Year
1997
Abstract
We have developed a new method to measure the reactivity and fissile content of highly radioactive nuclear materials. The fissile content in spent nuclear fuel is difficult to measure because of the high neutron and gamma-ray radiation backgrounds. We are using the intrinsic neutron emissions from curium and plutonium to self-interrogate the fissile material in the sample. The primary emission neutrons originate from the spontaneous fission of curium and plutonium as well as some (a,n) reaction neutrons from the oxides and other matrix materials. We are developing special reflecting surfaces to return a substantial fraction of the emission neutrons back into the sample to induce fission reactions in the fissile materials. The fissile material has a high-fission cross section for thermal-energy neutrons so we are returning a large fraction of thermal neutrons. We use neutron multiplicity counting together with the cadmium ratios to separate the primary emission neutrons from the fission neutrons induced in the fissile material. The cadmium ratio is obtained by measuring the sample twice. First with a cadmium liner (~1 mm thick) between the sample and the reflector and then a second measurement with the cadmium liner removed. We have used Monte Carlo Neutron Photon (MCNP) calculations to optimize the reflectors and sample geometry. Measurements on uranium fuel rods have been performed to test the concept using the high-efficiency passive neutron multiplicity counter (PSMC). The results of the MCNP calculations and the multiplicity measurements are presented.