Report of the Sub-Committee for theEvaluation of Fuel Debris Retrieval Methods(Outline)

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

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Sub-Committee for the Evaluation of Fuel debris retrieval methods

Table of Contents
1. Background of the study

2. Factors that make fuel debris retrieval
difficult3. Fuel debris retrieval methods
3.1 Partial submersion method
3.2 Submersion method
3.3 Partial submersion method option(Filling and solidification method)
4. Recommendations for the selection ofretrieval methods

5. Future Course of Action
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Sub-Committee for the Evaluation of Fuel debris retrieval methods

11. Background of the study◼ Although the fuel debris at the Fukushima Daiichi Nuclear Power Station isconsidered to be in a stable state based on the investigation of the state inside thereactors and analysis from plant parameters, there is no guarantee that this state willcontinue for a long period of time, and there is a concern that the risk will graduallyincrease resulting from age-related deterioration. Therefore, it is desirable to recoverfuel debris as soon as possible to ensure safety in the medium to long term.
◼ In preparation for the start of large-scale retrieval operations in the 2030s, it isnecessary to continue the work to make the retrieval method more reliable after
narrowing down the direction of the design study and proceeding with more
specific design studies than before, by extracting technical issues and proceeding withelaborate studies.
◼ Fuel debris retrieval is not only an unprecedented technological challenge, but also asocial and policy issue. In narrowing down the direction of the design study in the faceof great uncertainties, the government, TEPCO, and NDF should cooperate toadvance it, and conduct specialized and intensive studies by gathering technicalknowledge from inside and outside Japan.
◼ For this purpose, the “Sub-Committee for the Evaluation of Fuel Debris Retrieval Methods"(hereinafter referred to as the "Sub-Committee") was established under the NDF's
Decommissioning Strategy Committee, to conduct a comprehensive study andevaluation of the retrieval methods based on the basic premise of safety.
◼ The Sub-committee has reviewed and evaluated the followings;✓ Organizing out issues and measures in retrieval methods to address them
✓ Evaluation of each retrieval method (technological, etc.)✓ Recommendations for the selection of retrieval methods✓ Future Course of Action
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Sub-Committee for the Evaluation of Fuel debris retrieval methods

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1. Background of the study＜List of Committee members for
Sub-Committee for the Evaluation of Fuel Debris Retrieval Methods>(Titles omitted/in Japanese alphabetical order)

Chairperson

Toyoshi Fuketa

Previous Chairman, Nuclear Regulation Authority

Tatsuya Itoi

Associate Professor, School of Engineering
The University of Tokyo

Hiroto Uozumi

President & CEO, Atomic Energy Association

Koji Okamoto

Professor, Graduate School of Engineering
The University of Tokyo

Akira Kirishima

Professor, Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University

Takumi Saito

Professor, School of Engineering
The University of Tokyo

Nobuyuki Miura

Executive Director, Japan Atomic Energy Agency

Yoshihito Miyaike

Previous President, Central Nippon Expressway
Company Limited

Akio Yamamoto

Professor, Graduate School of Engineering,
Nagoya University

Hans Wanner

Previous Director General
Swiss Federal Nuclear Safety Inspectorate (ENSI)

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Sub-Committee for the Evaluation of Fuel debris retrieval methods31. Background of the study＜Date and agendas (1/2)＞
Time
１

DateAgenda

March 29 2023 • Procedures to study retrieval methods of fuel debris, etc.
• Policy of securing safety (Partial submersion method)• Overview and issues of Partial submersion method(water injection into RPV), etc.
• The groundwater structure and contaminated watermanagement of Fukushima Daiichi Nuclear Power Station• Overview and issues of Submersion method, etc.
• The necessity of fuel debris retrieval• Overview and issues of optional Partial submersion
method, etc.

２April 18 2023

3May 17 2023

4June 6 2023

5July 19 2023

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• Approach to securing safety ［Containment and shielding
September 22(relating to seismic resistance)］2023• Issues and measures -Seismic resistance(Containment/Shielding), etc.

• Clarifying issues on each retrieval method, etc.

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Sub-Committee for the Evaluation of Fuel debris retrieval methods

41. Background of the study＜Date and agendas (1/2)＞
TimeDateAgenda

7

October 112023• Approach to securing safety [Criticality control]• Issues and measures -Criticality control, etc.

8

November 162023• Issues and measures-High radiation dose/insufficient site information, etc.

9

December 152023• Approach to waste management• Evaluation of retrieval methods, etc.

