Report on "Sub-committee for the Evaluation of
Fuel Debris Retrieval Methods"

March 7,2024
Nuclear Damage Compensation andDecommissioning Facilitation Corporation
Sub-Committee for the Evaluation of Fuel Debris
Retrieval Methods

Table of Contents
1.

Background of the Study ................................................................................................ 1

2.

Overview of Fuel Debris Retrieval Method ..................................................................... 3

3.

2.1.

Factors that make fuel debris retrieval difficult ....................................................... 3

2.2.

Overview of fuel debris retrieval methods under considerations ............................ 4

2.2.1.

Partial submersion method............................................................................ 4

2.2.2.

Submersion method ...................................................................................... 6

2.2.3.

Partial submersion method Option (RPV filling and solidification)................ 8

Evaluation of Retrieval Methods ................................................................................... 10
3.1.

Prerequisites for the Evaluation ............................................................................ 10

3.2.

Basic policy for ensuring safety ............................................................................ 10

3.3.

Evaluation Approach ............................................................................................. 11

3.4.

Evaluation and issues of each retrieval method ................................................... 11

4.

Recommendations for the Selection of Retrieval Methods .......................................... 19

5.

Future Course of Action ................................................................................................ 23
5.1.

TEPCO's engineering study for large-scale retrieval method............................... 23

5.2.

Development of safety assurance concept ........................................................... 23

5.3.

Follow up by the Sub-committee........................................................................... 24

5.4.

Communications with local residents and governments....................................... 24

1.

Background of the Study
For decommissioning Fukushima Daiichi Nuclear Power Station (hereinafter referred to as

the “Fukushima Daiichi NPS”) of Tokyo Electric Power Company Holdings Co, Inc.
(hereinafter referred to as "TEPCO"), a fuel debris retrieval is regarded as a task that should
be tackled with thorough and careful preparation 1 and one of the most technicallychallenging issues, given that certain progresses have been made in fuel retrieval in spent
fuel pools and stagnant water in reactor buildings. In addition, although fuel debris isconsidered in a stable condition based on the internal investigations of the reactors andinterpretations of plant parameters, there is no guarantee that this condition will remain for along time, rather there is a concern that the risk will gradually increase due to degradationsof facilities and equipment etc. Therefore, it is desirable to recover fuel debris as promptly as
possible to ensure safety in medium to long term. Therefore, the fuel debris retrieval is anextremely important task for the success of the decommissioning over medium to long term.
Although TEPCO has been examining concepts for further expansions in scale for the fueldebris retrieval (Hereinafter referred to as "large-scale retrieval"), the evaluations of theretrieval concepts are difficult due to uncertainties in the conditions inside the reactors andthat engineering proof with unusual site conditions is still in progress. This makes it hard toadvance the detailed examinations of the technical problems in a specific retrieval method,which makes the evaluations and selection of the retrieval method further challenging.
Under the current situation we are facing to, it is appropriate to present the direction andpriorities of the efforts to be taken and accelerate the studies on the retrieval methods underthe existence of uncertainties in technology as well as conditions inside reactor so that largescale retrieval can be commenced in 2030’. It is necessary to advance the engineering
process that narrows down the retrieval options to conduct more detailed and specific studies
with identifications of technical issues in order to demonstrate reliable retrieval method.
Flexibility will also be necessary in this process. For example, if there are doubts cast overthe feasibility of the retrieval methods based on new information from internal investigationsetc., the direction of the engineering study will be revisited to narrow down retrieval options
again.
Since the fuel debris retrieval is an unprecedented technical challenge and also social andpolitical issues, in narrowing down the direction of the engineering study under great
uncertainties, it is necessary to carry out studies with the cooperation among the government(the Ministry of Economy, Trade and Industry and Nuclear Regulation Authority etc.), TEPCO

1 Quoted from " "Technical Strategic Plan 2023 for Decommissioning of the Fukushima Daiichi Nuclear

Power Station of Tokyo Electric Power Company Holdings, Inc.", Nuclear Damage Compensation andDecommissioning Facilitation Corporation1

and the Nuclear Damage Compensation and Decommissioning Facilitation Corporation(hereinafter referred to as "NDF"), as well as specialized and intensive examinations thatgather technical expertise from inside and outside Japan. Therefore, under the resolution of
the NDF Decommissioning Strategy Committee, the “Sub-Committee for the Evaluation of
Fuel Debris Retrieval Methods” (hereinafter referred to as the "Sub-Committee") has been
established to provide comprehensive examinations and evaluations mainly on technicalfeasibility and make recommendations for the selection of retrieval methods in Unit-3 forwhich engineering study should be advanced at this stage.
This report, as a deliverable of the Sub-Committee, intends to present an overview of
retrieval methods currently under considerations and summarize evaluations for each
retrieval method and recommendations for a retrieval method based on the evaluations.

