Storage of Spent Nuclear Fuel
One of the most controversial issues regarding nuclear power is the storage of used nuclear fuel once it has been removed from a nuclear reactor.
Currently, most spent nuclear fuel in America are being temporarily stored in pools of water at the reactor plant where it was used. Submerging the spent nuclear fuel in at least 20 feet of water has been proven to successfully safeguard the environment from any harmful radioactivity.
Back when reactor plants were first being designed and built, it was assumed that research would someday lead to a means of reprocessing the spent fuel or that some central permanent repository would be developed. Thus, there was no reason to develop an alternative means of storage other than these temporary storage pools.
However, America has yet to develop a reprocessing facility or a central repository for permanently storing the spent nuclear fuel. As a result, the pools at the nuclear facilities all across America are filling up with spent nuclear fuel with many reaching their maximum capacity.
The Nuclear Regulatory Commission (NRC) regulates the storage of spent nuclear fuel by issuing regulations as well as licenses for such storage. Since the storage pools have been reaching their maximum capacity, the NRC has been allowing some of the nuclear fuel that was previously stored in the pools to be transferred to above ground dry storage casks.
According to the NRC regulations, the spent nuclear fuel can be transferred after it has been in a pool after one to five years, depending on the type of nuclear fuel.
Typically, the storage casks are cylindrical and are made of steel and concrete. These casks are extremely heavy, weighing over 150 tons, and often have a height of over 16 feet. The spent fuel is placed inside a canister, which is itself placed inside the inner cavity of the cask.
Since the spent nuclear fuel still produces a considerable amount of heat, there must be some means to allow this heat energy to escape the cask while it is stored. Often, ventilation ducts are used to remove this residual decay heat.
As shown in the diagram of a typical storage cask, top ventilation ducts (15) and bottom ventilation ducts (16) are located around the circumference of the cylindrical body of the cask, allowing gas to flow through and remove the decay heat emitted from the spent nuclear fuel.
In order to sufficiently shield the environment from radiation, these casks are made of very thick steel and concrete which makes them extraordinarily heavy. The weight of these enormous casks often complicates simple procedures such as moving the casks from one place to another. To this end, the Department of Energy has approved certain methods of light-weight shielding that provides adequate protection to the surrounding environment while reducing the weight of each cask. U.S. Patent No. 6,372,157, issued on April 16, 2002, describes the various radiation shielding used in casks that can significantly reduce the weight of these casks.
This technology was originally developed by Science Applications International Corporation (SAIC), but was later assigned to the Department of Energy. The patent discusses that the spent fuel pellets themselves (depleted uranium dioxide pellets) can be mixed with a cement binder to form a material known as DUCRETE, which can be used as a shielding in the cask. Since the depleted uranium dioxide pellets have a greater density than the gravel aggregate normally used in concrete, the thickness of the shielding can be greatly reduced.
As a result, the cask’s diameter can be reduced by two-thirds, and the weight can be reduced from 150 tons to approximately 90 tons. However, DUCRETE does not transfer heat well and thus requires complicated ventilation ducts in order to properly cool the cask. Furthermore, the cement matrix has been shown to degrade after a period of time due to water-cement-uranium dioxide reactions at warm temperatures.
However, the patent describes a process of turning the used uranium dioxide pellets into much smaller particles, such as a powder or microspheres, which can be further processed into a shielding material. In order to form the shielding, several materials are added to the depleted uranium dioxide microspheres, such as resin, pitch, aluminum powder, alumina, hydrogen, boron, gadolinium, hafnium, erbium, and/or indium. The resulting mixtures is a liquid which can then be poured into the outer walls of the cask.
As shown in the diagram from the patent, the inner wall (22a) and the outer wall (24a) are made from thick forged steel. The liquid shielding material (28a) is then poured into the cavity between the inner wall (22a) and the outer wall (24a). After applying high temperatures to the outer wall, the liquid material forms a solid pyrolytic uranium compound (PYRUC). The PYRUC shielding is also layered into the cask lid (28b) as well as the cask bottom (28c).
The PYRUC shielding conducts heat much better than other available shieldings, which means minimum ventilation is required to cool the cask. In addition, the PYRUC shielding retains its physical properties up to 1000 C, which is much higher than is required by NRC regulations. Due to its high temperature resistance, an inner canister containing the spent nuclear fuel is not required, further reducing the overall size and weight of the cask. The patent explains that the overall cost of constructing the cask will be $600,000-$700,000, which is significantly less expensive than casks made of all concrete and steel.
The cost of the storage cask, as well as licensing fees, are one of the biggest factors faced by owners of reactor plants when determining which storage cask on the market to utilize. For example, a reactor plant owner would be required to pay the DOE a license fee for using the PRYUC shielded cask mentioned above. However, reactor plant owners can also use other cask designs which have been approved by the NRC.
One of the more predominant makers of these NRC-approved casks is Holtec International, Inc., who holds various patents relating to this technology. For example, U.S. Patent No. 7,590,213, issued on September 15, 2009, describes an improved cask which can be placed partially below ground. As can be seen by the diagram from the patent, only the top portion of the cask sits above ground level (23).
Above grade inlets (27) are located about 10 inches above ground level (23), which allow air to enter the cask while reducing the probability that rain or flood water will enter the cavity. The canister containing the spent radioactive fuel is placed inside the inner cavity (26) of the cask. The cavity is surrounded by a thick low carbon steel shell (34) and bottom plate (38) which are hermetically sealed to one another. A layer of insulation (37) surrounds the steel in order to prevent excessive transmission of heat decay from the spent fuel canister to the concrete (21). This insulation is made from blankets of alumina-silica fire clay (Kaowool Blanket), oxides of alumina and silica (Kaowool S Blanket), alumina-silica-zirconia fiber (Cerablanket), and alumina-silica-chromia (Cerachrome Blanket).
The bottom of the cask (22) made of a reinforced concrete slab. The lid (41) is also made of concrete and has four outlet ventilation ducts (42) which allows air to escape from the cavity (26). Filters or attenuators are placed in all ventilation ducts in order to prevent the escape of any harmful radioactive debris.
In conclusion, it is clear that demand for these storage casks will continue to grow rapidly as the number of spent nuclear fuel assemblies from reactor plants continue to rise. So far, America has been using the dry cask storage technology for 20 years with no incident. There is no doubt that this technology will continue to develop to meet the climbing demand.