Wednesday, April 11, 2012

Dangers of Spent and Melted Fuel at Fukushima: No Safe Method of Transport or Disposal

The spent and melted reactor fuel at Fukushima poses several big problems because spent fuel becomes much more radioactive after it has gone hot then when new, when it is only slightly radioactive and you can stand next to it. It has to be cooled and shielded by water. The most common reports are that standing 3 feet away from unshielded fuel for only a few minutes will give enough radiation to kill within days. Arnie Gundersen believes that even hundreds of feet away the radiation is enough to deliver dangerous if not instantly lethal amounts. 

The US had an unshielded device called Godiva II that was constructed inside a concrete building with 20-inch-thick (510 mm) walls and 8-inch-thick (200 mm) roof in a canyon a quarter mile away from the control room. The first version was used by people who stood next to it who noticed that light bulbs were lighting up with no power before they stepped away!
  • The melted down fuel has almost certainly fallen through the reactor pressure vessel (nuclear pressure cooker pot) to the floor of the containment room where it is barely covered by water as measured in Unit 2. That is why the radiation is so high at 72 SV/hr in Unit 2 containment. It should be in a water-filled RPV, shielded by water. Units 1 and 3 are too radioactive for people to even get outside the containment to do these measurements. Unit 4 core is empty
  • In an intact plant, hot or warm fuel is always kept under water for transfer across the narrow "cattle chute" into the spent fuel pool. It has to be kept under water for 10 to 20 years until it can be lifted out and placed into a dry cask for storage or transportation to long term storage. 
  • Since the Unit 2 rpv does not hold water, and the containment is not designed to be flooded (the water evidently drains down to the torus through the "downcomer" pipes), any damaged fuel would have to be lifted through the air where it would be dangerous to any people within hundreds of feet, and lethal within a few feet. Moreover, the refueling platform can only lift intact fuel assemblies by their handles. There is no existing crane device which can remove melted fuel, safely or not. There is also no pool or container which can safely hold damaged fuel, even if fuel pool #2 and refueling platform #2 and overhead crane works.
  • In unit 3, the rpv is shot, the overhead crane has collapsed, the refueling crane collapsed into the pool, and the pool is a wreck and filled with salt water. To remove the fuel, you would have to remove the roof wreckage, you would have to remove the massive collaspsed crane, lift off the reactor plug, remove the reactor cap which may be too damaged to remove by loosening the bolts, then you'd be looking down unshielded radiation, and you'd have to somehow dislodge cooled or melted fuel all over the containment floor, lift it unshielded through the air and put it somewhere. Removing fuel from the spent fuel pool would be easier, but still difficult because it would still be unshielded in the air, and most fuel assemblies probably cannot be lifted by their handles, if indeed they are not completely shattered into fragments with fuel pellets spilling all over the bottom of the pool without controlled spacing which could cause some criticalities.
  • In unit 4, the crane and refueling platform look intact, but tracks and power connections are probably too damaged to fix. It may be possible to remove fuel by the hooks, but fuel is still too hot to safely transport through the air into another storage container or pool at ground level, at least not with people standing around. 
  • Unit1 roof is still collapsed onto its pool, with high radiation all around which would make clearing wreckage difficult if not impossible. Crane and refueling platform are still there but probably unusable due to wreckage fallen on it and electrics probably shot.
  • In general, no safe method to move or remove fuel, whether intact or melted exists for any of the reactors, and after a year, zero progress has been made except for removal of some of the damaged roof of #4.

The Nuclear Information & Resource Service, an anti-nuclear group, highlights the big unsolved problem of nuclear power — radioactive waste:
Irradiated nuclear fuel rods discharged from commercial nuclear power plants are highly radioactive, a million times more so than when they were first loaded into a reactor core as “fresh” fuel. If unshielded, irradiated nuclear fuel just removed from a reactor core could deliver a lethal dose of radiation to a person standing three feet away in just seconds. Even after decades of radioactive decay, a few minutes unshielded exposure could deliver a lethal dose. Certain radioactive elements (such as plutonium-239) in “spent” fuel will remain hazardous to humans and other living beings for hundreds of thousands of years. Other radioisotopes will remain hazardous for millions of years. Thus, these wastes must be shielded for centuries and isolated from the living environment for hundreds of millenia.
Nuclear waste is the material that nuclear fuel becomes after it is used in a reactor. It looks exactly like the fuel that was loaded into the reactor -- assemblies of metal rods enclosing stacked-up ceramic pellets. But since nuclear reactions have occurred, the contents are’t quite the same. Before producing power, the fuel was mostly Uranium (or Thorium), oxygen, and steel. Afterwards, many Uranium atoms have split into various isotopes of almost all of the transition metals on your periodic table of the elements. 

