Nuclear Safety
NEA/CSNI/R(2022)7
June 2024
www.oecd-nea.org

Preparatory Study on Analysis

of Fuel Debris (PreADES Project)
Summary Report

Nuclear Energy Agency

NEA/CSNI/R(2022)7
Unclassified

English text only
11 June 2024

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PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  3

Foreword
The tsunami that followed the Great East Japan earthquake on 11 March 2011 led to beyond
design-basis accidents at Units 1, 2 and 3 of the Fukushima Daiichi Nuclear Power Plant
even though they had been shut down. Information available from ongoing investigationsof the reactor cores indicates that these units experienced severe accidents involving core
meltdown due to the total or partial loss of core cooling capabilities.
After the accident, the Inter-Ministerial Council for Contaminated Water, Treated Water
and Decommissioning Issues in Japan compiled the “Roadmap towards Restoration fromthe Accident at Fukushima Daiichi Nuclear Power Station” with the goal of ensuring timely
restitution of the site. Based on the roadmap, debris is being retrieved and other measures
are being implemented. Following a proposal from Japan, the Committee on the Safety of
Nuclear Installations (CSNI) initiated the “Benchmark Study of the Accident at the
Fukushima Daiichi Nuclear Power Plant” (BSAF Project), with Phase 1 in 2012 and Phase
2 in 2015. In recognition of the broad international interest in learning from post-accident
examinations and other activities related to this accident, Japan recommended to the CSNI
in 2013 that they develop a process to identify and follow up on opportunities to address
safety research gaps. The CSNI set up the Senior Expert Group (SEG) on Safety Research
Opportunities Post-Fukushima (SAREF). The SEG on SAREF held its first meeting in2013. In the SEG on SAREF, research proposals addressed the highest priorities for the
Fukushima Daiichi Nuclear Power Plant (e.g. debris sampling), even if its feasibility interms of technical details, cost, etc., was not known. Therefore, the proposals were “longterm considerations”, and the details were discussed when sufficient information became
available. Typically, this information related to the conditions inside the reactor building(RB), the primary containment vessel (PCV), the reactor pressure vessel (RPV), and soforth. In some cases, it was difficult to obtain the desired information, both technically andin terms of cost or worker dose. Another consideration was the anticipated timing of the
proposed examinations. Attention turned to “near-term projects” that could start relatively
quickly in a preparatory phase. One such activity was to collect and analyse basic
information and track information on the state of the damage, to maintain information
channels between the CSNI and relevant Japanese organisations, and to monitor thefeasibility of extraction, transportation, examination, etc. of the samples to be taken.
The Preparatory Study on Analysis of Fuel debris (PreADES) project was recommended
by the SEG on SAREF as one of several appropriate “near-term projects” to contribute tofurther international efforts to understand the Fukushima Daiichi accident and support
decommissioning efforts. The PreADES project aims to summarise the knowledge andexpertise related to debris characterisation collected from the different partners of the
project and identify the needs for debris analyses that will contribute to thedecommissioning of the Fukushima Daiichi Nuclear Power Plant. The goal of the projectis to contribute to a better understanding of severe accidents and reactor safety assessments
as well as to create appropriate and optimal methodologies for future debris sampling,
retrieval and storage. Consequently, the project outcomes are important for a futureinternational project of sample examination based on “long-term considerations”.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT4  NEA/CSNI/R(2022)7

Acknowledgements
The NEA thanks the experts of the task group and the technical supporting members who
provided valuable time and knowledge towards this activity. The NEA administrators
responsible for this activity were Yuji Kumagai, Didier Jacquemain, Andrew White andKentaro Funaki.
This document was approved by the CSNI at its 70th session on 8 December 2021
(NEA/SEN/SIN(2021)2/REV, not publicly available).
Operating Agent:•

Japan Atomic Energy Agency (JAEA), Japan

Participating organisations:•Japan Atomic Energy Agency (JAEA), Japan•

Nuclear Regulation Authority (NRA), Japan•

Central Research Institute of Electric Power Industry (CRIEPI), Japan•

Canadian Nuclear Laboratories (CNL), Canada•

European Commission Joint Research Centre (EC-JRC)•

Commissariat à l’Energie Atomique et aux Énergies Alternatives (CEA), France•Institut de Radioprotection et de Sûreté Nucléaire (IRSN), France•Electricité de France (EDF), France•

