Spent Fuel Handling Recommendations For Fukushima

SimplyInfo member Dean Wilkie reviewed a number of industry safety documents related to spent fuel handling safety as applicable at Fukushima Daiichi. This includes a list of suggestions how TEPCO should handle safety and processes related to spent fuel removal with selected sections from the source material.

 

RECOMMENDED GENERAL SPENT FUEL ASSEMBLY HANDLING RECOMMENDATIONS
APPLICABLE FOR ALL EVOLUTIONS AT FUKUSHIMA and SPENT FUEL HANDLING ACCIDENTS/RISKS

The referenced documents should be incorporated into the TEPCO design basis for the SFP4 operations. Every element of commonality should be integrated into planning done to date by TEPCO. A readiness assessment should be performed with representatives from all parties involved including TEPCO management to present what they will do at SFP4.

To date TEPCO has not established and made public the SAFETY LIMITS for SFP operation, any limiting safety system settings (or setpoints) to prevent approaching the safety limits, nor have they provided an exhaustive set of accidents, consequences and mitigation necessary to prevent fuel assembly damage or cladding failures

FIRE PROTECTION

  • Fire fighting extinguishers and extinguishing agents shall be arranged outside of the fuel removal building neat the entrance ways.

  • Only such extinguishing agents shall be provided which have been proved effective for the respective combustible materials

COMMUNICATION SYSTEM SHOULD BE ON A DEDICATED POWER SYSTEM

 

WATER QUALITY

  • A water purification facility shall be provided and designed such that

  • radioactive, ionic and solid impurities can be removed from the coolant

  • limit values for the fuel pool water quality that shall be specified in the respective boron concentration and pH value) can be achieved

  • suspended solids causing poor visibility can be removed.

  • Equipment or devices shall be provided with the help of which surface impurities can be removed from the fuel pool water.

  • With regard to work tasks during which, locally, an increase of the release of radioactive substances or suspended solids must be expected, e.g.,

    • during repairs of fuel assemblies, provisions shall be made, either, that

      • the water of the fuel pool can be extracted locally and passed through a purification system or

      • that the water can taken up by local purification devices.

 

CRITICALITY SAFETY

Specified normal operation under consideration of the coolant and of the permanent intermediate structures, the distance between the fuel assemblies in the storage racks for the fuel assemblies shall be chosen such that the neutron multiplication factor keff does not exceed 0.95 taking into account any tolerance and calculation uncertainty

Criciality monitors should be set up in the SFP4 BUILDING and Neutron monitors in the SFP

 

HANDLING, EXAMINATION and REPAIR TO FUEL ASSEMBLIES

If fuel rods are pulled out of a fuel assembly or if neutron absorbing inserts or structural parts are removed (e.g. with regard to repairs or inspections) then the associated change of the neutron multiplication factor shall be taken into account.

(4) With regard to containers with individual fuel rods, the following requirements shall be met:

a) The analysis shall be based on new (unirradiated) fuel rods. Here, burnable neutron poisons in the fuel material may not be taken into consideration.

b) The analysis shall be based on a moderation which would lead to the highest neutron multiplication factor for the given arrangement of fuel rods.

 

HANDLING EQUIPMENT

(1) Handling equipment for fuel assemblies and associ-ated items shall be designed to withstand external and internal events if their availability after such impacts is required for safety-related reasons or if there is cause to expect impermissible consequential damage.

(2) It shall be ensured by design measures that, regarding the use of lifting equipment and tools, there will always remain a sufficient water cover as shield from radioactivity.

 

EQUIPMENT OF FUEL POOLS AND STORAGE BUILDINGS

Set-down place for fuel assembly shipping casks. The set down place provided for fuel assembly shipping casks shall be located and designed such that

a) the transport route for the shipping cask is as short as possible and does not pass over the storage racks for fuel assemblies,

b) the rigging required for the transportation of the shipping cask can be mounted and removed without hindrance,

c) sufficient shielding is provided during loading of the shipping cask,

d) the shipping cask cannot damage the fuel pool or fuel assemblies if it should slip or tip over on account of any of the induced vibrations to be assumed

 

STORAGE AND HANDLING OF FUEL ASSEMBLIES AND ASSOCIATED ITEMS IN THE FUEL POOL AND DURING WORK

General requirements

(1) Fuel assemblies and other storage items shall be stored in the fuel pool only in the position specified for the individual item.

