http://enformable.com/2012/06/nuclear-engineer-identifies-mechanism-for-potential-catastrophic-drain-down-of-fukushima-unit-4-spent-fuel-pool/
Nuclear Engineer Identifies Mechanism for Potential Catastrophic Drain Down of Fukushima Unit 4 Spent Fuel Pool
14JUN2012
During a review of events concerning the status of Fukushima Unit 4, nuclear engineer Chris Harris identified the “weakest link” which may initiate a spent fuel pool draindown event.A major portion of the water in the Fukushima Unit 4 Spent Fuel Pool remains over the Fuel because of the, “Refueling Bulkhead and Bellows” Seal (see drawing above).Although this component has received little attention, its integrity is vital to maintaining Spent Fuel Pool Level. This is because in the current configuration (Refueling Mode) of Unit 4 and the known problem of leaking Refueling Slide Gates, the Drain Down of the Spent Fuel Pool could occur via Failure of the “Refueling Bulkhead and Bellows”.
TEPCO Analysis of Refueling Slide Gate
The Gate is a long rectangular “dam” in the side of the Fuel Pool which can be removed after the Reactor Refueling Cavity Well is filled so that Fuel can pass through the opening (Slot). The Fuel Handling Machine is then able to pass the fuel safely submerged. The Gate has seals so that the Fuel Pool doesn’t drain into the Cavity and dangerously expose Fuel Assemblies when not in Refueling Mode.TEPCO noted that the refueling gate seal is maintained by adequate pool water levels, and may be lost in the event of a LOCA sequence.The leakage through the Gate is caused by an apparent physical distortion which causes an unintended Flow Path between the Reactor Refueling Cavity Well and the Fuel Pool. Evidence of this condition includes a January incident where Fuel Pool water level had been maintained by water from the Reactor Refueling Cavity Well.If the Gate were undamaged and installed correctly, then there should have been no direct communication between the Well and the Pool.Because the damaged Gate provides a non isolable Flow Path, any loss of water from Reactor Refueling Cavity Well will Drain the Fuel Pool to the Bottom of the Gate.Reactor Refueling Cavity Well SealThe Reactor Refueling Cavity Well is formed by the Removal of the Drywell Dome, the Reactor Vessel Head, and the Installation of the Reactor Refueling Cavity Well Seal.The “Refueling Bulkhead and Bellows” or Reactor Refueling Cavity Well Seal is a thin flexible Stainless Steel ring that keeps the water from dumping down into the Drywell when filling the Cavity. When the Cavity level is equalized with the Fuel Pool Level, then the Gate can be removed.With the leaking Gate, the inventory of the Fuel Pool is relying on the Seal’s integrity. The Seal is not intended for long-term use. It is not able to be replaced if damaged while in use. It is not robust enough to withstand misalignment due to Reactor Building Structural damage or earthquakes. The Seal has been in service for greater than a year, presumably with no maintenance. Additionally, this Seal and all parts of the Primary Coolant System and Fuel Pool have been exposed to Salt Water, an environment for which its materials have not been analyzed.CONCERNIf the Seal were to fail, then the Fuel Pool would Drain to dangerously low levels right through the damaged Gate. Further, because the Seal Failure would cause drainage directly to the Containment Drywell, there would be no way to Refill the Fuel Pool.Such Seal failures are not uncommon. Below is a short Operating History of similar Seal failures. See: “Spent Fuel Draindown Events.”CONSEQUENCES:Estimating the Potential Impact of Failure of theFukushimaDaiichi Unit 4 Spent Fuel Pool
“A Local Problem for Japan or a Global Mega Crisis?”