Korea Atomic Energy Research Institute (KAERI), Korea•

Korea Institute of Nuclear Safety (KINS), Korea•Strålsäkerhetmyndigheten (Swedish Radiation Safety Authority, SSM), Sweden•

Paul Scherrer Institute (PSI), Switzerland•

United States Nuclear Regulatory Commission (NRC), United States•

United States Department of Energy (DOE), United States•

Electric Power Research Institute (EPRI), United States

Technical Supporting Organisation:•

International Research Institute for Nuclear Decommissioning (IRID), Japan•

Nuclear Damage Compensation and Decommissioning Facilitation Corporation(NDF), Japan•Tokyo Electric Power Company Holdings (TEPCO), Japan•

Institute for Safety Problems of Nuclear Power Plant (ISP NPP), Ukraine

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  5•

Kungliga Tekniska Högskolan (KTH), Sweden•

Studsvik Nuclear AB, Sweden•

Argonne National Laboratory (ANL), United States•

Sandia National Laboratories (SNL), United States

Leaders:
Task 1•

Leader: C. Journeau•

Figure of debris’ location (and severe accident phenomenology) Sub-leader:
J. Rempe, M. Barrachin•

Characteristic Table Sub-leader: C. Journeau•

Task 1-2 Sub-leader: A. Nakayoshi

Task 2•

Leader: D. Bottomley•

Task 2-1 Sub-leader: D. Bottomley•

Criticality control Group-leader: Y. Nauchi•

Establishing containment function Group-leader: E. Porcheron•

Maintaining cooling function Group-leader: W. Ma•

Reducing occupational radiation exposure Group-leader: A. Nakayoshi•

Task 2-2 Sub-leader: S. Koyama•

Task 2-3 Sub-leader: A. Morreale

Task 3•

Leader: S. Koyama, A. Morreale

List of Contributors
Co-ordinator:•

A. Nakayoshi (JAEA)

Authors:•

Canadao•

A. Morreale (CNL)

Franceo

C. Journeau, E. Excoffier (CEA)

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT6  NEA/CSNI/R(2022)7

•

•

•

•

o

M. Barrachin, E. Porcheron (IRSN)o

B. Tourniaire (EDF)

Japan
o

D. Bottomley, V. Krasnov, T. Washiya, S. Koyama, A. Nakayoshi(JAEA)o

K. Nakamura, T. Sonoda, Y. Nauchi (CRIEPI)

Korea
o

S. Anh, J. Song, S. Kim (KAERI)oY. Lee, Y. Cho, J. Lee (KINS)

Swedeno

P. Isaksson, N. Garis (SSM)oS. Bechta, W. Ma (KTH)oJ. Chen (Studsvik)

Switzerlando•Ukraineo•T. Lind, S. Nichenko (PSI)

V. Bezmylov (ISP NPP)

United StatesoR. Lee (retired), D. Marksberry, D. Algama (NRC)oD. Peko (DOE)oJ. Rempe (Rempe and Associates, LLC)oR. Yang, M. Nudi (EPRI)oR. Gauntt (retired), N. Andrews, D. Luxat (SNL)oM. Farmer (ANL)

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  7

Table of contents

List of abbreviations and acronyms ..................................................................................................... 8
Executive summary ..............................................................................................................................10
1. Task 1: Joint study on fuel debris expected properties and characterisation .............................15
2. Task 2: Identifying needs and major issues for future debris sampling, retrieval andanalysis ..................................................................................................................................................192.1. Task 2-1: Analytical table for debris analysis needs....................................................................192.2. Task 2-2: Major issues for safe handling and analysis of fuel debris ..........................................27
2.3. Task 2-3: Radioactive material “hot” analysis capabilities .........................................................28
2.4. Task 2 outcomes ..........................................................................................................................31
3. Task 3: Planning of future international R&D framework ..........................................................34
3.1. Criticality safety of fuel debris in test case study ........................................................................35
3.2. Cooling measures in test case study.............................................................................................35
3.3. Storage management in test case study ........................................................................................36
3.4. Ageing change in test case study .................................................................................................36
3.5. Test case study outcomes .............................................................................................................37
3.6. Recommendations on fuel debris characterisation.......................................................................37
3.7. Recommendations on practical aspects and safety issues related to debris retrieval
operations ............................................................................................................................................38
3.8. Recommendations on analysis plan to contribute to debris retrieval operations and toenhance severe accident knowledge ...................................................................................................38
4. Conclusion .........................................................................................................................................40
Annex A. Figures and Tables ..............................................................................................................42
References .............................................................................................................................................54

