In the United States, the national inventory of commercial spent nuclear fuel amounts to nearly 70,000 metric tons, which is stored at 75 sites in 33 states (see fig. 1). Fuel for commercial nuclear power reactors is typically made from low- enriched uranium fashioned into thumbnail-size ceramic pellets of uranium dioxide. These pellets are fitted into 12- to 15-foot hollow rods, referred to as cladding, made of a zirconium alloy. The rods are then bound together into a larger assembly. A typical reactor holds about 100 metric tons of fuel when operating—generally from 200 to 800 fuel assemblies. The uranium in the assemblies undergoes fission—a process of splitting atoms into fragments and neutrons that then bombard other atoms—resulting in a sustainable chain reaction that creates an enormous amount of heat and radioactivity. The heat is used to generate steam for a turbine, which generates electricity. The fragments created when fission splits atoms, or when bombarding neutrons bond with atoms, include hundreds of radioisotopes, or radioactive substances, such as krypton-90, cesium-137, and strontium-90. Furthermore, the neutron bombardment of uranium can also create heavier radioisotopes, such as plutonium-239. The radioisotopes produced in a reactor can remain hazardous from a few days to many thousands of years; these radioisotopes remain in the fuel assemblies and as components of the resulting spent fuel. Each fuel assembly is typically used in the reactor for 4 to 6 years, after which most of the fuel it contains is spent, and the uranium dioxide is no longer cost-efficient at producing energy. Reactor operators typically discharge about one-third of the fuel assemblies every 18 months to 2 years and place this spent fuel in a pool to cool. Water circulates in the pool to remove the enormous heat generated from the radioactive decay of some of the radioisotopes. As long as circulating water continues to remove this heat, pool water temperature is maintained well below boiling, typically below 120 degrees Fahrenheit. If exposed to air, however, recently discharged spent fuel could rise in temperature by hundreds or thousands degrees Fahrenheit. A pool is needed to ensure that heat generated from the decay of radioisotopes, particularly immediately after discharge from a reactor, does not damage fuel rods and release radioactive material. Figure 2 shows a fuel pellet for a commercial nuclear reactor and a fuel rod in an assembly. The pools of water are typically about 40 feet deep, with at least 20 feet of water covering the spent fuel, and the water is cooled and circulated to keep the assemblies from overheating. These pools are constructed according to NRC’s requirements, typically 4- to 6-feet thick with steel- reinforced concrete and a steel liner. The pools must be located inside what is known as the vital area of a nuclear power reactor, protected by armed guards, physical barriers, and limited access. Within the vital area, pools may be in one of two locations, depending on the type of reactor. In a pressurized water reactor, spent fuel is stored in a pool at or below ground level, but in a typical boiling water reactor, spent fuel is stored in a pool well above ground level, near the reactor vessel, as high as three stories above ground. Figure 3 shows the location of a spent fuel pool for a boiling water reactor, and figure 4 shows a typical spent fuel pool. As part of the construction permit and operating license application process for nuclear reactors, NRC requires companies licensed to operate these reactors to assess natural hazards, such as earthquakes, floods, hurricanes, and tidal waves that their reactors might face. Reactor operators must also show that their proposed pool designs would survive the most severe natural hazards, or combinations of less severe hazards, expected for that particular area.NRC has required reactor operators to reevaluate their original design criteria against more recent seismic information that has been developed since many of the nuclear power plants were first licensed. According to NRC documents, NRC developed its requirements with a concept of “defense-in-depth,” which is a way of designing and operating nuclear Since the Fukushima Daiichi disaster, power reactors that focuses on creating multiple independent and redundant layers of defense to compensate for potential human and mechanical failures so that no single layer, no matter how robust, is exclusively relied upon. All radioactive substances—referred to as radioisotopes—are unstable and spontaneously transform themselves into more stable isotopes by capturing or emitting atomic particles or by fission. The time it takes a radioisotope to decay into more stable substances is measured by a half-life. A half-life is the length of time it takes for one-half of a particular radioisotope to decay into a new isotope. After two half-lives, one-quarter of the