Ricochet is the best place on the internet to discuss the issues of the day, either through commenting on posts or writing your own for our active and dynamic community in a fully moderated environment. In addition, the Ricochet Audio Network offers over 50 original podcasts with new episodes released every day.
Ask The Expert: Spent Nuclear Fuel Handling and Storage
A basilisk is a mythical creature able to kill with a single glance. Used or “spent” nuclear fuel would have a similar effect, if you were to stand close to it without benefit of shielding: Within a matter of minutes, you would receive a lethal radiation dose. Unlike basilisks, however, spent fuel isn’t out to get you, and is handled and stored safely at every nuclear power station in the United States. In contrast, new fuel that has never been loaded into a nuclear reactor has a very low — almost negligible level of radiation — and can be touched and directly handled without incurring any significant radiation dose.
The smallest unit of nuclear reactor fuel is a fuel pellet, a cylinder of compressed uranium dioxide, enriched to about 3 – 4.5% of the U-235 isotope. Each fuel pellet is less than half an inch in diameter and less than an inch long. Fuel pellets are loaded into a slender tube (called cladding) about 12 feet long, usually made of Zircaloy (a metallic alloy); the sealed tube is called a fuel rod, which looks similar to a wooden dowel. Fuel rods are arranged in an array called a fuel assembly. A boiling water reactor (BWR) has a 7 x 7 or 8 x 8 array of fuel rods running parallel to each other, in an assembly about five and a half inches square, about 14 to 15 feet long, weighing about 600 pounds; a typical boiling water reactor core holds between 500 and 600 such assemblies.
In contrast, a pressurized water reactor (PWR) has larger fuel assemblies that contain significantly more fuel rods — between 14 and 17 per side, though hexagonal arrays also exist — and weigh 1300 lbs or more. A typical pressurized water reactor core holds fewer than 200 fuel assemblies. A few spaces in each assembly are fitted with guide tubes instead of fuel rods, to allow control rods or in-core instrumentation to be inserted.
A reactor is typically refueled every 18 months, during which time about a third of the fuel assemblies are replaced. The assemblies selected for removal are the oldest and most depleted fuel; while they are not completely “used up,” they will no longer support reactor operation at 100% power and cannot maintain a chain reaction. Spent assemblies still contain fissionable materials – both uranium and a number of lighter fission products, most of which are radioactive — and their contents still spontaneously decay and emit neutrons. Consequently, the spent fuel assemblies continue to emit heat and radiation, which gradually decrease over a period of years.
Irradiated fuel must be shielded to prevent the escape of radiation. When the fuel is not enclosed inside the reactor, water is used as a shield. Gamma rays — the most penetrating form of radiation — will be attenuated by half when passing through a mere 18 inches of water. A 20-foot layer of water will cut the gamma radiation in half about 13 times or more, which is generally good enough to handle the fuel safely.
The reactor vessel is the deep pit on the left and is, itself, within a larger pit called the refueling cavity.
The reactor is located at the bottom of a refueling cavity, a concrete “pit” at least 22 feet deep. The reactor head is unbolted and — as it is lifted — the cavity is slowly flooded with water so that the head is never more than a few inches above the water surface. When the water reaches a depth of at least 20 feet, the head is stored and an underwater tunnel between the cavity and the spent fuel pool is opened. One-by-one, fuel assemblies are lifted out of the core with a crane, underwater at all times, and laid down on a transfer cart which transports the assembly out of the containment and into the spent fuel pool in an adjacent building. There, a crane upends the fuel assembly, lifts it, and inserts it into a fuel rack in the spent fuel pool. The process is repeated until the required amount of spent fuel has been removed and stored, one assembly at a time. During the entire process, the fuel assembly is kept as far under water as possible to protect fuel handling personnel from radiation.
Storage of spent fuel must meet the following objectives:
- Protect the fuel handling personnel and the public from radiation, and prevent the release of radioactive materials;
- Dissipate the decay heat produced by the fuel and keep the fuel from overheating and damaging its cladding;
- Prevent the fuel from returning to criticality, i.e., restarting the nuclear chain reaction (return to criticality is possible if fuel assemblies are packed too close to one another without any moderating or neutron-absorbing materials); and
- Protect the fuel from mechanical damage.