FiguresFigure A.1. “Unique” analysis items allocated in the Figure of debris’ location

53

TablesTable A.1. Characteristic table: macro TMI (part of table)
Table A.2. Characteristic table: macro ChNPP4 (part of table)
Table A.3. Characteristic table: macro 1F (part of table)
Table A.4. Characteristic table: micro 1F (part of table)
Table A.5. Analytical table
Table A.6. Criticality control in analytical table (part of table)
Table A.7. Establishing containment function in analytical table (part of table)
Table A.8. Reducing occupational radiation exposure in analytical table (part of table)
Table A.9. Maintaining cooling function in analytical table (part of table)
Table A.10. Hot analysis capabilities table (part of table)

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT

42
43
44
46
47
48
49
50
51
52

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List of abbreviations and acronyms
1F
ALARA
ANL
ARC-F
BWR
CC
CEA
ChNPP4CNL
CPF
CRIEPI
CSNI
D&D
DSC
DTA
EC-JRC
EDF
EPRI
FACE
FCI
Keff
FIB
FP
HEPA
ICP-MS
INL
KAERI
KINS
LWR
LEPS
IRID
JAEA
LANL
MCCI
METI
NDF
NEA
NDA
NRA
OES
OM
ORNL
PCV
Phébus FP
PIE

TEPCO’s Fukushima Daiichi Nuclear Power Station
As low as reasonably achievable
Argonne National Laboratory
Analysis of Information from Reactor Buildings and Containment Vessels of Fukushima
Daiichi Nuclear Power Station
Boiling water reactor
Criticality control
Commissariat à l’Energie Atomique et aux énergies alternatives (Alternative Energies andAtomic Energy Commission, France)
Chernobyl Nuclear Power Plant Unit 4
Canadian Nuclear Laboratories
Chemical processing facility
Central Research Institute of Electric Power Industry
Committee on the Safety of Nuclear Installations
Decontamination and decommissioning
Differential scanning calorimetry
Differential thermal analysis
European Commission Joint Research Centre
Electricité de FranceElectric Power Research Institute
Fukushima Daiichi Nuclear Power Station Accident Information Collection and
Evaluation project
Fuel coolant interactions
Effective multiplication factor
Focused ion beam
Fission productHigh efficiency particulate air
Inductively coupled plasma mass spectrometry
Idaho National Laboratory
Korea Atomic Energy Research Institute
Korea Institute of Nuclear Safety
Light water reactor
Low energy proton spectrometer
International Research Institute for Nuclear Decommissioning
Japan Atomic Energy Agency
Los Alamos National Laboratory (United States)
Molten core concrete interaction
Ministry of Economy, Trade and Industry of Japan
Nuclear Damage Compensation and Decommissioning Facilitation Corporation (Japan)
Nuclear Energy Agency
Non-Destructive Assay
Nuclear Regulation Authority (Japan)
Optical emission spectroscopy
Optical microscopy
Oak Ridge National Laboratory (United States)
Primary containment vessel
Major in-pile tests
Post irradiation examination
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  9

PNNL
PreADES
PSI
PWR
RB
ROSAU
RN
RPV
SAFEST
SAREF
SARP
SEG
SEM
SEM-EDS
SIMS
SNL
TCOFF
TEM
TIMS
TMI-2
TG
TRU
US DOE
XPS
X-ray CTWDX
XRD