(2) Fuel arrangement diagrams or lists shall be provided for the reactor and the storage racks listing the identification codes and the respective storage locations of the fuel assemblies and the associated items. Other storage items present in the storage racks shall also be listed in the fuel arrangement diagrams.

(3) In case of a multi-zone fuel pool, the initial inventory of fissile material and the burn up shall be documented for each fuel assembly.

(4) Any handling of fuel assemblies and associated items in the reactor pressure vessel and in the fuel pool may only be performed following written work instructions, e.g. in the form of a step-by-step plan.

(5) Before commencing work, it shall be ensured that all tools and devices are in proper functioning order. All tests, modifications and repairs shall be documented.

(6) Without a specific proof of safety, no more than one fuel assembly at a time may be handled or transported inside the fuel pool. Simultaneous handling of fuel assemblies is permitted in the channel stripping machine, the inspection equipment and the repair devices.

(7) Fuel assemblies and associated items shall only be handled in the presence of a competent person in charge who shall coordinate the radiological protection measures required during handling with the radiological protection officer. At least two persons shall be present at all times during such handling procedures.

 

LOADING AND TRANSPORT OF SHIPPING CASKS

(1) Only such shipping casks may be loaded that are of a suitably designed structural type.

(2) The shipping casks may only be loaded on the basis of a fuel loading schedule and a written instruction which specify the intended relocation of the individual fuel assemblies (old position in the fuel pool, new position in the shipping cask) and which describe the individual steps of the required work tasks and testing steps

 

TESTS BEFORE TRANSPORTATION

 (1) Before transportation, tests and inspections regarding loading, leak tightness, safety devices, radiation level and contamination shall be performed to verify that the conditional provisions and limit values are adhered to that are specified for the type of transportation in the

a) operating instruction of the shipping cask,

b) Radiological Protection Ordinance,

c) technical receiving conditions of the incorporating .g., external intermediate storage or intermediate on-site storage) including the corresponding codes and specification in case of plant external transports,

d) transport license including the handling and operating instructions for the particular type of shipping cask as well as the associated servicing, assembling and testing instructions.

(2) The loading of the shipping cask shall be documented.

(3) Transportation shall only be carried out if the permissible limit values are not exceeded and all conditional provisions are fulfilled.

 

HEAT REMOVAL CAPABILITIES WITH OUT RESIDUAL HEAT REMOVAL SYSTEM

Is the current heat removal temporary system sufficient to handle the heat load for fuel handling evolutions?

Does the heat removal system have backup systems in the event of loss due to EQ/power loss?

Is diesel power available to all systems and are the diesels supplying primary power to ensure no power losses during handling evolutions?

SPENT FUEL ACCIDENTS

Spent fuel pool accidents, risks and consequences for Fukushima must cover a broader range since the operation is unique and first of kind following a major accident. The spent fuel pools have been exposed to the atmosphere and even the SFP4 covering, in the form of a temporary building, is not considered in accident analysis more than providing some delay in fission product releases. The following list of accidents are typical for normal spent fuel pools and some are more unique to the Fukushima facilities.

General Accidents/ Potential Consequences

Loss of Cooling due to Pump/ valve/piping failure and loss of heat sink
Consequence of this accident results in the slow heating of the spent fuel pool which as been estimated  to be ~   C/hr. This rate would result in the SFP reaching a boiling point in days.
Consequences of this accident results in the loss of water inventory. The assumed current boil off rate for SFP4 would take weeks to lower the water level with in the spent fuel pool. The significant impact of this event is to raise the radiation level above the water with in the pool which could inhibit workers to be in the area of the pool.  Ultimately, if no action is taken to restore cooling, the high area temperature and humidity, and low water level from boil off will become increasingly evident. Use of a single loop cooling for the SFP should not be allowed to prevent single failures which could allot heating of the SFP

Loss of of Coolant Inventory
Consequences are the same as above in that the radiation levels within the pool will elevated to extreme   values which will inhibit workers from being in the pool area. There needs to be a means to remotely align a makeup source to the SFP without entry to the refueling floor, so that make-up can be provided even when the environment is uninhabitable because of steam and/or high radiation.

