MOX Fuel - Americium Content

Americium Content

Plutonium from reprocessed fuel is usually fabricated into MOX as soon as possible to avoid problems with the decay of short-lived isotopes of plutonium. In particular, 241
94Pu decays to americium-241 (241
95Am) which is a gamma ray emitter, giving rise to a potential occupational health hazard if the separated plutonium over five years old is used in a normal MOX plant. While 241
95Am is a gamma emitter most of the photons it emits are low in energy, so 1 mm of lead, or thick glass on a glovebox will give the operators a great deal of protection to their torsos. When working with large amounts of americium in a glovebox, the potential exists for a high dose of radiation to be delivered to the hands.

As a result old reactor-grade plutonium can be difficult to use in a MOX fuel plant, as the 241
94Pu it contains decays with a short 14.1 year half-life into more radioactive 241
95Am which makes the fuel difficult to handle in a production plant. Within about 5 years typical reactor-grade plutonium would contain too much 241
95Am (about 3%). But it is possible to purify the plutonium bearing the americium by a chemical separation process. Even under the worst possible conditions the americium/plutonium mixture will never be as radioactive as a spent-fuel dissolution liquor, so it should be relatively straight forward to recover the plutonium by PUREX or another aqueous reprocessing method.

Also, 241
94Pu is fissile while the isotopes of plutonium with even mass numbers are not (in general thermal neutrons will usually fission isotopes with an odd number of neutrons, but rarely those with an even number), so decay of 241
94Pu to 241
95Am leaves plutonium with a lower proportion of isotopes usable as fuel, and a higher proportion of isotopes that simply capture neutrons (though they may become fissile isotopes after one or more captures). The decay of 238
94Pu to 234
92U and subsequent removal of this uranium would have the opposite effect, but 238
94Pu both has a longer halflife (87.7 years vs. 14.3) and is a smaller proportion of the spent nuclear fuel. 239
94Pu, 240
94Pu, and 242
94Pu all have much longer halflives so that decay is negligible. (244
94Pu has an even longer halflife, but is unlikely to be formed by successive neutron capture because 243
94Pu quickly decays with a halflife of 5 hours giving 243
95Am.)