The following Consequences are identified for a Loss of Spent Fuel Pool Level:* risk of a fire in fuel pool number 4 is real, and that the risks of contamination are so severe that an international effort is required.* a nominal release of 10% of the SFP 4 inventory of cesium and strontium would represent 3-10 times the March 2011 release amounts* Spent fuel pools are not protected in the same way as reactor cores, and the unit 4 building is seriously damaged* the fuel rods currently produce about one megawatt (MVV) or more of waste energy in the form of heat (1,000,000 Watts!)* If cooling water for the spent fuel pool is lost – either by collapse of the pool, formation of cracks in the pool, or other factors – a major release of radioactive material could result. Given the large amount of heat generated by the fuel rods, the temperature would rise quickly. These rods are surrounded by zirconium cladding, and at high temperatures, this cladding catalyzes hydrogen production, can generate additional heat, and even explode and burn (NRC, 2006).* The water surrounding the fuel rods in the spent fuel pools serves two purposes: First, it conducts heat away from the fuel assemblies to avoid overheating. Second, it provides shielding from the extremely high radiation levels near the rods.* loss of shielding by the cooling water could critically increase radiation levels in the entire Daiichi complex. High radiation is already a serious problem limiting worker and even robot access to the plant to perform repairs and mitigation, and to maintain cooling of the other spent fuel pools and reactors. Thus, a catastrophic failure of the unit 4 spent fuel pool could potentially cascade into additional releases from the other spent fuel pools and reactors.Conclusion:
The Fuel Pool Gates need to be repaired such that the Cavity Seal is not relied on solely for the maintenance of Fuel Pool level. Eventually, the Fuel must be removed from the Fuel Pool and moved to a safe location.Spent Fuel DetailsThe Unit 4 SFP had the greatest heat load because the entire core had been offloaded into the SFP to support ongoing outage reactor pressure vessel shroud work.The cavity gate was installed, isolating the spent fuel pool from the upper pools.A total of 1,535 fuel rods (97% of the total storage capacity) were in the SFP and their decay heat was relatively high (decay heat: approximately 2.26 MWt as of March 11).All 548 fuel rods had been transferred from the reactor to the spent fuel pool in December 2010, on an upper floor of the reactor building where they were held in racks containing boron to damp down any nuclear reaction.The safety function of the spent-fuel pool (SFP) and storage racks is to cool the spent-fuel assemblies and maintain them in a subcritical array during all credible storage conditions, to provide safe means of loading the assemblies into shipping casks, and were originally designed for limited storage of spent fuel until removed off-site.The safety of spent fuel in pools is achieved primarily by maintaining water inventory, geometry, and maintaining control of reactivity with boron.The fuel is stored in high-density racks inside of zirconium fuel rods under a pool of water under 38 feet deep.The racks stand 15 1/2 feet high leaving the fuel submerged under 23 feet of water if able to maintain water levels.Spent Fuel Pool Draindown EventsOne of the major postulated accident scenarios at a nuclear reactor involves a Loss-of-Coolant Accident (LOCA) which affects the ability to cool the reactor core, but a LOCA event can also occur with a spent fuel pool.In either event, the only measure for controlling coolant loss is to supply more coolant and sustain it.Coolant and power are essential, recirculation of water from a sump is necessary.Spent fuel pool (SFP) accidents involving a sustained loss of coolant have the potential for leading to significant fuel heat up and resultant release of fission products to the environment.Such an accident would involve decay heat raising the fuel temperature to the point of exothermic cladding oxidation, which would cause additional temperature escalation to the point of fission product release.In the case of absence of cooling, a time period ranging from a few hours to several hours (i.e., “grace period”) is needed to produce the lack of cooling conditions that damages the fuel bundles; the duration of the “grace period” depends, other than by the dimension of the pool (fixed and equal in the case of Units 1 to 4), upon the number of fuel bundles installed, their (average-equivalent) burn-up, and the period between the time of beginning-of-stay and the time of lack of cooling.Reference “grace period” duration is ten hours; this is a short time compared with the overall-equivalent time of the lack-of or of inadequate cooling (in the case of Fukushima, this is estimated as several days; see the following).The “grace period” could be shorter in case of loss of pool integrity. Any sudden change in pool water depth or temperature could have an adverse effect on shielding effect, and the ability of the release of radionuclides from the spent fuel assemblies.NRC research on Spent Fuel accidentsThe accident sequences that could result in water loss from the SFP, including beyond design basis earthquakes, various types of seal failures and dropped shipping casks, and the Zircaloy cladding fire issues have been studied by the NRC staff.Generic Issue 82 (GI-82)Generic Issue 82 (GI-82) relates to the concern that for a postulated accident sequence that results in the loss of water from a light-water reactor (LWR) spent-fuel storage pool, a Zircaloy cladding fire could occur and propagate to older stored fuel.This issue was identified during hearings concerning SFP reracking amendments in the late 1970s when licensees were starting to use high-density storage racks.High-density racks like those used at Fukushima Daiichi, are used to accommodate the storage of spent fuel in SFPs at reactor sites until the spent fuel can be removed from the reactor sites, or placed into dry cask storage. High-density racks mean that more radioactive material is stored in the pool, and so the potential dangers from releases of material are higher. If the fuel in the ponds has been discharged from the reactor relatively recently, then the risk of a fire is higher.The nuclear fuel in the Unit 4 spent fuel pool is not evenly spaced out, which is necessary to ensure that the heat generated from the most active fuel rods are dispersed evenly among the spent fuel pool.Maintaining a low-density storage configuration for recently discharged spent fuel would reduce the Zircaloy cladding fire probability by an order of magnitude, but at a greater cost for additional onsite storage space.However this requires careful planning and tracking of offload schedules and rotations with every refueling outage.