Loss of offside power from plant centered/grid power sources

  • consequences of this event can be multiple depending on equipment power supply

    • loss of SFP cooling system resulting in a gradual heating of the SFP

    • loss of crane power which could disrupt a fuel/cask handling evolution and result in a suspended load in or over the SFP

    • loss of lighting within the facilty hopefully would reduce lighting to emergeny lights

    • loss of power to the fuel handling machine could result in a suspended fuel element directly over a fuel rack or fuel transfer cask

    • loss of alarm instrumentation/ monitoring equipment etc which would render radiation or contamination detection and corrective actions

Failure of backup Diesel power

  • Loss of backup Diesel power with no plant or grid power would result in operations being suspended

    • suspended loads

    • loss of emergency action systems/alarms

 

Loss of offsite power from severe weather events

  • depending on severity of power loss the consequences would be similar to loss of offsite power from plant/grid section above

Internal fire

  • TEPCO has not divulged any fire suppression systems within the SFP building

    • A fire on the fuel handling machine or overhead crane could be difficult to immediately extinguished

    • combustible debris around the spent fuel pool needs to be policed and taken up to reduce fire loading

    • TEPCO should have an overhead mist spray system which would actuate in the event of a release of fission products. An overhead water mist spray tends to wash down the Iodines from the release and have a smaller potential Iodine release to the atmosphere

 

Seismic Events

  • Seismic events at Fukushima have not been clearly analyzed or if they have TEPCO has not shared any of the specifics

  • Consequences on SFP4 or U4 from a seismic event can be minimal or catastrophic depending on the EQ intensity

  • Seismic analysis has not been released for earthquakes that could impact the large supporting beam that the tracks for the 2 bridge cranes travel on.

  • Earthquakes with ground motion at the building could result in thespent fuel pool bridge moving/shifting which could impact operabilitie and handling of a fuel assembly

 

Requirement: Verify the Adequacy of Structural Steel (and Concrete) Frame Construction

Basis: At a number of older nuclear power plants, the walls and roof above the top of the spent fuel pool are constructed of structural steel. These steel frames were generally designed to resist hurricane and tornado wind loads which exceeded the anticipated design basis seismic loads. A review of these steel (or possibly concrete) framed structures should be performed to assure that they can resist the seismic forces resulting from a beyond-design-basis seismic event in the 0.45-0.5g pga range. Such a review of steel structures should concentrate on structural detailing at connections. Similarly, concrete frame reviews should concentrate on the adequacy of the reinforcement detailing and embedment.

Typhoon/Tornado  Events with high winds

The main threat from either of these events is high winds which, at Fukushima could damage safety systems such as SFP cooling which are external to the reactor buildings, electrical grid/in site power supply systems. in addition high winds can  result in projectile impact with the reactor and exterman supporting systems

The existing SFP4 building has been extensively damaged and does not have the outer protective concrete/steel framework to inhibit projective damage. In fact there are a number of debris items that have been pushed away fron the building which, if they became a projectile could penetrate the reactor building through openings caused by the earthquakes and explosion that could directly impact other components or wall. TEPCO should ensure that all loose debris is moved well away from the SFP4 area or secured to prevent them becoming projectiles. An F4 to F5 tornado would be needed to consider the possibility of damage to the a BWR SFP by a tornado missile. In addition, the SFP is a multiple-foot thick concrete structure. Based on the DOE-DOE-STD-1 020-94 information, it is very unlikely that a tornado missile would penetrate the SFP, even if it were hit by a missile generated by an F4 or F5 tornado.

