Nuclear Damage Compensation and Decommissioning Facilitation Corporation
The 1st International Forum on
the Decommissioning of theFukushima Daiichi NPS

Fuel debris retrieval strategy for
Fukushima Daiichi Nuclear Power Station
April 11th, 2016
Toshihiko Fukuda
Nuclear Damage Compensation andDecommissioning Facilitation Corporation (NDF)

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About fuel debris
What are "Fuel debris“ ?
“Solidified melted fuel distributed among fuel assemblies, control rods andsome other reactor materials”

Fuel debris of TMI-2

Fuel debris of Chernobyl

Fuel debris of Windscale

Features
Mixed with other materials not contained in the cladding such as nuclear fuel materials.
Risks to be considered with the plant status.
 Criticality, decay heat, containment, radiation, hydrogen explosion, support structure
Difficulties in risk management for the fuel debris
 Uncertainty: Lack of information on in-vessel conditions
 Instability: Fuel melted and facilities damaged by the accident Lack of risk management: Difficulty in accessing due to severe radiologicalenvironment
Institutional requirements and the rules

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Current condition of fuel debris Fuel debris inventory (Bq)
Reduced to one several hundredth from the days of the accident【Reasons】
 Release of volatile radionuclides by meltdown
 Release of soluble radionuclide by cooling water circulation and injection

 Radioactive decay over time Situation estimated by plant parameters
Reactor (fuel debris) in a stable state from the macroscopic observation
 Sub-criticality maintained: Noble gas concentration monitored by PCV gas control
system and sub-criticality confirmed
 Cooling: In-vessel temperature maintained by cooling water circulation and injection(20-40°C)
 Containment• Release of radioactive materials (gas phase): Maintain PCV pressure slight positive,0.003mSv/year at site boundary (Estimated additional dose)• Contaminated water (water phase): Contain water leakage from PCV by thebuildings.(Water level difference between inside and outside the building) Prevention of hydrogen explosion: Inert PCV by nitrogen injection
 Corrosion prevention: Inject corrosion inhibitor while filling the PCV with nitrogen3

無断複製・転載禁止 原子力損害賠償・廃炉等支援機構©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Fuel debris risk reduction strategy
 Two angles for fuel debris risk reduction
 Short & Mid-term risk reduction
Degradation of containment function, abnormal event initiated by instability of internal condition suchas possible criticality and flow out of radionuclide which may arise in a short & medium period of time.

 Long-term risk reduction
Prevent risks to the environment for a long period of time.

 Strategy for short & mid-term risk reduction1)

To understand fuel debris properties(decrease in uncertainty)

2)

To improve in-vessel conditions through fuel debris retrieval work
(elimination of instability)3)

To manage the storage of fuel debris in a stable state(improvement of risk management level)

Our Goal → “Assured, stable, and managed state“
However risk suppression during fuel debris retrieval work important.

"If you know your enemies (fuel debris) and know yourself (feasibility of technique),you will not be put at risk even if you have a hundred battles (fuel debris retrieval).
(Sun Zi's Art of War)4

無断複製・転載禁止 原子力損害賠償・廃炉等支援機構©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Logic tree for fuel debris risk reduction
Source：Strategic Plan 2015

Risk reduction offuel debris

Prompt retrieval of fuel debris withthorough preparations andtechnologies and storage in a stablecondition

Base

(3)
Maintaining and management of
stable condition of fuel debris
retrieval

Criticality
control

CoolingContainment(1)

Stable storage of retrieved fueldebris after collection andtransport

Safe retrieval of fuel debris

CanisterdesignMaintenance/reliability
improvement ofsafety facility(Future)Establishment oftransport system

Establishment ofstorage system

Processing and disposal ofretrieved fuel debris

Establishment ofaccountancy andcontrol system(2)
Understanding of location,amount, properties of fueldebris, FP distribution

Estimation byactual unit
investigation

Estimation Estimation basedon knowledge andbyexperiments
analysis

Safety during fueldebris retrieval work

Structural integrity of
PCV and buildings

Criticality Maintenance Establishment Dose reductionof coolingof containment
during operation
controlfunctionfunction

