Saturday, November 17, 2018

Nuclear Power? Yes, but what about…?

The Intergovernmental Panel on Climate Change (IPCC) recently released its special report on climate change with a clarion call for immediate action to reduce greenhouse emissions.[i] That has been clear for a long time, and the dire consequences of climate change are now expected to be felt as soon as 2030. The urgency is mainly because as a society we have been late in taking action.

The world also needs lots of energy, particularly electricity, for the well-being of its citizens. Unfortunately, the one source of clean electricity, nuclear power, has been off the table for many in the “environmental” movement. Readers of this blog know that I am a strong proponent of nuclear power. In my last post, I pointed out three reasons for supporting nuclear power: near-zero carbon emissions, fewest fatalities per unit of electricity, and the smallest environmental footprint. This post deals with several of the what-about questions I am asked online or after my talks.

What about the waste?
One question that I get asked most often has to do with the issue of “waste.” So, let’s talk about it. First, spent nuclear fuel is not “waste,” it is a resource for future nuclear plants. Importantly, the spent fuel is a solid; totally contained inside a steel rod. It is not something that is released to the environment. Contrast that with coal-fired power plants that emit billions of tons of gases and particulates into the air and equally large quantities of solid wastes, which can end up in streams and waterways.
Figure 1. Detail showing uranium pellets inside fuel rods and assembly of fuel rods.

To begin with the nuclear fuel consists of ceramic pellets of uranium oxide enriched in U-235, the fissile isotope. The isotopic composition of the fuel pellets is about 4% U-235 and the remaining 96% is U-238. After two years in service, the fuel pellets still contain about a quarter of the U-235. Also present are the fission products and 2% Pu isotopes. Between the remaining U-235 and Pu isotopes, about 50% of the original fuel value is still in the rods when they are removed from service. Two main reasons for retrieving the rods are that after about 2 years in the reactor core some of the cladding materials develop cracks and defects, and accumulation of fission products like cesium, iodine, strontium and barium, interfere with the maintenance of neutron balance.

The fuel assemblies are then withdrawn and stored under water. About 20 feet of water is needed to cool the rods and provide adequate shielding from the radiation. After 10 years under water, the highly radioactive fission products in have decayed to a point that the rods may be stored on site in dry concrete casks with passive air cooling. The spent fuel may alternatively reprocessed to recover the fissile elements (uranium and plutonium), and fashioned into pellets as is the practice in several countries like France, Japan, Russia. Because reprocessing could lead to opportunities for siphoning off plutonium and thus to proliferation of nuclear weapons, this practice was stopped in the US in 1980s. I will discuss the issue of proliferation later in this post, but let’s continue with the discussion of “waste.”

Figure 2: Storage of spent fuel in dry casks poses no radiation hazard.

Second, the total amount of spent fuel is tiny. According to the International Atomic Energy Agency, the total amount of spent fuel produced by all the nuclear power plants over the last 70 years is 370,000 tons.[ii] About 120,000 tons of it has been reprocessed. The total volume of all the spent fuel is 22,000 cubic meters, which would fill one football field (100 yards by 55 yds) to a depth of 13 feet! It is a small amount of “waste,” considering all the carbon-free electricity these plants have generated.

Storing the spent fuel in deep geological wellbores is technically feasible. As demonstrated in Finland there are no technological barriers to deep well storage, although there remain political barriers. In the United States, Yucca Mountain in Nevada was proposed as a repository, but after years of back and forth, the plans were abandoned. No politically acceptable site has yet been identified. To get around the political deadlock, Deep Isolation—a California company, started by daughter and father Elizabeth and Richard Muller, is proposing a modular and inexpensive option. Their solution exploits recent advances in fracking technologies to drill mile-deep wells with tilted horizontal holes to store the spent near the nuclear plants.[iii] Until such time as this or some other innovation is developed, on-site storage of the spent fuel in dry casks remains a perfectly viable option.[iv] Besides, on-site storage would make it easy to retrieve fuel for future fast reactors.