The spent fuel pool is deep enough to ensure that the top of the fuel racks are covered by at least 20 feet of water to provide adequate shielding. Heat is removed by a cooling system which circulates the pool water through a heat exchanger to remove decay heat. If this system were to fail, the pool would heat up gradually over a number of hours or days, and the heatup would be detected on instruments reading in the main control room. Cold water could be added using hoses to replace water lost by evaporation or boiling.
A spent fuel pool.
The fuel racks maintain proper separation between fuel assemblies and shield them from one another. The original fuel racks at most stations use physical space to separate fuel assemblies and control reactivity. As these fuel racks filled up over the years, they were replaced with new racks which packed fuel assemblies much closer together (saving space) but added divider plates between assemblies incorporating neutron absorbing materials containing boron, such as boron carbide. Boron-10 (about 20% of naturally occurring boron) is a powerful absorber of neutrons, and therefore it is useful in controlling the reactivity of the fuel. In a PWR fuel pool, boric acid will be added to the pool water to a concentration of up to 2000 ppm boron to further control reactivity and prevent a return to criticality (this is not an option in a BWR pool, which utilizes clean, demineralized water). The fuel racks keep the fuel oriented upright and protect it from mechanical damage. Administrative controls are used to prevent foreign objects from being dropped into the pool.
Fuel assemblies remain in the spent fuel pool until their decay heat production has reduced sufficiently that they may be moved to dry storage without danger of overheating. The NRC requires pool storage for a minimum of one year, but it generally takes five years or more for decay heat production to decline sufficiently. Customarily, utilities have been leaving fuel in the pool until the amount of space remaining in the pool becomes an issue. Most nuclear power stations have sufficient space in their spent fuel storage pool racks for approximately 20 to 25 years of spent fuel. When it becomes necessary to free up space in the spent fuel pool, the oldest fuel assemblies are removed and transferred to dry storage casks located a distance away from the plant, in an Independent Spent Fuel Storage Installation (ISFSI) to begin the second phase of their storage.
Storage casks
The dry storage cask system consists of two parts: a hermetically sealed metal canister to permanently contain the fuel, inside one of several types of overpack designed for transport, long-term storage, or both. The metal canister contains a storage rack inside, similar to the rack in the spent fuel pool. They typically hold between about 70 assemblies from a boiling water reactor, or between 24 to 32 from a pressurized water reactor, and weighs approximately 100,000 lb. However, the walls are too thin to provide adequate radiation and heat shielding, which is why the canister is kept inside an overpack at all times. The overpack is made of metal and concrete and is designed to provide enough shielding to allow personnel to work around the cask. The overpack for long-term storage weighs well over 100,000 lb empty. With a loaded canister inside, the whole storage cask weighs between 220,000 and 280,000 lb.
The canister is placed into a transport cask, lowered into the spent fuel pool, and loaded with fuel assemblies underwater. A lid is placed on the transport cask, which provides radiation shielding when the cask is removed from the water. The transport cask is removed to an area where it can be drained and decontaminated. The canister lid is welded on, all water and air is removed from the canister, and the interior is inerted with helium. The canister is transported to an outdoor cask storage area, and transferred from the transport cask to a storage cask. This is done by placing the transport cask on top of the storage cask, and lowering the canister from the transport cask to the storage cask while keeping the canister shielded at all times. The storage cask is then moved to its permanent location on a concrete slab. Though outdoors, the cask is protected from earthquakes by being bolted to the slab. The cask itself protects the fuel canister inside from wind-driven missiles and storms of all kinds.
Decay heat from the fuel assemblies is transferred to the wall of the canister (inside the cask) by natural convection of the helium inside the canister. The amount of decay heat produced by the fuel in one cask at the time of loading may vary typically from one to fifteen kilowatts or more, depending on how long the fuel assemblies have been out of the reactor core. The helium inside the canister can reach a temperature of several hundred degrees in temperature, and helps transfer the heat to the outer wall of the canister. The outer storage cask is a thick-walled concrete overpack which provides shielding from both radiation and heat, and is equipped with air vents near the top and bottom to allow the canister to cool inside the overpack by natural convection. The air vents have a labyrinth design to prevent radiation from escaping (radiation can travel only in a straight line).