Pacific Northwest Nuclear Laboratory (United States)
Preparatory Study on Analysis of Fuel Debris
Paul Scherrer Institute (Switzerland)
Pressurised water reactor
Reactor building
Reduction of severe accident uncertainties
Radionuclide
Reactor pressure vessel
Severe Accident Facilities for European Safety Targets
Safety Research Opportunities post-Fukushima
EU-Japan severe accident research programme
Senior expert group
Scanning electron microscopy
Energy dispersive X-ray spectroscopy, γ-spectroscopy
Secondary-ion mass spectrometry
Sandia National Laboratories
Thermodynamic Characterisation of Fuel Debris and Fission Products Based on Scenario
Analysis of Severe Accident Progression at the Fukushima Daiichi Nuclear Power Station
Transmission electron microscopy
Thermal ionisation mass spectrometry
Three Mile Island Unit 2
Thermogravimetry
Transuranic
United States Department of Energy
X-ray photoelectron spectroscopy
Ge-LEPS, γ scanning
Wavelength dispersive X-ray
X-ray diffraction

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT10  NEA/CSNI/R(2022)7

Executive summary
The Great East Japan earthquake occurred on 11 March 2011 at 14:46 (Japan time zone).
At the onset of the earthquake, the three operating units at TEPCO’s Fukushima Daiichi
Nuclear Power Station (hereafter referred to as “1F”) were shut down. However, thesubsequent tsunami led to beyond design-basis accidents in Units 1, 2 and 3. Although afull investigation of the reactor cores has not been completed, available information
indicates these units experienced severe accidents involving core meltdown due to the totalor partial loss of core cooling capabilities.
The Preparatory Study on Analysis of Fuel Debris (PreADES) project was recommended
by the Senior Expert Group (SEG) on Safety Research Opportunities post-Fukushima(SAREF) as one of several appropriate “near-term projects” to contribute to international
efforts to understand the 1F accident and support decommissioning efforts. The PreADES
project aims to:•summarise the knowledge and expertise related to debris characterisation collectedfrom the different partners of the project;•

identify the needs for debris analyses that contribute to the 1F decommissioning;•

improve understanding of severe accidents and reactor safety assessments;•

create appropriate and optimal methodologies for future debris sampling, retrieval
and storage;•

discuss a future international project of sample examination based on “long-term
considerations”.

The objectives of the project were accomplished by completing the following three tasks:
1. Task 1: Joint study on fuel debris expected properties and characterisation
Task 1-1 presented its compilation of severe accident knowledge in two tables with figures:•“Figure of debris’ location (including severe accident phenomenology)” focusedon the end state distribution and configuration of debris in the 1F units andcompiled relevant knowledge from severe accidents (TMI-2: Three Mile Island
Unit 2, ChNPP4: Chernobyl Nuclear Power Plant Unit 4), major in-pile tests(Phébus FP), and other large scale severe accident testing.

•“Characteristic table for debris” summarised knowledge from TMI-2 and ChNPP4debris analysis, 1F related experiments, 1F analytical efforts, and engineering
judgement. The first table summarised macro-properties of debris (e.g. visualobservations and composition). The second table summarised debris microproperties (e.g. fundamental characteristic properties such as melting point, heat
conductivity, mechanical properties) and the corresponding properties were
provided for species or “simple” materials (e.g. UO2, (U, Zr) O2, 316 and 304
stainless steel).

Task 1-2 focused on assessing which properties are important for understanding the 1F
accident and decommissioning work and for collecting information on debris
characteristics.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  11

