MOX fuel presents significantly greater radiological hazards than fresh UO fuel. The main concerns are:
1. Neutron Hazard from Spontaneous Fission
Spontaneous fission of Pu-240 (and to a lesser extent Pu-242) produces neutrons. This creates a small but measurable neutron dose field around MOX fuel assemblies. Neutrons are biologically more damaging than gamma rays (higher radiation weighting factor), so this is an important consideration.
2. Gamma Hazard from Am-241
The main gamma ray hazard from PuO comes from Am-241, which is formed by the beta decay of Pu-241:
Am-241 emits a characteristic 59.5 keV gamma ray with high abundance. Because Pu-241 has a relatively short half-life (14.3 years), the Am-241 content — and therefore the gamma dose rate — increases with time after the plutonium is separated. This sets a limit on the shelf-life of MOX fuel before a stage of americium elimination must be performed.
3. Self-Heating from Pu-238
The alpha decay of Pu-238 generates significant heat. This may lead to storage problems and must be accounted for in transport container design and interim storage.
4. Inhalation Hazard During Fabrication
The chief hazard associated with plutonium is the possibility of inhalation of airborne particulates generated during fabrication of the fuel pellets. Plutonium is one of the most radiotoxic substances known when inhaled. The main dose burden is normally to personnel involved in the maintenance of the automated plant. For this reason, MOX fabrication plants are highly automated with extensive containment and ventilation systems.