There are lots of concerns about the upcoming fuel removal work at unit 4. With this has come some passing mentions by various people that sort of grew a life of their own. We have broken this into two sections. There is also a companion article that focuses on the more technical issues of the fuel removal and related safety. First some answers to questions that have been circulating in the media. Second some clarfication on TEPCO’s statements about the fuel removal process.
Answers To The Big Questions
One mention was that “two fuel rods touching” could cause a massive uncontrollable nuclear reaction.
Charles Parrow made this comment in a Huffington Post article and his paper on Fukushima has been cited as the source of this statement. The peer reviewed paper does not include any discussion of the fuel at Fukushima and instead focuses on the health impacts of the radiation releases. His statement has been disputed by many even though it was quickly repeated by the media without any additional fact checking. Two rods touching as Parrow claims is unlikely to happen as fuel assemblies are handled one at a time and it takes far more than two assemblies nearby to create a reaction and certainly not the uncontrollable one Parrow describes. Individual fuel rods at TEPCO’s other plant have been routinely damaged and bent into configurations that caused these individual rods to touch. The only concern there has been the potential damage to the rod tubes themselves that could cause a fuel failure to be more likely. This technical document outlines that it takes 16-20 assemblies to create a situation where criticality could occur.
A criticality accident
While this is a potential, it requires some very specific conditions to get the right amount of fuel into a configuration that could make this happen. There has been debate even among experts about how much loose fuel pellets would be needed to create such a scenario. The potential for something going wrong that would release radiation into the defueling building causing it to need to be evacuated is a larger risk than a criticality accident.
Another concern has been having the pool drain or collapse.
A partial draining of the pool would be worse than a full draining of the pool. Full draining of the pool due to the current age of the fuel would actually be a better situation. Air convection would be in play. Our calculations that were independently verified indicate air cooling in this situation would be enough to prevent the fuel from melting. Of course there would be massive amounts of lethal radiation in the area around the unit and inside it making entry impossible. A partial draining of the pool could create a situation where fuel could heat up enough to melt. So either refilling the pool or draining it would be needed to stop that process. Of course refilling the pool would be the preferable action as the water provides shielding from the massive radiation levels of the spent fuel. If the pool were to completely collapse, the same problems of needing to cool the fuel and shield the massive radiation would be needed and present a major technical challenge. To date TEPCO has not provided any plan for such contingencies to the public or the media.
It is possible even in a worst case scenario that they might be able to cover or shield the fuel but it would be a major technical challenge. Any of these problems do not automatically result in a major release of radiation outside of the plant grounds. There are two methods of radiation impact, one being the radiation in proximity to the fuel where workers could not be near it. The other being situations where radiation is released to the environment. This would happen by either crushing or melting the fuel pellets.
The situations that cause melting or burning fuel become the scenarios where off site releases could be carried in the wind. So the concept of major releases from the plant would require a specific combination of events before any radiation that would travel long distances could happen. The other concern has been that the potentially lethal levels of radiation would prevent human access to the plant causing systems to go out of control. This is partially true as power for water circulation to the pools is still needed, if something could be done to mitigate the problem before those systems could somehow fail to operate they could regain control of the plant. The challenges and potential risk have been mis- characterized as if all situations automatically lead to a massive release of radiation, but that isn’t exactly true. Some clear contingency planning for some of these worst case scenarios is sorely needed and should be clearly conveyed to the public.
TEPCO released a report trying to assure the public the fuel removal process for unit 4 is safe. It does admit a number of things in the way they chose to word statements.
“Removing fuel from a spent fuel pool is a normal operation that has been done at any nuclear power station, even before the great earthquake.”
The work to be done is very similar to the routine process used to remove fuel from a reactor building. The routine process is not without risks. We documented in detail the wide variety of risks that exist in normal fuel handling operations. This process is complicated at unit 4 by having small debris in the fuel racks and suspected damage to some of the fuel.
“We are determined to take assured actions against these risks, and if a problem is recognized during the process, we will confirm it carefully and will carry out operation under safety-first principles.”
TEPCO provides no further details on the process or what actions are being taken so this boils down to TEPCO asking the public to just trust them.
“All 1,500 fuel units did not show fuel melting, but a hydrogen explosion, possibly caused by hydrogen coming in from Reactor 3, occurred.“
TEPCO has provided nothing to back up this claim that no fuel melted, they omit any statement about other kinds of damage. We disagree with TEPCO’s claim that all the hydrogen came from just unit 3. Our research indicates that unit 4′s spent fuel pool likely did play a contributing role to the hydrogen production.
“TEPCO has confirmed that the spent fuel pool and the buildings themselves are strong enough to withstand an earthquake with the strength equivalent to the 2011 earthquake off the Pacific coast of Tohoku (Seismic Level 6+).“
The west side upper floors of unit 4′s building are considered to be structurally failed by TEPCO. The earthquake resistance of the spent fuel pool of unit 4 only has that seismic rating for vertical shaking, not horizontal shaking as admitted by a TEPCO representative questioned at a symposium last year.
