Addressing NASA’s Plutonium Problem

Yet another positive aspect of NASA’s highly successful New Horizons probe is the fact that it is focusing attention on one of the biggest gaps in fulfilling some of the space agency’s long term plans; the lack of a coherent policy regarding space nuclear power. New Horizons, like Cassini at Saturn and the Curiosity rover on Mars, receives its electrical power from the heat generated by the decay of Plutonium 238 which is fed to a thermo-couple, a device which exploits the creation of a temperature variation between dissimilar metals to generate electricity. The overall apparatus consists of a variable stack of 1.5 KG PU-238 pellets which can configured to produce a certain power level, fins to radiate the heat and the thermo-couple itself. In recent years, NASA has standardized on a version called the Multi-Mission Radioisotope Thermoelectic Generator or MMRTG.

MMRTG: Credit NASA

With no moving parts to fail, it is as practically fool proof as anything made by humans can be. It is also woefully inefficient, converting only a small fraction of the available heat into usable electricity. This might not be such a problem if it weren’t for the fact that an even more inefficient element, i.e. U.S. government policy, is intimately involved.

After years of relying on Russia for a supply of PU-238 as its own infrastructure fell into disrepair, the U.S. is close to exhausting its dwindling stock of hockey puck sized pellets, which currently stands 35 kg, 17 kg of which is available to NASA. That limited inventory is only enough to mount two more missions beyond that required for the MMRTG which will go on a 2020 follow-up to the Curiosity Rover.

A belated effort to restart U.S. production headed by the Department of Energy but funded by NASA is underway, but it is fraught with bureaucratic turf issues, much of which boils down to the fact that NASA is being made to fund repairs to DoE facilities and is not happy about the costs. Once production, which comes from de-commissioned nuclear weapons and not new enrichment, resumes, it will be meager at best, resulting in an additional 1.5 kg per year.

One alternative is to use technology which makes better use of PU-238, thus stretching the supply further. The most obvious candidate is the Advanced Stirling Radioisotope Reactor, a $300 million dollar project managed by NASA Glenn from 2008 until it was cancelled in 2013.  Developed by prime contractor Lockheed Martin, the ASRG would have used the heat from PU- 238 to drive a conceptually simply but difficult to perfect piston engine offering four times the electrical output of the MMRTG for the same amount of fuel used.

Earlier this year, NASA released a study prepared by Johns Hopkins’ Applied Physics Laboratory at its request outlining issues related to the agency’s simultaneous pursuit of nuclear electric power for both the Science Mission and Human Exploration Operations Directorates. Labelled the Nuclear Power Assessment Study, the work examined possible overlap between lower power Radio Isotope Power systems (RPS) and comparatively higher power Fission Power Systems (FPS). Notably, the latter did not give any consideration to nuclear fission for the purpose of propulsion, only for electrical supply.

And while the study did conclude that NASA (just barely) has enough PU-238 at its disposal to conduct missions currently under consideration, it comes at the cost of a single mission maximum capacity of approximately  600 watts, a figure well below that used by the Cassini probe at the time of launch. As a result, a related finding is that for any new SMD missions requiring capacity significantly above the 600 watt threshold, it would demand either increased PU-238 production, or a change to a Stirling type generator.

Given the long time frames involved in developing most large NASA science missions, and the distinct lack of funding to do just that, the Johns Hopkins report might have quickly faded from consideration if it were not for the success of the New Horizons mission. Eight days after the Pluto fly-by, Ohio Senators Rob Portman (R) and Sherrod Brown (D) teamed up to propose the Efficient Space Exploration Act, which calls for NASA  to produce “a report on requirements and risks in connection with the use of radioisotopic power systems for space exploration beyond low-Earth orbit.”

Due within 180 days if passed, the act asks NASA to quantify just how much plutonium it really does need, and what the risks of running out, or just low, might be to space science. The motivation behind the requested report is not surprisingly, local politics. The Glenn Research Center is in Cleveland, and an Ohio company,  Sunpower Inc, recently acquired by Ametek, was a major sub-contractor to the ASRG project, and bills itself as the “world leader in free-piston stirling engines.”

While there is no doubt the Senators are hoping for a conclusion which builds the case for resuming the ASRG program or something similar, whatever its findings, a further examination of the PU-238 issue is certainly warranted. It almost defies belief that the United States could have achieved all that it has in robotic space exploration while at the same time badly undercutting its capacity to build on that legacy. It is all the more ironic that it comes at a time when launch costs are falling and spacecraft capacity is increasing.

It would almost be like going to the Moon in the 1960’s and then not returning for the next 60 years or so.