22.

Overview of Fuel Debris Retrieval Method

2.1.

Factors that make fuel debris retrieval difficult
Since the Fukushima Daiichi NPS is damaged reactors and in a special environmentthat differs from that of normal reactors, following factors that make fuel debris retrieval
difficult need to be adequately recognized when examining retrieval methods.
(1) Extremely high-level radiation dose in the primary containment vessel (hereinafterreferred to as "PCV") and the Reactor Pressure Vessel (hereinafter referred to as
the "RPV")The dose equivalent rate in the PCV/RPV is in the order of several Sv/h to several
hundreds of Sv/h, which hinders access by workers.
(2) High-level radiation dose inside reactor buildings
The dose equivalent rate inside the reactor buildings is in the order of several
mSv/h to several tens of mSv/h, and the entry by workers is limited to a short time.
(3)

Lack of the on-site information
Because of the constraints mentioned in (1) and (2) above, obtaining on-site

information is difficult hence engineering studies need to be performed with estimatedconditions under great uncertainties.
(4) Construction of containment boundary
When the existing reactor buildings or the PCV is used as containment boundaries,
damages due to the accident and deteriorations must be taken into consideration.
When a new containment barrier is introduced, appropriate containment properties
including seismic resistance that correspond to the on-site conditions need to beconsidered.
(5) Possibility of criticality
It is necessary to consider the possibility of criticality when PCV / RPV conditions,currently maintained under subcriticality, are changed (e.g., changing the distributionof fuel debris).
(6) Amount of waste generated
It is necessary to reduce the amount of solid waste generated by decommissioning
works as much as possible to alleviate overall burdens on storage management on
site.

3

2.2.

Overview of fuel debris retrieval methods under considerations
An overview of each method under consideration is provided below.
2.2.1. Partial submersion method
The partial submersion method is a method of retrieving fuel debris exposed in the
air or immersed at a low water level.
This retrieval method employes a combination of a “top access” in which the reactor
well is opened to access the PCV in vertical direction after the containment equipment
is installed on the operating floor and a “side access” in which the PCV is accessed in
the lateral direction after the containment equipment is installed on the side surface of
the PCV on the ground floor. Since a large opening and direct access can be made in
the top access, majority of fuel debris inside the RPV and the pedestal is assumed tobe retrieved by the top access. The side access will be utilized for the fuel debris outside
the pedestal where the accessibility from top is limited. The retrieval operations will be
conducted while pouring water into processing areas as necessary for suppression ofdust scattering. The side access can be made ahead of the top access to implement
safety measures such as preventions of criticality due to accumulations of cutting
particles during fuel debris retrieval and heavy drop objects from the RPV. Appropriate
containment barriers will be constructed to suppress radioactive dust dispersion andthe spread of contamination during retrieval. The basis of containment is that primarycontainment barrier comprises PCVs and other structures while the secondary
containment barrier comprises reactor buildings and other structures. In addition, a gas
phase containment system is installed at the primary and secondary containment
barriers to achieve active containment through difference pressure control inside andoutside the containment barriers.
In the fuel debris retrieval by the top access, since heavy objects such as a cell withthe containment functionality and equipment for retrieval etc. will be installed on theoperating floor, a large gantry will be newly required to support them with seismic
resistance. The gantry is structured with seismic isolation straddling reactor building ina north-south direction in order to avoid interference with the turbine building. In addition,
the additional building, in which facilities for safety systems (the gas-phase system andliquid-phase system), water treatment, and the pre-transfer treatment of debris/waste
are installed, will be located on the south side of the reactor building.
The retrieved fuel debris is transported to the additional building connected to thereactor building and storage in transfer containers and inspections such as hydrogenconcentration measurements is performed as pre-transfer treatment. The transfer
containers are transferred from the additional building to a pre-storage treatment facility4to analyze, sort, and dry fuel debris as pre-storage treatment. They are then stored incontainers and transferred in storage facilities.

Figure 1

Partial submersion method- Overall process flow for retrieval, transfer, and storage