Detailed records are kept on fuel handling. Each fuel assembly has a paper trail documenting where and when it was located in the reactor core for all three cycles; where it was located in the spent fuel rack and the dates of storage and removal; and which cask it was loaded into and the date of loading, among other information. A typical reactor refueling cycle produces enough spent fuel to require two or three casks. Approximately 200 such casks loaded annually nationwide.
The 11-minute video below shows the process of transferring fuel from the pool to casks at Diablo Canyon, a 2-unit PWR in California. The video also shows how at each stage of the process, seismic restraints are installed to protect the cask from possible earthquakes. This is required at all nuclear stations in the United States, not just in California.
https://www.youtube.com/watch?v=mILvWNgggfU
The third phase of storage would consist of long-term storage in a central repository, should one ever be constructed. In order to transport the fuel to such a repository, it would be necessary to transfer the canisters inside the storage casks to an overpack designed for rail transport, as the casks are too heavy to be transported by rail. These transport casks would likely have to be oriented horizontally in order to pass through underpasses along the route.
The Nuclear Waste Policy Act of 1982 required the Department of Energy to site, construct, and operate an underground waste repository in a geologically stable location. In 1987, DoE established Yucca Mountain as the the designated location. While more than $30 billion in taxes has been collected from nuclear utilities since 1982 — and approximately $9 billion has been spent on studies — no repository has been built. Utilities have sued the federal government to recover their costs of nuclear fuel storage. Collection of the fees was halted by a court order, and these payments stopped in 2014. The DoE attempted to withdraw its license application for Yucca Mountain in 2010, but this action was been opposed by lawsuits. Pursuant to a 2013 court order, the NRC resumed technical and environmental reviews and published the final volumes of its safety evaluation report in January 2015. Work continues on an environmental impact statement, and it is still unknown when a license may be issued. It is likely that outdoor dry cask storage will be the predominant method of spent fuel storage in the United States for decades to come.
Published in General
I am officially a geek. I loved this.
edit: and I live 5 miles from a nuclear plant. I’m glowing. Simply glowing. All along I thought they were removing the spent fuel from my backyard.
Thanks, I wasn’t sure if it was too “niche.”
I don’t know why the video wouldn’t embed. It’s a little slow but interesting.
I got to the Yucca Mountain visitors center years ago. Very impressive presentations for someplace that will never be used. Would be a very cool casino at this point.
What happens to the water in the pool? Does that become too radioactive and needs to be refreshed and disposed of.
You don’t need to replace the water. The spent fuel cooling system circulates the water through filters that remove any radioactive particles. As the pool is open on top, a certain amount of dust settles on the surface, and the filters take care of that too.
Yucca Mountain might still be constructed and used some day, but at the rate they are proceeding, it will likely take many more years. I have not been there, but I understand they have dug some tunnels that were used for study purposes. They have not dug the production tunnels that would house the spent fuel casks.
And, while we don’t do it, you can also take the spent fuel rods and reprocess them into new fuel, or into mixed oxide fuel.
But my favorite solution is the newer reactor designs that burn so hot -in the radioactive sense -that they cause all the highly radioactive isotopes, like Cessium and Strontium, to decay in the reactor.
I love that about energy production. Want to make it cleaner and safer? More Power!
Some mementoes from Yucca Mountain.
Great post! Thank you for going into all that detail!
The U.S. has fiddled with reprocessing off and on for decades. Work has started on a reprocessing facility at Savannah River, but like almost everything the federal government does, it is over budget and way behind schedule, and it might never be finished. US power plants would have to have license amendments and possible modifications to use mixed-oxide fuels, and they are obviously not going to bother unless they see an economic benefit, or if they cannot obtain fresh uranium fuel.
Hope you are glowing emotionally, and not literally.
Which plant, if you don’t mind saying?
The amount of radiation reaching you from the ISFSI is probably too low to measure.
Fascinating stuff, BSB. Thanks for posting it. I knew some of this, but your end-to-end presentation was enlightening.
I’m continually amazed that the enviros, whose major concern at the moment is carbon emissions, remain so adamantly anti-nuke. The NIMBYs (Not In My Back Yard) have become BANANAs (Build Absolutely Nothing Anywhere Near Anyone).
BANANAs! I love it!
I’ve thought they were “bananas” for years.
