Effect of the detector type on the performance of a simulated fast-neutron uranium collar for non-destructive assay of fresh fuel assemblies

摘要

We simulated four different versions of a Fast-Neutron Collar (FNCL) and compared their performance for the non-destructive assay (NDA) of fresh fuel. The four versions are based on different scintillation detector materials: liquid EJ-309, plastic EJ-276, trans-stilbene crystal, and small-molecule organic glass. This work was conducted within the framework of the Neutron Rodeo II. The International Atomic Energy Agency (IAEA) is currently testing an FNCL based on EJ-309 as a potential replacement of collars based on He-3 proportional counters to perform the NDA of fresh fuel assemblies. The IAEA FNCL is based on 12 cubic EJ-309 scintillation detectors (10×10×10 cm~3 volume) arranged in three panels, each including four detectors, surrounding the assembly on three sides. The fourth panel is a polyethylene bank holding two AmLi interrogation sources to induce fissions within the assembly. In the simulated FNCLs, we kept this fast-mode active-interrogation configuration constant and replaced only the detection material, to identify the benefits deriving from the use of the different scintillators. We used MCNPX-PoliMi, in conjunction with the MPPost detector response emulator, to model the FNCL response. The performance of each system was compared to the IAEA FNCL in terms of sensitivity to fissile mass and fuel form factor, assay penetrability, sensitivity to burnable poisons, and fuel pins' diversions. The U-235 linear density of assemblies containing 4 and 24 poisoned pins at 6% and 10% wt. concentration of gadolinium per pin was estimated to compare the sensitivity to burnable poisons of the four types of FNCLs. The four systems showed similar overall performance. The FNCL based on EJ-309 exhibits the highest sensitivity, because of the combined effect of the high neutron light output and the hydrogen concentration of this material. The stability over-time of newly developed materials, such as EJ-276 and organic glass, still needs to be demonstrated. This work was sponsored by the U.S. Department of Energy, National Nuclear Security Administration, Office of Nonproliferation and Arms Control.