled a spent fuel assembly assay method in its lead slowing-down spectrometer. The fuel assembly is interrogated with neutrons from a neutron source in the center of the lead. As the interrogation neutrons slow down in the lead, they create fissions in the fuel assembly. An array of 238U detectors can then detect the fission neutrons from the 235U and 239Pu in the fuel as a function of slowing down time. The focus of this MCNP modeling is to determine the sensitivity and self shielding effects in a 16 x 16 pin fuel assembly. The results show significant shielding of interrogation neutrons from the fuel pins located further from the source up to 80%. The shielding is more significant for slower neutrons than fast. Also, secondary fissions in the assembly greatly affect the detector response and create a nonlinear response to quantities of fissile materials. The system easily identified missing fuel pins, but the response is not proportional to the quantity of fuel missing and depends on the location of the missing pins. The error in determining the quantity of 239Pu was greater than 100% when using a linear fitting model. New fitting procedures and sources of data used to benchmark measurements must be further investigated