What causes plutonium 238 to fall apart

USA: Plutonium-238 sample produced for NASA

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December 28, 2015, 12:24 p.m.

The Oak Ridge National Laboratory (ORNL) announced on December 22, 2015 that they had produced a sample of around 50 grams of plutonium-238. The material was manufactured as part of a program largely funded by the US Aerospace Agency (NASA) to manufacture the material for space applications.

Plutonium-238 sample in a so-called hot cell
(Image: ORNL)
With the production of the plutonium-238 sample, a material of this kind suitable for use in radioisotope generators (Radioisotope Thermoelectric Generators, RTGs) was created in the USA for the first time in around 30 years.

The plutonium isotope plutonium-238 with a half-life of around 87.7 years can be used in RTGs on board space probes and landers to generate heat. The heat radiated by plutonium-238 from the spontaneous decay of plutonium nuclei provides the necessary high temperature difference with which thermocouples can generate electricity.

The sample now being produced in the USA at the ORNL in Oak Ridge in the US state of Tennessee consists of exactly the same oxide that has also been used in the USA to date in the construction of the heat sources for RTGs. Most recently, the USA won such material in a plant complex on the Savannah River, the so-called Savannah River Site (SRS), in the state of South Carolina, where production was stopped in the 1980s. In 1984 the Plutonium Fuel Form Facility (PuFF), a plant for forming plutonium pellets, ended. The reactor K required there was finally no longer available at the beginning of the 1990s.

Scientists now want to examine the material from the new sample for its chemical purity. The effectiveness of the manufacturing process is then put to the test. Adjustments may need to be made as the process is scaled to produce larger quantities of the product.

Bob Wham, project leader of the Nuclear Security and Isotope Technology Division of the ORNL, believes that a long-term ability to produce plutonium-238 for power systems, such as those used by NASA on board deep-space probes, is achieved when the manufacturing process takes place automated and upscaled.

The successful production of the sample is the result of a program started about two years ago in which NASA is supporting the United States Department of Energy (DOE) with around 15 million US dollars per year.

The production process as it is now envisaged begins at the Idaho National Laboratory (INL), which currently has a certain amount of stored neptunium-237. The INL provides the ORNL with a needs-based supply of a neptunium-237 oxide.

In the ORNL, the oxide obtained is mixed with aluminum and pressed under high pressure into pellets - tablets - with a high density. Typically 52 such tablets fill aluminum tubes, also known as targets, which are fed to the irradiation after they have been sealed.

The tablets are irradiated in the High Flux Isotope Reactor (HFIR) of the ORNL, whereby the isotope neptunium-238 is formed after a neutron has been absorbed. The latter quickly decays to plutonium-238 with the release of beta radiation.

The irradiated, plutonium-containing aluminum tubes are then first dissolved in the ORNL with caustic soda and a further chemical process enables the plutonium to be separated from the pellets using nitric acid from the remaining neptunium - which can be reused in the process - and other material. The plutonium obtained is then converted into oxide form.

The plutonia is then stored in Los Alamos National Laboratory (LANL) until it is needed to construct a heat source for a space mission.

According to current information from the ORNL, the USA currently still has an inventory of 35 kilograms of plutonium-238 for NASA missions. Only about half of these correspond to specifications with regard to the energy output. The plutonium currently available is likely to last well into the mid-2020s for two or three missions proposed by NASA.

With new material to be produced, one hopes to be able to stretch that which no longer meets the specifications. In this way, you want to quickly increase the usable inventory once production has started.

If the funding of the program continues as before, ORNL and INL could meet NASA's needs, the ORNL announced. Initially, between 300 and 400 grams of the required material would be produced per year. With a higher degree of automation, an average output of 1.5 kilograms per year could later be achieved.

Bob Wham is pleased that they have demonstrated that the selected manufacturing process works and is sure that they are ready for the next steps in the program.

The next NASA mission to use an RTG is that of the Mars 2020 rover. Among other things, this rover will continue the search for life on Mars, test technologies that could play a role in future manned Mars missions, and collect soil and rock samples that may later be brought to Earth.

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