10January 172024• Approach to securing safety, etc.

11February 212024• Technological feasibility and Business continuity of eachretrieval method• Repot of the Sub-committee for the Evaluation of Fuel
Debris Retrieval Methods（Preliminary draft), etc.

12February 292024• Repot of the Sub-committee for the Evaluation of Fuel
Debris Retrieval Methods（Draft), etc.

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Sub-Committee for the Evaluation of Fuel debris retrieval methods52. Factors that make fuel debris retrieval
difficult
Extremely high radiation① dose inside the PCV andpeople cannot enter.

④The containment vessel must
be opened while controllingthe diffusion of radioactivity

Spent fuelpoolon-site③ Insufficient
information due to ① and ②.

High radiation dose inside
the reactor building make it② impossible to work for long
periods of time

Responding to recriticality⑤ due to changes in the stateof fuel debris

Personnel entrance
Containment vessel(steel)
Reactor building

Fuel debris
Developing a plan for moving
and storing vast amounts of⑥
Estimated fuel debris distribution in Unit 3
contaminated structures andwaste Reproduction and Reproduction prohibited.
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Sub-Committee for the Evaluation of Fuel debris retrieval methods

63. Fuel debris retrieval methods
3.1 Partial submersion method
Top cell
Top accessWorking
platform
Fuel debris

To
additionalbuilding

Water
injection into
reactor coreSide access

Side cell

Examples of remote control equipment【Outline】
A method that is
retrieved with fueldebris exposed in theTo
air or submerged atadditionalbuilding
low water level.

Upperplatform
Lifting
strokeAbout
30m

Penetration
X-6

Approx.φ7.5m

Approx.φ4.2m

Lower
platform

Manipulator

CRD rail

PCV

An example of ROV

An example of long-arm
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Diagram providedby TEPCO

An example of platform-type access equipment

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3. Fuel debris retrieval methods
3.1 Partial submersion method

Strengths

Issues◆Since retrieving will be made in thesituation that is currently being
maintained without major change,
there are few pending issues
associated with the status change.

◆Because of the high radiation doseat the site, a wide variety of remotecontrol devices are requiredcompared to other methods, and it
takes a long time to develop, design,and verify the devices.

◆Fuel debris retrieval work can bestarted earlier than withsubmersion method◆Based on the past experience, ittakes a considerable period of time
to respond to recovery from remote◆Fuel debris processing and recovery
control device failures.
methods can be selectedaccording to the internal conditions, ◆As high-level dose fuel debris andsuch as selecting among multiple
waste are removed from the top,
retrieval methods for top access,which requires the installation of aand a combination of top and side
heavy cell and removal equipment
access.
on the operating floor, the scale ofthe platform for supporting thesecomponents becomes enormous.
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Sub-Committee for the Evaluation of Fuel debris retrieval methods

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3. Fuel debris retrieval methods
3.2 Submersion method【Outline】
A method for enclosing the entire reactor building with a
new structure called hull structure as a containment
barrier, submerging the building and retrieving fuel debris.
Upper unit
Top-access

Retrieval cell

Water level

Hullstructure

Ro ta t i n g ta b l e

Fuel debrisLower unit
A n exa m p l e o f p l a t fo r m t y p e a c c e s s e q u i p m e nt
Diagram provided
by TEPCO
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Sub-Committee for the Evaluation of Fuel debris retrieval methods

93. Fuel debris retrieval methods
3.2 Submersion method
Strengths

Issues◆Water shielding is expected toreduce radiation dose and also
suppress dust dispersion.
◆Strong containment barriers (hullstructure) allow for complete
isolation from the outside◆Rescue at equipment failure can be
manually operated from theoperating floor.

◆It is necessary to verify the on-site
workability in the construction ofhull structure in the underground of
the building (including groundstability at earthquake duringconstruction).
◆ Criticality and water quality control
need to be established for large
volumes of retained water◆ The scale of facility in the hull
structure is enormous.
◆ Longest preparatory construction
period and slowest start of fueldebris retrieval operations

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Sub-Committee for the Evaluation of Fuel debris retrieval methods103. Fuel debris retrieval methods

3.3 Partial submersion method option(Filling and solidification method)【Outline】
A method to reduce the on-site dose while stabilizing fueldebris by filler materials and to recover fuel debris,
structures inside reactor, by excavating through a relatively
small opening provided in the operating floor.
cover

