Q: A fresh UO fuel assembly (3.5% enriched) has a surface dose rate of approximately 0.2 Sv/h at 1 m. A MOX fuel assembly of similar geometry has a dose rate of 7 Sv/h at 1 m (5 Sv/h gamma + 2 Sv/h neutron). (a) By what factor is the MOX dose rate higher? (b) Explain the two main sources of the increased radiation from MOX fuel. (c) What additional precautions are required in a MOX fabrication plant compared with a conventional UO plant?
A:
(a) Factor = 7/0.2 = 35 times higher.
(b) The increased dose rate comes from: (1) Gamma radiation from Am-241, the daughter of Pu-241 ( = 14.3 years). Am-241 builds up in stored plutonium and emits a 59.5 keV gamma ray plus higher-energy gammas. (2) Neutrons from spontaneous fission of even-numbered plutonium isotopes (Pu-238, Pu-240, Pu-242). These neutrons require dedicated neutron shielding (hydrogenous materials).
(c) Additional precautions: gloveboxes for all Pu handling (alpha containment), neutron shielding (polyethylene or water), increased gamma shielding, remote/automated operations where possible, strict criticality controls (Pu is more reactive than U-235), enhanced security (separated Pu is a proliferation concern).