Viewpoint: European push for nuclear technology in space

There are places in the solar system that are cold and dark. Why is it that these are exactly the places scientists want to go? Well, for one: cold, dark environments promise to be well-preserved repositories of the past conditions of planets and minor bodies from the time when the solar system formed, as well as containing hidden treasures like samples of ancient solar activity or meteoroid impacts.

Exploring these regions requires missions with a long lifetime. Vehicles have to survive for a long time on planetary surfaces, astronauts require a consistent, robust source of heat and electrical power, and interplanetary spacecraft must work over decades travelling to the outer fringes of our solar system. What the future of sustainable space exploration and science requires is a new breed of power systems.

The sun (almost) always shines

Today, the space community relies mainly on photovoltaic power systems, a technology that was originally developed for the purpose of space applications and has found many terrestrial uses. However, these systems pose severe limitations for missions to places like the outer solar system. The available solar energy reduces with the square of the distance from the sun. For example, at Saturn the solar power density is a hundred times smaller than at Earth.

For this reason, space agencies have in the past relied on radioisotope power systems to power space probes like Cassini and Gailieo. These power systems have been implemented as Radioisotope Thermal Generators (RTGs), in which the heat from radioactive decay is converted into electrical power using the thermoelectric Seebeck effect. These RTGs have previously used 238Pu as a heat source due to its property of being an almost pure alpha-emitter when decaying into 234U. With a half-life of 90 years it is appropriate for most missions. Human missions have also used radioisotope power systems: The Apollo missions to the moon carried RTGs in order to power long-life experiment equipment.

Today, both approaches - solar photovoltaic and 238Pu-based RTGs - together can just meet the requirements of current missions in space exploration and science. However, new and more ambitious mission proposals are constrained by the availability of power sources. Realising these ambitions will require disruptive solutions.

Breaking through

The root cause of the constraints today are factors like the low levels of solar energy in the outer solar system, the 14-day-long lunar night, planetary winters and weather phenomena including dust storms - as well as the expensive and long process to produce the 238Pu fuel for RTGs. While we cannot do anything about how the sun works, breakthroughs are possible in the field of radioisotope fuel for RTGs.

Workshop to enable the new approach

The workshop discussion will involve stakeholders from the nuclear industry and research community, as well as space mission managers, space scientists, application developers, and service providers. The objective is to further advance a partnership between the public and the private sectors, the providers and the customers, and to understand the economic landscape of the technology. If successful, the workshop will mark a significant milestone towards improving the breakthrough technology of radioisotope hybrid power.

Over the last ten years, European researchers have grown more and more interested in the potential use of 241Am instead of 238 Pu. 241Am grows naturally as a result of the decay of Pu stockpiles that have over time been accumulated by nuclear power stations using the uranium cycle. Like 238Pu, 241Am is a pure alpha emitter, thus allowing it to be shielded easily for safe use around humans.

The down side of 241Am is its lower radioactivity by a factor of four, and the associated loss of thermal power density by the same factor. The biggest advantage of 241Am compared to 238Pu is its accessibility through a straight-forward chemical separation process, without the need for an intermediate nuclear irradiation step. This advantage opens a path to more affordable and more abundant access to radioisotope fuel for RTGs.

If combined with more efficient power conversion technologies, such as the Stirling cycle, even the disadvantage of lower power density can partially be compensated for. After a long study phase European research partners are growing more convinced that 241Am-based radioisotope power systems should be realised in the near term.

Added impetus come from the critical needs of near- and mid-term exploration and science missions. For the near-sun destinations like the moon and Mars, an optimal combination of photovoltaic, energy storage, and radioisotope technologies appears the best solution. The next step is to develop a chemical separation facility in Europe that can enable access to tens of kilogrammes of 241Am per year in parallel to the development of advanced power conversion and encapsulation technologies. On the political and legal side, the certification aspects of including radioisotope power systems on European launchers need to be addressed. If these steps are taken then a breakthrough in the exploration of our solar system will be within our reach: new, longer, more effective and affordable missions.

Customers expect more

Today, the customers of space missions - ultimately the citizens of countries involved in their realisation - expect more. They expect direct benefits, efficiency in implementation, and timely delivery. If radioisotope hybrid power becomes available in the future, missions such as the European concept HERACLES becomes even more technically possible. HERACLES could land on the moon in the mid-2020s, explore its surface for a whole year instead of just two weeks (the length of the moon's day), and return with samples of diverse origin. Future human missions to the moon and Mars could rely on a very robust source of thermal and electrical power. In particular the electricity produced by radioisotope hybrid power systems is very attractive to the human spaceflight community since a failure of a complex power storage subsystem during lunar night could spell disaster for the crew. Given this and other enabling and disruptive characteristics, it is quite possible that radioisotope hybrid power systems will accelerate a new and exciting phase in space exploration.

Dr Markus Landgraf

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