The University of Missouri (MU) is pushing boundaries in betavoltaic electricity generation. Patrick J Pinhero, professor in the chemical engineering department and the nuclear engineering programme, and Alan K Wertsching, senior staff scientist formerly with the Idaho National Laboratory discuss their research on water-based nuclear batteries.

Julian Turner: Please talk about the University of Missouri’s research into nuclear batteries.

Patrick J Pinhero: Several teams are pursuing nuclear battery research at the University of Missouri. Much of this work is focused on pushing the frontiers of nuclear battery technology by employing power sources using alpha or beta-particle decay based on a radioactive isotope that can be produced, separated and refined at the University of Missouri Research Reactor (MURR).

The approach adopted by my team and our collaborators in the area of betavoltaic emitters is more simplistic. Our idea is to focus on tritium, an isotope of hydrogen that possesses two neutrons in addition to the single proton and electron inherent to its common hydrogen form.

"A betavoltaic incorporated into a flight data locator could signal to search teams for years instead of months."

Tritium is a radioactive isotope that reduces its overall population by one-half every 12.32 years by the pure emission of a beta-particle, which is essentially an electron, an advantage over many other solutions, which produce harmful gamma radiation. There are other qualities of tritium that are appealing: it is the third lightest isotope, it possesses properties and reactivity similar to hydrogen, its path for production is very well-known and simple, and its hazards are well-known.

JT: What benefits does betavoltaic electricity generation offer compared with photon solutions?

Alan K Wertsching: The first betavoltaic batteries were developed during the 1950s and the basic design – an electron emitter coupled to a collector – remains the same to this day. Commercially available betavoltaic chipsets are low voltage and amp products for niche markets, such as the military, and in order to produce greater performance from betavoltaics, we looked at producing layered stacked arrays as a means of building to the needs of potential customers.

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Unfortunately, traditional materials were unsuitable for large stacked arrays because the mass and volume of a final battery would be excessive. Much thinner, lighter emitters and collectors were needed for an array design. Recent advances in the material science of graphene have yet to be incorporated into betavoltaic architecture, which, when incorporated into thin stacked betavoltaic arrays, would allow greater overall performance and wider utilisation.

There are many situations where betavoltaic power generation can provide superior performance over traditional chemical batteries and solar cell systems, including poor light conditions, physical extremes, inaccessible locations, disaster areas and irradiated locations.

JT: One of the devices under development at MU employs the radioactive isotope Strontium-90 and a water-based solution. How does this contribute to a more efficient nuclear battery?

PJP: Our device is not a water-based device but instead uses tritium, and requires assembly in a very dry, Argon-purged glovebox. Since it involves pure beat-radiation, shielding is quite simple and we do not require the use of a radiation hot cell.

JT: What are the potential commercial applications of advanced nuclear batteries?

PJP/AKW: The notion of a smart phone that recharges itself is appealing but initially the most likely customers are oil and gas and aerospace industries, and space flight companies, which need reliable power sources in inaccessible locations and physical extremes such as high or low temperature and pressure. For example, a betavoltaic incorporated into a flight data locator could signal to search teams for years instead of months.

The US nuclear industry is struggling to survive in tough US electricity markets flooded with cheap natural gas .

The odds of developing a commercially-viable product are reasonably good because ultra-thin collectors already exist, the science behind thin beta-electron emitters is strong and there is undoubtedly a growing international interest in these technologies.

PJP: In 2010, we began to formulate technologies that could solve a significant problem for the US military and potentially be commercialised. Our reasoning is that the military complex is often a better proving ground for developing technologies involving special nuclear material (SNM) due to the safeguards and security required to insure its non-proliferation.

The key objective of our project is a highly-portable continuous power source that can be used in micro-electromagnetic machines, nano-electromagnetic machines and near-term technologies, such as trickle charger units for extending the life of soldier ‘power packs’. This latter example is one of the primary goals of our product as our betavoltaic battery is lightweight, powerful and long-lived.

It is now at a stage where we can to transition from natural Li to isotopically pure 6Li, a regulated SNM. The advantage of doing this is that the thermal neutron absorption cross-sections are several orders of magnitude larger than the cross-sections for natural Li, meaning that the team can examine real manufacturing techniques. In addition, the team has filed multiple patent applications on designs furnishing much higher capacities for powering vehicles.

JT: What safety measures are incorporated into the new batteries to prevent radiation exposure?

PJP: Our devices keep the radiation in the package. One might assume that tritium will be quite mobile because it is so small and could diffuse throughout the graphitic matrix and potentially effuse out due to the difficulty is sealing it. However, the scientific literature has demonstrated that it remains within the matrix at temperatures up to 900K (627°C). Most of the ambient operating environments in which we are promoting this technology are far below this temperature. Our only enemy is moisture. For this reason, we are employing a robust, hermetically-sealed package.

JT: What are the MU project timescales and can we expect to see a prototype in the near future?

PJP: Major proof-of-principle analysis begins with the emitter and we are currently irradiating our higher-power, grapheme-based beta-emitters. Once this is optimised, we can tweak the design of our array device and make button cells of various sizes.

"With enough financial support to fund both our irradiation and packaging, we could have a commercial-ready device in three years."

The major hurdles are in the regulations for the handling and transport of these devices. We desire to work with the Defense Advanced Research Projects Agency (DARPA) or another military sponsor to help us with developing geometry, packaging and field-testing of our devices.

AKW: With enough financial support to fund both our irradiation and packaging, we could have a commercial-ready device in three years, or a little less, depending on size and application. Of course this timeline is predicated on whether there is the regulatory framework in place to allow its use.

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