52.2.2. Submersion method
The submersion method involves enclosing the entire reactor building with a new
structure, called a shell structure, and flooding inside the reactor building to retrieve fueldebris. The water shielding enables to reduce radiation dose in working areas. The shellstructure is constructed with a complex support structure made by three layers of thicksteel plates, and is designed as containment barriers that support all the loads of thereactor building and water with sufficient seismic resistance.
Remote operated equipment is lowered under water from inside the cell installed on
the operating floor to dismantle reactor internal components and retrieve fuel debris
inside the RPV / pedestal and outside the pedestal. Fuel debris outside the pedestal isretrieved via a worker access route on the basement floor.
The three-layer structure consists of the inner wall in contact with water and a cell
installed on the operating floor as a primary containment barrier, the shell structureinterior as a secondary containment barrier, and the outer wall as a water shielding
containment barrier. In addition, a gas phase containment system is installed at the
primary and secondary containment barriers to achieve active containment throughdifference pressure control inside and outside the containment barriers.
In the retrieval cell at the top of the shell structure, the top access-related facilities
such as the debris retrieval equipment, equipment for safety systems (the gas-phasesystem, the liquid-phase system), equipment for water treatment and pre-transfer
treatment of debris/waste are installed. Pre-transfer treatment of the retrieved fueldebris is carried out in water until it is stored in a shielded container, and cleaning,draining, lid-closing and inspection of the container are carried out in air. The amountof storage shall be limited in order to guarantee the hydrogen concentration at or below
the flammable limit for radiolysis of moisture contained in the transfer container. The
transfer container is transferred from the shell structure to the pre-storage treatment
facility, and the fuel debris is analyzed, sorted, and dried in the process of pre-storage
treatment. It is then stored in a storage container and stored in the storage facility.

6Figure 2 Retrieval process flow of the Submersion method7

2.2.3. Partial submersion method Option (RPV filling and solidification)
Partial submersion method option (RPV filling and solidification) is a method thatstabilizes inside the pedestal, outside bottom of the pedestal, inside RPV, reactor welletc. by liquid filling materials which will be solidified after certain period of time, andthen retrieves fuel debris and reactor structures by remote operated equipment throughrelatively small opening at an operating floor. The solidified part is recovered by drilling
equipment etc., and unsolidified part is recovered by an appropriate remote operated
jig.
Filling materials are injected into the inner bottom of the pedestal by a side access tostabilize and solidify fuel debris, and then the filling materials are injected into the space
where solidification is deemed appropriate below the reactor well (Lower part of PCV
head, inside RPV, remaining part inside pedestal, etc.) by a top access for stabilization.
As for the retrieval operations, a remote operated jig is inserted from the operating
floor through the opening, and retrieval operations are carried out by drilling fuel debris
and filling materials to the bottom of the pedestal. For the fuel debris retrieval throughunfilled spaces, casing is set to carry out drilling and recovery operations.
The spaces to be filled and solidified are where temporary stabilization is urgently
needed, drilling retrieval is deemed efficient and reduction of radiation dose due toshielding effect against γ-rays by the filling materials is expected etc. The extent of fillingand solidification will vary depending on the results of the internal investigations andthe situations inside reactor that will be confirmed after the commencement of theretrieval operations. There will be various cases including the partial filling scenarios
such as filling the entire space below the reactor well, only a small part of RPV, theentire space inside RPV, the entire pedestal space or the only small part of pedestal,etc. Evaluations will be made as appropriate based on the results of internal
investigations and accessibility for drilling retrieval etc.
For drilling filling materials, two retrieval options can be considered – one is to retrieve
as a solid such as core boring or crushing and the other is to circulate drilled cuttings
with water and retrieve by filter container.
Since a cell with the containment functionality, equipment for retrieval etc. and a cellcover will be installed on the operating floor, a gantry will be newly required to support
their loads.
A double-layered containment barrier is constructed for suppression of radioactive
dust dispersion and spread of contaminations during retrieval operations. As with thePartial submersion method, the primary containment barrier consists of the PCV, and8the secondary containment barrier consists of the reactor building. However, comparedto the Partial submersion method, suppression of radioactive dust dispersion andspread of contaminations will be enhanced since fuel debris is covered by fillingmaterials. In addition, a gas-phase containment system is installed in the
primary/secondary containment barriers, and dynamic containment is implemented by
the pressure difference inside and outside the containment barriers.
Regarding transfer and storage of fuel debris stored in the recovery container, a study
should be conducted referring to the concept of the Partial submersion method and theSubmersion method.

Figure 3 Overview of Partial submersion method Option

9

3.

Evaluation of Retrieval Methods

3.1.

Prerequisites for the Evaluation
In carrying out the evaluation of the retrieval methods, the Sub-committee has set out

the following conditions.
・Unit 3, which is expected to be the first reactor for a large-scale retrieval, will be thescope of this evaluation. For Units 1 and 2, the retrieval concepts will be examinedand selected based on individual conditions with reference to the results of this study.
・Retrieval method will be evaluated based on the assumptions of fuel debris
distribution
・The scope of this study is up to retrieval and transferring of fuel debris, excluding
storage of retrieved materials. Dismantling of reactor buildings and the process anddisposal of waste will be examined after scrutinizing the status and properties of theretrieved waste materials.
・For the public exposure dose level at the site boundary, current regulatory system isreferred as the initial setting for this study.
Regarding fuel debris distributions, it is possible that new issues that are different
from the above prerequisites are identified from the information obtained through futureinternal investigations. In such a case, it is possible to change the retrieval scenarios oreven to revisit the selection of retrieval method itself in the middle of the engineering
study.
3.2.

