By Bob Evans,
and Steve Tumminelli,
Enercon Services Inc.
Utilities are facing a thorny problem in managing the highly radioactive fuel that is discharged from reactors used to generate electricity. The utilities, design firms, industry groups, legislators, community leaders, and manufacturers of specialized fuel storage containers are working together to ensure that the country’s nuclear power plants stay on line producing electricity and preventing any further deterioration of the country’s electric power supply.
Nuclear power plants in the U.S. produced 20 percent of the nation’s electrical power in 1999. Fission of uranium atoms produces heat which, in turn, is used to make steam that drives turbines connected to generators. Since the uranium, which is contained in fuel assemblies in the reactor pressure vessel, is consumed by the fission process, the fuel assemblies must be periodically replaced. Typically, about a third of the fuel assemblies are replaced every 18 to 24 months depending on the individual plant’s operating cycle and fuel design. When fuel is removed from the reactor pressure vessel, it is termed “spent fuel,” is highly radioactive, and requires cooling to remove heat from decaying radioactive isotopes.
The spent fuel is stored temporarily in spent fuel pools at nuclear power plants. When nuclear power plants were built in the 1960s, 1970s and 1980s, it was expected that the spent fuel would be sent to reprocessing plants where remaining fissionable materials in the fuels would be recovered and re-used in new fuel. Most utilities designed their spent fuel pools to hold about 10 years’ worth of spent nuclear fuel.
In the late 1970s, President Carter established a new national policy dictating that spent fuel not be recycled because of concerns over possible diversion of fissionable material for nuclear weapons production. In 1982, Congress passed the Nuclear Waste Policy Act which mandated that the U.S. Department of Energy (DOE) assume responsibility for disposing of the spent nuclear fuel by 1998. DOE has been studying a site in Nevada at Yucca Mountain for nearly 20 years as a possible location for a deep geologic repository where the spent nuclear fuel could be stored for thousands of years without adversely affecting the environment. However, studies at the site have taken far longer than expected and political problems associated with siting a permanent repository have substantially delayed the program. DOE currently does not expect to be able to begin receiving spent nuclear fuel from utilities until at least 2010.
For almost 20 years now, utilities have been working to address the issue of storing spent nuclear fuel at their nuclear power plant sites for longer periods of time. Most plants have redesigned the racks in their spent fuel pools to increase the number of assemblies that can be stored there. Still, many plants are faced with the prospect that their spent fuel pools will not be able to accommodate all the spent fuel that will be discharged from the reactor until DOE can begin accepting spent fuel shipments. It is estimated that 78 of the country’s 103 reactors will have no room left in their spent fuel pools by 2010.
After the spent fuel is removed from the reactor, heat and radioactivity in the fuel drop significantly. Many utilities are taking advantage of the decrease in heat load to remove “cooled-down” assemblies from the spent fuel pool and transfer them into dry storage casks that can be maintained at the reactor site with minimal risk to the plant and the public. A group of these dry storage casks is called an Independent Spent Fuel Storage Installation (ISFSI). It is expected that most nuclear power plant sites will have to construct an ISFSI in order to safely store the volume of spent fuel that will be produced by their reactors before DOE can begin accepting shipments of their spent fuel for permanent disposal.
According to Alan Nelson, a Project Manager at the Nuclear Energy Institute, 16 reactor sites in the U.S. had constructed ISFSIs as of August 2000. From 2001 to 2006, there are firm plans to construct an additional 18 ISFSIs that will handle spent fuel from 26 nuclear reactors. A total of 198 casks had been loaded with spent nuclear fuel as of August 2000. This number is projected to increase to 380 casks by 2005 and to 950 casks by 2010.
As part of the Nuclear Waste Policy Act in 1982, Congress directed the Nuclear Regulatory Commission (NRC) to develop a streamlined process for utilities to use in moving spent fuel into dry casks. NRC amended its regulations to allow nuclear power plant owners to store spent nuclear fuel in pre-approved containers (casks) without having to go through a potentially lengthy and contentious licensing process. Under this streamlined process, storage cask vendors submit their cask designs to the NRC for detailed technical reviews. Once a vendor’s design is approved, utilities can order and install casks that meet their individual needs. To date, the NRC has approved 12 spent fuel cask designs. Cask designs include those that rest horizontally in racks and vertically. The various cask designs can accommodate different sizes and types of fuel, heat loads and weight restrictions on plant equipment used for handling casks.
