The NRC's implicit definition of transuranic (TRU) waste as that which contains alpha-emitting transuranic radionuclides with half-lives greater than 5 years in concentrations greater than 100 nanocuries per gram.¹ It also has separate definitions for two important transuranic radionuclides that do not qualify as TRU waste, but which have decay products that do: plutonium-241 and curium-242.

Plutonium-241 has a half-life of 14.4 years, but its main decay mode is beta, not alpha radiation. However, it decays into americium-241, which is an alpha-emitting radionuclide with a half-life of 432 years, and which does fall into the TRU waste category. Hence the NRC defines waste containing more than 3,500 nanocuries per gram of plutonium-241 as equivalent to TRU waste because it decays into waste containing slightly above 100 nanocuries per gram of americium-241.

Similarly, waste containing more than 20,000 nanocuries per gram of curium-242 (half-life 163 days) decays into waste containing about 100 nanocuries per gram of plutonium-238 (half-life 87 years). Hence, this is also treated as equivalent to TRU waste.

The EPA and DOE definitions of TRU waste, however, include only elements containing alpha-emitting TRU elements with half-lives greater than twenty years in concentrations greater than 100 nanocuries per gram (see Table 4). The EPA-DOE definition is far less stringent than the NRC definition on several grounds:

• It excludes curium-244, which is an alpha-emitter with an 18-year half-life.²

• It does not take into account the fact that high concentrations of plutonium-241 and curium-242 decay into transuranics that meet all EPA-DOE criteria for TRU waste. This means that waste defined as "low-level" by the DOE (because it contains TRU elements with half-lives less than 20 years) could be disposed of in shallow pits. But after several years or decades of storage, some of these wastes could be classified as TRU wastes due to the build up of americium-241 and/or plutonium-238, and hence require deep geologic disposal!

In sum, not only is the TRU waste classification system inconsistent between various bureaucracies, but the contradictions are such that they also imply serious differences in how the same wastes would be managed depending the jurisdiction in which they were created, and the time which is allowed to elapse before disposal.

1. The NRC defines Greater-Than-Class-C (GTCC) waste as that which exceeds low-level waste limits and which must be disposed of in a repository. Therefore, NRC's definition of GTCC waste with only TRU elements should be the same as EPA's TRU waste definition (EPA requires repository disposal for TRU waste).

2. It also excludes another alpha-emitting transuranic, californium-250 (half-life 13 years). This may be an issue with some wastes at Oak Ridge National Laboratory (and elsewhere?).