Basic policy for ensuring safety
The special characteristics of the Fukushima Daiichi NPS include: a large amount ofradioactive materials are in a various state in various forms (atypical); there is a large
uncertainty in on-site information on the status of radioactive materials and containment
barriers; Prompt actions are required because the current radiation dose levels are highand there are concerns about further deterioration of the containment barriers. On theother hand, the intrinsic energy that causes the release of radioactive materials is low,
so the progress of an event is generally considered to be slow. In considering retrieval
methods, the priority should be given to the safety with the full recognition of these
special characteristics. Based on the above, we have established the following basic
policy for ensuring safety.
< Basic policy for ensuring safety >・Bring on-site conditions more stable state as promptly as possible and reduce theresidual risks as low as possible.

10・Ensure to meet safety standards for radiation effects during decommissioning and try
to reduce radiation effects as low as reasonably practicable (Special focus should begiven to ensuring safety of workers as unusual radiation work conditions are
expected)・Take all possible measures to prevent negative radioactive effects as well as any
psychological or social negative impact on local residents and governments.
3.3.

Evaluation Approach
The evaluation items shown in Table 1 were set out from 5 basic concepts (Safe,

Proven, Efficient, Timely, Field-oriented).
Table 1. Criteria used in this evaluation
Items
Safe

ProvenEvaluation criteria based on 5 basic concepts
Compliance with approach and criteria for ensuring safety such as
containment, shielding, criticality, and seismic resistance
Reliability (technological maturity)
Flexibility against uncertainty

Efficient

Efficiency

TimelyTime consciousness (Early start of fuel debris retrieval and time period
for implementations), this criterion is evaluated based on time requiredfor construction, operations and maintenance

Fieldoriented

Easiness of installations, operations and manipulations for equipment
Maintainability

In addition, resources to be required, time (construction, operation and maintenance),
flexibility of changing to another retrieval method and possibility of applications to Units
1 and 2.
3.4.

Evaluation and issues of each retrieval method
Three retrieval methods presented in Section 2.2 were evaluated based on theevaluation criteria above. Although the study period for the Submersion method isrelatively short and the study period for the Partial submersion method Option is evenshorter compared to the Partial submersion method which has been studied for a longtime, the evaluations were carried out based on the information obtained at present andcountermeasures to the issues that are assumed reasonable.

11Each retrieval method was evaluated by analyzing the series of work processes frompreparations, constructions, fuel debris retrievals, transfer/storage to maintenance, withidentifications of the issues and countermeasures. As a result of the evaluations, it was
concluded that there was no retrieval method that is deemed technically unfeasible atthis stage, although all methods still have issues to be addressed. The main issues areshown in Table 2. Table 3-1 and Table 3-2 shows the results of the comparativeevaluations of each method for resources, time (construction, operation andmaintenance), flexibility to change to other methods, and the possibility of deployment
to Units 1 and 2 are also considered.

12

Table 2 Main issues
Criteria

Partial submersion method

Submersion method

・Protection of workers
・ Protection of workers
・ Feasibility of the liquid-phase system in aCompliance・Feasibility of seismic isolation structures
boric acid solution environment
with approachfor the gantry
・ Measures to maintain subcriticality underand criteria for・ Countermeasures to deterioration of
normal water conditions
ensuring
existing structures
・ Feasibility of the emergency boric acidsafety・ Sorting of fuel debris and waste
injection system
・ Sorting of fuel debris and waste

13

Reliability(Technology
Readiness
Level)

・Troubleshooting and recovery
・ Further studies required on equipment
and operations for criticality control
・ Adequate demonstrations of remote
operated devices

Partial submersion Option

・Protection of works
・ Seismic resistance of the equipment
installed on reactor buildings
・ Sorting and storage scenario of fueldebris and waste

・Troubleshooting and recovery
・Troubleshooting and recovery
・ Further study required on equipment to・ Further studies required on equipment andbe newly installed
operations for criticality control
・ Further studies required on equipment
・Verifications on operations for remoteand operations for criticality control
operated equipment under water・ Method for filling evaluations
・ Examinations on water circulation andrecovery systems
・Internal investigation is required inadvance・ Examinations on countermeasures todecay heat

Flexibility touncertainties

・ Internal investigation is required inadvance・ Internal investigation is required inadvance

Efficiency

・ Selection of fuel debris processing andrecovery methods
・ Reduction of loads on upper structures(weight of shielding)・ Optimization of new gantry structure
design・ Study on retrieval methods other than・ Ensuring visibility
drilling
・ Examination of retrieval operations outside
・ Method of treating the water used forthe pedestal
drilling and recovery
・ Optimization of new structure design
・ Amount of fuel debris and high-level・ Amount of waste generated during thewaste generated
preparation work
・Process of sludge waste due to drilling