What about fuel supply?
Another question I often face is that there isn’t enough uranium to power the expanded nuclear fleet. At the current rate of consumption (56,000 tU/yr), global supply of reasonably assured resources (RAR) of uranium at $80/tU could last about 40 years, but if the nuclear fleet were to be expanded substantially, these resources would not last very long. The thing to note is that apart from uranium RAR there is a much larger uranium resource base, which is accessible at higher price. Doubling the price of uranium ore from $80/ton to $160/ton would greatly increase the supply of uranium while raising the cost of electricity by only a fraction of a cent per kWh, because each ton of U produces 1.2 TWh of electricity. With more than adequate supplies in hand, current market conditions do not favor efforts to explore other sources of uranium, but with appropriate price signals these secondary sources could become part of the RAR.

Nuclear weapons are another significant source of uranium. Under the megatons-to-megawatts program, many nuclear warheads are being dismantled. The highly enriched uranium and plutonium of the warhead is diluted with depleted uranium to make mixed oxide (MOX) fuel suitable for nuclear power plants. Further, if we replace the current practice of using the fuel in a once-through mode by reprocessing waste fuel, the current supply of uranium could last several-fold longer. Finally, if we develop breeder technologies to use thorium the supply of fissile fuels would be virtually inexhaustible.

What about nuclear proliferation?
Nuclear proliferation is another reason people object to developing nuclear power plants. The underlying assumption is that countries with nuclear power plants will use their technology and facilities for developing nuclear weapons. This assumption is not justified. There are many countries which have nuclear power, but no plans to develop weapons. Argentina, Belgium, Canada, Japan, Germany, South Korea, and Sweden are prime examples of nations in this category. Nor is nuclear power a prerequisite of weapons development. USA and Soviet Union developed weapons long before nuclear power plants were built.

The reason why countries acquire nuclear weapons technology is largely geopolitical, and proliferation is best addressed through political solutions. Countries acquire them primarily as a deterrent against a more powerful neighbor. Other factors include national pride and exercise of hegemony. France was concerned about Soviet weapons in its neighborhood. It developed nuclear weapons despite protestations from the Kennedy administration. Even assurance of a nuclear umbrella from the US did not dissuade France from its path.

Uranium-based nuclear weapon has a relatively straightforward gun design that shoots one subcritical mass of uranium into another subcritical mass. Together the pieces exceed the critical threshold and a nuclear explosion ensues. However, getting the uranium fuel and enriching it to weapons grade (i.e., greater than 80% U-235) is a substantial challenge. Recall that natural uranium has only 0.7% U-235. Fuel grade uranium requires increasing the ore about 6 fold to between 3% and 5% U-235. Enriching it to weapons grade requires an additional 20-fold enrichment, something that requires access to a vast arrays of high speed centrifuges. Non-nuclear nations could not follow this path surreptitiously as these facilities would be readily detected and are also subject to sabotage through cyber-attacks—the U.S. successfully set Iran back in its drive to acquire weapons grade uranium by deploying the Stuxnet worm.

Plutonium-based devices require Pu-239, which is present in the spent nuclear fuel. Chemical extraction of plutonium is a relatively simpler process than enriching isotopes, and hence the fear that countries could divert some of the spent fuel for making a weapon. However, the construction of the plutonium device is much more challenging and presents a major hurdle.[v] Besides, the plutonium in spent fuel also contains other isotopes of plutonium, notably Pu-240, which must be removed, or the device would simply fizzle. Weapons-grade plutonium are best obtained from research reactors in which the uranium is “burnt” for very short periods before reprocessing—a practice not conducive to electrical power production.

What about dirty bombs?
Another concern is the possibility of nuclear waste being used by terrorists to create a “dirty bomb” which causes radioactive damage. The damage that a dirty bomb can really create is one of fear of contaminating a vital area so as to cause mass disruption. A dirty bomb will also cause a small increase of cancer risk for the population as a whole, but it does not lead to immediate loss of life. Terrorists are more likely to try something more spectacular, including conventional bombs. 

