Cincinnatus BLOG *** Political Commentary - Social Commentary

Coal-fired plants produce approximately 50% of the electricity in the United States and 82% of power-generated carbon dioxide emissions. If electric vehicles are charged exclusively by coal-fired electricity they produce more greenhouse gases than a traditional gasoline-powered combustion engine car. In the future, electricity must be generated cleanly if we expect automotive electric-drive technologies to reduce our carbon dioxide burden. Clean energy alternatives like Wind and solar power will probably make a significant contribution to clean energy generation, but realistically, we cannot count on these two sources for more than 20-30% of our electricity needs in the next 20 years. Even reaching these modest goals will require a major investment in energy infrastructure and fundamental advances in technology. In light of these realities, it would appear that nuclear power may be the only clean alternative.

Nuclear power plants provide about 17% of the world’s electricity. A few countries have made it the centerpiece of their power-generating policy; e.g. France generates more than 75% of its electricity from nuclear reactors. In the United States, the 104 commercial nuclear generating units produce 20% of the nation’s electricity. It has been 20 years since a new nuclear plant has been proposed in the U.S, but interest has increased recently because nuclear power is one of the few proven ways to produce utility scale electricity without concurrently increasing carbon dioxide emissions. One benefit of a nuclear power plant is it can run at peek capacity 24 hours a day and the excess energy produced in off- peak times can be used to charge electric vehicles at night or generate hydrogen from water to power the future hydrogen economy.

At the moment, there does not appear to be the political will to augment existing nuclear power generation. Fears about nuclear waste transportation and storage have virtually paralyzed the industry. The U.S. however, faces a dilemma: the existing nuclear infrastructure is aging and will ultimately come off line leaving a gap representing a loss of 20% (>725 billion kilowatt-hours) in our existing energy production. When added to the expected 29% increase from 3,659 billion kilowatts-hours to 4,705 billion kilowatt-hours of electricity needed in 2030, the problem magnifies. If we were to abandon nuclear power, we would have to build new clean energy facilities capable of producing an additional 1,800 billion kilowatt-hours by 2030 to fill the gap. Electric cars would only exacerbate this problem.

So how can we overcome the greatest obstacles to embracing nuclear power - safe transport and disposal of radioactive waste?

First, a brief review of how the process works: Heavy water reactors and graphite-moderated reactors can use natural uranium, but the vast majority of the world’s reactors require enriched uranium. The U-238 isotope makes up more than 99% of naturally occurring uranium ore with U-235 accounting for 0.711%. There is also an even more rare form present, U-234, formed by the decay of U-238. (The numbers refer to the atomic mass of the isotope, i.e. the number of protons and neutrons in the atomic nucleus). U-235 will fission when stuck by a free neutron, thus U-235 is called a “fissile isotope.”(Fission - a nuclear reaction in which a heavy nucleus splits spontaneously or on impact with another particle, thereby releasing energy.)

The probability of a U-235 atom capturing a neutron as it passes is very high. In a reactor the “critical“ state is achieved when one neutron ejected from each fission causes another fission to occur. U-238 on the other hand, will absorb the free neutron when it is struck and yield an atom of the isotope U-239. The U-239 undergoes natural radioactive decay and yields the fissile isotope of Plutonium (Pu-239). The U-238 isotope is said to be fertile because it yields a fissile Pu-239. Plutonium is the isotope that causes the greatest concern for nuclear waste disposal because it has a half-life of over 24,000 years and can be used for nuclear weapons.

