Search Results for spent fuel

UCS in Science: The NRC Must Act to Reduce the Dangers of Spent Fuel Pool Fires at Nuclear Plants

, senior scientist

In a Policy Forum article published in this week’s Science magazine, I argue, along with my co-authors Frank von Hippel and Michael Schoeppner, that the U.S. Nuclear Regulatory Commission (NRC) needs to take prompt action to reduce the alarmingly high potential for fires in spent fuel pools at U.S. nuclear plants. Read more >

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Spent Fuel Damage: Pool Criticality Accident

, director, Nuclear Safety Project

Disaster by Design/Safety by Intent #29

Disaster by Design

Disaster by Design/Safety by Intent #26 described a progression leading to overheating and damage to a reactor core, often labeled a meltdown. Disaster by Design/Safety by Intent #27 described the damage to a reactor core that can result from reactivity excursions. Disaster by Design/Safety by Intent #28 and #29 mirror those commentaries by describing how irradiated fuel stored in spent fuel pools can experience damage from overheating and reactivity excursions. Read more >

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Nuclear Spent Fuel Damage: Pool Accident

, director, Nuclear Safety Project

Disaster by Design/Safety by Intent #28

Disaster by Design

Disaster by Design/Safety by Intent #26 described a progression leading to meltdown of a reactor core. Disaster by Design/Safety by Intent #27 described damage resulting from reactivity excursions.

This commentary describes a progression leading to overheating damage of fuel in a spent fuel pool. Next week’s post will describe how fuel in a spent fuel pool could experience a reactivity excursion. Read more >

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Crowded Spent Fuel Pools

, co-director and senior scientist

Spent fuel pools pose a much bigger threat to public safety than they should because of the large amount of radioactive material they contain, which could be released to the environment in a severe accident.

While concerns tend to focus on the nuclear fuel contained in reactor cores, cooling pools in the U.S. typically contain much more fuel than the core. Currently, U.S. pools overall contain over 5 times more radioactive fuel than is in all the reactor cores, and some individual reactor pools contain more than 8 times as much fuel as the reactor core. Yet the pools don’t have the same level of protection or safety systems as the reactor cores.  Read more >

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Radiation from Accelerating the Transfer of Spent Fuel from Pools to Casks

, co-director and senior scientist

Contrary to claims by some in the nuclear industry, accelerating the transfer of spent fuel from cooling pools to dry casks would not pose a significant risk to workers from increased radiation exposure, and does not outweigh the benefits of such transfers. Read more >

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UCS Comments on NRC’s Draft Spent Fuel Storage Study

, director, Nuclear Safety Project

In late June 2013, the Nuclear Regulatory Commission released a draft of its long-awaited study on spent fuel storage methods.

We thought the NRC’s study would answer the question of whether it is safer to store irradiated fuel in spent fuel pools or in dry storage at nuclear plant sites. This question arose after 9/11 for security reasons and resurfaced after Fukushima for safety reasons. The question is simple:

Read more >

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New NRC Study Shows Benefits of Transferring Spent Fuel to Dry Casks

, co-director and senior scientist

The draft study released Monday by the Nuclear Regulatory Commission (NRC) on the potential consequences of an earthquake on spent fuel pools reinforces our concerns about spent fuel pool safety and fails to address some key issues. Read more >

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Fission Stories #112: If I Only Had a Drain: Trouble at the Wolf Creek Spent Fuel Pool

, director, Nuclear Safety Project
Wolf Creek spent fuel pool

Wolf Creek spent fuel pool

In December 1987, an operator at the Wolf Creek nuclear plant near Burlington, Kansas, forgot to close a valve in the pipe connecting the spent fuel pool to the refueling water storage tank. The open valve allowed gravity to drain water from the spent fuel pool to the tank. Read more >

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UCS Comments on Expedited Transfer of Spent Fuel from Pools to Dry Casks

, senior scientist

On June 15, UCS submitted comments to the NRC on the recommendations of the post-Fukushima Task Force for expedited transfer of spent fuel to dry casks.

UCS supports the accelerated transfer of spent fuel from pools to dry casks. A chief advantage of such transfer is to increase the safety margin for events (either severe accidents or terrorist attacks) that cause a loss of water from the pool and result in heating of the spent fuel to the ignition temperature of the fuel’s zirconium alloy cladding, a self-sustaining zirconium fire, fuel damage, and massive radiological release.

The safety margin could be increased through enhancing defense-in-depth by strengthening the passive safety response of a pool to such events. It is remarkable that the nuclear industry and the NRC point to the so-called passive safety features of new nuclear reactors as a major advantage over the current generation, yet for operating reactors they oppose making modest changes that could enhance the passive safety response and reduce reliance on active measures to respond to a spent fuel pool fire. Such passive safety measures would be desirable for situations, such as Fukushima-type events, when active mitigative measures might not be able to be used quickly or effectively.

To increase safety, we find that:

  • the spent fuel in spent fuel pools should be configured to minimize the risk of a zirconium fire  if active safety measures are not available, by maximizing the potential for cooling by natural air circulation,
  • the spent fuel should be configured so that even if ignition does occur in hotter fuel assemblies, there is little risk that the fire will propagate to cooler assemblies,
  • the amount of spent fuel in the pool should be reduced to limit the radiological release, in the event the fire does propagate to cooler assemblies.

The NRC does not believe that expedited transfer of spent fuel to dry casks is necessary, given that following the 9/11 attacks it required licensees to disperse hotter spent fuel throughout the pools in a so-called “1×4” configuration (if feasible) and to install emergency means for providing makeup cooling water.