The spent fuel storage casks are made from steel or steel-reinforced concrete as much as 18 inches thick. Lead is generally incorporated into the design to provide additional shielding from the radiation still produced by the spent nuclear fuel. Figure 1 shows one type of spent fuel storage cask. The spent fuel assemblies are placed into stainless steel canisters. The canisters are sealed and the air is purged from the canister with helium. The helium provides an optimum mechanism for transferring heat from the assemblies to the canister interior surface by natural convection. The inert helium atmosphere in the casks also prevents corrosion of fuel assemblies. The canister is placed into the storage cask where vents provide natural convection cooling for the canister. – Figure 1. Spent fuel storage cask
The casks, which are designed to sit on concrete pads outside the nuclear power plant’s structures but inside a secured area at the site, are designed to withstand a broad range of extreme natural phenomena including earthquakes, floods, fires, explosions, tornadoes, and temperature extremes. The entire ISFSI includes the casks, pads where the casks sit, instrumentation for monitoring the casks, security systems and equipment, and any other supporting structures. Parameters typically monitored for an ISFSI include cask exit air temperatures and area radiation levels. High exit temperatures may indicate blockage of airflow that requires cleaning.
The typical life cycle for an ISFSI starts with a process to select a cask vendor. Bid specifications are developed that define site specific parameters relating to fuel design and heat load, site design considerations for natural phenomena, and site specific requirements regarding quality assurance.
Figure 2. Three-dimensional architectural rendering of a typical ISFSI design.
Once a cask vendor is selected, detailed design of the ISFSI can proceed. This includes the design of the storage pad, haul path evaluations and upgrades (cranes and roads for bringing casks to the ISFSI from the plant), equipment for lifting and handling, security systems, electrical power supplies, and any operational support facilities, such as monitoring buildings and stations. Figure 2 shows a three-dimensional architectural rendering of a typical ISFSI design.
When the NRC approves a spent fuel cask design, they issue a Certificate of Compliance (CoC) for the Vendor’s system. This CoC is issued for reactor owner use under the General License provisions of the Code of Federal Regulations. The ISFSI designer and the reactor owners must verify that site specific conditions fall within the design limits established in the CoC before use. This may require analysis based on specific characteristics of the spent fuel, analysis of the cask response to natural phenomena such as an earthquake, or comparison of bounding site parameters with cask design parameters. If the site specific conditions are within the “envelope” of the Certificate of Compliance, then the reactor owner and designer only need to perform a safety evaluation that verifies that the ISFSI does not create the potential for a new or different kind of accident at the site.
Figure 3. Planned ISFSI at Diablo Canyon Nuclear Plant
If, however, the design conditions for the site fall outside the design envelope established for the cask, the utility must obtain a site specific license before using the cask. At the Diablo Canyon Nuclear Power Plant, Enercon Services is providing support to Pacific Gas and Electric Company in designing an ISFSI. The installation will eventually accommodate 140 casks. Because of the high seismicity of the Diablo Canyon Site, Enercon, the cask vendor, and Pacific Gas and Electric are working together to demonstrate that the combination of the pad supporting the casks, seismic restraint systems, and the casks will withstand the site’s Design Basis Earthquake. Documentation substantiating this conclusion will have to be submitted to the NRC, and the NRC will have to issue a separate plant specific license for the ISFSI at Diablo Canyon. Figure 3 shows the planned site for the ISFSI at the Diablo Canyon Nuclear Plant.
At the Oyster Creek Generating Station in New Jersey, where the site seismic demand is not as severe, Enercon is providing services to AmerGen Energy to support the selection of a storage system under a General License. The ISFISI at Oyster Creek is expected to consist of 20 casks.
In addition to design issues, considerable technical support is also required for operational activities to load the spent fuel from the spent fuel pool into the canisters that will in turn be placed in the casks. This may include special analyses of equipment to be used for lifting the spent fuel and the loaded canisters, nuclear criticality and shielding analysis, and thermal evaluations. Support may also be required in completing environmental assessments of proposed changes in the plant’s technical specifications and in addressing issues identified by local stakeholders.
Bob Evans is Enercon Services’ D&D Services Program Manager. He has 23 years of experience in commercial and governmental nuclear power programs with expertise in licensing, engineering and technical support. Evans holds a bachelor’s degree in nuclear engineering from the University of Missouri at Rolla.
Ron Miranda is Enercon Services’ Senior Project Manager for Independent Spent Fuel Storage Installation Projects. He has 28 years of experience in the nuclear power generation industry as a civil/structural design engineer for new power plant construction, and as a project manager for existing plant modification and maintenance tasks. Miranda holds a master’s degree in civil/structural engineering from the Newark College of Engineering/New Jersey Institute of Technology.
Steve Tumminelli is a Consulting Engineer at Enercon Services and is currently assigned as the Project Engineer of the Diablo Canyon Independent Spent Fuel Storage Installation project. He has 32 years of experience in industry, 23 of which have been in the design and evaluation of nuclear power plant structures and components. Tumminelli holds a Ph.D. in civil engineering from Lehigh University.