・ Properties of fuel debris are unknown
・ On-site workability of shell structures
・ Properties of fuel debris are unknown(including ground stability in case・ Measures to prevent water level reversal
earthquake occurs during construction,
during ground improvement
measures to prevent water level reversal,
Workability
・Installation procedure for interface
and constructions with contaminations)and operability
design between new structures and the ・ Handling leakage of water from the shell
operating floor
structure
・ Emergency recovery method when・ Transfer site of the stabilization equipment
installing access facility
other than the turbine building, method ofrerouting, and operation (including during
construction period)

Maintenability
14・ Maintenance and troubleshooting ofinterface new facilities and existing
facilities
・ Troubleshooting and recovery methodby full remote operations

・ Detection of water leakage between
containment barriers and subsequent
restoration measures

・ Properties of fuel debris are unknown
・ Method of installing a filling hose inside
the pedestal
・ Examination of filling materials and filling
methods
・ Interface design between new structures
and the operating floor

・ Prevention of drilling water leakage intoor out of a cell

Table 3-1 Other evaluations (except for flexibility for changeover to other retrieval methods)
Criteria

Partial submersion method

Submersion method

Filling and solidification method

15

Resources

・ Preparation works: Removal of・ Preparation works: Removal of・ Preparation works: Removal ofinterfering buildings is required
interfering buildings is required
interfering buildings is required
・ Construction works: Installation of a・ Construction works: Installation of shell ・ Construction works: Installation of aworking platform / extension building is
structures is required
working platform is required
required・ Waste: Removal of interfering buildings ・ Waste: Removal, etc. of interfering・ Waste: Removal of interfering buildings
is requiredbuildings is required
is required

Work period(Constructions,operations andmaintenance)

・ Preparation works: Medium-term
・ Construction works: Medium-term
・ When retrieving: Assumed to be thesame level for all methods
・ Recovery in case of remote control
device failure: Long-term

Changeover toother method

Described in the following table (A summary of the contents and degree of impact to change one retrieval method to another ineach operational status)

Application to
Unit 1 and 2

Application to single unit: Possible
Simultaneous application for unit 1 / 2:
Possible・ Preparation works: Long-term
・ Construction works: Long-term
・ When retrieving: Assumed to be thesame level for all methods
・ Recovery in case of remote control
device failure: Short-term(Manual works inside a cell is possible)

Application to single unit: Possible
Simultaneous application for unit 1 / 2:
To be examined

・ Preparation works: Medium-term
・ Construction works: Short-term
・ When retrieving: Assumed to be thesame level for all methods
・ Recovery in case of remote control
device failure: Short-term(Manual works inside a cell is possible)

Application to single unit: Possible
Simultaneous application for unit 1 / 2:
PossibleTable 3-2. Other evaluations (Flexibility for changeover to another retrieval method)
Original
retrievalmethodPartialsubmersion
methodSubmersion
methodStatus before thechangeSubmersion method
Details
ImpactPartial submersion method option
Details
Impact-Changeable

Small

Changeable

Small

-

New equipment needs
to be removed

Large

Changeable

Small

Small

-

-

Changeable

Small

Same facility can be used for
top access; if side access isrequired, interference facility
needs to be removed.
After draining, same as above
If filling and solidification ispossible underwater, it may beleft submerged.

Middle--Middle

-

-

Small

Changeable

Small

-

-

MiddleNew equipment needs
to be removed

Middle--Removing
Changeableinterference buildings

Small

Changeable

Small

-

-

After the start of new
construction

Small

New equipment needs
to be removed

Small

-

-

16

Partialsubmersion
methodoption (nogantry)Partial submersion method
Details
ImpactRemoving
interference buildings
After the start of new
construction
Removing
Changeableinterference buildings
Same facility can be used for
After the start of new top access; if side access isconstruction
required, interference facility
needs to be removed.
After submersion

