Nuclear Spent Fuel Damage: Pool Accident

, director, Nuclear Safety Project | April 19, 2016, 6:00 am EST
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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.

U.S. reactors shut down for refueling every 18 to 24 months. During a refueling outage, workers discharge one-quarter to one-third of the fuel assemblies from the reactor core into the spent fuel pool and replace them with new fuel assemblies. Spent fuel assemblies contain large amounts of unstable radionuclides, byproducts from the atomic fissions that powered the reactor. The radioactive decay of these radionuclides generates heat. Spent fuel assemblies also contain large amounts of uranium and plutonium atoms, fissionable material that could reignite a nuclear chain reaction. Consequently, fuel assemblies stored in spent fuel pools are vulnerable to damage by overheating and reactivity excursions.

Each spent fuel assembly is 12 to 14 feet long and about 10 inches square. Spent fuel assemblies are placed into metal storage racks occupying the lower third of the 45-feet deep spent fuel pools. The racks sit on feet providing a few inches clearance between the bottoms of the racks and the reinforced concrete floor of the spent fuel pool (Fig. 1).

Fig. 1 (Source: UCS)

Fig. 1 (Source: UCS)

Like most in-ground swimming pools, spent fuel pools have openings—called scuppers—at the normal water level. Water enters the scuppers and flows into the spent fuel pool cooling system. This system cools the water before returning it to the spent fuel pool. To lessen the risk of accidentally draining water, the spent fuel pool’s walls and floor have no penetrations below the normal water level.

The decay heat emitted from spent fuel assemblies warms the water filling the racks. The warmed water rises due to convection. The rising warm water pulls in cooler water from beneath the racks to replace it (Fig. 2). This natural circulation flow cools the irradiated (spent) fuel assemblies.

Fig. 2 (Source: UCS)

Fig. 2 (Source: UCS)

If the water were to rapidly drain from a spent fuel pool, the spent fuel pool cooling system obviously stops working. But the “chimney effect” takes over. The decay heat emitted from spent fuel assemblies warms the air filling the racks. The warmed air rises due to convection. The rising air pulls in cooler air from beneath the racks to replace it (Fig. 3). Studies performed by the NRC indicate that, except for irradiated fuel removed from an operating reactor core within the past two months, air cooling may be sufficient to protect the spent fuel from overheating.

Fig. 3 (Source: UCS)

Fig. 3 (Source: UCS)

But what happens if a spent fuel pool only partially drains? The partial drainage disables the spent fuel pool cooling system and also blocks the “chimney effect” air flow (Fig. 4). The irradiated fuel assemblies may continue to be adequately cooled, even when the spent fuel pool water level drops below the top of the racks. The decay heat emitted from spent fuel assemblies heats water filling the racks to the boiling point. The steam vapor flowing past the exposed upper portions of the fuel assemblies can be enough to protect them from overheating.

Fig. 4 (Source: UCS)

Fig. 4 (Source: UCS)

At some point, the water level in a partially drained spent fuel pool can drop low enough to prevent adequate cooling (Fig. 5). The decay heat emitted from the uncovered portion of the spent fuel assemblies is not carried away by the steam flowing past. Instead of getting removed, the heat increases the temperature of the metal rods containing the fuel pellets. Insufficient cooling can damage the spent fuel assemblies similar to how overheating damages fuel assemblies within the reactor core.

Fig. 5 (Source: UCS)

Fig. 5 (Source: UCS)

Safety by Intent

If inadequately cooled, irradiated fuel assemblies in spent fuel pools can be damaged similar to how irradiated fuel assemblies in reactor cores can be damaged by overheating.

There are differences between irradiated fuel in spent fuel pools and reactor cores. Some of the differences make spent fuel pool accidents less likely to occur than reactor core accidents. But some of the differences make the consequences from spent fuel pool accidents greater than from reactor core accidents.

When insufficient cooling water flows through a reactor core, workers have minutes to hours at most to restore adequate cooling before fuel damage occurs. When insufficient cooling flows through irradiated fuel assemblies in a spent fuel pool, workers have hours to days to restore adequate cooling. The longer time frame does not guarantee they can successfully intervene, but it certainly gives them a much higher chance of success.

On the other hand, the average spent fuel pool has about six times as many spent fuel assemblies as exists in the reactor core. Thus, a spent fuel pool accident could damage more irradiated fuel assemblies than can be damaged in a reactor core meltdown. And the spent fuel pool is located outside the thick containment walls, making it more likely that radioactivity released from damaged fuel escapes to the environment.

Which is preferable? A reactor core meltdown or a spent fuel pool accident?