NUREG-1353
The results of other studies are provided in NUREG-1353, “Regulatory Analysis for the Resolution of Generic Issue 82, Beyond Design Basis Accidents in Spent-Fuel Pools”.
Although these studies conclude that most of the spent-fuel pool risk is derived from beyond design basis earthquakes, this risk is not greater than the risk from core damage accidents due to these beyond design basis earthquakes.
Basically, the conclusions were that any earthquake strong enough to lead to a severe accident in a spent fuel pool would probably also pose a significant threat to the reactor.
The argument was due to the low probably of such events, it made little sense to focus on the spent fuel pool when the reactor would normally pose the more significant challenge.
These conclusions will almost certainly be revisited in light of recent events at Fukushima.
Two accident sequences of note have been considered:
(1) refueling cavity seal failure resulting in serious transfer canal drainage; and
(2) seal failure resulting in spent fuel pool drainage.
For the spent fuel pool drainage scenario (Case 2), core-melt was not assumed, being limited to a small LOCA in which the ECCS functions as intended and no fuel melting occurs, but some fuel cladding ruptures.
For the refueling canal event, melting of the fuel assembly in transit was assumed.
The scenario began with a rapid drainage of the refueling canal with the transfer of a spent fuel assembly in process could subject plant personnel in the containment building to radiation exposure prior to their evacuation of the building.
Analysis performed by Pacific Northwest Laboratory (PNL) indicates that, for the case of a single fuel assembly lying in the bottom of a dry refueling canal, exposure levels on the order of 10,000 R/hr on the operating deck at the extreme edge of the canal are possible.
Extrapolation of the PNL analysis indicates a potential general radiation level on the order of 100 to 500 R/hr on the operating deck due to reflection from the containment dome.
Considering the possible canal drainage time, alarm levels, and evacuation times, it appears unlikely that any operator would receive a lethal dose prior to evacuation; however, it is likely that workers could receive a whole-body dose on the order of 50 to 100 man-rem.
In terms of offsite public exposure, this dose is about comparable to some of the higher probability/lower consequence core-melt scenarios.
Radiation exposure to the public was assumed to occur only in the event of a coincident refueling cavity seal failure and an open fuel transfer canal i.e., a drainage path for the spent fuel pool.
The frequency of refueling cavity seal failure resulting in serious spent fuel pool drainage, is estimated to be the product of the frequency of spent fuel pool drainage and the probability of no recovery.
National Research Council of the National Academies studies on Spent Fuel risks
In a 2006 study conducted by the National Research Council of the National Academies, some startling facts were discovered about Spent Fuel Pool draindown events:
“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….”
Haddam Neck Spent Fuel Pool Draindown
On August 21, 1984, the Haddam Neck plant experienced failure of a refueling cavity seal during preparations for refueling. The seal assembly consists of an annular plate with two pneumatic seals.
The refueling cavity water level (23 feet) decreased to the level of the reactor vessel flange within 20 minutes which flooded the containment, with approximately 200,000 gallons of water.
Fortunately, no fuel was being transferred at the time.
The seal assembly was subject to a gross failure due to lack of interference between the width of the seal annulus and the width of the opening, which allowed the seal to be significantly displaced.
If a similar seal failure were to occur at a plant during fuel transfer, fuel elements could be partially or completely uncovered and could result in high radiation exposure to plant personnel, possible fuel cladding failure, and release of radioactive material.
Also, because the refueling cavity is connected to the spent fuel storage pool, the potential exists for this seal failure to initiate drainage of the spent fuel pool to a level which would have uncovered the top of the fuel, if the fuel transfer canal were open at the time.
If newly discharged fuel had been placed in the spent fuel pool, a postulated draindown of the spent fuel pool could have led to even higher radiation levels in the spent fuel pool building than the radiation levels postulated by the licensee.
If a seal failed and spent fuel pool water were lost while a fuel assembly was lifted, fuel could be uncovered and fuel cladding could fail.
Response to the Haddam Neck Draindown
Following the event at Haddam Neck, on August 24th, 1984, IE Bulletin No. 84-03 was issued requiring licensees to investigate the potential for refueling cavity seal failures at their plants.
The responses also showed that many plants use a pressurized bladder of similar configuration to that used at Haddam Neck, which are most vulnerable to failure as they are susceptible to misalignment, improper inflation, puncture, and rupture.
Surry Unit 1 inadvertent spent fuel pool Draindown
On October 2, 1988, with Surry Unit 1 in cold shutdown, the licensee was preparing to test the fuel transfer system (see attached figure), before fuel off-load.
- The transfer canal door was in place and the single door seal was inflated.
- The fuel transfer canal was dry.
- The fuel transfer tube was open, the blind flange was removed on the containment side, and the gate valve was open on the spent fuel pool side.
