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Article by Vadim Sablo

Get ready for Nuclear Batteries !

Currently at the University of Missouri, researchers are developing a nuclear energy source that is smaller, lighter and more efficient than any other devise currently in existence. Jae Kwon, assistant professor of electrical and computer engineering, and his research team have been working on building a small nuclear battery, currently the size and thickness of a penny, intended to power various micro/nanoelectro-mechanical systems (M/NEMS).

This invention seems to be very interesting and really promising, however at the same time the word "Nuclear" has connotations of something more dangerous.

We decided to investigate this technology to find out a little bit more about it and to see whether we could really expect to see it on the shelves of the local supermarket in the near future.

 I approached Dr. Mark J. Sarsfield, who is employed in the UK nuclear industry and he agreed to explain and present his views on nuclear battery development to me.

VS - Hi Mark, could you first give us an insight into your views on nuclear energy generally?

MS - At the end of the 19^th century the science community were on the verge of discovering radioactivity; a process that, at the time, appeared to be energy for free. Over the next century the understanding that followed contributed to the discovery of the atom, defined modern day chemistry and nuclear physics, saw the development of the nuclear bomb and use of nuclear fussion for the supply of almost 16% of global electricity needs.¹

 At the core of this technology lies the simple fact that nuclear processes release over 100,000 times more energy than chemical combustion.².  

  Nuclear power sources:

There are many different types of nuclear power sources but they all fall under the umbrella of electrical power generated from nuclear decay events or radioactivity. The longevity of the device will be dependent on the half-life of the radioisotope i.e. the time it takes for half of the material to decay. There are a number of types that work on different principles. The details of each technology can be found on the web: 

Thermoelectric

Thermoionic

Thermoacustic

Thermophotovoltaics

• Dynamic Heat Engines (See Advanced Stirling Radioisotope Generators developed by NASA)
• Alkali Metal Thermal to Electric Converter (AMTEC)
• Beta-Voltaic Systems

 Of all of these technologies thermoelectric generators are the most developed.  These electrical power sources (Radioisotope Thermoelectric Generators - RTGs) work on the basis of converting the heat generated by nuclear decay into electricity by the ‘Seebeck effect'. The technology is well established and devices built by Russia and the USA are used as a reliable power sources for space craft, Arctic bases, remote lighthouses and military use. 

 

 VS - How do the nuclear batteries developed by researchers at University of Missouri work?

MS - These nuclear batteries fall into a category of small devices that generate electrical power by converting energy from a nuclear decay event directly into electricity i.e. not via thermal energy. Beta voltaic technology is by no means new. For example, in the 1970s ¹⁴⁷Pm (promethium) was used in combination with a semiconductor to generate small currents of electricity sufficient to power a heart pacemaker. 

VS - What would be the greatest technological advancement in this particular device?

 MS - The technology proposed by the group at Missouri is interesting and an important step forward for the development of beta voltaics. The big idea here is a reduction in the radiation damage of the semi-conductor by using a liquid semi-conductor. Radiation damage to a solid semi-conductor reduces its energy conversion efficiency over time.

VS - The technology looks promising, however what about the biggest challenges for nuclear batteries?

MS - used in these batteries is a beta emitter, has a very short half life of 87.3 days and will be ineffective in less than half a year. While these studies are an excellent proof of concept longer lived isotopes must be demonstrated to exploit the benefits of extended battery life. It is possible that batteries could last for hundreds of years if they can be made compatible with e.g. ²⁴¹Am, which has a half-life of 433 years and can deposit 0.11 W_th/g.

VS - In general terms, you are saying that these devices have limited benefits over current battery alternatives? But there is the potential to develop batteries that last hundreds of years?

 MS - Yes, the power output of the Missouri devices is very low (nW - 1x10^-9 Watts) and needs to be increased by many orders of magnitude to be of practical use. For example the ¹⁴⁷Pm pacemaker mentioned above had a power output of 23,000 times larger. Having said that there may be low power applications where these devices can be applied as M/NEMS.

The maximum beta decay energy of ³⁵S is 0.167 MeV. Compare this to most alpha decays of around 5 MeV and there is obviously an opportunity to increase the energy density by a factor of ~ 30 by using an alpha emitter. It must be demonstrated that the liquid semi-conductor is capable of harvesting the higher energy densities from alpha particles.

 The device needs to operate at above 100°C as a Se/S eutectic to be in liquid form. A room temperature liquid semi-conductor may be more appropriate although with higher energy density radioisotopes the decay heat may well self-maintain the higher temperatures.  

 The authors report problems with leaking devices and this will not go down well with public opinion. The devices must be robustly contained to strict regulatory guidelines. Also, if a highly radioactive material is required the device may need to be registered as a "radioactive source".

VS - Word "Nuclear" sounds dangerous, are there really any safety issues?

 MS - The power output will be restricted by the amount of radioactivity allowed in each device. For example, to produce 1 we the devices may contain enough radioactive material to become a security threat. It is entirely feasible that many devices could be collected and combined into a dirty bomb-spreading radioactivity in populated areas. Special nuclear material such as this will be strictly regulated.

VS - How about the cost of such a project?

MS - The costs of producing the nuclear material may be prohibitive depending on how much is needed for each device and what its lifetime will be. For example, the cost to build a dedicated reactor to produce 238Pu, used in space batteries, is in the $billions and chemical separations in the $100 millions.

 

VS - In this case what could be realistic steps to make this technology feasible and practical for everyday use?

MS - In my opinion useful targets to aim for are: 

• Generate devices that will last for > 20 years (cf some Li batteries can last > 10 years).
• Reduce radioisotope production costs to less than £50 k / 1 Watt electric (W_e).
• Increase the energy conversion efficiency from the current to > 30%.
• Use less than 38 kBq of alpha radioactive material (amount of 241Am used in smoke detectors) for public acceptability.

 

VS - So, do you think that nuclear batteries will go to the mass market in near future?

 MS - While it is possible to generate safe devices that could be of use to society within the next 10 years, in my opinion they will fill a niche market (possibly military) and be strictly regulated if operated in the public domain. Low power output devices using less than 38 kBq of alpha material may become more freely available

VS - Dr. Sarsfield, thank you very much for your contributions to this article.

Further reading can be found on this subject:

http://munews.missouri.edu/news-releases/2009/1007-mu-researchers-create-smaller-and-more-efficient-nuclear-battery/

http://news.bbc.co.uk/1/hi/8297934.stm

http://www.boingboing.net/2009/10/07/tiny-nuclear-battery.html

http://www.nibsltd.com/