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IRID Part 5; Fuel Debris Research

The current working plan is still to try to plug leaks via the torus and torus room, flood containment and remove fuel from the top of the reactor well, similar to Three Mile Island.

Part of the plan for spent fuel at Fukushima Daiichi would be to reprocess the spent fuel after it has been put into storage (assumed to mean cask storage). After that they would reprocess any of the damaged assemblies. This reprocessing scheme may have some very big hurdles. Japan’s reprocessing plant at Rokkasho is now 19 years behind schedule with no completion date planned. There is also discussion in Japan that it is time for them to end both the nuclear fuel cycle program and the MOX program. The only other option would be to send the fuel to France for reprocessing. The idea of shipping the spent fuel from the Fukushima disaster site to France to make more nuclear fuel and waste byproducts to deal with would probably be hugely unpopular in Japan and abroad.

Preparation work towards fuel removal include the ongoing decontamination work to reduce radiation doses in the reactor buildings. They also plan to establish additional power, water and communications service within the reactor buildings for use in the future work. They hope to have this preparation work completed by 2019.

Measures toward stability of the reactor sites includes confirming the structural integrity of the containment vessels by 2016. Since these are still intended to be filled with water, the stability of the containment vessels is critical. IRID also lists conducting corrosion inhibition measures by 2017 but doesn’t specify what those would be. Our research indicated considerable corrosion is expected in the piping and systems due to salt water injection and lacking ongoing maintenance. By 2019 they expect to be doing research and implementation of criticality controls on the fuel debris.

Work is currently underway to examine the torus structures in each reactor. At the same time laboratory work is underway to find a way to plug portions of the torus to make the containment structure water tight. They hope to begin repair work by 2017. To date they have been unable to do any of the inspection work inside the unit 3 torus room. TEPCO ceased efforts to further investigate the torus room of unit 3 after losing a robot inside. They also refused to release most of the video related to that work.

Planned for 2017 to 2018 is work to determine how to seal off cooling pipes, plumbing penetrations and to deal with the upper hatch of containment. Any effort to continue to pour water into the reactors would have to cease by then. There has been debate if they still need to continue to pour water into the reactors at all. TEPCO did attempt to gradually reduce the water injection rate but stopped when hydrogen levels began to rise in some of the units.

Attempts to detect fuel debris in the suppression chamber (torus) is dated for 2014-2015. Fuel debris detection for the reactor (and likely containment at the same time) is dated for 2014-2016. Internal study of the reactor is slated for 2016. Internal study of the containment structure for 2019. This work to study the containment structure will include obtaining samples of the melted fuel. Fuel debris study into behavior, characteristics and how they may be able to deal with it to be studied in 2019. They still plan to begin fuel debris removal in 2020.

unit1_torus_failuresFor unit 1 they are still considering the failure points between the containment drywell and the torus or torus room to be the sand pocket and the vacuum breaker line.

In the schematic provided by IRID they also confirm that the pipe used to pump out the torus exits the torus tube and routes to the corner room of the torus room level of the basement. This may play a role as they look further to determine how much damaged fuel may have relocated into the torus or elsewhere.

 

 

 

 

unit2_torus_failures For unit 2, IRID reports the containment water level to be below the downcomers and shows the sand pockets to be intact. Yet they assume the lower side of the torus or the torus drain pipe to be broken.

 

 

 

 

 

unit3_torus_diagram

Unit 3′s assumptions are largely based on the detection of a leak in the MSIV room found in 2014. This has caused TEPCO to assume the water level inside unit 3′s containment structure is as high as reported here. TEPCO has done no other containment or torus inspections outside of a brief robot survey around 2012. That inspection caused the robot to become stranded in the torus room. TEPCO never released the entire video or photos for this survey. Only a very short highly edited snippet of video was released from that inspection even though the video is streamed into the robotic control room. The missing video should have been fed in the same manner as the video made available. TEPCO tried to dismiss the lack of video on the robot failure.