Uranium ores mined in the United States contain about .05% to 0.3% uranium oxide, U₃O₈ (Triuranium octaoxide). Uranium ore mined in other countries can have a somewhat higher concentration of uranium oxide than that of the United States. Regardless of the source, the following steps are needed to convert naturally mined uranium to nuclear fuel for use in a reactor:

• Milling - Mined uranium ores are processed by grinding the material to a uniform particle size and then the uranium is leached out most often using sulfuric acid to produce 90 to 95% pure uranium. This milling process commonly yields a dry powder form called “yellowcake” which is sold commercially as U₃O₈.
• Conversion - Milled uranium oxide must be converted to uranium hexafluoride, UF₆, the form that is required by most commercial enrichment facilities. During the conversion process, impurities are removed and the uranium is combined with fluorine to create the UF₆ gas. Although a solid at room temperature, uranium hexafluoride can be converted to the gaseous state at a moderately high temperature - 134^oF (57^oC). The uranium hexafluoride contains only natural uranium, which is yet to be enriched.
• Enrichment - The fuel for a nuclear reactor must contain a higher concentration of the critical U-235 isotope, because it is the key to starting a nuclear reaction and keeping it going. For civilian reactors, uranium is “enriched” to contain 3 to 5% U-235 and 95% to 97% U-238. (Weapons grade uranium is composed of 90% or more of U-235.) Gaseous diffusion is the only commercial process being used in the United States.  The solid uranium hexafluoride (UF6) from the conversion process is heated in its container until it becomes a liquid. The container becomes pressurized as the solid melts and UF6 gas fills the top of the container. The UF6 gas is slowly fed into the plant’s pipelines where it is pumped through special filters called barriers or porous membranes. The holes in the barriers are so small that there is barely enough room for the UF6 gas molecules to pass through. The isotope enrichment occurs when the lighter UF6 gas molecules (with the U-234 and U-235 atoms) tend to diffuse faster through the barriers than the heavier UF6 gas molecules containing U-238. One barrier isn’t enough, though. It takes many hundreds of barriers, one after the other, before the UF6 gas contains enough uranium-235 to be used in reactors. At the end of the process, the enriched UF6 gas is withdrawn from the pipelines and condensed back into a liquid that is poured into containers. The UF6 is then allowed to cool and solidify before it is transported for fuel fabrication. Other forms of enrichment such as gas centrifuge and laser separation can also be used but will not be discussed because they are not used for commercial purposes in the United States.

Source: United States Nuclear Regulatory Commission

• Fuel Fabrication - For use in a nuclear reactor the enriched uranium hexafloride is converted into uranium dioxide (UO₂) powder that is then processed into pellet form. The pellets are sintered - fired at high temperature to create hard ceramic pellets of enriched uranium. The pellets are loaded into tubes and constucted into feul assemblies. Depending on the cofiguration of the light water reactor, fuel assembly may contain up to 264 fuel rods and have dimensions of 5 to 9 inches square by about 12 feet in length.

Inside the reactor the rods are collected into bundles. The bundles are usually submerged in water inside a pressure vessel - the water acting as a coolant. The bundle must be slightly “supercritical” which means that on average more than one neutron is free to hit a U-235 atom and if left unchecked the uranium would eventually overheat and melt. To prevent this, control rods that absorb neutrons are inserted into the bundle. By raising or lowering the control rods the operator can control the rate of the nuclear reaction. The rods can be raised to increase the rate of the reaction or lowered into the uranium bundle to slow it down. The rods can also be lowered completely into the uranium bundle to shut the reactor down to prevent an accident or to change the fuel.

The uranium bundle acts as a source of high-energy heat to convert water into steam. As in traditional power plants, the steam drives a turbine that spins a generator to produce electricity. In some reactors the steam goes through a heat exchanger to convert another loop of water into steam, which then drives the turbine. This later design has the advantage of not allowing the radioactive steam to come in contact with the turbine. Other experimental reactors use carbon dioxide gas or liquid metal (sodium or potassium) as the coolant in contact with the reactor core thereby, allowing the reactor to be run at a higher temperature. In effect, once you get past the reactor itself there is little difference between a nuclear power plant and coal-fired plant except the source of the heat used to create steam.