Task 1 provided recommendations for future examinations that could inform graphicaldepictions of the debris end states in the reactors for 1F Units 1, 2, and 3, and characteristicsof the debris. Similarities and differences in the integrated remediation processes
implemented at TMI-2, ChNPP4, and 1F were assessed with a recognition that remediation
still requires large efforts at ChNPP4 and 1F. Initial efforts at 1F focused on establishingand maintaining a stabilised and controlled state of the damaged plant (e.g. ensuring
damaged reactors’ cooling and limit radioactive release) and implementing provisions
against further failures (e.g. reinforcing weakened structures). Then, efforts focused on siteclean-up. Similar potential hazards (e.g. re-criticality, pyrophoric reactions, radiation
exposure, radiation release) were identified that must be addressed throughout remediation
activities. Prior clean-up efforts at TMI-2 and ChNPP4 illustrated the need to prioritise
future examination requests, emphasising examinations required to minimise radiation
releases or hazards at the site, to ensure safe and efficient decontamination anddecommissioning (D&D), and, as resources allow, provide knowledge related to accident
progression and enhancement of reactor safety (e.g. knowledge to improve severe accident
system codes and accident management measures).
The characteristics of various types of debris expected to be found at 1F (as indicated in
the figure on the debris' location) were reported. Mechanical and thermal properties wereorganised from microscopic/macroscopic points of view. These characteristics areimportant for decommissioning, especially for completing the retrieval and storage ofdebris.
2. Task 2: Identifying needs and major issues for future debris sampling, retrieval, andanalysis
Task 2-1 organised an “Analytical table for debris analysis needs”, which arranged thesample and analysis items by priority, considering cost, availability, timing of thedecommissioning work in practice, and interest for decommissioning and severe accident
analyses. The analytical table produced in this task considered four key requirements forthe decommissioning processes: “Criticality control”, “establishing containment function”,“reducing occupational radiation exposure”, and “maintaining cooling function”. For each
aspect, required sample and analysis item needs were prioritised, considering theircontribution to 1F decommissioning, cost and timing of sampling.
In the criticality control discussion, most analysis items received a high priority score,especially mass ratios for U + Pu isotopes, 157Gd / (U+Pu) and homogeneity. Prior TMI2 core bore sample examinations showed that rare earth FPs (La, Pr, Ce, Nd, and Pm) weremore significantly correlated with Zr than with U. This suggested that these lanthanides
may be more prevalent in the Zr-rich tetragonal (Zr, U) O2 phase rather than the cubic
phase. However, these phases were intimately mixed in debris. Appropriately sized samples
will avoid this problem and should still deliver reasonable estimates for fuel concentrations
and burnups. For establishing containment function, scores indicate that needs related tolimiting airborne contamination during debris retrieval should be prioritised. With respect
to reducing occupational radiation exposure, scores for activities to be completed before
transport and storage (e.g. waiting period for retrieval and retrieval steps) were highest.
Tasks related to the use of videos and tasks that could characterise airborne particles andaerosols were also prioritised. Finally, regarding the maintenance of cooling functions
topic, activities to quantify the heating power by calorimetry and to characterise the debris
geometry, porosity, and fragment size distribution by photography, SEM, etc., weredeemed important for completing retrieval, transport and storage tasks.
Task 2-2 compiled knowledge and information related to practicalities and the limitationof risk for future debris retrieval and even prior trial retrieval (or sampling) operation (e.g.
maintaining safe heat removal and the confinement of radioactivity, limiting risks of rePREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT12  NEA/CSNI/R(2022)7
criticality and of hydrogen combustion and optimising radioprotection). The task involves
three steps: transport, storage, and handling for analysis. For the transport step, theexperience and information obtained from the Severe Accident Facilities for European
Safety Targets (SAFEST), Phébus FP, International Research Institute for Nuclear
Decommissioning (IRID) evaluations, TMI-2, and the Japanese, Korean and European
regulations were collected and utilised. Casks, details of transport container specifications,and safety limits or boundary conditions for material to be transported were summarised(e.g. design and geometry, containment specifications, limits in water content related tohydrogen formation by radiolysis, limits in U/Pu and other actinides content related to
criticality safety, limits in radioactive content in relation to heat removal and radiologicalprotection). For the storage step, TMI-2 experience, evaluation examples from the Okuma
Analysis and Research Center of JAEA, experience from the JAEA Chemical Processing
Facility (CPF), evaluations from the Korea Institute of Nuclear Safety (KINS), andrequirements in Japanese regulations were reviewed. These included, for example,
requirements for hydrogen management, criticality safety, heat removal, radiologicalprotection and safeguards. As regards the handling for analysis step, evaluations by the
Okuma Analysis and Research Center and requirements in Japanese regulations forcriticality safety, radiological protection, and safeguards were summarised.
Task 2-3 investigated the availability and capabilities of possible facilities for theradioactive sample preparation and analyses from 1F (e.g. radioactive material “hot” cells).
This task identified international radioactive material “hot” analysis facilities and theircapabilities for potential use in possible future projects to analyse debris samples from 1F.
The following viewpoints were considered and organised in the collection of the facility
information:•

facilities and techniques for debris analysis;•

experience related to severe accident research, including the advantages andlimitations of techniques;•

technical requirements for sample preparation and analysis;•

feasible analysis and measurements to be performed on samples from 1F.