It is also questionable if this one partial pipe leak could be the sole cause of the containment structure to not fill even further with water. The leak from this pipe was discovered to be flowing out of the MSIV room via a destroyed steel door and into a floor drain. Some of the floor drains in a reactor building are routed to a controlled waste water system in case radioactive water is released. It is not clear where this floor drain specifically routes to or if any such waste water catch system is still in operation and functional.

fuel_debris

This rather outdated assumption of the reactors condition was included. The illustration provided by TEPCO. The report does mention that assumptions are subject to change as they gather more inspection information. Starting from such a best case scenario assumption is problematic as it could cause research contractors to have to make significant changes along the way. There are however, alternate plans by IRID that seem capable of doing fuel removal under more of a worst case condition, so planning is at least somewhat considering such possibilities.

scoping

 

The rough plan for inspecting the containment and pedestal is to proceed in a series of steps. First to try to inspect the grating floor (seen in green) using the X-100B penetration opening. They plan to investigate the condition of the control rod drive rail as part of this. The CRD rail was attempted to be used for running a scope camera into unit 2′s pedestal. They discovered during that work that the end of the rail was missing.

After the grating floor is successfully inspected they hope to insert a robot into a hole in the grating floor to inspect the concrete floor of containment outside the pedestal. This would give a considerable amount of information. They could learn if the melted fuel exited the pedestal, collect other visual evidence and radiation readings. Unit 2 was found to have very little water in the bottom of containment on previous scope inspections. TEPCO and IRID also assume that there was no failure of containment to the torus room via the sand pockets or broken downcomers. With the water level inside containment below the downcomers it isn’t clear how or where the water poured into containment is escaping. The third phase involves less defined work that would try to identify the condition of the melted fuel. The first work is expected to be done yet in fiscal year 2014 with the last work by 2017. Unit 2 is the first reactor that will have this work done with the assumption they would later try it at the other two units that suffered meltdowns.

shape_robot_containment

 

Unit 2 inspection work using the control rod drive rail was also updated.
A1: This was the scope attempt on the CRD rail in 2013. This was ended without looking into the pedestal after they discovered the end of the CRD rail was missing. There was also evidence of charring or corium around the pedestal opening.
A2: This is the updated inspection using the CRD rail, slated for 2014 (fiscal year).  IRID and contract companies have been working on a way to run a scope camera and bridge the gap in the CRD rail.
A3: Planned for 2015, this effort would again run the scope into the pedestal but would look around the bottom of the pedestal and platforms.
A4: This would look specifically into the bottom of the pedestal and is scheduled for 2016
u2_crd_inspections There was also a brief mention about research work to create instruments to locate and measure the melted fuel. While few details were given, they did mention that the equipment would be expected to work in small areas under conditions of high radiation, fog, dripping water etc.

Efforts to inspect inside the reactor vessels could take place in phases until 2018. It appears the preferred method may be to drill a hole through the reactor well cover, containment cap, reactor cap and any remaining internal equipment to then insert a scope into the vessel. This work was not clearly defined but will require installing shielding before any actual work could be done.

rpv_inspection_ideas

Another tactic IRID is exploring is to possibly try looking into the reactor vessel by way of some of the water or instrument lines. The ones they are considering are:
Water supply lines N4A and N4B
Core spray line N5A
Jet pump instrumentation line N8

Computer modeling of the meltdowns continue. Work using MAAP5 computer models is being done. The newer release of MAAP includes some code improvements:

MAAP code improvements increase the accuracy of debris estimation and plant behavior
Improvement of the MAAP5 code and validation
 Following improved item and its advanced specification-based
(Held at the EPRI commissioned United States Code improvements)
 Reactor core damage propagation model improvements
 multiple account migration pathway of molten fuel 
 Lower plenum debris behavior model improvements
 (Deposits form and structure interaction)
 Containment debris behavior model improvements
(Expansion behavior, concrete interaction)
 Improved code validation
 Validate the model of individual behavior by element tests, etc.
 Plant whole behavior is validated by test, etc.