However, the NRC has not substantiated this belief by providing the public with sufficient technical information to demonstrate that these measures are sufficient to reduce the risk to an acceptable level. The NRC has conducted both analytical and experimental studies on spent fuel pool fires in the last decade but most of the results of these studies are classified or otherwise withheld from the public.

UCS believes that the scant information that has been released on these issues to date supports our view that the uncertainties in pool fire analysis are so large that substantial safety margins are needed to maintain defense-in-depth. These margins can be achieved only by reducing the pool inventory well below the densely packed configuration that the NRC currently advises licenses to maintain.

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Fission Stories #95: Deep Spent Fuel Pool Fishing

, director, Nuclear Safety Project

                                                       Figure 1

During a refueling outage in December 1979, workers at the Pilgrim nuclear power plant south of Boston (and north, way north, of Miami) transferred new (un-irradiated) fuel bundles into the spent fuel pool. They used the reactor building’s overhead crane to pick up a new fuel bundle from the inspection stand on the refueling floor and lower it into a storage rack in the spent fuel pool.

Figure 1 shows a new fuel bundle in the inspection stand next to the new fuel vault. Figure 2 shows the rails for the reactor building’s overhead crane (the crane itself is on the rails behind the photographer) and the refueling platform that straddles the spent fuel pool and reactor cavity. The refueling platform is used to transfer fuel bundles underwater between the spent fuel pool and the reactor core.

                                                     Figure 2

Workers placed one new fuel bundle into a storage rack and began pulling the crane’s hook out of the spent fuel pool. Suddenly, radiation alarms on the refueling floor sounded. Operators discovered an irradiated fuel bundle at the end of the crane’s hook and quickly returned it to a storage rack.

The new fuel bundle had been placed in a storage rack near an irradiated fuel bundle. The crane’s hook wedged between the lifting bail and fuel channel on the irradiated fuel bundle. The operators did not realize they had snagged an irradiated fuel bundle until it came close enough to the pool’s surface to set off the radiation alarms on the refueling floor.

The refueling platform normally used to transfer fuel bundles in the spent fuel pool and the fuel preparation machine (equipment used to conduct maintenance on and inspections of fuel bundles) mounted on a side wall of the spent fuel pool are designed not to permit a fuel bundle to be raised closer than 8 feet from the spent fuel pool’s surface. This amount of water shields workers from the intense radiation emitted from irradiated fuel bundles.

However, the overhead crane they were using—which is not supposed to move irradiated fuel—could have lifted the spent fuel bundle out of the pool, and would have if they had not stopped as soon as they heard the radiation alarms.

Our Takeway

Using an unbaited hook, workers at Pilgrim nearly managed to land a big one. It’s extremely fortunate they didn’t get a closer look at this one that got away. The radiation levels from an exposed irradiated fuel bundle can cause a fatal dose in mere seconds.

This potentially lethal hazard is precisely why the refueling platform and fuel preparation machine are designed to prevent a fuel bundle from getting too close to the pool’s surface. Of course, these design features afford zero protection when workers use other equipment to move fuel bundles in the spent fuel pool. They should not have used the overhead crane close to irradiated fuel in the spent fuel pool and nearly paid a high price for their mistake.

“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.

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Susquehanna Spent Fuel Pool Concerns, and How I Ended Up at UCS

, director, Nuclear Safety Project

In November 1992, Don Prevatte and I submitted a report to the NRC regarding our concerns with spent fuel pools at boiling water reactors (BWRs), of which 35 are operating in the US. We had been consultants working on a team to evaluate the proposed increase in the maximum power level of the two BWRs at the Susquehanna nuclear plant in Pennsylvania. My assignments included the spent fuel pool cooling and cleanup system while Don’s assignments included the reactor building ventilation system. While reviewing each other’s work, we uncovered a problem.

The spent fuel pool for nearly all US BWRs is located inside the reactor building, which also fully encloses the reactor containment building. The reactor building ventilation system was designed to cool rooms and areas in event of an accident to protect emergency equipment from damage caused by high air temperatures.

The design calculation for the reactor building ventilation system considered heat emitted by operating motors, heat emanating from piping filled with hot water, and heat given off by incandescent light bulbs. Collectively, these heat sources amounted to 5.2 million BTUs per hour (a British Thermal Unit, or BTU, is defined as the amount of heat needed to increase the temperature of one pound of water by one degree Fahrenheit).

The cooling system for the reactor building ventilation system was sized to accommodate this amount of heat removal, thus ensuring that emergency equipment would not overheat and fail.

But the design heat load from irradiated fuel stored in the spent fuel pool was 12.6 million BTUs per hour, meaning the spent fuel could emit up to that much heat. Under normal operation, that heat would be carried out of the building by the cooling system. However, safety analyses assume the spent fuel pool cooling system will not be operating during a reactor accident. In that case there would be no heat added to the reactor building from the spent fuel pool pump motors and piping, but without cooling the spent fuel pool water would heat up, boil, and release heat into the reactor building air. A lot of heat—considerably more heat than that present in the reactor building from all other sources, and far more than the cooling system could handle.

The water boiling off the spent fuel pool would condense and drain down into the basement of the building where it would submerge and disable emergency equipment—at least the emergency equipment that had not already been disabled by excessive temperatures in the building. In addition, as water boiled out of the pool and exposed the fuel, the radiation levels inside the reactor building during an accident would prevent workers from entering to open the manual valves that supply makeup water to the spent fuel pool.

Hence, a reactor accident would lead to a spent fuel pool accident. And the boiling spent fuel pool would create conditions inside the reactor building that would disable the emergency equipment needed to cool the reactor core.

