This article was printed in CONTROL's September 2009 edition.
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) .
After at least six months, the rods can be moved from these ponds into steel-reinforced, helium-cooled casks (Figure 2), but often stay in the ponds for years. After five years of cooling, they usually are entombed in concrete in bunkers. In the United States, nuclear waste is stored at 122 temporary sites. This practice is unsafe because casks can be penetrated by conventional weapons and, in that case, can release radioactive cesium gas. All nuclear waste the United States is in temporary storage because no permanent underground facilities have been opened to date. Therefore, the temporary storage of high-level nuclear waste is continuing. Some of these storage facilities are 50 years old and are filling up.
Decommissioning, Transportation and Permanent Storage
When the nuclear power plants reach the limit of their useful life, they have to be decommissioned. On average, the world's fleet of 439 nuclear power plants has been operating for over 20 years, and the design life of a nuclear power plant is typically for 30 or 40 years, although they can operate in excess of their design lives. The technology required for the permanent and safe disposal of the components of the decommissioned plants is yet unresolved. Low-level wastes (all materials that were exposed to radioactivity) are usually stored at dedicated dry burial sites, while the plant itself is entombed in concrete.
Transportation distances to the temporary storage sites (Figure 3) are usually short, as these sites are usually near the plant. On the other hand, if the residual fuel values are to be recovered (as is often the case outside the United States), high-level wastes need to be transported over long distances to reprocessing plants.
“Permanent” Depository: Hardened interim storage of spent fuel or of high-level waste in dry casks are not permanent solutions. To date, because of the risk of leakage due to earthquakes, volcanic activity or other causes, no permanent disposal site (Figure 4) has been found. The United States government planned to build a nuclear waste repository in the Yucca Mountains to open 1998, but because of public resistance that target date has now been moved to 2020, and some believe that it will never be opened. Finland is also working on a disposal site at Olkiluoto, but as of this writing, no permanent storage facility is in operation anywhere in the world.
Monitoring and Process Control
In previous articles, I have described the controls needed to make the operation of nuclear power plants safer. Here I will concentrate on the controls required to reliably detect radiation, not only from power plants, but also from any other sources. Radioactivity monitoring requires sophisticated systems that will protect the public, not only from the accidental release of radioactive materials (during power plant operation, dismantling, transportation or storage), but also from the release of radioactivity as more and more nations are building and transporting both conventional and dirty bombs.
Dosimeters can be pen-like devices clipped to the operator's clothing. They measure and display the cumulative dose of radiation received. These dosimeters can also transmit their readings to the central control room where the exposure of all employees is continuously monitored. Similarly, central displays and alarms can be provided to monitor the stationary radiation sensors distributed throughout the plant. These monitors can be the Standard Radiation Environment Monitors (SREM), Proton Monitors (PM), Scintillation Fiber Detectors (SFD) or real time RadFet units. The readings of both the dosimeters and of the stationary monitors can be transmitted (via satellites) to centralized monitoring hubs around the world, where integrated area displays and automatic alarm systems are provided.
The unit of measurement of radiation intensity and dosage is the Roentgen (R). Of the three forms of radiation (alpha, beta and gamma), it is gamma (X-ray) that is the most harmful. R is the radiation exposure that is equal to the quantity of radiation that will produce one electrostatic unit of electricity in one cubic centimeter of dry air at 0°C and at atmospheric pressure. Instruments that measure exposure can be rate, radiac, radiation, fallout, Geiger counters and remote monitors. If a radiation range meter reads 10 R/hr, that means that a person in that area will receive 240 R in a day. At such a dosage, death occurs in about 10% of the cases (at 500 R in about 50% and at 800 R about 99% of the cases).
Satellite Monitoring is performed by the International Satellite Monitoring Agency (ISMA) of the U.N.' International Atomic Energy Agency (IAEA). Such monitors can provide a ground resolution of about one meter. The IAEA monitors detect the worldwide flow of nuclear materials using some 350 cameras producing 150,000 images that are fed to 50 radiation sensing stations and 90 surveillance systems. Some sites (such as the ones in Armenia, Brazil, Chernobyl and Hungary) are already connected to the central hub in the IAEA headquarters. Process control can help in integrating all these systems into a single worldwide satellite communication-based network.
Special sensors (developed by Science Applications International Corp. – SAIC) can detect high-velocity spin-off particles and can pinpoint the locations of enriched uranium throughout the world. These sensors can also detect the leakage of radioactive materials and the transport of nuclear devices.
Until total nuclear disarmament and the decommissioning of all nuclear power plants is completed, only sophisticated automatic controls can provide reasonable safety in the nuclear age. Like the monitoring systems for personnel protection and for surveillance during the nuclear age, once this age ends, the monitoring of the permanent storage facilities will have to continue for many thousands of years and will also require sophisticated process control and safety systems.