Another computer modeling program, Sampson, is being used. The release being used listed these features and improvement:

SAMPSON model additions and improvements
(a) model of the phenomenon of temperature stratified storage container pressure suppression pool
-Develop a natural circulation of water 3-dimensional flow model coordinate system (Cartesian and cylindrical)
-Analysis of unit 2 in, validate the analysis features of RCIC
(b) in the runoff pathways model to the lower plenum and lower plenum
Melt and structural materials and cooling materials interaction model
-Path to the corium of the lower plenum and evaluated by analysis, improved model
-Melt and structural materials and cooling materials interaction model todevelop and verify the functionality
(c) high temperature eutectic reaction in terms of oxidation reaction model improvements
-Add the B4C and iron oxide reaction model
-B4C and iron with eutectic reaction model to develop and validate thefunctionality
(d) and melt in the bottom of the pressure vessel
Structure material interaction model
-Interaction model to develop and validate the functionality 

The MAAP example given in the presentation does not specify what reactor unit it is in reference to or if mitigating factors like cooling are included or not in these time events. It could be a hypothetical generic “what if” considering a direct meltdown without intervention. They do give a corium volume and flow amount for containment.

Reactor water  effective fuel, top 11 / 3 / 2011 17:50
Core damage  fuel bar maximum temperature greater than 1200 ° c  2011 /3 / 11 18:40
Meltdown  fuel bar maximum temperature  2200 ° c  2011 / 3 / 11 18:50
RPV corruption 12 / 3 / 2011 10:00
Pedestal floor: 91 ton
Drywell floor: 89 ton

MAAP_corium They also covered some of the muon detector research. There are two methods being considered. One uses a single set of detectors but can only determine fuel sources larger than 1 meter. The angled pair of detector sets can detect fuel amounts as small as 30cm. The single variety could be used as is. The pair method, they consider still in need of development. They have done a successful use of the paired system on the reactor at Tokai.
muon_detectorsThe muon scanning at Tokai used the spent fuel pool for imaging as there was no fuel in the reactor. The identification of fuel shows as blue in the images.

tokai_muon_sfp

Another test used a 1/10th model based on Fukushima Daiichi and tried to image a fuel source. They were able to identify it down to 30cm.
muon_model_testA plan for how the muon system would actually be installed at unit 2 Fukushima Daiichi showed where the units would be located. One would be above ground outside, the other in the upper floors of the turbine building. They gave the rough size of the units as:
Muon detection equipment 2: 7x14m2
Muon detection instrument 1: 14x7m2

u2_muon_plan Fuel debris storage canisters are in development. Some of the factors they cite that make the fuel at Fukushima different from the fuel from Three Mile Island is the location (in containment), a larger amount of melted fuel and higher radioactivity in the buildings. The canisters will have to deal with cooling, criticality control, proper shielding to protect workers and be safe from accidental destruction. A set of safety tests will be developed along with a way to do mock tests on the canisters.

The image below shows an example of neutron criticality within reactor containment for the purposes of the criticality control studies.

neutron_criticality_containment The image below shows locations they consider important for criticality control and detection.
criticalityEfforts to come up with a plan to flood the containment structures include inspections of the buildings and equipment to determine if they can handle the extra load of flooding containment with water. There is also a plan to do some sort of corrosion inhibiting prior to flooding. Two plans are being considered based on reactor condition. One is to plug the downcomer pipes with sealant then flood containment. In the situation where the torus structures are assumed to be water tight both the torus tube and containment would be flooded together. This would likely require some amount of cementing and sealing to assure no future leaks would erupt.

containment_floodingFor fuel debris removal they cite a set of common challenges.

Regardless of the method, fuel debris extraction work of common challenges
In the main ones are as follows.
(1) cutting fuel debris
(2) remote work
(3) pollution control
(4) shielding
(5) criticality prevention

The research work was listed as these.
(1) fuel debris cutting
(1) cutting of ceramic test specimens
(B) test specimen production
(2) remote work
(1) long arm control technology
(2) remote workprototype arm (including within a cell)
(3) pollution control
(1) selection of isolation sheets

fuel_debris_removal_plan

The diagram below shows an alternative plan for fuel removal if containment can’t be flooded. It includes a ventilation system to pull gasses and contaminated air out of the containment structure but in general removes fuel the same way.

alternative_fuel_removal

A third process uses a grappling system to remove fuel in a situation where the containment can’t be flooded

alternative_fuel_removal2

 The forth option for sideways removal of fuel debris that includes an isolation cell around the containment hatch door.
alternative_fuel_removal3

 

 

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