As Don and I investigated further, more problems surfaced. Susquehanna’s owner initially justified the situation by saying that the non-safety-related spent fuel pool cooling system would remove the heat, even though it was not credited as doing so in the safety studies. Indeed, we found that emergency procedures directed the operators to open two electrical breakers within an hour of an accident to shut down all non-emergency systems inside the reactor building.

We also found that the standby gas-treatment system—a ventilation system located inside the reactor building that processes air discharged to the atmosphere to reduce its radiation levels by a factor of 100—would shut down if the spent fuel pool water approached boiling because the warm vapor evaporating from the pool would trick sensors into thinking there was a fire, causing inlet dampers to close. And we found that if the spent fuel pool cooling system was not operating, the operators would have no indications of the level or temperature of the water in the spent fuel pool.

The NRC failed to take our report seriously. They didn’t even read it. We had attached all the relevant correspondence between us and the plant’s owner to the report. I made two-sided copies of many of the 35 attachments to save postage costs. But when I took the original report to a copy shop, they mistakenly made single-sided copies and left out every other page. The NRC dismissed our concerns at Susquehanna and every other similarly designed nuclear plant without even noticing that roughly half of the report was missing.

Don and I wrote letters summarizing the spent fuel pool problems to the governors and US senators in the states with BWRs like Susquehanna. We also sent letters to the three congressional committees that oversee the NRC. Congressmen Phil Sharp wrote several letters to the NRC about our concerns, as did several governors and US senators. The NRC granted our request for a public meeting for us to communicate our concerns to the agency. About 15 minutes into that meeting on October 1, 1993, the NRC project manager for Susquehanna was sound asleep and snoring in the first row.

The issues were resolved at Susquehanna by the owners’ commitment to always operate with the spent fuel pools connected to each other. In case of an accident involving the Unit 1 reactor core, the systems on Unit 2 could be used to cool both spent fuel pools without adversely affecting conditions inside the Unit 1 reactor building, and vice-versa. The owner also took steps to install additional instrumentation to enable operators to monitor spent fuel pool water levels and temperatures and resolve the standby gas treatment system design issues.

However, little to nothing has been done to address the spent fuel pool vulnerabilities at other BWRs in this country.

Following this incident, I authored Nuclear Waste Disposal Crisis, a book about spent fuel storage issues. It was released by PennWell Publishing in January 1996. Chapter 8 outlined spent fuel pool safety issues. Chapter 9 detailed our spent fuel pool concerns at Susquehanna. And Appendix A summarized actual spent fuel pool problems that occurred at U.S. nuclear power reactors.

The tragedy at Fukushima Dai-Ichi involved many of the same concerns Don and I raised at Susquehanna. It appears that irradiated fuel in at least two of the site’s seven spent fuel pools has been damaged due to overheating.

The media attention to our efforts to get the NRC to resolve the spent fuel safety issue made nuclear workers across the country aware of our concerns. I started getting calls from both colleagues and strangers asking if I’d champion their safety concerns. I distinctly recall one man telling me, “I don’t want to raise this safety concern and put my job on the line, but since your career is already toast, I thought you’d raise it for me.” I still had a job in the industry at the time, but I appreciated his point. Raising safety concerns in the nuclear industry invokes the gangplank more often than it involves the corporate ladder.

Fortunately for me, Bob Pollard retired from the Union of Concerned Scientists in January 1996. Jim Riccio and Paul Gunter, who I’d met in 1994 during the campaign to call attention to the spent fuel pool problems, suggested I apply for the job. I did, and was hired by UCS. I’ve been working to get the NRC to resolve specific safety issues since then.

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Lochbaum Senate Testimony on Spent Fuel: March 30

, director, Nuclear Safety Project

Dave Lochbaum, Director of UCS’s Nuclear Safety Project, is testifying again this morning, this time to the Senate Energy and Water Development Appropriations Subcommittee. His testimony on the threat posed by spent fuel storage in the US and what can be done about it is available here.

Today, tens of thousands of tons of irradiated fuel sits in spent fuel pools across America. At many sites, there is nearly ten times as much irradiated fuel in the spent fuel pools as in the reactor cores. The spent fuel pools are not cooled by an array of highly reliable emergency cooling systems capable of being powered from the grid, diesel generators, or batteries. Instead, the pools are cooled by one regular system sometimes backed up by an alternate makeup system.

The spent fuel pools are not housed within robust concrete containment structures designed to protect the public from the radioactivity released from damaged irradiated fuel. Instead, the pools are often housed in buildings with sheet metal siding like that in a Sears storage shed. I have nothing against the quality or utility of Sears’ storage sheds, but they are not suitable for nuclear waste storage.

The irrefutable bottom line is that we have utterly failed to properly manage the risk from irradiated fuel stored at our nation’s nuclear power plants. We can and must do better.

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Where Did the Water in the Spent Fuel Pools Go?

, co-director and senior scientist

Since early in the current crisis—a few days after the earthquake and tsunami  damaged the reactors at the Fukushima Dai-Ichi facility—people have been concerned about lack of water in the spent fuel pools at the reactors. The water is needed to cool the spent fuel rods, which continue to generate significant heat for years after being removed from the reactor core due to their radioactivity.

Each of the six reactor units at Fukushima has its own spent fuel pool. These pools are about 12 meters deep. The spent fuel occupies roughly the bottom 4 meters of the pool and the surface of the water is typically about 7 meters above the top of the rods. In normal operation, electric pumps continually circulate the water in the pool, pulling out heated water, which is then cooled and sent back into the pool.

