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.
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