Partialsubmersion
methodoptionNew retrieval method after change

After draining water, same as
above

Removing
Changeableinterference buildings
After the start of new
construction

Additions and modifications to
the platform should beconsidered in designing

New equipment needs to be
removedMiddleMiddle(1) Evaluation of the Partial submersion method< Advantages >・Since the retrieval operations will be conducted without significant changes on thecurrent conditions, it is possible to mitigate concerns on changing the status of thereactor.
・Fuel debris retrieval works can be commenced earlier than the Submersion
method.
・Suitable processing and retrieval method can be selected from multiple options of
top access as well as combinations of top access and side access depending on
the status of the reactor.
< Issues to be addressed >・A wider variety of remote operated equipment are required compared to othermethods due to high-level radiation dose environment, which would require a long
period for design, development and demonstration. Furthermore, there is apossibility that design changes are required after installations at the site. In
addition, significant amount of time would be needed for rescue of remote operated
equipment in case of failure, recovery methods under a high-level radiation dose
environment and/or prevention measures against an increase in radiation levels
inside a cell will need to be considered.
・In order to retrieve fuel debris and waste that have high radiation dose from the top
opening, it is necessary to install a heavy cell and retrieval equipment on theoperating floor, therefore, the gantry structure to support them with sufficientseismic resistance becomes large in scale.
(2) Evaluation of the Submersion method< Advantages >・Radiation level at operating floor can be reduced by water shield against strong
gamma rays from inside the reactor, which makes remote operations from top
easier and margin for worker exposure will be extended. Prevention of dust
scattering can also be expected.
・A rigid containment barrier (the shell structure) allows complete isolation from theoutside.
・Manual recovery operations of remote operated equipment from the operating floor
is possible in case of failure17< Issues to be addressed >・Verifications of workability at the site in the basement of reactor building (e.g.,ground stability in case earthquake occurs during constructions, countermeasures
to inflow of groundwater and quality assurance on large-scale constructions of theshell structure) and secure transfer locations for the stabilization facilities outside
the turbine building.
・Careful criticality management as well as management of a large amount ofretained water (water quality control, leakage prevention, corrosion control, etc.),
・The shell structure is extremely large in scale.
・The longest preparation period is required therefore the commencement of the fueldebris retrieval work is the latest.
(3) Evaluation of Partial submersion method Option< Advantages >・The fuel debris on bottom of RPV and pedestal can be stabilized by solidifications
with filling materials. Also handling of fuel debris can be standardized and simplified
by solidification.
・Reduction of radiation level can be expected due to shielding effect by a fillingmaterials and the relatively small opening to access the reactor, etc. As a result, it ispossible to simplify the cell to be installed in the operating floor, the cell cover, andthe gantry structure to support them. In addition, manual recovery operations in theoperating floor would be possible in case of equipment failure.
・The drilling equipment is simple and remotely operated with vertical movement in
one axis. In addition, it is flexible as a bit or jig can be changed depending on theobject to be drilled.
・Compared to the Partial submersion method and the Submersion method,equipment is smaller in scale, and it is likely that the fuel debris retrieval work can
be commenced at the earliest timing.
< Issues to be addressed >・The selection of the filling materials (Fluidity, adjustment of curing time, mechanicalproperties after solidification, thermal conductivity, chemical stability, degradation byradiation, etc.), and the establishment of the filling method and the filling evaluation
method are required.
・It is necessary to select and verify the bit for the object to be drilled.
・The larger the extent of filling and solidification becomes, the more amount of waste
will be generated. Special considerations will be required for handling sludge.

18

4.

Recommendations for the Selection of Retrieval Methods
From the evaluations of retrieval methods in Section 3.4, it was confirmed that all

retrieval methods have advantages and issues to be solved, and that the technicalelements in each retrieval method are accompanied by uncertainties (in terms ofengineering, fundamental data, demonstration, etc.). It is difficult to make a clear
judgment on three retrieval methods unless certain engineering study and research anddevelopment (Trial design and manufacturing, and demonstration of useful technologyetc.) are carried out.
On the other hand, given that delays on commencement of fuel debris retrievals could
increase the overall risks in the decommissioning project, it is deemed adequate topresent a retrieval method for which the focused engineering study should becommenced, based on the current evaluations of technical characteristics for each
retrieval method. It is desirable not to stick to one retrieval method, but to consider the
scenarios in which the strengths of each retrieval method are combined to complement
the weaknesses (Hereinafter referred to as “retrieval scenario").
Internal investigation, sampling and analysis are essential to promote more realistic
engineering studies. Information such as damages on existing structures, the amount anddistributions of fuel debris (including existence of stump-shaped damaged fuels in theRPV) and radiation maps inside a reactor will be provided by internal investigations. That
information can be reflected in the engineering studies for the establishment of accessroutes to fuel debris, retrieval methods and equipment for them. In addition, properties offuel debris can be obtained through sampling and analysis, and can be utilized as inputs
for engineering studies of fuel debris processing, criticality management, packaging,transfer and storage, waste management, and inventory management, etc.
The direction of retrieval methods and scenario that should be focused on in the futurewere examined according to the following concepts.
< Basic Concept of Retrieval Method Selection >A retrieval method or scenario should:① Be simple without an excessive scale of facilities and equipment at the initial stage andcan be modified to reflect information and experience that will be obtained.
② Be flexible to change the direction of retrieval method or scenario as necessary basedon internal investigations.
③ Put emphasis on efficiency of the entire decommissioning project to avoid the situationin which continuity and feasibility of the project might become unstable due to19