It’s like debating whether it’s better to be poked in the left eye or the right eye.

—–

UCS’s Disaster by Design/Safety by Intent series of blog posts is intended to help readers understand how a seemingly unrelated assortment of minor problems can coalesce to cause disaster and how effective defense-in-depth can lessen both the number of pre-existing problems and the chances they team up.

 

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  • n_coast

    Reacting to the sentence, “Spent fuel assemblies also contain large amounts of uranium and plutonium atoms, fissionable material that could reignite a nuclear chain reaction.”

    Spent fuel pools are designed to prevent any possibility of sustained fission. Poison is provided by boron in the water and the steel in the rack structure to capture stray neutrons. Some designs include rods containing boron.

    Boiling off the water would not start a chain reaction. Water is required around low enriched uranium to slow the neutrons to to raise the probability of capture by the U235.

    In March, 2011 we could read all kinds of imaginative dire warnings about what had happened or what could happen to the Unit 4 spent fuel pool. The truth was much less exciting. About a year ago the fuel elements were removed from the pool, placed in storage casks, and moved to a storage area.

    • jharragi

      An explosive device or aircraft encountering a fuel pool could cause a pool to drain and would likely change the geometry of the fuel assemblies. These situations could indeed bring about a nuclear chain reaction and with it devastating consequences. Large earthquakes have similar risks. If the quake that unleashed the Fukushima event had centered near the plant rather than 100 miles away, there would have likely been 6 fuel pools collapsed complete with the dire consequences you brush away.

      >The truth was much less exciting.
      That depends on what you call exciting. The truth is there were large numbers of people exposed to radioactive materials due to the Fukushima event. The nature of this type of exposure is to cause horrific maladies years or decades down the road. Tepco, the Japanese government and nuclear industries around the world have used their financial might to downplay these consequences. So I would say the conspiracy behind the media suppression of the consequences of the event and the engineering shortcomings that contributed to the disaster may be exciting. As for the people who will actually experience those consequences, I suspect they would rather do without that excitement – and in fact never should have experienced it.

      • n_coast

        I don’t know if anybody has modeled the effect of a large airplane or a missile aimed at a BWR spent fuel pool, but it could not trigger a chain reaction. Operation of a reactor with low enriched fuel requires water, graphite, or another moderator to slow neutrons to increase the capture cross section, and poisons like boron have to be carefully limited. Try disconnecting a rack from the building, hauling it away, getting rid of any boron, and submerging it in fresh water.

        No pools collapsed at the Fukushima plants..

        The effect of radiation exposure depend on the dosage. You are exposed to radioactive material when you eat a banana or any of a list of foods that contain trace amounts of radioactive materials. The most horrible effects of radiation exposure are associated with a diagnosis of acute radiation sickness. This occurs at radiation exposures upward of 2 Sv in a short time interval. Death is likely at exposure above 4 or 5 Sv and occurs within a month.

        I don’t think the nuclear industry is rolling in money or otherwise has the power to control the news media. If they did, couldn’t they figure out how to silence their detractors on the internet?

        • jharragi

          > I don’t know if anybody has modeled the effect of a large airplane or a missile aimed at a BWR spent fuel pool, but it could not trigger a chain reaction. Operation of a reactor with low enriched fuel requires water, graphite, or another moderator to slow neutrons to increase the capture cross section, and poisons like boron have to be carefully limited. Try disconnecting a rack from the building, hauling it away, getting rid of any boron, and submerging it in fresh
          water.

          This is an interesting statement. I’ll begin by addressing your first point. Since the 911 hijackers modeled a PWR strike, I’m sure the NRC will model this as well. I might point out that we are not discussing a reactor nor a specific type. All of the things you suggest I try are not likely to lead to criticality. However if I were to change the geometry of the spent fuel pellets in relation to each other, it could – not that criticality is required to cause a catastrophic situation. If by ‘not triggering a chain reaction’ you mean ‘not create an explosion like a nuclear bomb’, that is correct – still there are many situations in which a damaged fuel pool could disperse huge amounts of hazardous materials over wide areas.

          >No pools collapsed at the Fukushima plants..

          Obviously. As I pointed out the earthquake that unleashed the Fukushima incident was centered 100 miles away. If it was close, you would have been knocked off your soapbox at the same time that the fuel pools collapsed –
          seeing as how the pools were designed to withstand only a small fraction of the seismic intensity that occurred at that quake’s epicenter.