PreADES project member organisations provided information about “hot” analysis
facilities and their capabilities for 19 commercial and national laboratory facilities in eight
countries including Canada (1), Germany/European Union (1), France (3), Japan (5), Korea(1), Sweden (1), Switzerland (1) and the United States (6).
3. Task 3: Planning of future international R&D framework
Since discussions on implementing an international collaborative research framework forthe actual debris analyses are important and should be conducted, the framework was
expected to be defined and commonly agreed upon prior to starting actual debris analyses,with a shared understanding of the objectives for the proposed debris analysis plans. Thus,
within the PreADES project, in addition to the knowledge collected in Tasks 1 and 2, test
case studies (to define analysis plans focused on specific aspects such as analyses to address
the re-criticality risk during debris retrieval operation in Unit 2) were initiated with the aimof furthering the discussions on the proposed analysis plans.
Below is a summary of the main outcomes of the project and recommendations for futuretasks related to debris characterisation, practical and safety aspects of debris retrieval
operations and analysis plans to contribute debris retrieval operations and enhance severe
accident knowledge.

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Fuel debris characterisation
Significant efforts had been conducted in Task 1 in compiling and sharing knowledge toassess expected debris characteristics at 1F. Though the knowledge compilation is of value
to the preparation of debris retrieval operations at 1F and for enhancing understanding of
the accident’s development, it is, however, fully acknowledged that significant
uncertainties remain related to debris compositions, physical properties, distribution andconfiguration in each of the three damaged reactors. For instance, knowledge gained from
TMI-2 and ChNPP4 accidents and core melt accident testing is only relevant for certain
aspects of the 1F accident. The reactor type (BWR) and core material composition at 1F
are different from those at TMI-2 and ChNPP4. Furthermore, the three 1F reactors havedifferent accident progressions and end states, so various in- and ex-vessel debris
distributions and configurations are estimated, which makes them not directly comparable
to those at TMI-2 and ChNPP4.
Knowledge gained through preliminary analyses of actual samples collected at 1F will be
key for reducing these uncertainties. A preparatory discussion was started in the PreADES
project with the joint task force between the PreADES, the ARC-F (Analysis of Informationfrom Reactor Buildings and Containment Vessels of Fukushima Daiichi Nuclear Power
Station), and the TCOFF (Thermodynamic Characterisation of Fuel Debris and Fission
Products Based on Scenario Analysis of Severe Accident Progression at the Fukushima
Daiichi Nuclear Power Station) projects to evaluate the formation mechanisms of Ubearing particles and other components contained in actual deposit samples obtained at 1F.
In addition to providing those data on actual debris characteristics, the joint task force also
started to investigate the formation mechanisms of the U-bearing particles with theintention to provide additional knowledge on the accident development.
Such co-operative efforts should continue after the PreADES project completion, using
data and information from samples collected during “trial” retrieval operations, as they are
expected to provide highly valuable data and information for implementing safe debris
retrieval operations and for enhancing severe accident analyses.

Practical aspects and safety issues related to debris retrieval operations
Significant efforts had been conducted in Task 2 to compile relevant knowledge andexperience regarding some practical aspects and safety issues related to debris retrieval
operations. The efforts had addressed in particular the risk of re-criticality, the risk of dust
emission and how to appropriately manage cooling of debris, confinement of radioactivity
and radiological protection in debris sampling and retrieval operations. This information isvaluable to develop guidance and recommendations regarding the design and safe
management of debris retrieval operations but also to recommend necessary R&D andanalyses on actual 1F samples to further limit the identified risks. Views on practicalaspects and risks assessment could be refined in the future with knowledge gained throughactual 1F samples analyses.

Plans of analyses to contribute to debris retrieval operations and enhance severe
accident knowledge
The project proposed a first approach to defining an analysis plan intended to adequately
support safe debris retrieval operations and provide important information and data forsevere accident analysis. It is recognised that more work is required to achieve a fully
practical analysis plan that will adequately prioritise the needs and interests for debris
retrieval and severe accident analysis. Analysis plans will certainly be revised with theprogress and practicalities of debris retrieval operations, as was the case at TMI-2.