Fuel debris
retrieval method

Worksafety

Establishment ofDevelopment ofequipment/device for fuel access route to fueldebris
debris retrieval

Establishment ofsystem equipment
and areas5

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Strategy on the in-vessel Investigation（comprehensive evaluation/estimation)
Policy Decision forFD Retrieval

Selection of FDRetrieval Method

Start of FD
Retrieval▽▽▽

Overall Assessment of in-vessel Conditions (Estimation)
Heat Balance, Review from Plant Parameters(FD (Fuel Debris) Distribution）

In-vessel* Visual
Inspection/Measurement(Fuel debris conditions)

Visual Inspection/Measurement by in-PCV inspection＊(Pedestal, RPV Bottom etc.)

Accident Propagation Code Analysis （Location of FD, Amount, Composition, FP Distribution）

Measurement by Muon＊(FD in RPV or not?)Simulated Debris /Historical Insights（FD characterization)*Investigation depends on the Balance between Cost of Development andInvestigation, Time needed and Dose Rate vs. Information obtained

 “How sure is sure enough?” is the function for decision.

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Prioritization in the fuel debris retrieval method assessment
Information needed for fuel debris retrieval method
 Location of the fuel debris (rough estimation)
 Presence of fuel debris with higher risk
 Damage condition of structures
 Rough fuel debris estimation;
core regions, bottom of the vessels(in/outside the vessels), bottom of thePCV (in/outside the pedestal).
 Unsolved fuel (fuel rod stub) at core
periphery
Criticality risk.

(1) Core region(1) a (fuel rod stub(1) b (Molten fuels)(2) Bottom of vessel(2) a (inside RPV)
(2) b (outside RPV)

(3) Bottom of vessel(3) a (inside pedestal)(3) b (outside pedestal) Status of MCCI at the bottom of PCV andRPV pedestal damage(1) a: Fuels remaining in the core region without being melted (fuel rod stub)(1) b: Fuels remaining in the core region that melted with cladding etc.
(2) a：Fuel debris accumulated on the inner side of the bottom of RPV(2) b：Fuel debris accumulated on the outer side of the bottom of RPV(adhering to the CRD housing etc.)(3)(4)a:Inside
Fuel debrisof S/C fallen on the bottom of PCV and accumulated inside the pedestal(3) b: Fuel debris fallen on the bottom of PCV and leaked to the outside of pedestal

Fig. Estimated located where fuel debris are distributed

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Plant inspection status and estimated fuel debris locations
Current estimated plant status

Feedwatersystem

Core
spraysystem

Estimated fueldebris location

Plant investigation
situation

Unit

Source：Strategic Plan 2015

Unit 1Core
spraysystem

Feedwatersystem

Unit 2Core
spraysystem

Feedwatersystem

Unit 3-D/W water level approx. 30 cm from PCVbottom.
-D/W water level approx. 3 m from PCV bottom.
-S/C water level close to the center,-D/W water level approx. 6.5 m from-S/C is mostly filled with water.
almost the same water level to that ofPCV bottom. (estimated from the-Leakage from the sand cushion piping identified
the torus.
pressure differential of D/W and S/C)
-Leakage from the expansion-joint cover of the-No evidence of leakage in the torus upper S/C mostly filled with water.
vacuum break line connected to the W/W venting
section.
-Leakage from the expansion-joint ofpiping confirmed.
-Photo of RPV pedestal inside taken fromthe main steam piping D confirmed.
-Hot dose rate (several Sv/h) spot in the southeast
the opening confirms the structure ofarea of the reactor building 1st floor.
RPV lower region, which shows that thevessel failure will not be so large.
-Almost all molten fuel dropped down to the RPV
lower plenum and no fuel debris remains in thecore.
-Some molten fuel dropped to the RPV lower plenum and on the pedestal floor,-Dropped fuel debris into the lower plenum fallen
and the remaining stays in the core (no fuel debris estimated outside theon the RPV pedestal bottom.
pedestal).
-Dropped fuel debris in the pedestal bottom flew
outside of the pedestal (probable attack to theshell).