          >The effect of radiation exposure depend on the dosage. You are exposed to
          radioactive material when you eat a banana or any of a list of foods that contain trace amounts of radioactive materials. The most horrible effects of radiation exposure are associated with a diagnosis of acute radiation sickness. This occurs at radiation exposures upward of 2 Sv in a short time interval. Death is likely at exposure above 4 or 5 Sv and occurs within a month.

          Some might suggesting that deaths caused by radiation that strikes someone in 10 years are more moral than those that strike in 10 days. Perhaps power plant
          owners make these distinctions, I don’t. I recommend that you read John Gofman to get a better understanding: http://www.ratical.org/radiation/CNR/PP/

          >I don’t think the nuclear industry is rolling in money or otherwise has the power to control the news media. If they did, couldn’t they figure out
          how to silence their detractors on the internet?

          Hiring people to post statements crafted to minimize the hazards of nuclear power on sites like this would be a very cost effective investment…

          • n_coast

            I specified the BWR because their spent fuel pools have less structural protection than is provided by PWR containment structures.

            A chain reaction in low enriched uranium oxide fuel requires neutrons in the electron-volt energy range where the capture cross section is high, but fission kicks out neutrons in the MEV energy range with low probability of capture. No moderation = no chain reaction regardless of how the fuel elements are bent, folded, or mutilated.

            The Chernobyl explosions and fire were very effective in spreading highly radioactive material over a wide area. Because of the rapid decline in the energy emitted from nuclear fuel after reactor shutdown, I don’t think you could do any worse with the contents of a spent fuel pool, and you would have to somehow remove the spent fuel from a deep reinforced concrete pit. The effects of the Chernobyl disaster to the public would have been much less if the USSR had evacuated nearby residents sooner and taken measures to reduce the effect of the I-131.

            Symptoms of acute radiation sickness include severe nausea, vomiting, cramps, and diarrhea. Severe damage is done to bone marrow, the cardiovascular system, and the digestive system. Once exposed, the sickness is inevitable, whereas cancer is not. Look at the statistics for bombing survivors in Japan.

            Do you think I could get paid for my efforts?

          • jharragi

            “A chain reaction in low enriched uranium oxide fuel requires neutrons in the electron-volt energy range where the capture cross section is high, but fission kicks out neutrons in the MEV energy range with low probability of capture. No moderation = no chain reaction regardless of how the fuel elements are bent, folded, or mutilated.”

            Another irrelevant factoid followed by an attempt to baffle us with BS resulting in a rather misleading statement.

            There is a curve of the likelihood for neutrons at a particular energy to induce fission in a sample of a particular fissionable material. You have to sum the products of neutron densities of a given energy and the probability of fission at that energy level.

            Brand new fuel pellets do not emit many neutrons so yes, you can pile many up without incident. However we are not discussing new fuel, but spent fuel – which will consist of an array of isotopes each of which will be emitting characteristic decay products. Each of these isotopes will also have their own curves describing what will induce nuclear reactions. Many of them will be more reactive to this flux at various energies than is uranium – additionally, some of these materials emit radiation (including neutrons) that can induce further reactions. Spent fuel is exceedingly more complex then a simple level of enrichment in new fuel.

            This list should be of interest:
            https://en.wikipedia.org/wiki/Criticality_accident#Incidents
            Each of these incidents involve tiny inventories of nuclear material compared to the vast amounts stored in spent fuel pools, but they should give a notion of the possibilities.

            “Do you think I could get paid for my efforts?”

            Hopefully not so much on this particular thread…

            *******************

            All this thinking about waste – and watching an interesting video regarding tritium on http://www.fairewinds.org inspired a question that Dave might explore as a future topic:

            Tritium is constantly being created in a running reactor. Tritium is also created in fuel pools. My understanding is that a large fraction (all – except for what has already decayed) escapes or is dumped or otherwise discharged into the environment. My question is this, what is the difference in tritium created in dry cask storage vs. immersion in a fuel pool? If there were an aggressive program to move this fuel after the minimum period of immersion would this make a significant reduction (or increase) in the amount of this dangerous isotope that is created?

          • n_coast

            “Another irrelevant factoid followed by an attempt to baffle us with BS resulting in a rather misleading statement. . . . Hopefully not so much on this particular thread…”

            You are hurting my feelings, and you are going ad hominem.

            Delayed neutrons are released within a few seconds after fission of U-235. Significant numbers of them could not exist in spent fuel. Even if they did, moderation would be required to initiate and sustain a fission reaction.

          • n_coast

            A readable report on tritium was posted on the Scientific American site about 2 years ago; http://www.scientificamerican.com/article/is-radioactive-hydrogen-in-drinking-water-a-cancer-threat/. Readers’ comments offer further views.