Unlike the cooling systems for the reactor core, the pumps for these pools typically don’t have backup power, so once electricity from the power grid was cut off, the pumps stopped operating.

Once that happened, the heat from the fuel rods began raising the temperature of the water in the pools. Once the water in a pool reached the boiling point, it would begin to boil away and lower the level of the water.

As long as the water, even though boiling, continues to cover the fuel rods it protects them from damage. But once it drops far enough to expose a meter or so of the rods, the exposed sections can become hot enough to damage the cladding on the rods and release radioactive gases. If the fuel continues to heat up, the cladding can begin to burn, which produces hydrogen. If enough hydrogen collects above the pool, it can explode.

The earthquake took place on the afternoon of Friday, March 11. Four days later, on Tuesday, March 15, press reports of a fire in the Unit 4 spent fuel pool and hydrogen released from the spent fuel rods suggested that water had dropped well below the top of the fuel rods in the Unit 4 pool.

The next day, Wednesday, March 16, there was enough concern about lack of water in some of the spent fuel pools that workers began trying to drop water into them from helicopters and spray water from fire hoses.

These remote efforts to add water were necessary because of the high radiation levels around the pools, which is additional evidence for the loss of water from the spent fuel pools. The water, in addition to cooling the fuel rods, also shields against the radioactivity being emitted from the fuel. As the water levels in the fuel pools dropped, the radiation levels around them increased and workers could not get close to them.

Since Japan has reported the level of heat generated by the spent fuel in the pools, we can estimate how long it would take for the water in the pools to boil off and begin to expose the fuel, assuming there were no leaks.

These estimates show that the time required to boil off water is too long to account for very low water levels and exposed fuel in the pools by March 16, assuming the cooling systems in the pools were working correctly before the earthquake struck.

The calculation assumes that cooling of the pools stopped at the time of the earthquake and tsunami, and at that point the spent fuel pools were filled with water at 20 degrees C (with the amounts given in the table below). Column 5 in the table shows the time it would take for that amount of water to be heated to boiling temperature (see the end of the post for details).

                         Data on the pools is from here.

All the pools are reported to be about 12 meters deep. The pools at Units 2, 3, 4, and 5 all have dimensions of 12.2m x 9.9m x 11.8m deep. The fuel assemblies are about 4 meters long. They sit on racks in the pool, slightly off the pool’s floor.

If the pools are filled with water there would be about 7 meters of water above the top of the fuel assemblies, which corresponds to the amount of water given in column 6 of the table. Once the water in the pool has been heated to boiling, column 7 gives the additional time required for boiling to cause the water level to drop to the top of the fuel assemblies, just covering them.

Press reports state that the water in the pool at Unit 4 was thought to be approaching the boiling point late on March 14, three days after the earthquake. This roughly agrees with these estimates, but it is not clear what information those reports on based on. If correct, this information suggests the water level in this pool had not dropped significantly by this time, since otherwise the smaller amount of water would have heated more quickly.

Reports also said that workers began taking initial steps on March 15 to cool the water in the pools at Units 5 and 6, and that by March 17 temperatures were still rising, reaching between 60 and 70 degrees C. This is also roughly consistent with the numbers in the table, assuming these pools were full of water. By March 19, cooling at both pools 5 and 6 appeared to be working to bring the temperatures down.

But the times in the table indicate that boiling of the water in the pools would take much too long to account for the low water levels and exposed fuel reported at the spent fuel pools in Units 2, 3, and 4. So something else must have caused the low water levels.

One cause of low water levels may have been that water splashed from the pools during the earthquake. I haven’t seen reports suggesting significant water loss by splashing or that the water levels in the pools were low shortly after the earthquake. Also, as noted above, the reported rate of heating of the pools at Units 4, 5, and 6 suggests that the water level was not significantly reduced early on.

Moreover, except in the case of the Unit 4 pool, even if the water levels in the pools had dropped by several meters during the earthquake—corresponding to hundreds of tons of water being spilled—the heating and boiling times to expose the fuel would still be too long to account for very low water levels in the pools.

A second possible cause of the low water levels is that the pools at Units 2, 3, and 4 all developed significant leaks. Some reports said that a leak was suspected in the pool of Unit 4 and possibly in Unit 3. This analysis suggests that all three pools may be developed leaks.

Dave Lochbaum has suggested a common failure mode for leaks in the spent fuel pools. Large doors in the side of the pools are equipped with rubber tubes that are inflated to seal around the door. Even if these seals were not damaged, without power to run the pumps that keep the seals inflated, they can lose air over time and create leaks around the door. Such leaks may not show up immediately since it could take some time for the seals to lose air pressure.

The pumps for these seals currently do not have backup power so leaks of this kind can result from an extended loss of power from the grid. This is a vulnerability of this type of plant design that could happen elsewhere, including at a number of plants in the the US, and needs to be addressed.

Details of the calculation

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More on Spent Fuel Pools at Fukushima

, co-director and senior scientist

Masa Takubo this morning sent information he had collected from briefings and press reports in Japan, and the TEPCO website, about the spent fuel pools. Here is an updated table:

The pools at Units 2, 3, 4, and 5 all have a volume of 1,425 cubic meters, with dimensions of 12.2m x 9.9m x 11.8m deep.

The Unit 1 pool is somewhat smaller (1,020 cubic meters) and the Unit 6 pool slightly larger (1,497 cubic meters).

Each fuel assembly consists of roughly 60 fuel rods and has a mass of about 170 kg.

The fuel assemblies are about 4 meters long. They sit on racks in the pool, slightly off the pool’s floor. The water level is typically kept 7-8 meters above the top of the assemblies.