The hurdles for making a dirty bomb are also significant for a terrorist organization. For starters, amassing the radioactive material without killing themselves before they can make the device is an insurmountable challenge for the terrorists. While the radiation from the dirty bomb after it has been exploded—and the radioactive materials dispersed—poses only a small risk to the large population, in a concentrated form, as would be necessary for the bomb, the radiation levels will be high enough to cause illness and death. The terrorists could decide to make dirty bomb from  alpha-emitters, because they can protect themselves against this non-penetrating radiation. For this type of dirty bombs the radioactive materials can be obtained from a variety of medical and other devices that have nothing to do with nuclear energy, and so this threat is not reduced by turning off nuclear power plants.

There will always be violent groups with grievances, and therefore the risk of having such a group acquire a small fissionable bomb can be minimized with adequate protection of important secrets and technology. The real risk would be from a rogue nation providing a terrorist group with a small nuclear device and a means for delivering it. A one-kiloton “suitcase” device does not wreak as much damage as many conventional explosive charges do. 

I do not mean to take the threat of terrorist attacks using a nuclear device lightly, for it can cause mass economic disruption. Ours is not a situation with easy choices, and the growth of nuclear power is important for the energy security of the world. This is a problem that needs a political solution. Greater involvement of the IAEA and internationalization of the enrichment and fuel reprocessing is a path proposed by ElBaradei, former Director General of the IAEA and winner of 2005 Nobel Prize for peace. Internationalization of fuel processing could provide assurance to countries pursuing nuclear power that they will get the fuel for their plants. Stable energy supplies is a deterrent of war. This scenario also provides the safeguard that nuclear material is not being diverted to military uses, because the material would be watched over by personnel from many different countries.

[i] Summary for Policymakers of IPCC Special Report on Global Warming of 1.5°C approved by governments; Press release. Oct. 8, 2018;
[ii] Status and Trends in Spent Fuel and Radioactive Waste Management, IAEA Nuclear Energy Series No. NW-T-1.14 (2018)
[iv] U.S. Nuclear Regulatory Commission:
[v] In order to successfully explode a plutonium bomb it is important to keep the Pu-239 from poisoning itself by its own neutrons. To achieve this, plutonium is fashioned into a thin-walled sphere so that the neutrons mostly escape and are not absorbed by the neighboring Pu atoms. In order to explode the device, the Pu sphere has to be compressed together into one lump. This feat is achieved by a process of implosion. A number of chemical explosives are set off around the Pu sphere, and their shock wave compresses the sphere. The timing of the explosions has to be very precisely controlled, or the nuclear device simply fizzles.

Tuesday, August 21, 2018

Why I Favor Nuclear Power

The world needs energy
There is little doubt that the world needs lots of energy—cubic miles of oil worth of energy—just to afford the current population a decent standard of living. More than 80% of the primary energy we currently use is derived from fossil fuels, and the immediate consequence is a higher concentration of CO2 in the atmosphere, ocean acidification, and an increased risk of climate change. The need for producing much more clean energy, in particular electricity, becomes evident when one considers the 3 billion people living at or near-poverty conditions.

Use of electricity produces no environmental pollutants, and thus its use should be encouraged wherever possible, such as for heating and transportation instead of using natural gas and oil. These transitions only increase the demand for clean electricity. As a rough estimate global production of electricity has to increase at least four-fold its current level of about 25,000 TWh per year. Of course, electricity is only as clean as the method for generating it in the first place.

As some deeply concerned about the state of our environment, I realize that we have to stop using fossil fuels as soon as possible, and start relying on zero-CO2 sources like nuclear, wind and solar. On a life cycle basis, all these sources produce only a few grams of CO2 per kWh of electricity compared to nearly a kilogram of CO2 per kWh for coal power plants.

Nuclear power produces CO2-free electricity
Unlike generation electricity by burning fossil fuels, generation by nuclear, hydro, solar, and wind are not associated with direct emissions of CO2. That is not to say that these sources are carbon-free. Varying amounts of CO2 are released in the processes used for producing the requisite materials, construction, and ultimate dismantling. Comparison of various electricity generation technologies must account for both direct and indirect emissions. The figure below was taken from a paper by Markandya and Wilkinson, which compares the direct and indirect emissions of CO2 for various power generation technologies.[1] On a life-cycle basis, CO2emissions from nuclear are the lowest, about 30 g/kWh compared to 1.3 kg/kWh for coal, and even lower than solar and wind because they require substantially greater amounts of materials such as steel, concrete, and glass for producing equivalent amount of electricity.