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Road to achieve the fuel debris retrieval(3) Start retrieving fuel debris for the initial unit
(1) Determine fueldebris retrieval policyHigh(2) Select method
for initial unit

Fuel debrisRetrieval

Preparatory workEquipment
SystemMock up and training
Performance guarantee

TechnologyReadiness
Level*1(TRL)

For･Critical technology･Important performancerequirements

Element test
Feasibility study/concept design
Conceptual study
Screening

Low

Conceptual
investigation

Prioritization
SafetyResource
Site

Understanding the plant and in-vessel conditions

Prerequisite

Practical application/verification test
Basic/detailed design

Milestone for Mid- and long-term(1) Determine the fuel debris retrieval policy
Around Summer 2017(2) Select the method for initial unit
First half of FY 2018(3) Start retrieving fuel debris for the initial unit
During FY 2021
*1 The vertical axis represents an imageof the Technology Readiness Level ofthe overall retrieval system instead ofindividual technology.

TimeSafety requirements (Regulatory requirement)
Securing accessibility (Decontamination, etc.)9

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Toward the selection of fuel debris retrieval method
 Method of fuel debris retrieval consists of :
 Access route and systems(including decontamination and securing the space) Transportation of equipment
 Visualization and measurement
 Cutting and dust collection
 Packaging and transfer
 Safety functions required when retrieving the fuel debris
 To control Sub-criticality
 To maintain cooling function
 To provide the containment function
 Liquid phase (Prevention of the contaminated water leakage) Gas phase (Dust dispersion prevention)
 To secure structural integrity for PCV/buildings
 Occupational dose reduction (shielding) To ensure industrial safety Five Guiding Principles for debris fuel selection method:"Safe," "Proven," "Efficient," "Timely" and "Field-oriented"

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Water level inside PCV for debris retrieval

Water level inside PCV critical for selecting fuel debris retrieval method
 Advantage of increase of water level inside the PCV up to the upper part of the core
(To flood fuel debris etc by cooling water is an ordinary and assured method. ) Dust dispersion prevention (when cutting fuel debris) Dose reduction by water shielding (activated Co, Cs contamination, fuel debris) Margin for cooling function

 Disadvantage of increase of water level inside the PCV (advantage of not to increasethe level) Large scale and difficult PCV repair work (water leak blockage) required Measures assuming large-scale leakage accident required (relating to thecontaminated water management).
 Sub-criticality during the increase of water level required.
 Verification of structural integrity during the earthquake required for theincrease of the water load.
Water level inside the PCV to be determined by the balance of the advantage anddisadvantage of the increase of water level inside the PCV, their feasibilities andfuel debris distribution.

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Access direction and water level inside the PCVAlternative directions (side/bottom) should be studied considering that the fueldebris may have been dispersed to the bottom of the PCV in addition to normal
access to the core fuels from the operating floor.
Method should be selected focusing on feasibility
Combination of access direction and water levels inside PCV*(Full submersion, Submersion, Partial Submersion, and Dry) methods to bestudied for the feasibility are narrowed down as follows.

Combination of access direction and water levels inside PCV
Entry direction
TopSideSubmersion
Partial submersionDrya.
b.

: Possibility of water flowing
out from the openings: Difficulty of constructing
new access route

Full submersion
Waterlevel

Bottomc.

: Difficulty of evaluating
cooling performance

Selection of the methods based on the above.
Methods to be focuseda. Full submersion/Submersion method – top entry method  “Submersion –Top entry”
b. Partial submersion– top entry method  “Partial submersion – Top entry”
c. Partial submersion – side entry method  “Partial submersion – Side entry”＊Definition of water level insidethe PCV
Full submersion
Water is filled up to thereactor well
Submersion
Fuel debris are submerged
The portion above the lower
part of the core issubmerged (mage)Partial submersion
Partial fuel debris areexposed to the air
Only the bottom of the PCV
is submerged (image)Dry
No water12