The sixth column of this table gives the heat generated by the fuel in the spent fuel pool. According to these numbers, the Unit 4 pool is the biggest concern for overheating.

We know that 548 fuel assemblies in the Unit 4 pool were removed from the reactor and placed in the pool three months ago when the reactor was down for maintenance. They joined 783 assemblies that were already in the pool. The value given for the amount of heat being generated in the pool is roughly equal to an estimate of the heat from the 548 fuel assemblies transferred from the core of the reactor 3 months ago, so the heat released by the remaining 783 assemblies in the pool must be relatively low.

Adding Water to the Pools

People are concerned about the level of water in the spent fuel pools because if the water level drops and the fuel is exposed to air for long enough, the temperature of the fuel rods can increase to the point they suffer damage and release potentially large amounts of radioactive gases. Cooling has been restored to the pools in Units 5 and 6, and efforts to restore cooling at Units 1 and 2 are underway.

The biggest concern is at Units 3 and 4, and in both cases explosions have damaged the reactor buildings that surround the spent fuel pools, so that gases released from the spent fuel pools would go directly into the atmosphere.

Japanese press over the past few days reported attempts to add water to the pools by helicopters and fire hoses. The first attempts were relatively small, either dropping or spraying 50-60 tons of water, with little idea of how much of that may have gotten into the pool. (A metric ton of water equals about 275 gallons.) More recently, hoses have been used at Unit 3 for many hours at a stretch.

The Japanese Nuclear and Industrial Safety Agency (NISA) updates on March 20 and 21 said that in the past day workers attempted to add 40 tons of seawater to the Unit 2 pool, 1137 tons of sea water to the Unit 3 pool, and 90 tons of fresh water to the Unit 4 pool. There has been no report of how much of this water has actually gotten into the pools.

To put that amount into perspective, the total amount of water in each of the Unit 2-5 pools is 1,300-1,400 tons. The amount of water needed to fill a pool up to the top of the assemblies—to keep them just covered—is about 500 tons.

So the amount of water workers have attempted to add to the Unit 3 pool yesterday and the day before would have more than filled the pool if a significant fraction of what was being shot at it actually entered the pool Since the heat load in this pool is low, it is not immediately clear why officials are focusing so strongly on this pool.

Because the fuel in the Unit 4 pool is hot, the water in that pool will evaporate water rapidly. If the water in the pool had been heated to boiling temperature, then heat from these assemblies would be enough to boil off about 3 tons of water every hour, or about 70 tons per day.

This amount of water would have to be replaced each day in Unit 4 to make up for that boiled away, and leaks in the pool—which have been reported—would increase the amount required. If the fuel is already partially uncovered, significantly more water would need to be added to cover it again.

The Common (Shared) Storage Pool

In addition to these individual pools, there is a larger common spent fuel pool that is used to store spent fuel from all 6 reactors once it has been out of the reactor for 19 months and has cooled down. It has a volume of 3,828 cubic meters (29m x 12m x11m deep) and currently has 6,375 spent fuel assemblies in it. It is located 50 meters west of Unit 4. Reports also say that this pool continues to have water supplies but its cooling system is not functional.

Because this pool has water, and because the fuel is much less radioactive, it has not been a concern in the current crisis.

Change of Decay Heat with Time

The plot below shows how the decay heat from spent fuel decreases with time. The hottest spent fuel in the Unit 4 pool is about 3 months, or 0.25 years, old.

Decay heat as a function of time from 0.01 years (about 4 days) to 100 years for spent-fuel burnups of 33, 43, 53 and 63 MWd/kgU. The lowest burnup was typical for the 1970s. Current burnups are around 50 MWd/kgU

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Possible Source of Leaks at Spent Fuel Pools at Fukushima

, director, Nuclear Safety Project

A current focus of concern in Japan now is the pools at the reactors where spent fuel is stored. Some of this spent fuel is still very radioactive since it was only removed from the reactors a few months ago, and it must be covered by water and cooled to keep from overheating. If the spent fuel rods get too hot, they can suffer damage and release significant amounts of radioactive gases into the atmosphere, and could eventually catch fire.

Since several of the reactor buildings that surround these pools have been damaged by explosions, the radioactivity released from the pools in those buildings would get directly into the atmosphere. Similar fuel damage within the reactor cores would be surrounded by the reactor’s primary containment so that a much smaller fraction would get out, unless there was a significant breech of the containment.

Water needs to be added to the spent fuel pools at Fukushima since heating by the spent fuel causes the water to evaporate and boil off.

In addition, reports from Japan say that the spent fuel pool at reactor Unit 4 is leaking, which further increases the need for additional water.

A possible source of the leak in the Unit 4 pool may be the seals around the doors (or “gates”) on one side of the spent fuel pool. These gates are shown in the diagram below. They are located between the pool and the area above the reactor vessel. They are concrete with metal liners, and are roughly 20’x 3’.

When fuel is moved between the pool and vessel, this whole region is filled with water, the gates are opened, and the fuel can be moved to or from the reactor core while remaining under water. The water not only keeps the fuel rods cool but acts as a radiation shield.

                Boiling Water Reactor (BWR) Spent Fuel Cooling System

When the gates are closed, they are made watertight by an inflatable seal, similar to a bicycle innertube, that runs around the sides and bottom of the gates. Electric air pumps are used to inflate these seals and keep them inflated as air leaks out of them over time.

These pumps are powered by electricity from the power grid, and not by backup diesel power or batteries. So once the power grid in Japan was knocked out, these seals could not be inflated if they lost air over time. If these seals lost air they could lead to significant water loss from the pool, even if there were no direct physical damage to the pool from the earthquake or tsunami. This may be what happened at pool 4, and could affect the other pools as well.

