As Fukushima Daiichi reminds us, Rancho Seco’s spent fuel rods remain ‘hot’ for generations to come
Talk about commitment.
Almost 22 years ago, by a vote of its citizens, Sacramento turned off the Rancho Seco nuclear power plant. But an embrace of nuclear power lasts a long, long time. Since the closure of Rancho Seco, SMUD has removed all radioactive components from the nuclear reactor, and the property where the plant was located is now available for other use. However, directly next to the remains of the nuclear plant sits a rectangular block of a building that contains all of the spent nuclear fuel used to produce electricity from 1975 to 1989.
With the recent tragedy at the Fukushima Daiichi nuclear complex, legitimate questions are being asked about how utilities, the nuclear industry and governments are caring for spent nuclear fuel that remains dangerous for living beings for 100,000 years or longer. Unused nuclear fuel is made up of uranium mixed with low concentrations of the radioactive isotope of uranium (U-235) encased in hardened zirconium tubes. At some power plants, as is the case in one of the Fukushima reactors, U-235 is mixed with plutonium recovered from nuclear weapons. In either case, a portion of the fuel is “used up” in about 18 months and has to be replaced with fresh fuel.
The remains of uranium and plutonium fission are a cocktail of radioactive isotopes, including iodine, cesium, strontium, cobalt, krypton, molybdenum and about 20 other elements. Some of the elements are gases like iodine and krypton, while most are solids. Some of the radioactive materials decay quickly, while others last a very long time. Some are metabolized by living beings while others are inert. When removed from the nuclear reactor, the spent fuel is physically hot as well as radioactive.
Spent fuel tubes have to be kept cool enough to prevent the zirconium alloy from melting and releasing the radioactive materials they contain. To keep the spent fuel cool, workers at nuclear power plants like Fukushima and Rancho Seco put it in spent fuel pools, which look like swimming pools but are about 40 feet deep. In the spent fuel pools the radiation can break up water molecules and cause the build-up of highly explosive hydrogen. Spent fuel buildings have equipment to monitor and treat the hydrogen to avoid explosions.
However, unlike the nuclear-power reactor which is housed in a hardened containment building, spent fuels pools are usually housed in a building that cannot withstand a high-energy explosion. That means, it is critically important that the spent fuel remain covered with water and that hydrogen be removed. Otherwise, there is the potential that an enormous quantity of radioactive materials will be released into the surrounding environment. To carry out these protective activities, it is necessary that there always be a source of electricity to power water pumps and ventilation equipment.
It is still not clear, after more than two weeks, what precisely occurred at each of the four damaged nuclear power plants in Japan. However, it seems very likely that the spent nuclear fuel in at least two of the facilities was uncovered and that there were several hydrogen explosions. It is possible that some of the spent fuel melted down. The solidarity and the sacrifice of the Japanese people in the face of this tragedy have been impressive. Unfortunately, it appears that the consequences of the damage at the Fukushima nuclear plants will continue for months if not years.
At Fukushima, we know that the combination of the earthquake and tsunami caused the reactors to lose off-site power. It appears that the diesel-fuel storage tanks for the backup generators were washed away in the tsunami. The second backup system of battery power either did not work or ran down in a short period of time. The cascade of failures left the reactors and spent fuel pools without a source of electricity for an extended period of time.
No nuclear power plant currently in operation anywhere on the planet is fail-safe. The design philosophy for the existing generation of nuclear power plants is defense in depth. That is, they are designed to have multiple and diverse systems to prevent a system or component failure from becoming catastrophic. The most troubling aspect of what happened at Fukushima is that no one—not the Tokyo Electric Power Company, not the Japanese government nor the International Atomic Energy Agency—anticipated a tsunami near the magnitude of the one that occurred on March 11 or considered what it might do to the defensive systems.
Many nuclear power plants are built near the ocean, since many people live in coastal areas and nuclear power plants require large quantities of water to produce electricity. In California, the San Onofre Nuclear Generating Station and the Diablo Canyon Power Plant are located very near the Pacific Ocean. Diablo Canyon is also located near the San Andreas and Hosgri faults. The U.S. Nuclear Regulatory Commission determined in 1978 that Diablo Canyon can withstand a 7.5 magnitude quake. SONGS is built to withstand a magnitude 7.0 earthquake.
With the closure of Rancho Seco in 1989, SMUD was faced with the decision of what to do with the spent nuclear fuel at the power plant. Leaving it in the spent fuel pool would require SMUD to operate and maintain multiple mechanical systems for an unknown period of time. That would be very costly and potentially dangerous. Some at SMUD thought the spent fuel might soon be shipped to an underground repository at Yucca Mountain in southern Nevada. Others, including myself, thought that Yucca Mountain might not open for a long time, if ever.
The alternative to storage in a spent fuel pool is dry cask storage. SMUD was one of the first utilities in the United States to license a dry cask storage system for spent nuclear fuel. In 2001 and 2002, SMUD removed the 493 spent fuel tubes from the spent fuel pool and put them into steel storage canisters. Each canister was filled with helium, welded shut and tested for leaks. The canisters were then inserted into steel-reinforced concrete storage vaults, which are also welded shut. Unlike other utilities that have free-standing casks which could be breached by a determined terrorist, SMUD’s facility is considered to be secure.
How long will SMUD be responsible for the spent nuclear fuel?
No one really knows. The Independent Spent Fuel Storage Installation at Rancho Seco is licensed by the Nuclear Regulatory Commission through 2027. It probably will be relicensed. While SMUD has certainly proven that it can meet the power needs of Sacramento without nuclear power, it will find it difficult to free itself from the responsibility to take care of the remains of nuclear power for many generations to come.