This article was printed in CONTROL's September 2009 edition.
Read Bela Liptak's six part series "Process Controls Prevent Nuclear Disasters," to learn how process controls could have prevented past nuclear accidents and how it could improve the safety of the nuclear power industry. Visit www.controlglobal.com/liptaknuclear.html
By Béla Lipták, PE, Columnist
In the previous five articles on nuclear safety, I have written about past nuclear accidents and the ways how process control could have prevented them. Besides meltdowns and leaks, accidents can also occur due to earthquakes, ageing and terrorist attacks. I have also discussed the relative costs of fossil, renewable and nuclear power plants and noted that renewable cost are dropping while the costs of building traditional ones is rising. Here I will concentrate on the role of process control can play in reducing the risks associated with the transportation and storage of nuclear wastes.
We cannot be sure if the problem of nuclear waste storage will be ever solved, but we can be sure that the cost of permanent storage could exceed both the building and the decommissioning costs of nuclear power plants. The yearly waste production of each nuclear reactor is about 20 tons of high-level nuclear waste. In the United States, there already are in temporary storage some 30,000 tons of spent fuel rods and some 380,000 cubic meters of other highly level radioactive wastes.
A Distant Dream
As a distant dream, let me also mention the idea of nuclear waste transmutation. It is somewhat similar to the dream of the medieval alchemist's dream of "transmuting" lead into gold. Some nuclear industry people hope that someday it might be possible to change the high-level nuclear waste into much less dangerous wastes. Under the U.S. Accelerator Transmutation of Waste Program, Los Alamos and other Department of Energy laboratories are studying this and developing such accelerator-driven technologies. If you want to read about this "pipe dream," go to: http://www.lanl.gov/orgs/pa/science21/ATW.html
I do emphasize that transmutation is only a dream, because right now the American nuclear fuel technology is actually going backwards: An example of this is that the only American fuel producer (USEC) uses 55-year-old gas diffusion technology, which is not competitive with the new European centrifuging technologies, because it requires 20 times the energy to produce the same fuel.
Weapons-grade plutonium from the dismantling of American nuclear weapons is presently stored in locations like Amarillo, Texas. It takes about 20 pounds of separated plutonium to build a dirty bomb. This means that just in the United States there is enough plutonium in these storage facilities to build over 15,000 dirty bombs.
By definition, radioactive waste can be low-level (LLW), intermediate-level (ILW), high-level (HLW) and transuranic (TRUW). Here I will concentrate on long-lived HLV fission products, which include Technetium-99 (Tc-99, half-life 220,000 years), iodine-129 (I-129, half-life 17 million years), TRUW Neptunium-237 (Np-237, half-life two million years) and Plutonium 239 (Pu-239, half life 24,000 years).
The Nuclear Non-Proliferation Treaty permits all nations to enrich (concentrate) U-235 to ~3%, which is the concentration needed for nuclear power plants, but allows only the members of the U.N. Security Council to further concentrate to 90%, which is needed to build nuclear weapons. Non-members, such as India, Pakistan and Israel, also acquired nuclear weapons and others (Iran, North Korea and probably more) are in the process of getting them.
It seems that for at least another century we will be living with various concentrations of radioactive wastes and that during this period, process control should focus on making the decommissioning, reprocessing, transportation and storage of nuclear waste safer than it is today.
Reprocessing and Temporary Storage
Reprocessing: The residual fuel values of spent reactor fuel and radioactive materials from the dismantling of nuclear weapons can be recovered in reprocessing plants. The first reprocessing plants (Hanford, Wash.) used bismuth phosphate and served to recover only plutonium and only for use in nuclear weapons. This design was followed by the solvent extraction process using nitric acid, which can also recover uranium (Savannah River, S.C.).
Reprocessing increases the volume of radioactive waste over twenty-fold because of the addition of chemicals, and it also requires long-distance transportation from the source of the waste to the reprocessing plant. For these reasons and because of the risks of plutonium being stolen to build dirty nuclear bombs, reprocessing in the United States was stopped some three decades ago, but France, India, Japan, Russia and the U.K. continue to operate such plants. Today, an international partnership is evolving to reprocess spent nuclear fuel in such a way that the product will be useable only for fuel in power plants, but not to build nuclear weapons.
Temporary Waste Storage: 99% of all HLW waste comes from spent fuel rods. After the fuel pellets are loaded into fuel rods and assembled, the core assembly is installed into the nuclear reactor, and the initial fuel assemblies are used for a year or two, while subsequent ones might last 5 to 6 years. At that time the used fuel rods are replaced, while the extremely hot spent rods are usually sent to poor water ponds that are slightly borated with boric acid that absorb radiation (Figure 1) .