We saw an example of this in the US at the Hatch nuclear plant in Georgia in December 1986. This reactor is very similar to the reactors at Fukushima. In the Hatch case, the line supplying air to the inflatable seal was accidentally closed, the seal lost pressure and created a leak, and by the time the problem was identified several hours later some 141,000 gallons of water leaked from the pool—about half the water in the pool Fortunately, the source of the problem was discovered and fixed before the water level uncovered the fuel.

An NRC document on the leak gave this description of the event:

A valve in the single air supply line to the seals was mistakenly closed. Although water level dropped about 5 feet and low-level alarms in the spent fuel pool worked, the leak was not specifically identified for several hours because a leak detection device was valved out and none of the seals were instrumented to alarm on loss of air pressure.

The NRC document goes on to note that if the water level had gotten low enough to expose the fuel the high radiation level around the pool would have made it difficult for workers to fix the problem.

The closed air line in the Hatch case had the same result that lack of electric power the air pump inflating the seals in Japan could have.


The spent fuel pool appears on the right side of this diagram. The reactor vessel and its reactor core appear on the left side. The refueling platform is used to move fuel assemblies one at a time between the reactor core and the spent fuel pool through an opening in the spent fuel pool wall created by removal of a gate.


Overhead view of an irradiated spent fuel bundle being transferred from the reactor core (lower right) to the spent fuel pool (upper left) through what is called the “cattle chute” at the Browns Ferry Nuclear Plant in Alabama. The spent fuel pool gate has been removed to connect the spent fuel pool water with the water in the reactor well area above the open reactor pressure vessel.


Looking down at the fuel transfer canal at the Grand Gulf Nuclear Station in Mississippi. Grand Gulf is a BWR with a Mark III containment. It features a fuel transfer canal that does not exist in the BWR Mark I and Mark II containment designs. However, all three Containment designs feature gates that are removable from the spent fuel pool walls to allow underwater transfer of spent fuel assemblies. In this picture, the fuel transfer canal is in the center, the spent fuel pool is to the left, and the cask loading area is to the upper right and is used when fuel is transferred from spent fuel pool to casks.


Looking down into the spent fuel pool at the Grand Gulf Nuclear Station in Mississippi before the plant commenced operation. The spent fuel pool is drained of water. The fuel storage racks can be seen in the lower region of the spent fuel pool. The beams used to hold the racks in place against forces from an earthquake can been seen between the racks and the pool walls. In the lower right portion of the picture, the opening in the fuel pool wall created by the removal of the spent fuel gate can be seen.


A cross-section fuel of a typical BWR spent fuel pool. The fuel pool gate appears on the left side of the pool. The bottom of the opening created when the gate is removed (or its seals leaking) is about 5 feet above the top of spent fuel in the storage racks at the bottom of the spent fuel pool.

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Spent Fuel Pools at Fukushima

, co-director and senior scientist

Because of their high radioactivity, fuel rods continue to produce very significant heat even after they are no longer useful for generating electricity and are removed from the reactor core. Such “spent fuel” rods need to be continually cooled for many years to prevent them from heating to a level where they would suffer damage.

To cool the rods after they are removed from the reactor core, they are placed on racks in a spent fuel pool that circulates cooled water around them. This water is circulated by pumps that are run using electricity from the power grid. Typically these pumps do not have backup power from deisel generators or batteries, so if power from the grid is interrupted, as it is in the case of the Japanese earthquake, they will stop operating.

Once the cooling pumps stop, the water in the spent fuel pools will begin to heat up and will eventually start to boil off. The pools are typically 45 feet deep with the fuel rods stored in the lower 15 feet of the pool, so 30 feet of water would have to boil off before exposing the rods. That could take several days, so this issue may only be appearing now.

The pool at Fukushima Dai-Ichi Unit 4 is a particular problem since the fuel rods in it were only removed from the reactor core during a refueling of the reactor in December 2010. Therefore, they still have a very high level of radiation and are generating more heat than the spent fuel at the other reactors at the Dai-Ichi site.

The spent fuel pools for the Dai-Ichi reactors are located on an upper floor in the reactor building (see red circle in figure). The pools are outside the primary containment of the reactor, but inside the reactor building, which is the secondary containment. Typically any radioactivity released from the pool will be contained by the reactor building, which is maintained at less than atmospheric pressure so air flows into the building rather than out. The air in the building is filtered to remove the radioactivity before it is released outside.

As the water in the pool heats up and evaporates, the vapor will carry some radiation with it. This includes tritium and radioactive particles in the water.

If the water level in the pool drops low enough to expose the spent fuel, the fuel rods can suffer the same kinds of damage as fuel in the reactor core that is expose. If only a small length of the rods is exposed, they will get hot enough to create steam, but the steam flowing along the exposed surface of the rod will cool it enough that the rod’s cladding will not reach the high temperature required to react with the steam and create hydrogen.

Once the water has dropped low enough to expose several feet of the length of the fuel rods, they can become hot enough that the zirconium cladding of the rods will react with the steam and release hydrogen.

Tokyo Electric Power Company (TEPCO) has said there was a hydrogen explosion that damaged the Unit 4 reactor building on Tuesday morning in Japan (Monday afternoon U.S. time), reportedly blowing a 26-foot wide hole in the side of the building. If this explosion was due to hydrogen, that hydrogen very likely came from the spent fuel since there is no other clear source (this reactor was not operating when the earthquake hit). And if the spent fuel produced the hydrogen, that indicates that the water level in the spent fuel pool must have been low enough to have exposed a significant fraction of the fuel rods.

