Production of 238Pu is a complex process. At the top level, this process involves the following steps:
Processing of materials prior to irradiation.
Purify neptunium-237 (237Np).
Fabricate 237Np targets.
Irradiation of targets in a nuclear reactor to transform 237Np into 238Pu.
Processing of materials after irradiation.
Extract, separate, and purify 238Pu and the remaining 237Np from the irradiated targets.
Recycle the extracted 237Np so that it can be used to make more targets.
Process the 238Pu so that it can be used to fabricate RPS fuel pellets, which are then assembled into GPHS modules.
The capabilities of existing facilities and the expertise of existing staff at the DOE’s Idaho National Laboratory (INL) and Oak Ridge National Laboratory (ORNL) make them the best places to carry out the above steps. In particular, there are just two operational reactors in the United States that can enable the production of large amounts of 238Pu (on the order of kilograms per year) in a timely fashion: the Advanced Test Reactor (ATR) at INL and the High Flux Isotope Reactor (HFIR) at ORNL.
The ATR and HFIR reactors are light-water fission reactors that use enriched uranium as fuel. Both have numerous cylindrical voids at various locations in and around the reactor core where targets can be inserted and irradiated. The rate at which 237Np is transformed into 238Pu will vary greatly according to the location of the 237Np targets in the reactor.
There are nine primary test positions (flux traps) in the ATR.5 Six of these are dedicated full-time to the DOE’s Office of Naval Reactors. This office is responsible for developing reactors to power submarines and aircraft carriers for the U.S. Navy. Naval Reactors is the primary customer for the ATR and the primary source of funds used to sustain the ATR.
There are also many other usable positions in the ATR where 237Np targets could be irradiated, although the outer positions have neutron and gamma fields that are an order of magnitude lower than the positions nearest the center of the core. If 237Np targets are placed in all of the core positions except for the six flux traps that are dedicated to Naval Reactors, ATR is thought capable of creating up to 4.6 kg of 238Pu per year using proven, cylindrical 237Np targets and standard reactor operating conditions. Advanced targets with a more complex geometry, which could be introduced later as a process improvement, would increase the yield, perhaps as high as 5.8 kg/year. A yield of 3 to 4 kg/year would allow ATR to produce 238Pu while still supporting the Office of Naval Reactors as well as other users, such as the National Scientific User Facility.
Like the ATR, HFIR also has multiple positions where targets can be irradiated. The DOE’s Office of Science is HFIR’s primary user. Assuming that HFIR will continue to support its primary mission of neutron science, HFIR can create, at most, about 2 kg/year of 238Pu using standard target designs and reactor operating conditions. However, this would reduce the amount of support that it can provide to secondary activities, such as production of medical and industrial isotopes.
Some test positions tend to produce unacceptably high concentrations of an unwanted Pu isotope (236Pu) in irradiated targets. Unlike 238Pu, the natural decay of 236Pu produces significant gamma radiation, which makes handling and processing of irradiated targets much more difficult and hazardous. Because 236Pu has a half-life of just 2.9 years, if irradiated targets are determined to have too much 236Pu, they are stored until the 236Pu decays sufficiently so that radiation levels are within acceptable limits.
Ultimately, the total amount of 238Pu that the United States can easily produce is limited by the availability of 237Np. Trace amounts of 237Np occur naturally in uranium ores, but as a practical matter, 237Np used for 238Pu production must be artificially produced. 237Np is not currently being produced in the United States, and it would not be easy to restart production. (The existing stockpile was created as a byproduct of Cold War production of nuclear weapons material.) However, the United States has enough 237Np in storage at INL to produce 5 kg of 238Pu per year for more than 50 years.
There are four primary options for initiating domestic production of 238Pu in a timely fashion. All of these options (1) rely exclusively on existing reactors (ATR and/or HFIR) to irradiate 237Np targets, (2) would require new or refurbished processing facilities to fabricate 237Np targets and extract 238Pu from the irradiated targets, and (3) would ship extracted 238Pu to Los Alamos National Laboratory for encapsulation in fuel pellets.6
Flux traps are areas with high levels of thermal neutron radiation, which is ideal for converting 237Np to 238Pu with minimal impurities.
The 238Pu encapsulation facilities at Los Alamos National Laboratory are currently operational and have been used to prepare fuel for past missions as well as the Mars Science Laboratory. All four programmatic options for domestic production of 238Pu assume that 238Pu encapsulation facilities will remain at Los Alamos National Laboratory because it would not be cost-effective to relocate them to another location such as INL.