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March 20, 2011 • 0 Comments
Dry-Cask Storage vs. Spent-Fuel Pools| by Lisbeth Gronlund | nuclear power | nuclear power safety | Japan nuclear |
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 (Note that the discussion of Yucca Mountain is out of date.)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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March 19, 2011 • 2 notes • 1 Comment
Good Sources on Fukushima| by David Wright | nuclear power | nuclear power safety | Japan nuclear |
These sites are providing very useful, timely information on the situation in Japan:
-The Bulletin of the Atomic Scientists is posting periodic updates from Tatsujiro Suzuki, a Japanese expert on nuclear power, who is following events in Tokyo.
-Jeffrey Lewis at armscontrolwonk is posting daily updates from the Washington DC Office of the Federation of Electric Power Companies of Japan (FEPC) .
-The Japan Atomic Industrial Forum (JAIF) is posting updated status charts on the Dai-Ichi and Daini nuclear plants.
Translating times between Japan and the US:
JST = Japan Standard Time = GMT + 9
EDT = Eastern Daylight Time = GMT - 4
so H:00 JST = (H:00 – 13:00) EDT
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March 19, 2011 • 5 notes • 3 Comments
Possible Source of Leaks at Spent Fuel Pools at Fukushima| by Dave Lochbaum | nuclear power | nuclear power safety | Japan nuclear |
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.
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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.
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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.
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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.
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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.
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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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March 18, 2011 • 2 notes • 1 Comment
Transcripts of Press Briefings on Fukushima by UCS Technical Experts| by David Wright | nuclear power | nuclear power safety | Japan nuclear |
We have been providing daily phone briefings for reporters by our technical experts on the evolving situation surrounding the crippled reactors in Japan, and will continue these briefings through the weekend.
To make this information available to the public, we have been transcribing and posting these briefings, including the Q&A sessions. The transcripts, and voice recordings of the opening remarks remarks by the briefers, are available here, and we will add new transcripts daily to that same site.
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March 18, 2011 • 1 note • 3 Comments
More on KI Pills| by Lisbeth Gronlund | nuclear power | nuclear power safety | Japan nuclear |
We’ve gotten some questions asking for clarification about our statement on potassium-iodide (KI) pills. In particular, why are KI pills effective in the case of inhalation of radioactive iodine, but not considered an effective countermeasure to ingesting it via, for example, milk?
According to the 2004 National Academy of Sciences study on Distribution and Administration of Potassium Iodide in the Event of a Nuclear Incident:
Exposure to radioactive iodine is possible through the ingestion pathway, so it is important that plans address this situation. Monitoring of the environment and food products controls this route of exposure. Removing contaminated products from the market and isolating contaminated products until the radioactive iodine decays to safe levels are the most effective way to eliminate radiation exposure and damage to the thyroid. That also eliminates the need for the use of KI by the general public as a protective action.
Potassium iodide can only reduce the risk from radioactive iodine that has entered the body, not eliminate it. People in the radioactive plume do not have the option of not breathing, so taking KI is an effective countermeasure against inhalation. However, people have the option of not drinking contaminated milk or eating other contaminated food products. In comparison, taking KI would be less effective.
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March 18, 2011 • 20 notes • 17 Comments
Possible Cause of Reactor Building Explosions| by Dave Lochbaum | nuclear power | nuclear power safety | Japan nuclear |
Dramatic videos show the explosions that severely damaged the reactor buildings at first Unit 1 and then Unit 3 at the stricken Fukushima Dai-Ichi nuclear plant in Japan. The explosions are attibuted to the ignition of hydrogen gas that collected within the reactor buildings. This was early in the crisis, and before the spent fuel pools are thought to have lost water and started producing hydrogen.
The hydrogen was likely produced by damaged fuel rods in the reactor core. To reduce pressure in the reactor vessel, some of that hydrogen was released from the vessel into the primary containment structure of the reactor.
A key, unsolved riddle is how a significant amount of hydrogen escaped from the primary containment into the reactor building, and how this low-probability event would have happened in mulitple reactors.
How Hydrogen Got into Primary Containment
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