Shortly after that—at 9:38 am Tuesday (8:38 pm EDT Monday)—TEPCO discovered a fire on the fourth floor of the building in the spent fuel pool, which reportedly burned for three hours. That fire may have been the oxidation of the zirconium cladding, as it was continuing to produce hydrogen.

The other effect of heat damage to the fuel rods is that radioactive gases such as iodine-131 and cesium-137, which are produced in the fuel during the operation of the reactor, can be released. The hole in the secondary containment at Unit 4 means that any emissions from the spent fuel will be vented directed to the outside.

If water cannot be added to the pool, or if the pool has been damaged and is leaking, the fuel may remain uncovered. The exposed fuel can get hot enough to melt, depending on how long it has been out of the reactor. If the fuel melts, it would release significant additional radioactivity into the air.

This same scenario could occur at Units 5 and 6 if the water in the spent fuel pools is not replenished, although the fuel there has apparently been in the pools longer and is not as radioactive as at Unit 4. For rods that have been in the pool for long enough, their decay heat will have dropped sufficiently that they will not undergo the same rapid oxidation as newer fuel rods will, and would not produce as much hydrogen.

Thus, depending on the age of the spent fuel in Units 5 and 6, there may be less hydrogen produced if the water level in the spent fuel pool drops. There may still be enough heat to damage the fuel and release radioactive gases, but if the secondary containment is not damaged by a hydrogen explosion, that gas may not be released to the atmosphere.

If mechanisms to fill the pool at Unit 4 are broken, or if there is a need to repair the pool, it will be difficult to get workers close enough to do this. If spent fuel has been in the pool for a relatively short time, even if the water level is at the top of the fuel rods, the radiation dose to someone at the railing of the pool would give them a lethal dose in well under a minute. This would explain why there have been reports of requests to use helicopters to deliver water to the pools. However, it appears that this is not a practical way of delivering water.

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Dry-Cask Storage vs. Spent-Fuel Pools

, co-director and senior scientist

The nuclear crisis in Japan has started discussions about the safety and security advantages of storing spent fuel in dry casks (see photo) rather than spent fuel pools. UCS has long recommended that spent fuel be transferred from the pool to dry cask storage once the fuel has cooled enough, after about five years. This is a major issue in the U.S. because U.S. pools are becoming increasingly packed with spent fuel.

Here are some links for more information on this issue:

(1) Chapter 5, “Ensuring the Safe Disposal of Nuclear Waste,” from UCS’s report Nuclear Power in a Warming World (2007), which covers interim and long-term waste storage, and discusses why reprocessing is neither an effective nor desirable waste management strategy. (Note that the discussion of Yucca Mountain is out of date.)

(2) A 2003 paper Ed Lyman co-authored, followed by links to comments on the paper by the NRC, and the authors’ response:

Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States,” by Alvarez, R., Beyea, J., Janberg, K., Kang, J., Lyman, E., Macfarlane, A., Thompson, G., and von Hippel, F.N., Science and Global Security, Vol 11, 1:1-51 (2003)

Here’s the abstract of the paper:

Because of the unavailability of off-site storage for spent power-reactor fuel, the NRC has allowed high-density storage of spent fuel in pools originally designed to hold much smaller inventories. As a result, virtually all U.S. spent-fuel pools have been re-racked to hold spent-fuel assemblies at densities that approach those in reactor cores. In order to prevent the spent fuel from going critical, the fuel assemblies are partitioned off from each other in metal boxes whose walls contain neutron-absorbing boron.

It has been known for more than two decades that, in case of a loss of water in the pool, convective air cooling would be relatively ineffective in such a “dense-packed” pool. Spent fuel recently discharged from a reactor could heat up relatively rapidly to temperatures at which the zircaloy fuel cladding could catch fire and the fuel’s volatile fission products, including 30-year half-life 137Cs, would be released. The fire could well spread to older spent fuel. The long-term land-contamination consequences of such an event could be significantly worse than those from Chernobyl.

No such event has occurred thus far. However, the consequences would affect such a large area that alternatives to dense-pack storage must be examined—especially in the context of concerns that terrorists might find nuclear facilities attractive targets. To reduce both the consequences and probability of a spent-fuel-pool fire, it is proposed that all spent fuel be transferred from wet to dry storage within five years of discharge. The cost of on-site dry-cask storage for an additional 35,000 tons of older spent fuel is estimated at $3.5–7 billion dollars or 0.03–0.06 cents per kilowatt-hour generated from that fuel. Later cost savings could offset some of this cost when the fuel is shipped off site.

The transfer to dry storage could be accomplished within a decade. The removal of the older fuel would reduce the average inventory of 137Cs in the pools by about a factor of four, bringing it down to about twice that in a reactor core. It would also make possible a return to open-rack storage for the remaining more recently discharged fuel. If accompanied by the installation of large emergency doors or blowers to provide large-scale airflow through the buildings housing the pools, natural convection air cooling of this spent fuel should be possible if airflow has not been blocked by collapse of the building or other cause. Other possible risk-reduction measures are also discussed.