For sites, like Fukushima, where Typhoons/tornadoes are considered a viable threat, to account for objects or debris. The same 2×4 inch timber is considered but for heights above ground to 50 ft. The tornado missiles are (1) the 15 Ibs, 2×4 inch timber with a horizontal speed of 150 mph effective up to 200 ft above ground, and a vertical speed of 100 mph; (2) the 3-inch diameter, 75 lbs steel pipe with a horizontal speed of 75 mph and a vertical sped of 50 mph effective up to 100 ft above ground; and (3) a 3,000 lbs automobile with ground speed up to 25 mph. For the straight wind missile, an 8-in CMU wall, single width brick wall with stud wall, or a 4-inch concrete (reinforced) is considered adequate to prevent penetration. For the tornado missile, an 8 in CMU reinforced wall, or a 4-to-1 0-inch concrete (reinforced) slab is considered adequate to prevent penetration (depending on the missile).
Fukushima unit 4 SPF Unique Accidents/Potential Consequences

SFP surge tank discharge blockage

The spent fuel pool surge tank is integral to the collection of water which overflows into the tank. The water collects in the tank and is then pumped through the remainder of the SFP cooling system. Failure of this tank resulting in loss of supply water. blocking of the piping discharge by debris would cause the inability to circulate water essential for cooling.
Loss of air for the fuel assembly handling hook

Bottled air is used to pneumatically lock the hook when attached to a fuel assembly. loss of this air would result in 1 of 2 locking measures being lost and place the fuel assembly in a reduced condition of positive backed up connection for removal.

 

Heavy load drops

Transfer cask

The cask drop event analysis exclusively considered drops severe enough to catastrophically damage the SFP so that pool inventory would be lost rapidly and it would be impossible to refill the pool using onsite or offsite resources. There is no possibility of mitigating the damage, only preventing it. The dose rates in the pool area could be tens of thousands of rem per hour, making any recovery actions (such as temporary large inventory addition) very difficult. Load Drop consequence analyses should be performed for facilities with non-single failure-proof systems. The analyses and any mitigative actions necessary to preclude catastrophic damage to the SFP that would lead to a rapid pool draining should be performed with sufficient rigor to demonstrate that there is high confidence in the facility’s ability to withstand a heavy load drop. TEPCO should demonstrate that the lift cranes etc or the type with non-single failure-proof systems to reduce the chance of crane failure from a single fault

Transfer cask lid

TEPCO has not revealed in detail when the transfer cask lid will be removed. Typically it would be in the spent fuel pool which, if dropped it could impact a fuel rack and damage the tops of fuel assemblies. Consequences could be impact damage to the fuel lifting bail and compressive forces against the fuel assembly which would result in some bending of fuel rods within the upper region of the assembly. Fuel rods have a spring at the top of the pellet stack which would help to absorb the impact loading from a dropped cask lid. It is uncertain if any fuel pellet cladding failure would occur but, if it did fission gases would be released into the SFP4 building

Fuel assembly

BWR_Fuelassembly

 

  fallingfuelassembly assemblytop

A BWR fuel assembly dropped from the crane hook during outage and clashed against the rack bottom plate of spent fuel pool. The area monitoring system indicated no radiation release, however, damage at the top of fuel channel was found in the following inspection. As fuel integrity is essential for further management, a finite element model was established to evaluate the damage condition. Several component elements including fuel rods, tie plates, and channel were set up and integrated into a full assembly. The analysis results provided the impact force on the fuel assembly and the dynamic response of each component element. The event did result in the damage of fuel channel yet fuel rods fracture was not expected. A BWR fuel assembly dropped from the crane hook by accident during outage handling. Before the event, it was 38cm in depth inside the rack of spent fuel pool. It then clashed against the rack bottom; the traveling distance was estimated to be 4.19m. Fortunately, there was no indication of radiation release. The event was illustrated in Fig. 1

