Plutonium power to Pluto

4 January 2016

The USA’s Savannah River site in South Carolina produced the Pu-238 oxide used by NASA’s New Horizons probe to fly within 12,500km of Pluto in July 2015. By Corrina Thompson

The Pu-238 was used in three radioisotope thermoelectric generators (RTGs) to power the mission, which provided the first close-up images of the ice dwarf planet. The generators take heat from radioactive decay and convert it to electricity. New Horizons generators produce about 200W of electric power.

Savannah River restarted the Pu-238 oxide line in the late 1990s, after a short period of shutdown, to produce oxide for NASA's Cassini space mission to explore the Saturn system. Space power sources have to be compact, reliable and effective over very long periods, and where solar power is not practical, Pu-238 can be used. RTGs have no moving parts to wear out or break, making them suitable for the 9.5 year, 3-billion-mile journey to fly by Pluto.

Savannah River's HB Line, located on top of the site's H Canyon reprocessing plant, was constructed in the early 1980s for Pu-238 production, both for space exploration and to recover materials stored in H Canyon.

HB Line has three process lines. Phase I is the scrap recovery processing line. Phase II is the production line for Pu and Np oxides. Phase III was originally the Pu-238 oxide production line, but is now used to prepare excess Pu and U materials for disposition.

Phase I became operational in the late 1980s and was used to dissolve and dispose of legacy plutonium materials, and to dissolve legacy uranium for blending into low enriched uranium for Tennessee Valley Authority. The process converted solid nuclear materials into nitrate solutions and transferred solutions to H Canyon for disposition. In 2011, the Department of Energy told Savannah River to stop chemical processing in Phase I and use gloveboxes to do dry downblending of high-impurity plutonium oxide for disposal at the Waste Isolation Pilot Plant in New Mexico.

Phase II can produce oxide material from Np-237 or Pu-239 nitrate solutions. It started operations for the first time in November 2001. The Pu was shipped to the site's FB Line for packaging in containers for long-term storage, and then to the K Area for interim storage. The Np material went to Idaho National Lab for further processing and conversion to reactor targets for future Pu-238 production and space exploration.

Phase III has been converted into a processing facility, to open storage containers and oxidise metals to allow them to be dissolved in the Phase I area or H Canyon dissolvers.

H Canyon, which became operational in 1955, treats spent HEU fuel from the USA and overseas research reactors that have been converted to LEU or shut down.

In 2014, H Canyon started a programme to reprocess aluminium-clad and aluminium alloy spent fuel from research reactors, such as Oak Ridge's High Flux Isotope Reactor, and to downblend the uranium product for use in power reactors. It has also recovered Np-237 and Pu-238 from irradiated targets, and recovered U and Pu for military purposes.

The H Canyon facility has also completed reprocessing 147 bundles of long-stored U and Th metal fuel from the 20MWt Sodium Reactor Experiment (SRE), which operated in California from 1957 to 1964 and had a high proportion of U-233.

As part of an upgrade programme, 16 modifications were made to H Canyon recently. These included sealing abandoned piping, upgrading filters in the central exhaust system and replacing exhaust building doors. Safety analyses, reference documents and procedures were revised and employees trained accordingly, to keep the facility up-to-date with modern standards and practices. The site also completed repairs and installation of the H Canyon Acid Recovery Unit's reboiler, which recycles nitric acid vapours released during processing for reuse.

H-Canyon is primarily used as the only operating large-scale, radiologically-shielded chemical separations facility in the USA, but it is also a test bed for new technologies in the spent nuclear material field.

Beyond non-proliferation uses, the plant's role has expanded to include special nuclear material accountability, environmental monitoring and compliance, and improved process control. In 2013, Argonne National Laboratory (ANL) became the first outside organisation to use H Canyon to test analytical equipment. ANL developed an ultraviolet visible spectroscopy system that works by using absorption or transmission of light to measure Pu concentration in reprocessing plants. The tool was developed for safeguarding purposes.

With respect to using nuclear power for space missions, there is a limited supply of Pu-238 in the world, with limited budget and production facilities. John Hopkins Applied Physics Laboratory produced a report for NASA, released in June 2015, which concluded that radioisotope power systems may be enough to fulfil currently projected space flight needs, at projected Pu-238 production rates and with current technology. However, significantly increased capability in the rate of electrical power available for missions is possible only with increased Pu-238 production rates or flight qualification of a dynamic converter.

The report said higher efficiency converters, e.g. thermo-electric enhanced multi-mission radioisotope thermoelectric generators or Stirling technology must be matured for space flight, so that they can provide the same power from less Pu-238. If not, an increase in Pu-238 production will be required. ¦

Savannah River Site’s H Canyon building, where the Pu-238 destined for space was produced. Photo courtesy of US DOE.
Inset: NASA’s New Horizons space mission is powered by Pu-238 produced at the US DOE Savannah River Site. Photo courtesy of NASA.

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