Figure 1. Nuclear power emits the least amount of carbon dioxide.

Nuclear power has a low environmental footprint
Since nuclear fission releases a million times the energy than chemical reactions (combustion), it takes that much less fuel. We burn billions of tons of coal, oil and gas, but to generate an equivalent amount of energy we need only a few thousand tons of uranium, which greatly reduces the environmental burden of mining for the fuel.

The high energy density means that nuclear plants are compact. They do not require much land and can be located close to centers of power consumers. The Diablo Canyon Nuclear Power Plant with its twin 1100 MW units take up only 940 acres (less than 1.5 square miles). Area required for comparable nameplate generation capacity for solar and wind farms is 10 to 100 times greater, and another three times larger to compensate for their low capacity factor. Because of the large area requirement, large renewable power generation systems have to located further away from city centers, and hence have a greater need for easements for  more transmission lines.

The tonnage of commodity materials such as concrete, steel, cement, and glass required for the construction of power generating facilities are also much smaller for a given capacity of nuclear power than other power generation systems, further reducing the environmental burden. Nuclear plants produce power 24 X 7 with occasional scheduled shutdowns for maintenance and refueling. They thus have capacity factors in excess of 90%. Renewable sources such as wind and solar have capacity factors between 25 and 30%. Whereas nuclear plants are built to last 60 or more years, wind and solar facilities last only 30 years. The higher capacity factor and longer life means that nuclear plants produce six times as much electricity as comparably sized wind and solar facilities. Figure 1, taken from the Department of Energy’s Quadrennial Technology Review, shows the tons of materials required for producing 1 TWh of electricity from different power generation systems. As is evident from the figure, the environmental burden of nuclear plants is the least of all other sources.

Figure 2. Nuclear power has the lowest materials intensity.

Nuclear power is safe
If we look at the fatalities associated with mining, installation, and operation from power generation of various energy sources, nuclear is the safest. The following chart, compiled by the Canadian Nuclear Agency includes deaths from the pollutants emitted by fossil fuels that cause asthma and other respiratory ailments.1 The figure of 161 deaths per TWh for coal is a global average, and it is worth noting that in the US it is 15, while in China it is 278. There is no doubt that measures to control the emissions of particulate matter and other pollutants from coal plants in the US have been very beneficial.

The figure of 0.04 deaths per TWh for nuclear plants includes the estimated 4000 deaths from radiation exposure following the Chernobyl nuclear disaster. This estimate is an upper limit for the number of fatalities as it is based on the linear no-threshold model (LNT). This model assumes all ionizing radiation is harmful, and its effects are cumulative; in other words the model assumes that our bodies have no repair mechanisms. We live with constant exposure to ionizing radiation and have evolved DNA-repair mechanisms to deal with certain levels of radiation exposure. A more realistic estimate would further reduce the number of fatalities from nuclear power to about 0.013 per TWh.

Figure 3. Nuclear power has the fewest fatalities per unit of electricity.

The prospect of climate change and ocean acidification are real, and the long time it takes to implement corrective measures means that we must rapidly decarbonize our energy systems. Our fears of radiation are largely unfounded and have had the deleterious effect of continued use of fossil fuels. Even as we deploy wind and solar—the nominally low-carbon sources—the absence of large scale storage systems have forced us into using natural gas power for back up. The design of natural gas power plants used as spinning reserves are selected on the rapidity with which they can be brought online. These designs are among the least efficient of gas-fired plants, with thermal efficiencies around 33%, and thus high carbon emissions. Gas-fired power plants that operate with a combined steam cycle have thermal efficiencies in excess of 50%. Analysis by Larsen and Rez shows that we would do better in terms of carbon emissions if instead of installing low capacity factor wind or solar systems and backing them with natural gas, we simply used a combined cycle natural gas plant.[2]

[1] Markandya and Wilkinson, Lancet 2007, 370, 979-90.
[2] T.C. Larsen, P Rez, Journal of Sustainable Energy Engineering, 2017, 194-206, (