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Development of fuel debris retrieval method
 For each step (e.g. rise of PCV water level, removing upper structures, andretrieving fuel debris retrieval) of three methods requires to: Extract systems to secure the safety Investigate the concept of required equipment/device for fuel debris retrieval work
 Select and implement element test to confirm the feasibility of the functions required
for equipment/device above Request for proposal (RFP) with multiple manufacturers in Japan and overseas
to review the on-site applicability Partial submersion method (top/side entry method) Cutting/dust collection and visualization and measurement technologies commonlyused for the methods.
Three methods to be focused onSubmersion–Top entry

Partial submersion–Top entry

Partial submersion–Side entry

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Feasibility study for the overall fuel debris retrieval system
Systems constituting the fuel debris retrieval method(for three major methods) : Radioactive dust processing system Negative pressure management system/HVAC
 Cooling water recirculates andRadioactive
dust processingsystem

Internal gascontrol system(negativepressure ctrl)

injection system (including leakage

water collection/purification)
 Criticality control system(monitoring system/boron injection)

Criticalitycontrolsystem
Clean upwatersystem Contaminated water treatment system Plant data measuring system

Circulating
injectioncooling Communication system Electrical system etc.

Review the required safety technical specification underlined aboveand feasibility and possible placement
Each system will be required for three major methods but its scale/type are subject to change
depending on the method. Feasibility for each method will be confirmed.

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Toward the stable storage of fuel debris
Source：IRID Symposium

Case of TMI-2

Vacuumdryingequipment
Storage canister
handling crane(Shielding unit)

storage canister
handling

Debris loading
Closing lid
Install plague

Canisterlid

Transfer
Canister
Body

Vessel for heating
vacuum drying

Crane(shielding unit)

Storagecanister

Storagecanister

Welding on the
preliminary/secondary lid

vent pipe
Transport outside
the premises(railway/vehicles)

DebrisStoragecanister

Heatingvacuum drying

Transportcask

Fuel canalHorizontal silo
Canister body
Storagecanister

PCV
Loading of fueldebrisCover

FilterRemove storage canister(Partial Submersion, remote)・Wet storage

Loading cask instorage canister

Canisterlid

Sampling
port

Dry storage
TMI-2 Storage canister

Issues toward stable storage Difference from TMI-2
 High burn-up and concentration
 MCCI products are expected to exist.
 Melting with seawater injection, instrumentation cable etc.

 Storage method and place for fuel debris
 Method for safeguard and counting of fuel debris

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R&D promoted for the fuel debris retrieval
Naraha Remote TechnologyDevelopment Center R&D for the practical/on-site application (e.g. IRID)
 R&D leads to the comprehensive evaluation ofin-vessel conditions
 R&D regarding the fuel debris retrieval technology

 Decommissioning R&D Partnership Council(Secretariat of NDF)

Okuma Analysis andResearch Center Integrated management of the development from thebasic and fundamental to the practical technology.
 Clarification and matching of on-site needs and R&D seeds
 Human resource development

 Development of R&D center (JAEA) Naraha Remote Technology Development Center Okuma Analysis and Research Center CLADS*

CLADS

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無断複製・転載禁止 原子力損害賠償・廃炉等支援機構©Nuclear Damage Compensation and Decommissioning Facilitation CorporationConclusion


Risk reduction for fuel debris will be promoted in short and mid-term as below
aiming at the “assured, stable and managed state.”
 To understand the status and characteristics (decrease in uncertainty)
 To improve in-vessel conditions by the fuel debris retrieval(elimination of instability)
 To manage the storage of fuel debris at a stable state(improvement of risk management level)To have comprehensive analysis and assessment of in-vessel situation and to challenge to
upgrade the accuracy of estimation in pursuit of successful fuel debris retrieval

The most appropriate fuel debris retrieval method to be selected based on the conditions ofeach unit, securing the safety functions and in line with the fundamental philosophy("Safe," "Proven," "Efficient," "Timely" and "Field-oriented") .

R&D for the fuel debris retrieval including stable storage of the fuel debris. Integrated
alliance is to be promoted among the related organizations for both the development ofbasic and fundamental technologies and establishment of the centers to support the R&D.

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