- The refueling cavity seal was not in place.
An accidental pinhole puncture of the single air supply line to the transfer canal door pneumatic seal was promptly detected and the air leak quickly stopped before it could lead to a loss of seal integrity.
A review of this event by the licensee showed that, given the configuration of the transfer canal, the transfer tube, and the refueling cavity in place at the time of the event, an inadvertent draindown of the spent fuel pool could occur to a height of only 13″ above the top of the fuel assemblies (see attached figure).
The licensee estimated that the dose rate, based on the spent fuel inventory at the time of the event, could have reached 50 R/hour on the operating deck.
Risk stemming from decay heat in Spent Fuel Pools
For the larger loss-of-coolant-inventory accidents, water addition through the makeup pumps does not successfully mitigate the loss of the inventory event unless the location of inventory loss is isolated.
NRC analyses show that it is not feasible, without numerous constraints, to define a generic decay heat level (and therefore decay time) beyond which a zirconium fire is not physically possible.
Heat removal is very sensitive to these constraints, and two of these constraints, fuel assembly geometry and spent fuel pool rack configuration, are plant specific.
Both are also subject to unpredictable changes as a result of the severe seismic, cask drop, and possibly other dynamic events which could rapidly drain the pool.
For these calculations, the end state assumed for the accident sequences was the state at which the water level reached 3 feet from the top of the spent fuel.
This simplification was used because of the lack of data and difficulty in modeling complex heat transfer mechanisms and chemical reactions in the fuel assemblies that are slowly being uncovered.
However, the NRC estimated that recoverable events such as small loss of inventory or loss of power or pool cooling would evolve very slowly, and hoped that many days would be generally available for recovery whether the end point of the analysis is uncovery of the top of the fuel or complete fuel uncovery.
The NRC determined that the extra time available (estimated to be in the tens of hours), was a key reason for the low risk of a severe accident, as they claimed that the water in the spent fuel pool would surely boil off at a slow rate.
Shortly thereafter, they deemed would not impact the very high probabilities of fuel handler recovery from these events.
Safety Significance
A refueling cavity seal failure is itself considered to be an initiating event for an accident sequence.
The immediate result of a refueling cavity seal failure during fuel transfer is the loss of water from the refueling cavity.
The consequences involving the spent fuel pool are based on the assumption that the fuel transfer canal connecting the refueling cavity to the spent fuel pool is open at the time of the initiating seal failure and that the canal cannot be closed.
The possible safety consequences are as follows:
- high radiation levels in the containment due to uncovering of spent fuel in transfer;
- radioactive material release in the containment building due to rupture of fuel pins (by self-heating after uncovering);
- Increased radiation levels in the spent fuel pool building would severely limit stay time in the building and impede swift recovery efforts.
Unit 4 was in an outage which the full core to be offloaded, which was stored in one central area in the SFP. This is commonly understood to increase the risk of a Severe Accident in the event of a sudden Loss of Coolant Accident (LOCA).
Potential Spent Fuel Pool drain down could lead to uncovered fuel, heat-up of the fuel in the pool, which can lead to “zirconium fire” initiation and propagation of the large spent fuel pool inventory of Cs-137 and other radionuclides.
There is also potential for significant long term heat transfer to surrounding structures.
This type of event would not potentially affect the accessibility of the entire Fukushima No. 1 nuclear plant site, but also could lead to the abandonment of the Fukushima No. 2 plant located nearby.
Worst-case scenarios revolving around the Unit 4 Spent Fuel Pool by the governments in Tokyo and Washington DC after the March 11th disaster involved the evacuation of residents from the Tokyo metropolitan area.
Effective use of probabilistic safety assessment (PSA) in risk management
PSA has not always been effectively utilized in the overall reviewing processes or in risk reduction efforts at nuclear power plants.
Japan has not made sufficient efforts to improve the reliability of the assessments by explicitly identifying the uncertainty of these risks.
Without a safety culture, there will be no continual improvement of nuclear safety.
Reflecting on the current accident, the nuclear operators whose organization and individuals have primary responsibility for securing safety should look at every item of knowledge and every finding and confirm whether or not they indicate a vulnerability of a plant.
They should reflect as to whether they have been serious in introducing appropriate measures for improving safety, when they are not confident that risks concerning the public safety of the plant remain low.
Also, organizations or individuals involved in national nuclear regulations, as those who responsible for ensuring the nuclear safety of the public, should reflect whether they have been serious in addressing new knowledge in a responsive and prompt manner, not leaving any doubts in terms of safety.
We should be prepared to confront difficulty during restoration from the accident, but also sure to remember that we will only be able to overcome this accident by uniting the wisdom and efforts of not only Japan but also the world.
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