The anti-nuke protestors would have a lot more credibility if they’d stop invoking Chernobyl (outdated design), Fukushima (9.0 earthquake, tsunami, and backup generators sited too close to the ground), and Three Mile Island (scary, but nobody died, plus it happened 36 years ago).
Wow.
Limerick.
I actually don’t really mind. When I first moved there in ’97, the security was more flexible, and I could drive on the site after hours, and listen and watch the water fall through the cooling towers. I’m fascinated.
Since 2001, I am a little more concerned that I live so close. It was months or even a year after September 11 before my mind let me believe the plant wasn’t going to be a terrorist target.
And the monthly drill with the siren always stops my heart.
A nuclear plant really is a hard target. It’s much easier to fly an airplane into a tall office building. The containment walls are very thick; I doubt that an aircraft could penetrate them. They actually are designed to withstand an aircraft crash (I don’t know how large an aircraft); prior to 2001, everyone assumed that such a crash would be accidental. And the security at most plants now reminds me of a federal prison. There are guards everywhere armed with Glocks and AR-15s. You actually don’t see most of them, but they are watching. I can’t go into all of the security features, but suffice it to say that a terrorist would have a hard time even thinking of a way to get in and do major damage that would release radioisotopes. There are much easier ways to kill a lot of people.
It’s been a while since I did the research, but my recollection is that civilian reprocessing is illegal in the US. Carter era decision, because of course having a civilian reprocessing system would encourage more military nuclear weapons.
Re-read anonymous’s post to see the stupidity of that logic.
But was that security in place pre-September-11?
I don’t suppose my admiring of the cooling towers was anywhere near the actual reactor, but I never intended to go in.
I don’t worry now, as I expect the security is as strong as necessary, and I am more likely to be a terror target at my place of work than 5 miles from the nuclear plant.
Thanks for the clarification. The article I was reading was not 100% clear on that point.
I don’t imagine we will reprocess commercial nuclear power fuel until the economics change. One estimate I read was that uranium would have to reach $360/kg (about $160/lb) for reprocessing to be economical. The current price is around $35/lb.
Carter did make it illegal. Reagan reversed that decision, but did not restore funding. Little or nothing has been done since.
If there is ever a uranium shortage (say, because Russia controls our domestic uranium supplies), we will have to start opening casks and reprocessing fuel. It will be waiting.
Security has always been tight at nuclear plants, much tighter than at airports, but after September 11 additional measures have been taken to make sure terrorists would have a hard time infiltrating without being picked off. Cooling towers have always been a lower security area, as they have nothing to do with nuclear safety.
Yes, I’m sure that’s true. The security perimeter is now expanded, and I can no longer get anywhere near the cooling towers.
Your information brings facts to my current comfort level. :)
From the inimitable Randall Munroe:
Very cool post – thank you! And “thank you” to the editors for promoting it, as I missed this on the Member Feed.
Not exactly – part of the reason the US went to such thick containment structures for nuclear reactors is that three nutcases hijacked a plane in 1972 and threatened to crash it into a nuclear reactor…
Very informative! When I was a kid, I heard a man speak, who had some experience in France dealing with the waste material from their national nuclear power program. This was in the 1990’s. He said that the entire store of all the waste from all of their plants was kept in a closet in a building in downtown Paris. That stuck with me ever since as such a terrifying thought! Now I understand why it really isn’t such a big deal.
Information is a good thing. But there are many who will create fear, out of nothing, to accomplish their own agenda.
This post is one of the any reasons I like Ricochet. Great post. Very informative. Very anonymous-like.
Is there a reason why the head is drawn above the surface? Why not keep it underwater as well?
I am surprised we still have a nuclear industry. As taxpayers we pay environmental groups (reimburse their legal fees) to harass any activity related to nuclear energy. An experienced admin law lawyer puts numerous carefully crafted objections on the docket when the process starts, then litigates the adequacy of the formal agency responses in hopes of having a federal court order the process back to square one, often after time-consuming (and expensive) appeals. The technical legal term for this process is “cha-ching!”
The long lead time also means that the designs change, costs shift, bids no longer apply and there will need to be a new set of legal fees for the inevitable litigation over the allocation of the overruns.
Honestly, the coolest part of my job is that I get to take a post like this — from someone who writes clearly and who knows what they’re talking about — give it a little boot shine, and help share it with others.
I love editing stuff from our contributors, but this is the kind of thing that makes my day.