original radioisotope will be left, but three-quarters will have changed to the new isotope. After 10 half-lives, only 1/1,000 of the original radioisotope is left. nuclear emergency.years and will take over 300 years to decay to negligible amounts. Cesium-137 contributes to the decay heat in a spent fuel pool and is a significant land contaminant if released. ) In contrast, cesium-137 has a half-life of 30.2 Typically, according to NRC officials, spent fuel must remain in a pool for at least 5 years to decay enough to remain within the heat limits of currently licensed dry cask storage systems. Spent fuel cools very rapidly for the first 5 years, after which the rate of cooling slows significantly. Spent fuel can be sufficiently cool to load into dry casks earlier than 5 years, but doing so is generally not practical. Some casks may not accommodate a full load of spent fuel because of the greater heat load. That is, the total decay heat in these casks needs to be limited to prevent the fuel cladding from becoming brittle and failing, which could affect the alternatives available to manage spent fuel in the future, such as retrieval. In recent years, reactor operators have moved to a slightly more enriched fuel, which can burn longer in the reactor. Referred to as high-burn-up fuel, this spent fuel may be hotter and more radioactive coming out of a reactor than conventional fuel and may have to remain in a pool for as long as 7 years to cool sufficiently. In the original designs submitted for spent fuel pools, fuel assemblies were packed in relatively low densities, but operators have replaced these low-density racks with higher-density racks to store more spent fuel. According to NRC officials, NRC accepts high-density storage of spent fuel if certain conditions are met, such as adequate cooling, the maintenance of structural integrity, and the prevention of a critical chain reaction. Neutron-absorbing materials can be used to keep closely packed assemblies from starting a chain reaction.in the 1980s, NRC conducted several safety studies on the impact of increasing the density of spent fuel in pools and determined that the risk of a potential release from overheating or igniting, or even of a critical chain reaction from the dense geometric configuration, was small, particularly if certain steps were taken to reduce the risk. Even with re- As pools began to fill racking to a dense configuration, however, spent nuclear fuel pools are reaching their capacities and may contain several thousand assemblies each. As reactor operators have run out of space in their spent fuel pools, more operators have turned to dry cask storage systems. These systems consist of a steel canister protected by an outer cask made of steel or steel and concrete to provide shielding from the heat and radiation of spent fuel. In one typical process of transferring spent fuel to dry storage, reactor operators place a steel canister inside a larger steel transfer cask and lower both into a pool. Spent fuel is loaded into the canister, a lid is placed on the canister, and then both the canister and transfer cask are removed from the pool. The lid is welded onto the canister, and the water drained. Then the canister and transfer cask are aligned with a storage cask and the canister is maneuvered into the storage cask. The storage casks, in either vertical or horizontal designs, are usually situated on a large concrete pad surrounded by safety systems and a security infrastructure, such as radiation detection devices and intrusion detection systems. The transfer process has become routine at some power plants (see fig. 5). In addition to regulating the construction and operation of commercial nuclear power plants, NRC also regulates spent fuel in dry storage. NRC requires that spent fuel in dry storage be stored in approved systems that offer protection from significant amounts of radiation. NRC evaluates the design of passively air-cooled dry storage systems for resistance to certain natural disasters, such as floods, earthquakes, tornado missiles, and temperature extremes. NRC may require physical tests of the systems, or it may accept information derived from scaled physical tests and computer modeling. For example, dry storage systems must be able to withstand, among other things, being dropped from the height to which it would be lifted during operations; being tipped over by seismic activity, weather, or other forces or accidents; fires; and floods. NRC has also analyzed the performance of dry storage systems in different terrorist attack scenarios. Once a dry storage system is approved, NRC issues a certificate of compliance for a cask design. Currently, NRC may issue a cask certificate for a term not to exceed 40 years. Similarly, NRC may renew a cask certificate for a term not to exceed 40 years (see fig. 6). The length of time that spent fuel can safely be stored in dry casks is uncertain.