protracting fuel retrieval period.
④ Provide better confidence on engineering, research and development to address
important technical issues that are key to technical feasibility.
⑤ Have simple remote operated equipment which is easy for maintenance and allow both
remote operated operations and manual works in a complementary way.
⑥ Be able to minimize the radiation exposure risk to the public, as well as easily
implement the measures to reduce worker exposures.
⑦ Be able to present an approach of ensuring safety with confidence and not require
excessive safety margins.
Based on <Basic Concept of Retrieval Method Selection> above, we consider asfollows:
The Partial submersion method does not require significant changes on current status
for fuel debris retrieval, hence it can avoid concerns on the changes in current conditions.
In addition, suitable processing and retrieval methods can be selected such as
combinations of a top access and a side access depending on the status of the reactor.
On the other hand, however, due to large system in scale and complete dependence on
remote operated equipment, there are great concerns over conflicts with yard planning,
feasibility of the retrieval sequences and overall performances of remote operated
equipment such as operability and operating rate. To address these concerns, it isnecessary to conduct engineering study for the Partial submersion method with realistic
conditions and constrains. Furthermore, there is a chance to resolve some of the issues
in the Partial submersion method by partially introducing concepts of the Partial
submersion method Option. For example, it is expected that applying some of conceptsof the Partial submersion method Option would reduce the radiation dose at an operating
floor and improve some of issues identified in the Partial submersion method to someextent. For engineering study going forward, it is appropriate to seek options to combine
the Patial submersion method and the Partial submersion method Option incomplimentary way.
As for the Submersion method, although this method has a potential to solve various
issues associated with high radiation dose at the site due to the effect of water shielding,there are issues regarding to: large scale of equipment and construction works, thelongest time required to commence retrieval operations, its irreversibility as a project,uncertainty in the site workability of constructing shell structures, and the difficulty ofhandling large amounts of water. Therefore, it is deemed difficult to select the Submersion

20

method for a full-scale retrieval option at this stage. On the other hand, however, there isgreat advantages of handling materials with high-radiation dose under water which may
make fuel debris retrieval operations easier in the areas where it is difficult to retrieve by
the Partial submersion method, and considering the work efficiency, a future transition
from the Partial submersion method to a retrieval method that can utilize water shielding
functionality is deniable. Therefore, an option that can utilize water shielding functionality
should continue to be examined in parallel with the Partial submersion method including
the ground investigations around and below reactor buildings.

As for the Partial submersion method Option, entire facility is smaller in scale and theequipment is simple, and the effect of filling and solidification can be expected totemporarily stabilize the fuel debris. However, it cannot be denied that technicalexaminations have not been sufficiently conducted yet, e.g., the selection of the fillingmaterials to meet requirements has not been completed, the technical examinations havenot been matured yet on how to fill the spaces in reactor, and conceptual designs
regarding to drilling the solidified areas and handling of the recovered materials have notbeen sufficiently explored. In addition, the overall operation sequence and retrieval works
need to be more specified, hence it is considered too early at this stage to judge thefeasibility of this retrieval method alone. If a certain level of confidence can be obtainedon the selection of filling materials and the recovery method through engineering study
and research and development to demonstrate the feasibility of this retrieval method, thiswould enable to improve Partial submersion method further by applying the functionalityof filling and solidifications. The retrieval scenario that combines Partial submersion
method and Partial submersion method Option should be investigated going forward.
For the evaluation of three retrieval methods, special focuses were given to criticality
control, seismic resistance and waste management in terms of realistic safety measuresin response to the magnitude of risks. For criticality, the premises of the evaluations isthat fuel debris retrieval will be conducted in subcritical conditions by motoring anddetection of status that are close to criticality. For seismic resistance, given that inherent
energy is low and too conservative assumptions would increase the overall risks byprotracting the period of fuel debris retrieval, it is necessary to conduct optimum seismic
resistant design in line with site specific conditions from the view point of the impacts tothe site boundary and workers if containment is lost. For waste management, the amountof waste would not be a decisive factor for comparative evaluations although it will be
aimed to minimize the amount of waste generated as much as possible. On the other21

hand, however, it should be noted that characteristics of wastes (such as sludge) would
impact on feasibility and technical issues of each retrieval method.
In addition, for any retrieval method, an enough understanding of situations inside PCV/ RPV is a prerequisite for engineering and safety assurance. The internal investigations
that have been carried out so far have provided some information on a part of bottom of
PCV, but little information is available on the top of the PCV and inside the RPV.
Accelerating progress of internal investigations are quite important for the selection of theretrieval method, however, considering the targeted time frame, it is deemed inevitable to
make parallel progresses on the engineering study for fuel debris retrieval and internal
investigations.
An "access to the inside of the RPV through a small opening on the well shield plug" in
the Partial submersion method Option is similar to the operational concept of internal
investigations of the RPV by top access. A series of tasks in a top access is composed of:establishment of an access route to the inside of the RPV with partial filling andsolidifications as necessary through a small opening that allows manual works, andinternal investigations with small camera and fuel debris sampling by core boring. Thesetasks are expected to enable smooth transition to fuel debris retrievals while expanding
the extent of filling and solidifications and core boring recovery.
Based on all discussions made so far, it is considered appropriate to:Commence engineering study, research and development combining the Partial
submersion method and the Partial submersion method Option.



