Fukushima Daiichi Draindown Risks

7/31/2019 Fukushima Daiichi Draindown Risks

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Nuclear Engineer Identifies Mechanismfor Potential Catastrophic Drain Down of

Fukushima Unit 4 Spent Fuel PoolFukushima Daiichi Unit 4 in Refueling Mode with Leaking Fuel Pool SlideGate


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During a review of events concerning the status of Fukushima Unit 4, nuclear engineer Chris Harrisidentified 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 1). Although this component has received little

attention, its integrity is vital to maintaining Spent Fuel Pool Level. This is because in the currentconfiguration (Refueling Mode) of Unit 4 and the known problem of leaking Refueling Slide Gates, theDrain Down of the Spent Fuel Pool could occur via Failure of the "Refueling Bulkhead and Bellows".

TEPCO Analysis of Refueling Slide GateThe Gate is a long rectangular dam in the side of the Fuel Pool which can be removed after theReactor Refueling Cavity Well is filled so that Fuel can pass through the opening (Slot). The Fuel HandlingMachine is then able to pass the fuel safely submerged. The Gate has seals so that the Fuel Pool doesn'tdrain 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 lostin the event of a LOCA sequence.


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Radiation exposure to the public was assumed to occur only in the event of a coincident refueling cavityseal 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 tobe 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 risksIn a 2006 study conducted by the National Research Council of the National Academies, some startlingfacts were discovered about Spent Fuel Pool draindown events:

The ability to remove decay heat from t he 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 thefuel assemblies to rise, accelerating the oxidation of the zirconium alloy (zircaloy) cladding that encasesthe uranium oxide pellets.

This oxidation reaction can occur in the presence of both air and steam and is strongly exothermic thatis, the reaction releases large quantities of heat, which can further raise cladding temperatures. Thesteam reaction also generates large quantities of hydrogen.

These oxidation reactions [with a loss of coolant] can become locally self- sustaining at hightemperatures (i.e., about a factor of 10 higher than the boiling point of water) if a supply of oxygenan d/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 thefuel 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 cancause the cladding to balloon out and rupture.

At higher temperatures (around 1800C [approximately 3300F]), zirconium cladding reacts with theuranium 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 gasesand some of the fuels 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 fuelassemblies in the pool, producing a propagating zirconium cladding fire.

The high-temperature reaction of zirconium and steam has been described quantitatively since at leastthe early 1960s.
http://www.nap.edu/catalog.php?record_id=11263http://www.nap.edu/catalog.php?record_id=11263http://www.nap.edu/catalog.php?record_id=11263http://www.nap.edu/catalog.php?record_id=11263


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Surry Unit 1 inadvertent spent fuel pool DraindownOn 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.

1. The transfer canal door was in place and the single door seal was inflated.2. The fuel transfer canal was dry.3. 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.4. The refueling cavity seal was not in place.

An accidental pinhole puncture of the single air supply line to the transfer canal door pneumatic sealwas 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, thetransfer 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 (seeattached 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 notsuccessfully 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 heatlevel (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 assemblygeometry 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 possiblyother dynamic events which could rapidly drain the pool.

For these calculations, the end state assumed for the accident sequences was the state at which thewater 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 transfermechanisms and chemical reactions in the fuel assemblies that are slowly being uncovered.


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However, the NRC estimated that recoverable events such as small loss of inventory or loss of power orpool cooling would evolve very slowly, and hoped that many days would be generally available forrecovery whether the end point of the analysis is uncovery of the top of the fuel or complete fueluncovery.

The NRC determined that the extra time available (estimated to be in the tens of hours), was a keyreason for the low risk of a severe accident, as they claimed that the water in the spent fuel pool wouldsurely boil off at a slow rate.

Shortly thereafter, they deemed would not impact the very high probabilities of fuel handler recoveryfrom these events.

Safety SignificanceA 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 therefueling cavity.

The consequences involving the spent fuel pool are based on the assumption that the fuel transfer canalconnecting the refueling cavity to the spent fuel pool is open at the time of the initiating seal failure andthat 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 theSFP. This is commonly understood to increase the risk of a Severe Accident in the event of a suddenLoss of Coolant Accident (LOCA).

Potential Spent Fuel Pool drain down could lead to uncovered fuel, heat-up of the fuel in the pool, whichcan lead to zirconium fire initiation and propagati on of the large spent fuel pool inventory of Cs-137and 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 nuclearplant site, but also could lead to the abandonment of the Fukushima No. 2 plant located nearby.


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Worst-case scenarios revolving around the Unit 4 Spent Fuel Pool by the governments in Tokyo andWashington DC after the March 11 th disaster involved the evacuation of residents from the Tokyometropolitan 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 effortsat nuclear power plants.

Japan has not made sufficient efforts to improve the reliability of the assessments by explicitlyidentifying 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 haveprimary responsibility for securing safety should look at every item of knowledge and every finding andconfirm whether or not they indicate a vulnerability of a plant.

They should reflect as to whether they have been serious in introducing appropriate measures forimproving safety, when they are not confident that risks concerning the public safety of the plant remainlow.

Also, organizations or individuals involved in national nuclear regulations, as those who responsible forensuring the nuclear safety of the public, should reflect whether they have been serious in addressingnew 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 toremember that we will only be able to overcome this accident by uniting the wisdom and efforts of notonly Japan but also the world.

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Fukushima Daiichi Draindown Risks