Poolside examination was performed immediately to evaluate the extent of damage. The initial visual inspection, in Fig. 2 , showed that a triangular piece at the top of fuel channel was missing. We could imagine the violent mechanical interaction between fuel channel and tie plates. As de-channel was not recommended, there was no further information of fuel rods the acceleration, velocity and drag force of the fuel assembly were calculated using the weight of fuel assembly as well as the floating force. The initial acceleration was 8.61m/s2 , decreased with the falling of the fuel assembly. The fuel assembly accelerated at the beginning of the event. However, the acceleration slowed down as the travel distance increased. The velocity of the fuel assembly was at 7.46m/s when it struck against the rack bottom plate

A finite element model was established to simulate the fuel assembly drop event. The analysis results are consistent with the inspection observations. The conclusions are as follows:

(1) The peak impact force on the fuel assembly occurred at the time 5.8ms after the impact, while for fuel cladding it was at the time 7 ms.

(2) The calculated channel stress revealed the damage of the triangular piece, while fuel rod damage was not expected due to the drop event.

(3) Compression spring played an important role in the impact process and the travel of fuel channel was well demonstrated.

FUEL ASSEMBLY INTEGRITY ACCIDENTS

  • Separation of fuel assembly during removal from rack

  • Stuck fuel assembly during removal from rack

  • Loss of fuel assembly integrity

  • Failure of spent fuel rod

  • Damage to fuel pellet

  • Fission gas release into the SFP 4 building

The primary focus on these examples of fuel assembly loss of integrity focuses on the fuel pellet. Each of the examples stated above can impact the condition of the fuel pellets which are inside the sealed fuel rods that are housed in the fuel assembly framework. Worst case accidents concerning fuel pellets is cracking or fracturing the cladding material which surrounds the U oxide. This accident releases the pressurized internal content of fision gases and particulate material directly into the spent fuel pool water and ultimately to the SFP4 building then out to the atmosphere.

At the present time, TEPCO has not informed us on how they have made preparations to monitor for this event happening, tracking the monitoring of activity levels in the air or water, what effect it could have on the external fuel cooling systems and inhabilitiy of the fuel building as well as re-entries into the area. TEPCO has also not shown what types of SEALING the SFP4 building from the outside to contain the fission gas/particulate release.
Failure of the SFP building HVAC

Failure of the HVAC at the SFP4 building would resut in degraded working environment due to heat or cold, it would eliminate the capabilty to remove any air activity that could accumulate during the fuel assembly handling evolotions. In addition, affective monitoring of gases with in the building or removal of air activity would be eliminated. Internal monitoring would be necessary but entrance to the building may be inhibited. TEPCO needs to demonstrate that a single failure will not fail the HVAC system, leaving the building with no back up HVAC.

 

Spent Fuel Pool Penetrations

Requirement: Verify the Adequacy of Spent Fuel Pool Penetrations

Basis: The seismic and structural adequacy of any spent fuel pool (SFP) penetrations whose failure could result in the draining or siphoning of the SFP must be evaluated for the forces and displacements resulting from a beyond-design-basis seismic event in the 0.45-0.5g pga range. Specific examples include SFP gates and gate seals and low elevation SFP penetrations, such as, the fuel transfer chute/tube and possibly piping associated with the SFP cooling system. Failures of any penetrations which could lead to draining or siphoning of the SFP should be considered.

For BWR pools (and PWR pools that are not at least partially embedded), the seismic capacity is likely to be somewhat less and the potential for out-of-plane shear and/or flexural wall or base slab failure, at beyond-design-basis seismic loadings, is possible. A structural assessment of the pool walls and floor slab out-of plane shear and flexural capabilities should be performed and compared to the realistic loads expected to be generated by a seismic event equal to approximately three times the site SSE. This assessment should include dead loads resulting from the masses of the pool water and racks, seismic inertial forces, sloshing effects and any significant impact forces.

Sources:

http://www.kta-gs.de/e/standards/3600/3602e.pdf

http://ocw.mit.edu/courses/nuclear-engineering/22-06-engineering-of-nuclear-systems-fall-2010/lectures-and-readings/MIT22_06F10_lec18.pdf

http://www.tandfonline.com/doi/pdf/10.1080/18811248.2006.9711196

 

 

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