In parallel, conduct internal investigations by a top access through a small opening.

Continue study for a retrieval option utilizing water shielding functionality too, inparallel with the tasks above.

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5.

Future Course of Action

5.1.

TEPCO's engineering study for large-scale retrieval method
As mentioned above, it is necessary for TEPCO to start more detailed engineering

study for a large-scale retrieval based on the recommendations in this report. In
proceeding the engineering study, the first action will be an establishment of theorganization gathering expertise inside and outside TEPCO and it is also necessary toconsider the parties to be involved in constructions of facilities and operations for fueldebris retrievals in the future.
In making the recommendations of this report, we narrowed down the direction of theengineering study and identified the issues to be addressed under a great uncertainty.
TEPCO is required to tackle the issues presented in this report such as internal
investigations and research and development, as well as proceed more detailed
engineering studies. In addition, the direction of the engineering studies presented inthis report needs to be continuously reviewed at every opportunity that can reduce
uncertainties, e.g., internal investigations, research and development, and analysis offuel debris samples. In case there are any doubts raised based on new information frominternal investigations etc., it needs to be considered to revisit the direction ofengineering and re-evaluate fuel debris retrieval method.
In addition, it is also recommended not to stick to the original plan due to concerns
such as accountability for resources already invested. If new information is obtained
through internal investigations etc. that creates need for reconsiderations of a retrieval
method, the retrieval method should be reviewed without hesitation of changing theplan.
Furthermore, the studies need to be parallelly conducted to identify incidents that canbe assumed in fuel retrieval operations, assess the impact of those incidents (safetyassessment) and consider the criteria to resume the works.
5.2.

Development of safety assurance concept
Since the fuel debris retrieval is an unprecedent task to be conducted under unusual

conditions and environments and risk pictures are different from those in normal
reactors, it is necessary to establish safety standards and criteria specific to the
Fukushima Daiichi NPS. In carrying out more detailed engineering studies for largescale retrieval, it is necessary to set out safety standards and criteria by the operator
itself for safety functionality such as containment, shielding and subcriticality
maintenance.

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Safety standards and criteria will greatly affect the scale of equipment and operations
and overall time, and therefore need to be appropriately set out to avoid the risk levelremains high by protracting engineering process and retrieval operations. To avoid
delay on the commencement of fuel debris retrieval, it is recommended to set out safety
requirements and criteria by the operator and present them to Nuclear Regulatory
Authorities at as accurately and as early stage as possible, then reflect reviews andsuggestions of the regulator to basic and detailed design.
Examples of major discussion topics for safety requirements are shown in ① to ③
below.
[Examples of major discussion topics regarding to safety]① Concept for seismic resistant design for the facilities in large-scale retrieval method
Design basis in line with realistic conditions for seismic resistance for specificengineering study by TEPCO.
② Concept for criticality management during fuel debris retrieval
As a part of safety assessments, identification of the scenarios in which criticality
occurs, the assessment of probability of occurrence and impact, prevention measures,monitoring, mitigation measures etc.
③ Concept of waste storage
The storage method with the considerations of hydrogen gas generations andcriticality control
5.3.

Follow up by the Sub-committee
Since this report presents recommendations based on the evaluations with great

uncertainties, TEPCO needs to continuously review the direction of engineering study
presented in this report through the engineering, internal investigations, and researchand development conducted by TEPCO. Prior to acceleration of detailed engineering for
concrete retrieval system and equipment, the assessment on the feasibility needs to bemade within one or two years. Therefore, it is recommended to keep this Sub-committee
in near future to follow up TEPCO’s engineering, research and development activities.
5.4.

Communications with local residents and governments
The recommendations in this report were made based on the evaluations withtechnical and specialized expertise. Indeed, from the viewpoint that the safety is the firstpriority, this activity should be carried out responsibly by experts who own the technicalknowledge and experience. On the other hand, it is important to lead the activities and24

deliverables in this Sub-committee to the sense of security of local governments andresidents.
Therefore, it is necessary to fully explain and share the contents of this report and theimplementation status of TEPCO's engineering study by communications with local
governments and residents.

End of the Report25