The pressure vessel of the reactor is typically housed inside a concrete liner that acts as a radiation shield, and then this liner is further housed in a much larger steel containment vessel. This larger containment vessel contains the core and the working parts of the reactor such as cranes needed to refuel and maintain the reactor. The purpose of this steel containment vessel is to prevent leakage of any radioactive material from the plant. Finally, an outer concrete building with the strength to survive traumatic events such as a jet liner crash protects the steel containment vessel. It was the absence of a secondary containment structure that permitted the radioactive material to escape at Chernobyl.  Unlike old Soviet-era nuclear plants, all nuclear plants in Europe and the United States have robust secondary containment structures.

Spent fuel discharged from a nuclear reactor contains appreciable quantities of fissile U-235 and Pu-239 (about 1%), fertile U-238 along with other radioactive materials. The recovered Uranium and Plutonium can be recycled as nuclear fuel. MOX fuel, mixed oxide, is a blend of reprocessed uranium and plutonium oxides that can be used as an alternative to the enriched uranium fuel used in light water reactors that currently predominate in nuclear power generation.

Although Europe reprocesses nuclear fuel in France and uses it in about 30 reactors, reprocessing is not permitted in the U.S. due to the perceived danger of nuclear proliferation. However, a special MOX facility is being built in South Carolina as a means of rendering weapons-grade plutonium less hazardous. The end result of this process would be to convert the weapons grade plutonium to MOX fuel assemblies that would contain approximately 95% uranium oxide and 5% plutonium oxide, which would then be irradiated in a commercial reactor. The spent fuel from this process would make it less dangerous and difficult to recover weapons grade plutonium.

The major drawback to nuclear power is the safe transportation and disposal of either the spent fuel or if reprocessed, waste from the reprocessing plant. Under the Nuclear Waste Policy Act of 1982 and its amendments, the Department of Energy has planned to dispose of this waste in a deep, stable geologic structure, aptly called a deep geological repository. Yucca Mountain, Nevada, has been chosen as the site of this repository. Originally, The Department of Energy was to begin accepting spent nuclear fuel at the Yucca Mountain Repository by January 31, 1998 but has yet to do so because of a series of delays due to legal challenges, concerns over how to transport the nuclear waste to the facility, and political pressures. As a result, currently only temporary storage exists for radioactive waste in the United States. It is stored in specially designed, water-filled basins or dry casks at the commercial reactor sites or special reactor storage facilities.

The 104 nuclear reactors in the U.S. produce 2,200 tons of radioactive waste annually and, even if no new reactors are built the U.S., will have produced approximately 95,000 tons of spent nuclear fuel by 2050. The repository at Yucca Mountain, which is now slated to be open in 2020, has a capacity of only 77,000 tons. As an alternate approach to disposal of commercial nuclear waste, The Department of Energy has created, The Global Nuclear Energy Partnership (GNEP), to deploy advanced nuclear recycling and reactor technologies. The stated purpose GNEP is to: “help provide reliable, emission-free energy with less of the waste burden of older technologies and without making available separated plutonium that could be used by rogue states or terrorists for nuclear weapons. These new technologies will make possible a dramatic expansion of safe, clean nuclear energy to help meet the growing global energy demand.”

GNEP believes that they can turn current nuclear waste into fuel for a new bread of reactors that could produce 100 times as much energy as a conventional nuclear reactor while producing 40% less waste.  If successful, this technology would take at least two decades to implement and unfortunately would not provide a short-term solution to the radioactive waste problem.

Though still in the early experimental stage, the GNEP has drawn its share of critics who believe that the concept is unworkable. Critics believe reprocessing is costly and would increase the risk of nuclear proliferation. It has been suggested there is no urgency to follow the reprocessing path, and until a better solution is elucidated,  storage in dry casks either at current reactor sites, regional facilities, or in a single national facility will give us the 50 years needed to find that better solution.

Despite the obvious problems, there are no utility scale alternatives to clean nuclear power and so it must be kept on the table if we are to reduce our carbon footprint. Without increased nuclear power it is virtually imposable to achieve the promised reduction of carbon dioxide emissions as electric-drive technologies come on line. If in 10 to 20 years we are still using coal to generate half our electricity, there is no benefit to investing large sums in electric-drive vehicles.