Review of ‘Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States’,” by the Nuclear Regulatory Commission (NRC), Science and Global Security, Vol. 11, 2-3:203-212 (2003)

Response by the Authors to the NRC Review of ‘Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States’,” by Alvarez, R., Beyea, J., Janberg, K., Kang, J., Lyman, E., Macfarlane, A., Thompson, G., and von Hippel, F.N., Science and Global Security, Vol. 11, 2-3:213-223 (2003)

(3) Ed also co-authored a paper addressing issues related to the potential effects of a radiation release from spent fuel:

Damages from a Major Release of 137Cs into the Atmosphere of the United States,” by Beyea, J., Lyman, E., von Hippel, F. N., Science and Global Security, Vol. 12:125–136 (2004)

Here’s the abstract:

We report estimates of costs of evacuation, decontamination, property loss, and cancer deaths due to releases by a spent fuel fire of 3.5 and 35 MCi of 137Cs into the atmosphere at five U.S. nuclear-power plant sites. The MACCS2 atmospheric-dispersion model is used with median dispersion conditions and azimuthally-averaged radial population densities. Decontamination cost estimates are based primarily on the results of a Sandia study. Our five-site average consequences are $100 billion and 2000 cancer deaths for the 3.5 MCi release, and $400 billion in damages and 6000 cancer deaths for the 35 MCi release. The implications for the cost-benefit analyses in “Reducing the hazards” are discussed.

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Perils of New Nuclear Fuel: Part 4 – Grand Gulf’s Painted Water

, director, Nuclear Safety Project

Fission Stories #189

The previous two Fission Stories commentaries (#187 and #188) described problems at the Grand Gulf Nuclear Station near Port Gibson, Mississippi involving new fuel bundles stored in the new fuel vault and later in transferring them into the spent fuel pool. This post caps the trilogy by describing a problem encountered with the new fuel bundles in the spent fuel pool. Read more >

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Perils of New Nuclear Fuel: Part 3 – Grand Gulf’s Safety Suspension

, director, Nuclear Safety Project

Fission Stories #188

The previous Fission Stories commentary described a problem at the Grand Gulf Nuclear Station near Port Gibson, Mississippi that prevented workers from transferring fuel bundles from the new fuel vault to the spent fuel pool. This post describes a problem encountered shortly after that problem was eliminated and the transfers began. Read more >

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Perils of New Nuclear Fuel: Part 2 – Grand Gulf’s Mighty Pen

, director, Nuclear Safety Project

Fission Stories #187

The previous Fission Stories commentary described a problem with new nuclear fuel encountered at Browns Ferry in the early 1980s. This commentary sustains that theme by describing another problem with new nuclear fuel, this time at the Grand Gulf Nuclear Station near Port Gibson, Mississippi. Read more >

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Perils of New Nuclear Fuel: Part 1 – Browns Ferry’s Midnight Ride

, director, Nuclear Safety Project

Fission Stories #186

Much has been written about the perils of nuclear fuel when it resides in the core of a nuclear power reactor and later when it is spent fuel stored here, there and everywhere. But new nuclear fuel has proven problematic at times, too. This is the first of a quartet of stories about new fuel problems. Read more >

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Fuel Amounts at Fukushima

, co-director and senior scientist

NOTE: On March 21, an updated set of numbers was posted here.

Based on Japanese press stories, we have compiled a table of the amount of fuel in the cores of the reactors and the spent-fuel pools in the 6 reactors at the Fukushima Dai-Ichi nuclear facility.

While BWR fuel comes in various sizes, the last column assumes 170 kg per assembly. Each fuel assembly consists of roughly 60 fuel rods.

Thanks to readers for confirming that the fuel rods in Unit 4 had been moved from the core to the spent fuel pool during maintenance.

Units 5 and 6 were reported to be producing power in January, but are now shut down for mainenance. Reports say that 130 assemblies from each core were recently transfered to the pools, but those were included in the previous numbers in the table for the spent fuel for those reactors. If anyone has additional information about these reactors, please let us know.

A New York Times article states that 32 assemblies in the spent fuel pool of Unit 3 are MOX. The MOX fuel rods were stored in the pool but TEPCO announced they were being loaded into the core last fall, so we think those are currently in the core.

The same article says that a total of 11,125 spent fuel assemblies are stored at the Fukushima Dai-Ichi facility. However, not all of those are stored in the pools in the reactor buildings. Several hundred are currently in dry cask storage, and more than half of the total are stored in a common storage pool.

Thanks to Masa Takubo for his help in compiling these numbers.


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The “Versatile Fast Neutron Source”: A Misguided Nuclear Reactor Project

, senior scientist

The Union of Concerned Scientists (UCS) supports a moderate level of Department of Energy (DOE) research funding to make nuclear power safer and more secure—for example the agency’s program to develop accident tolerant fuels for nuclear reactors. Conversely, UCS does not support programs that not only would cost a lot of money, but also could make nuclear power more dangerous and less secure. That’s why the organization is troubled by a bill that was passed by the House of Representatives on February 13. Read more >

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Florida’s Nuclear Plants and Hurricane Irma

, senior scientist

Will Florida’s two nuclear plants, Turkey Point and St. Lucie, be able to withstand Hurricane Irma?

Florida governor Rick Scott, the utility Florida Power & Light (FP&L), and the US Nuclear Regulatory Commission (NRC) have all provided assurances that they will. But we are about to witness a giant experiment in the effectiveness of the NRC’s strategy for protecting nuclear plants from natural disasters. Read more >

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The Pyroprocessing Files

, senior scientist

The article by Ralph Vartabedian in the Los Angeles Times highlights the failure of the Department of Energy’s decades-long effort to chemically process a stockpile of spent nuclear fuel at Idaho National Laboratory, ostensibly to convert the waste to forms that would be safer for disposal in a geologic repository. A secondary goal was to demonstrate the viability of a new type of processing spent fuel—so-called pyroprocessing. Instead, it has demonstrated the numerous shortcomings of this technology. Read more >

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