“Inspections confirmed that the building is not tilted.”
TEPCO only confirms the reactor containment structure and spent fuel pool for being level. The outer portions of the building were not confirmed and can move without impacting the condition of the containment structure or the pool. NRA’s investigation showed that the individual outer floors of the building are uneven.
“Inspections confirmed that the building is free of any damage that may collapse the building“
This isn’t the concern. The concern is that if something like the west wall were to fall away it could take nearby structures with and potentially damage the spent fuel pool or systems tied to it with in the failure. Any new damage or failure to the south side of the building could compromise the spent fuel pool.
“Confirmation has been made on the integrity of the reactor building and spent fuel pool, by conducting approaches of building slanting measurement and regular inspection, including visual checks and concrete strength verifications“
While some of this is true, they ignored reporting the documented failure of the west sides of the building in the upper floors and other sections of the building that are considered structurally failed. TEPCO is not being forthright in this report.
“Fuel pool walls and floors are constructed extremely thickly. Above that, the entire pool is supported by an extremely thick seismic resistance wall. Therefore, even should there be damage to the other outer walls and floor, the same seismic resistance as before the earthquake is secured, and thus the floor will not fall out.”
This is TEPCO’s response to the question if the pool can hold the fuel already in the pool. This really isn’t a question anyone was asking. A more appropriate question is can the pool hold the fuel AND the transport cask without damage to the pool? Spent fuel casks can weigh as much as 50,000 pounds. Can the pool handle this extra weight and can the pool handle the impact if the cask were to be dropped in the pool? The US NRC considers crane failure and cask drops to be genuine risks that can happen during fuel handling.
“Ventilation units will be installed inside the cover, which will discharge air to the outside of the cover through a filtering unit to minimize emission of radioactive substances out of the cover“
TEPCO does not explain the effectiveness of these filters or the capacity of air contamination they could actually handle.
“Confirmation has been made that a single fuel cannot lead to a critical condition. Work will be performed in a careful manner, one fuel at a time.”
The standard industry practice is to move one fuel assembly at a time. They are also constrained by the fuel machine that can only move one assembly at a time. There has been a rumor going around about “two fuel assemblies touching” and creating some cataclysmic event. This is not within the technical nature of spent fuel. One of our research team spoke to the “expert” that originally made that comment that was repeated in the media recently. They were unable to explain or defend their statement..
“Debris in the pool will hinder the handling of fuel, and thus will be removed by special machines. Fuel pellets*1 are covered by fuel covering tubes*2 , as well as a channel box made of extremely strong zirconium alloy, separating the pellets and debris. Therefore the debris will not directly contact fuel pellets“
The unused fuel assemblies removed from unit 4′s spent fuel pool earlier for analysis did show small pieces of debris lodged between the fuel rods of the assembly so debris IS able to get down into the fuel and could cause a problem removing some assemblies. The fuel handling crane does have a load limit that can be set to stop pulling if it meets a certain amount of resistance. They would be able to stop the process but how they then free the assembly has not been explained by TEPCO.
“The fuel handling machine has multiple safety measures by doubling wires and brakes. Even if power is lost during the work, the hook would not open and would not drop fuel. Confirmation has been made that a single fuel assembly cannot lead to a critical condition even if it is dropped“
Most modern nuclear industry cranes have fail safes built in to try to prevent a “single failure” from causing damage. Other types of failures such as structural or mechanical failures can still impact these kinds of cranes. This is a known risk addressed in the NRC and other fuel handling risk documents.
“It is thought that the hydrogen generated at Reactor 3 flowed into the Reactor 4 Stand-by gas treatment system / Building Ventilation System during venting (*1) , which caused hydrogen explosion at Reactor 4.”
Our analysis showed that it was impossible for unit 3 to be the sole cause of the hydrogen explosion at unit 4 and that unit 4 itself likely played a partial role in the hydrogen production that lead to the explosion.
“Corrosion of the vessels and components was a concern because of seawater injection into the fuel pools. Therefore, we implement appearance checks for unused fuel removed from the Reactor 4 fuel pool, as well as inspection for corrosion of the fuel components. These inspection showed that there were no corrosion which have effects on fuel integrity.”
TEPCO did admit there was corrosion but has provided no data to confirm their assumptions that this did not damage all assemblies. It is also not confirmed if corrosion could be worse on spent fuel assemblies that may have had defects or damage from operation.
“”Dropped Debris Collision Tests” were conducted regarding the concern of deformation of “fuel” in the situation in which debris fell from above during earthquakes. As a result, although there were handle deformations and bends in the fuel, elevation performance and safety of fuel sealing performance is ensured.”
This does indicate damage. Without looking at all assemblies they can not truly confirm the performance of every assembly in the pool. This is simply an assumption by TEPCO. These damaged assemblies will still present a challenge to remove from the pool. TEPCO has made a jig to try to remove those without a usable handle but have not admitted any testing being done with this jig.
Please also see our write up on what should be done to improve safety in this process.
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