What passes for normal at the Fukushima Daiichi plant today would have caused shudders among even the most sanguine of experts before an earthquake and tsunami set off the world’s second most serious nuclear crisis after Chernobyl.
Fourteen months after the accident, a pool brimming with used fuel rods and filled with vast quantities of radioactive cesium still sits on the top floor of a heavily damaged reactor building, covered only with plastic. The worries picked up new traction in recent days after the operator of the plant, Tokyo Electric Power Company, or Tepco, said it had found a slight bulge in one of the walls of the reactor building, stoking fears over the building’s safety.
Government critics are especially concerned, since Tepco has said the soonest it could begin emptying the pool is late 2013, dashing hopes for earlier action.
Senator Wyden, whose state could lie in the path of any new radioactive plumes and who has studied nuclear waste issues, is among those pushing for faster action. After his recent visit to the ravaged plant, he said the pool at No. 4 poses “an extraordinary and continuing risk” and the retrieval of spent fuel “should be a priority, given the possibility of further earthquakes.”
According to Tepco, the pool at the No. 4 reactor, which was not operating at the time of the accident, holds 1,331 spent fuel assemblies, which each contain dozens of rods.
This isn’t true. Yes all waste generates its own heat, but you can bury it in e.g. concrete because that conducts heat far better than water. Providing it’s in contact with the air or ground, the heat cannot build up massively. Only in a vacuum (like space) can higher temperatures be reached, but not those you talk about as a substantial amount of heat is lost as infrared heat.
Can't bury the site as the spent fuel has to be continually cooled. If not decay heat builds up and it reaches a point that will melt everything it comes in contact with and if it contacts a significant source of water, you get a steam explosion:
4. Spent fuel becomes exposed in the pool at Unit 4, and the damaged fuel begins to melt. This melted fuel interacts with the concrete of the pool itself, producing a molten fuel-coolant interaction (MFCI) 2 and releasing radioactive materials.3
3. It is unlikely that the pool would have melted even if left uncooled. It is likely, however, that it would have become hot enough to release its contained cesium-137.
Hydrogen is generated in a nuclear reactor if the fuel in the reactor loses its cover of cooling water.
The same phenomenon can occur in a spent fuel pool in case of a loss of cooling water.
The ability to remove decay heat from the spent fuel also would be reduced as the water level drops, especially when it drops below the tops of the fuel assemblies. This would cause temperatures in the fuel assemblies to rise, accelerating the oxidation of the zirconium alloy (zircaloy) cladding that encases the uranium oxide pellets. This oxidation reaction can occur in the presence of both air and steam and is strongly exothermic—that is, the reaction releases large quantities of heat, which can further raise cladding temperatures. The steam reaction also generates large quantities of hydrogen….
These oxidation reactions [with a loss of coolant] can become locally self-sustaining … at high temperatures (i.e., about a factor of 10 higher than the boiling point of water) if a supply of oxygen and/or steam is available to sustain the reactions…. The result could be a runaway oxidation reaction—referred to in this report as a zirconium cladding fire—that proceeds as a burn front (e.g., as seen in a forest fire or a fireworks sparkler) along the axis of the fuel rod toward the source of oxidant (i.e., air or steam)….
As fuel rod temperatures increase, the gas pressure inside the fuel rod increases and eventually can cause the cladding to balloon out and rupture. At higher temperatures (around 1800°C [approximately 3300°F]), zirconium cladding reacts with the uranium oxide fuel to form a complex molten phase containing zirconium-uranium oxide. Beginning with the cladding rupture, these events would result in the release of radioactive fission gases and some of the fuel’s radioactive material in the form of aerosols into the building that houses the spent fuel pool and possibly into the environment. If the heat from one burning assembly is not dissipated, the fire could spread to other spent fuel assemblies in the pool, producing a propagating zirconium cladding fire.
The high-temperature reaction of zirconium and steam has been described quantitatively since at least the early 1960s….3
3 National Research Council, Safety and Security of Commercial Spent Fuel Storage: Public Report. Washington, D.C.: National Academies Press, 2006, on the web at www.nap.edu..., pp.38-39. This report addressed the issue of terrorist attacks on spent fuel pools and the precautions that might be taken in light of the potential severity of the problem.