My friends at EnerChemTek, a Toronto-based consulting company, recently brought to my attention a technology by a company called Infinite Power for harnessing the energy from the radioactive decay of a strontium[1] isotope, 90Sr. (Infinite? Immediately, my skepticism goes on high alert!) The company’s website promotes the technology, which was invented by scientists at the University of New South Wales, as a solution to global energy crisis. It promises abundant (almost inexhaustible) source of inexpensive clean (i.e., carbon-free) electricity that can power the world! My curiosity was piqued.
If true, and it is a big IF, the claims by Infinite Power would constitute a giant step forward for the power-hungry planet. To be sure, deployment of any technology at global scale takes decades, but having a scalable clean technology is a step in the right direction.
Devices for harnessing energy from decaying radioisotopes are often referred to as nuclear batteries, although they are not electrochemical cells. Such batteries produce electricity through various processes: thermoelectric, thermionic, or photovoltaic. Radioisotope thermoelectric generators of RTGs have been used for long duration space exploration missions. Cassini and Galileo probes were powered by a plutonium oxide RTG. The device described by Infinite Power is different and falls under the category of a photoelectric nuclear battery. It uses the electromagnetic emissions resulting from the radioactive decay to produce electricity with photovoltaic (PV) materials.
There were not many details about the technology on the company’s website, which is understandable. After all, if the company is sitting on a gold mine, they would not want to disclose all their secrets. They do mention the basis of their technology, which is that they have fabricated “hardened PV cells” to produce electricity from the radiation emanating from radioactive strontium. The “hardening” allows these PV cells to capture and convert even high energy radiation, exposure to which degrades the garden-variety PV cells.
90Sr has a half-life of 28.3 years and thus it would take a century for its radioactivity to subside to less than a tenth its original intensity. 90Sr decays by emitting a beta-particle (an electron) and forms 90Y, an isotope of yttrium. In turn, the daughter 90Y decays by another beta-emission to stable 90Zr, zirconium. 90Y is extremely radioactive and has a half-life of only 64 hours. The energy of emitted b-particles from 90Sr and 90Y are 0.5 and 2.5 MeV (million electron-volts). Combined, the energy from the radioactive decay of the two isotopes amounts to about 3 MeV. Not all of this energy could be converted to electricity, but this figure places an absolute limit to what is realizable. So how much energy does that represent? How many cubic miles of oil? Or how many TWh of electricity could possibly be generated by this source?
To answer these questions, we need to know (i) how much 90Sr is available—the resource potential, (ii) how the energy of the emitted particles is transformed into electromagnetic radiation, and (iii) the fraction of that energy that is captured by the “hardened PV device,” and finally convert the MeVs into kWh or other commonly used units of electrical energy.
90Sr is not present in natural strontium, but it is present in the spent nuclear fuel. Nuclear power plants have been operating commercially since 1956 when the Calder Hall plant in England began operation. This was a small facility, only 50 MW of capacity; since then, over 400 nuclear power plants have been built and the global installed nuclear capacity has ramped up to over 350 GW. A typical 1-GW nuclear plant generates about 25 to 30 tons of spent fuel each year. While about a third of the spent fuel is being reprocessed in countries like France, Russia, Japan, and the United Kingdom, most of the spent fuel is stored in dry casks at the power plants. The World Nuclear Association estimates the global stockpile of spent nuclear fuel in 2013 to be 250,000 tonnes (metric tons).[2] Isotopes of uranium and plutonium comprise over 95% of the spent fuel. The amount of 90Sr in the spent fuel is on the order of 1%, for a total of 2,500 tonnes. At 3 MeV per decay, the total energy amounts to 2,230 TWh.[3] While 2,230 TWh is a lot of energy (potentially worth over $100 billion), it is less than a tenth of annual global electricity consumption. In 2018 alone, the world consumed 26,000 TWh of electricity.
Clearly, the technology is not the “(a)nswer to climate change” as proclaimed by Infinite Power. However, the fact that I do not know enough about their technology and could be missing a big factor, I am not inclined to simply dismiss their claims mere corporate hype. This technology can still be a significant component of a broader portfolio of clean technologies. As described, the technology is highly modular and lends itself to addressing energy needs under a wide range of situations, such as powering remote villages, industries, and charging stops for electric vehicles, including trucks. There are probably many applications for which this nuclear battery would be particularly suited and therefore be of interest to investors and technology developers.
In this analysis I have assumed that the entire inventory of Sr in the spent fuels is accessible. While methods for chemical extraction of Sr from spent fuel have been developed—for example, the Idaho National Lab has published a processes for extracting almost 100% of 90Sr from the spent fuel with dilute nitric acid—access to the spent fuel will likely present several jurisdictional barriers, and thus limit the overall supply. Reprocessing has been banned in the USA since 1976. Furthermore, a major factor in the cost of the extraction process will be the implementation of safety protocols for handling radioactive materials.
Another important consideration would be the fraction of energy from the decay that could be converted into electric power. I reviewed a patent underlying the company’s technology, but it was very light on details. It says that their technology as being able to use high energy photons from the decay. As mentioned above, 90Sr and 90Y both decay by emitting high energy electrons. The emitted electrons can cause excitation of electrons in other atoms, which would then emit photons that could excite electrons in a PV cell to produce electricity. However, the patent gives no idea of the spectrum of the radiation from their 90Sr source nor the bandwidth of the PV material. Such information would be essential for determining the fraction of the total available energy that can be realized as electric power.
All in all, I find this technology extremely intriguing and hope that further R&D is conducted to answer some of the questions before the technology is advanced to commercialization.
[1] Strontium has chemical properties similar to calcium and is situated right below it in the periodic table. Natural strontium is not radioactive and non-toxic. However, certain isotopes of Sr that are formed during nuclear fission of uranium are radioactive. Although the b-emissions are readily blocked and can penetrate only about 1 cm through the skin, the chemical similarity of strontium and calcium allows 90Sr to get incorporated in the bones, where it can do damage and thus potentially cause cancer.
[2] https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-wastes/radioactive-wastes-myths-and-realities.aspx
[3] One eV is the energy released one a charge of a single electron drops by 1 Volt. It is equal to 4.45 x 10-23 Wh. To obtain an estimate of a gram mole of eV we multiply the 4.45 x 10-23 Wh by the Avogadro’s number, 6.02 x 10-23 , to get 26.8 Wh. The 2,000 tonnes of 90Sr correspond to 22.2 million g-moles. Multiplying the moles of 90Sr with the amount of energy per eV, and then multiplying that with 3 x 106 (for 3 MeV per decay) I get 1,800 TWh as the energy released from decaying 90Sr and its daughter 90Y.
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