The waste, sometimes called spent fuel, is dangerously radioactive, and remains so for thousands of years. When it first comes out of the reactor, it is so toxic that if you stood within a few meters of it while it was unshielded, you would receive a lethal radioactive dose within a few seconds and would die of acute radiation sickness [wikipedia] within a few days. Hence all the worry about it. 

In practice, the spent fuel is never unshielded. It is kept underwater (water is an excellent shield) for a few years until the radiation decays to levels that can be shielded by concrete in large storage casks. The final disposal of this spent fuel is a hot topic, and is often an argument against the use of nuclear reactors. Options include deep geologic storage and recycling. The sun would consume it nicely if we could get into space, but since rockets are so unreliable, we can’t afford to risk atmospheric dispersal on lift-off.

Combating weapons of mass destruction: the future of international ... - Google Books Result E. Busch, Daniel Joyner - 2009 - History - 395 pages
"without special handling gear, anyone near unshielded spent nuclear fuel can absorb a lethal radiation dose within minutes"

Even after ten years of cooling, spent nuclear fuel emits dangerous levels of gamma and neutron radiation. A person standing one yard away from an unshielded spent fuel assembly could receive a lethal dose of radiation (about 500 rems) in less than three minutes. A 30 - second exposure (about 85 rems) at the same distance could significantly increase the risk of cancer and/or genetic damage. Defense high-level waste, which contains even higher concentrations of gamma-emitting fission products, is similarly dangerous. The surface dose rate of spent fuel is so great (10,000 rem/hour or more), that shipping containers with enough shielding to completely contain all emissions would be too heavy to transport economically. Federal regulations allow shipping casks to emit 10 millirems/hour at 2 meters from the cask surface, equivalent to about one chest x-ray per hour of exposure.
Routine exposures become especially problematic in situations where the transport vehicle is caught in heavy traffic with cars and other vehicles in close proximity for extended periods. Routine exposures also are of concern when the cask vehicle is stopped for repair, fueling, inspections, etc.

Spent nuclear fuel is both highly radioactive and thermally hot. Nuclear fission inside a reactor transforms a small percentage of the original uranium fuel into additional uranium isotopes, isotopes of plutonium and other transuranic elements, and fission products such as strontium-90 and cesium-137. Fission products, which account for most of the radioactivity in spent fuel during the first hundred years after removal from a reactor, emit both beta and gamma radiation. Reactor operations may also coat the exterior of the fuel rods with corrosion products, or "crud", containing radioactive isotopes of cobalt, nickel, and iron.
A typical ten-year old spent fuel assembly from a Pressurized Water Reactor (PWR) contains about 26,000 curies of strontium-90 (plus many thousands of curies of other dangerous isotopes). The strontium-90 in just one spent PWR assembly would be sufficient to contaminate twice the volume of water in Lake Mead (23 trillion gallons). While the strontium -90 and most of the other dangerous radionuclides are part of the solid pellets that make up the fuel, and therefore not easily dispersed, a severe accident or series of human errors could cause a release of fuel and/or crud particles mixed with smoke accompanying a fire. They could then be inhaled or enter the soil and contaminate the food chain. Other isotopes that remain highly radioactive for decades are so hazardous that inhalation or ingestion of amounts too small to be seen can lead to cancer, radiation-induced disease, and death.
A 1985 DOE contractor report concluded that a maximum severe, credible accident involving a single, current-generation rail cask could result in release of radioactive materials to the environment. The study assumed a severe impact followed by a massive fire fed by large quantities of fuel. According to the study, release of only a small fraction (1380 curies) of the cask's contents would be sufficient to contaminate a 42 square mile area. The costs of cleanup after such an accident would exceed $620 million, and the cleanup effort would require 460 days, if it occurred in a rural area. An alternative analysis by an Agency contractor estimated cleanup costs for the same rural accident ranging from $176 million to $19.4 billion, depending primarily upon permissible post-accident soil concentrations of cobalt-60, cesium-134, and cesium-137, and upon regulatory requirements for disposal of the contaminated soil. Cleanup after a similar accident in a typical urban area would be considerably more expensive and time consuming (perhaps $9.5 billion just to raze and rebuild the most heavily contaminated square mile or so). Much more detailed studies are necessary to estimate accident cleanup costs for a specific urban location in metropolitan Las Vegas or elsewhere in Nevada.
The conditions under which a worst-case accident could occur are poorly understood.

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