Cell
Drilling
equipment

Rod changer rack
Transfer/Storage method
has not been
considered

Bit changer rack

RecoveryEquipment(Filter)Casing

Filling
Material

Drilling
equipment

GantryStructures needto be drilled firstto install casing

Inside RPV/Pedestal(Partially filled)
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Filler receiving
tank

Drilling watertank
Water supply
pump

Waste liquid
receiving tank

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3. Fuel debris retrieval methods

3.3 Partial submersion method option(Filling and solidification method)

Strengths

Issues◆Once stabilized by solidifying withfiller materials, solid form can behandled uniformly and simply
during retrieval.
◆Dose reduction is expected due toshielding effect by filler materialand access to fuel debris throughsmall openings.

◆Selection of filler materials(flowability, adjustability of curingtime, mechanical properties after
solidification, thermal conductivity,
chemical stability, degradability withradiation, etc.), filling method, andmethod to check filling condition
must be established.

◆Manual operation is also expectedin the rescue in case of equipment
failure.

◆Selection and verification of tip bits,etc. depending on the drilling
targets are necessary.

◆Drilling equipment is remotelyoperated, but the structure issimple, and the tip bit and otherparts can be replaced dependingon drilling targets.

◆The amount of waste generated
increases according to the fillingrange.

◆Smaller facilities may be theearliest to start retrieval.
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◆When collecting waste in the form ofsludge, care must be taken in its
handling.

Sub-Committee for the Evaluation of Fuel debris retrieval methods

124. Recommendations for the selectionof retrieval methods◼ 3 retrieval methods have issues, and it is desirable to consider scenarios that takeadvantage of the strengths of each retrieval method.

MethodEvaluation and direction of each retrieval method
in terms of method selection

Partialsubmersion
method• Although it does not require significant changes on current status to retrieve fuel debris and it canavoid concerns on the changes in current conditions, there are significant performance concerns
such as the feasibility of the construction sequence, remote operability, and overall availability, etc.
Therefore, it is necessary to conduct design studies that take into account the realistic
constraints and conditions at the site.
• Since merging the partial submersion method option with this method may solve some ofthe issues of this method, it is appropriate to explore to establish a complementary
relationship with partial submersion method option in future design studies.

Submersion
method• There are problems such as uncertainty in the on-site workability of hull structure and difficulty inhandling a large amount of water, so it is difficult to select retrieval method as the one to be
started for full-scale construction at present.
• The advantage of handling high-level dose materials in water is great, and considering thepossibility that it will be easier to retrieve from difficult parts in partial submersion method and theefficiency of the work, it cannot be ruled out that it may be shifted from partial submersion
method to submersion method, which can utilize water shielding functions in the future.
• The retrieval method, which can utilize water shielding functions, should be studied inparallel, for example, by investigating the nearby ground including the lower part of the reactor
building.

Partialsubmersion
methodoption• It is too early to judge the feasibility of this method at this point, as insufficient technical study isundeniable.
• When the selection of filler material and the injection/recovery methods of filler material
become clear, since it is expected that this method can develop partial submersion method,a scenario with partial submersion method by applying the features of partial submersion
method option is assumed.

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4. Recommendations for the selectionof retrieval methods◼ Enough understandings of the situation inside the reactor are aprerequisite for its design and for ensuring its safety at anyretrieval method.
◼ Accelerating progress of internal investigations is important in thefuture, however, it is essential to make parallel progresses on theselection of a retrieval method and its engineering.

◼ Start design studies and research and development utilizing partial
submersion method Option functions based on partial submersion
method◼ In parallel with this, internal investigations on a small-scale throughtop-access will be conducted.
◼ A retrieval method utilizing water shielding functions will also bestudied.

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5. Future Course of Action(1) TEPCO's engineering study for large-scale retrieval method✓ TEPCO will start more detailed engineering study based on therecommendation in this report.
✓ It is important to tackle the issues presented in this report such as internal
investigations and research and development, as well as engineering studies.
(2) Development of safety assurance concept✓ The concept of securing safety/judgment criteria and their basis should beclarified at an early stage, then reflected in the basic design and detailed design
based on the views and suggestions from the regulatory side.

(3) Follow-up by the Sub-committee✓ It is recommended to keep this Sub-committee in near future to follow up
TEPCO’s engineering, research and development activities.
(4) Communications with local governments and residents✓ The contents of this report and the implementation status of TEPCO's engineering
study will be fully shared through dialogue with local governments andresidents in relevant regions.

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