A New Life for Spent Nuclear Fuel

 

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.

 

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[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.

Role of Bioenergy in Achieving Sustainability

I was recently invited to give a keynote address at an international conference on Bioenergy and Sustainability. Because of the Covid-19 pandemic the conference was held virtually over Zoom. What follows is an abstract of my presentation; the full lecture can be accessed here.

The word sustainability shares its root with sustenance. In the case of modern society sustenance comes from use of energy, which is derives from many sources: oil, coal, natural gas, hydroelectric, nuclear, wind, solar, and biomass. Annual consumption of global energy is equivalent to 4 cubic miles of oil (cmo), about 3 of which are obtained from fossil sources: oil, coal, and natural gas.

 

The dominance of fossil energy in the global mix has been longstanding—ever since the dawn of the industrial revolution in the mid nineteenth century. As a result, the concentration of carbon dioxide in the atmosphere has increased from 280 ppm to over 400 ppm and continues to rise. CO₂ is a greenhouse gas and it and now threatens life as we know it from the resulting climate change. To avert devastation from climate change or constrained energy supply, the world desperately needs sources of clean, carbon-free energy that together can scale to cmo levels.

 

Much emphasis has been placed in recent years on resources like wind and solar to provide clean electricity. Technological advances have led to dramatic reductions in their costs and their advocates now propose a future powered entirely by them. However, these costs do not include the cost of storage, currently provided by natural gas, nor do they consider the environmental cost of mining for the materials needed for their installation. Scaling them to a 100%-renewables scenario will strain the global supply of commodities like steel, concrete, glass, and aluminum; clearly not a sustainable scenario.

 

Burning biomass has been proposed as a fuel source; indeed, prior to the industrial revolution the world once derived 100% of its energy from bio sources. Unlike wind and solar, bioenergy sources are storable and do not suffer from intermittency. However, biomass use also results in emitting CO₂. The only reason these emissions are not counted is that the regrowth of the biomass would take an equivalent amount of CO₂ out of the air. For this assumption to hold, it is important that we consider harvesting only rapidly growing biomass or annual crops.

 

Global biomass production is substantial; it is estimated that 75 Gt (gigatons, or 10⁹ tons) of biomass are produced annually. Most of the biomass is in the forests and oceans and not readily recoverable, nor is it desirable to cut down this “sequestered” carbon and burn it. The estimate for recoverable biomass resource is only 3 Gt/y. At a heating value of 15 GJ/t (gigajoules/ton) the energy from these 3 Gt of biomass would correspond to only 0.3 cmo. The low energy density of biomass translates into large areas over which the biomass to be harvested and transported to the power plant: 160 sq. miles of fast growing trees each year to power a single 100 MW plant.

Clearly, we cannot rely on biomass to meet global energy demand for clean energy. Yet, there are some applications where energy from biomass is uniquely suited. Production biofuels is one such example, and many conversion of starch in grains into bioethanol is a thriving business—thanks in large part to the support the industry receives from various state agencies. There are also processes for converting lignocellulosic wastes into biofuels, although there deployment has been hampered by high costs. The main reason for using biofuels is to reduce greenhouse gas emissions; however, on a life-cycle basis the biofuels reduce greenhouse gas emissions between 20% and 40%!

Co-firing biomass, particularly waste biomass, may provide only a limited amount of energy, but it would help enormously with waste management since many municipalities are running out of landfill space. Likewise, utilizing agricultural waste in an engineered system rather than open-field burning would go a long way in reducing urban pollution in many countries.

True sustainability demands a scalable source of clean and cheap electricity. Nuclear power can deliver that. It has the smallest environmental footprint and the best safety record, but public concerns over plant safety, long-term storage of waste, and cost are considerable obstacles. Getting the public to embrace nuclear power is a Herculean task, but it must be undertaken. We have to (i) educate the public (ii) stop closing functional nuclear power plants; (iii) expand the fleet of nuclear power plants; and (iv) develop and deploy the next generation of walk-away safe plants that can also use the spent fuel as a resource.

Planet of the Humans: A Review

Michael Moore’s documentary Planet of the Humans was released on YouTube in conjunction with Earth Day, 2020—a casualty of the Covid-19 pandemic as a theatrical release was precluded. I had heard of the movie and knew that it was about the limitations of renewable energy sources. I was glad that a renowned documentarian like Moore was putting a spotlight on the shortcomings of “renewables,” because the public has been largely seduced into believing that wind, solar and biomass can provide us with all our energy needs while averting climate change. Last weekend, I finally got to see the movie.

