Spent nuclear power fuel accumulated over the past 50 years is bound up in more than 241,000 long rectangular assemblies containing tens of millions of fuel rods. The rods, in turn, contain trillions of small, irradiated uranium pellets. After bombardment with neutrons in the reactor core, about 5 to 6 percent of the pellets are converted to a myriad of radioactive elements with half-lives ranging from seconds to millions of years. Standing within a meter of a typical spent nuclear fuel assembly guarantees a lethal radiation dose in minutes.
Heat from the radioactive decay in spent nuclear fuel is also a principal safety concern. Several hours after a full reactor core is offloaded, it can initially give off enough heat from radioactive decay to match the energy capacity of a steel mill furnace. This is hot enough to melt and ignite the fuel’s reactive zirconium cladding and destabilize a geological disposal site it is placed in. By 100 years, decay heat and radioactivity drop substantially but still remain dangerous. For these reasons, the US Government Accountability Office (GAO) informed the Congress in 2013 that spent nuclear fuel is “considered one of the most hazardous substances on Earth.”
US commercial nuclear power plants use uranium fuel that has had the percentage of its key fissionable isotope—uranium 235—increased, or enriched, from what is found in most natural uranium ore deposits. In the early decades of commercial operation, the level of enrichment allowed US nuclear power plants to operate for approximately 12 months between refueling. In recent years, however, US utilities have begun using what is called high-burnup fuel. This fuel generally contains a higher percentage of uranium 235, allowing reactor operators to effectively double the amount of time the fuel can be used, reducing the frequency of costly refueling outages. The switch to high-burnup fuel has been a major contributor to higher capacity factors and lower operating costs in the United States over the past couple of decades.
High burnup significantly boosts the radioactivity in spent fuel and its commensurate decay heat. Of particular concern is the effect of high-burnup fuel on the cladding that contains it in the fuel assemblies used in commercial reactors. Research shows that under high-burnup conditions, that cladding may not be relied upon as the primary barrier to prevent the escape of radioactivity, especially during prolonged storage in the “dry casks” that are the preferred method of temporary storage for spent fuel. Resolution of these problems remains elusive.
The NRC and the nuclear industry do not have the necessary information to predict when storage of high-burnup fuel may cause problems. To err on the side of caution, high-burnup fuel might have to be left in cooling pools for 25 years—as opposed to the current three to five years for lower burnup spent fuel— to allow cladding temperatures to drop enough to reduce risks of cladding failure before the fuel is transferred to dry storage. Also, the cooling pools at US commercial reactors are rapidly filling, with more than 70 percent of the nation’s 77,000 metric tons of spent fuel in reactor pools, of which roughly a fourth is high burnup. So far, a small percentage of high-burnup used fuel assemblies are sprinkled amid lower burnup fuel in dry casks at reactor sites. But by 2048—the Energy Department’s date for opening a permanent geologic disposal site—the amount of spent fuel could double, with high burnup waste accounting for as much as 60 percent of the inventory.
These concerns were given greater prominence in May of this year by a National Academy of Sciences panel established by Congress to review the response of the NRC to the Fukushima nuclear accident. In its report, the panel warned the NRC about terrorist attacks for the second time since 2004 and urged the agency to “ensure that power plant operators take prompt and effective measures to reduce the consequences of loss-of-pool-coolant events in spent fuel pools that could result in propagating zirconium cladding fires.” Allison Macfarlane, then chair of the U.S. Nuclear Regulatory Commission (NRC), noted in April, 2014 that “land interdiction [from a spent nuclear fuel pool fire at the Peach Bottom Reactor in Pennsylvania] is estimated to be 9,400 square miles with a long term displacement of 4,000,000 persons.”