I had mixed reactions as I watched the documentary. Although Michael Moore is the Executive Producer, he does not appear in the film. The film is narrated by Jeff Gibbs who tells his journey of disillusionment with the green movement. I liked how effectively Gibbs dispenses with the myth that wind and solar lead to reduced emissions! Reducing greenhouse gas emissions should be our main objective, not simply installing renewable energy sources. Gibbs points out the massive amounts of steel, concrete, and other materials required for any wind turbine, which would last for only 20 years. Similarly, large quantities of commodities are also needed for a solar farm. Production of the commodity materials requires tons of fossil resources, which cut into the “greenness” of these installations. All the mining necessary to obtain the raw materials also degrades the environment.

Gibbs loses credibility when he asserts that it takes more energy from fossil sources than what is produced by wind and solar power plants. Many reputable life-cycle analyses show that these sources are net positive energy producers after five to seven years. By overstating his case against wind and solar and not acknowledging their contribution to carbon-free electricity, he leaves himself open to easy criticism by advocates of renewable “green energy.” And he has been criticized for that by many environmentalists.

That said, Gibbs is right in his overall criticism. Life-cycle analyses do not count emissions from sources needed to back up wind and solar. Those emissions are counted as emissions from coal or natural gas plants, even though in many instances these plants are expressly built to support a new wind or solar farm. The necessity to back up intermittent sources is the reason why Germany’s greenhouse gas emissions did not decrease even though the country spent over $500 billion on its Energiewende (Energy Transformation) program.

Another fallacy of the “renewables” is that it considers biomass carbon neutral. Burning wood chips (actually forests, as the movie points out) is not good for the environment and is ruining habitat. It also can take a hundred years or more for new trees to grow and offset the carbon dioxide emitted during the burning of wood chips. Gibbs dwells a lot (indeed, too much for my liking) on the environmental damage done by growing biomass for fuel or devoting farmland for growing crops to make biofuels.

Gibbs next turns his attention to the moneyed interests behind the “green” movement. He exposes many promoters of green energy as deeply vested in fossil energy sources. The reason fossil companies support wind and solar installations is that they know the renewables will have to be backed up by new natural gas plants or the predominately coal-based grid. [i]

Gibbs portrays many leaders of the environmental movements such as Bill McKibben and Al Gore as being in the pockets of fossil energy companies. Without convincing evidence, such portrayal is sure to raise the ire of the green movement. The movie seems to make the case that nothing good will come if we rely on market forces, forgetting the counterpoint that profit motive is exactly what drives innovation and solutions.

I was particularly disappointed by the documentary because it offered no solutions. It completely ignored nuclear power and just harped on the Malthusian theme that we humans are the problem since we have so increased in numbers and each of us is consuming too much! The last part of the movie showed scenes of rampant environmental damage and an orangutan clinging on to a lone tree surrounded by devastated landscape. Pure environmental porn; ugh!  

The movie leaves the viewer hopeless, and with a sense of despair that can lead to inaction, when what the world needs is rational thinking and concerted action. In an interview with Stephen Colbert, Michael Moore said that his objective in producing this film was to raise the alarm and motivate the younger generation into action. I am afraid, by not offering any hope the movie fails on that front.

So, what are we supposed to do?  Revert to 17^th century lifestyles? That seems to be the message but recall that back then life expectancy was barely 40 years and infant mortality was ten times higher than today, and people spent almost all their time in the drudgery of procuring food and fuel rather than other pursuits. The world population in the 17^th century was less than 1 billion; today it is 7.5 billion and about half are living in unacceptable poverty without access to electricity, clean water, and adequate food. It will take energy to lift them out of poverty so they too can lead healthy, productive lives. Where is that energy going to come from?

Incidentally population growth, a theme that the movie touches, is best addressed by raising living standards. The need for copious quantities of clean energy is evident.  The movie makes it amply clear that the required energy cannot be produced through the magical thinking of 100% renewables. It will require a substantial expansion of nuclear power—the one clean source that is safe, scalable, and cheap enough for global deployment.

As an antidote to the movie, I suggest watching this interview by Michael Killen of me and Alex Cannara. https://youtu.be/IjhnE-hgx0M

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[i] Battery storage just does not scale to provide the required back up; besides it too is fraught with issues of materials supply.