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In Task 3, a first test case study was conducted for evaluations using results from isotopic
analysis of debris containing U and Pu. The results provide key input for: criticality safety;
cooling measures; storage management; and ageing changes. Required analysis items andevaluation viewpoints were expanded. Other test case studies are expected to be discussed
in the future; where warranted, a specific analysis plan may be developed.
In addition, the development of an international round robin analysis exercise using
samples from past severe accident experiments on representative corium mixtures werediscussed during the project. It was recognised that the activity would have several possible
benefits such as:•

developing and sharing experience in the analysis and transport of debris samples;•

establishing optimised procedures for sample preparation and analysis for future
work on 1F debris sample analysis;•contributing to the enhancement of debris analysis capabilities;•

analysing the variability in analysis results and quantifying uncertainties;•

establishing facility and analytical tool capability for debris characterisation (micro
and macro-analysis, chemical analysis, dose and radiation analysis);•establishing a framework for a future international research implementation.

It is recommended that efforts to develop the international round robin analysis exercise becontinued after the project is completed.

Future plans
Start of trial debris retrieval is currently planned after 2024. The start of future international
projects using actual debris is foreseen approximately in the mid-2020s at the earliest
because a five to ten-year period of preparation is expected until full‐scale debris retrieval
can be initiated. New on-site facilities being built for analysing, characterising andconditioning debris should also be operational at that time. Considering this situation, it is
proposed to establish, soon after the PreADES project completion, a new project for
maintaining international co-operation before the full‐scale debris retrieval starts, toaddress the efforts which should be continued on debris characterisation, on providing
guidance and recommendations for establishing practical analysis plans and on conducting
an international round robin. This is foreseen to be done in the proposed project Fukushima
Daiichi Nuclear Power Station Accident Information Collection and Evaluation (FACE)
project. Continuous information sharing and discussion at the international level should bemaintained to progress in a timely manner towards full-scale debris retrieval operations.
The organised expertise and results in the project should be updated in a timely manner.
That is why it is important to maintain communication and discussion among PreADES
participants, relevant Japanese organisations, and other 1F relevant projects under theCommittee on the Safety of Nuclear Installations (CSNI). Japan also recognises a
responsibility to share analysis results of obtained samples in 1F with international experts.
The activity of the joint task force can be used as a model case for future international
research frameworks using actual debris. Therefore, participants are expected to continue
to share and discuss information on the latest 1F situation and to share expertise with theinternational community.

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1. Task 1: Joint study on fuel debris expected properties and characterisation

The objective of Task 1-1 was to review relevant information and provide
recommendations for future examinations that could inform graphical depictions of thedebris end states in the reactors for 1F Units 1, 2 and 3. To accomplish this, two activities
were completed. Firstly, relevant knowledge from TMI-2 and ChNPP4 was reviewed,
along with results from prototypic tests and hot cell examinations. Secondly, the current
debris end state figures for the damaged reactors at 1F were reviewed along with supporting
information that was used to generate these figures. In completing these tasks, several
findings and insights were obtained that will inform future 1F examinations:•

Knowledge regarding severe accident phenomena has increased considerably since
the accident at TMI-2 (28 March 1979). This knowledge was obtained from postaccident examinations, large integral tests, separate-effects tests, and analyses using
severe accident codes with improved models based on insights gained from priorevents and prototypic testing. However, uncertainties remain when extrapolating
this knowledge to different accident scenarios and reactor types.

•

Similarities and differences in the integrated remediation processes implemented at
TMI-2, ChNPP4 and 1F were assessed with the recognition that remediation stillrequires large efforts at ChNPP4 and 1F. Initial efforts at 1F focused on establishingand maintaining a stabilised and controlled state of the damaged plant (e.g. ensuring
damaged reactor cooling and limiting radioactive release) and implementing
provisions against further failures (e.g. reinforcing weakened structures). Then,efforts focused on site clean-up. Similar potential hazards (e.g. re-criticality,
pyrophoric reactions, radiation exposure, radiation release) have been identified
that must be addressed throughout remediation activities.

•

Examinations provide essential information for completing remediation. Similartypes of examination information have been relied upon at TMI-2, ChNPP4 and
1F, including radiation surveys, visual images, non-destructive methods and,ultimately, destructive examinations to obtain detailed properties. Occasionally,
more advanced technologies have been implemented at 1F (e.g. muon tomography
at 1F versus ultrasonic methods at TMI-2).

•

Debris end state figures, which are periodically updated, are effective for
integrating information from post-accident examinations, plant-specific analyses,
plant instrumentation and severe accident knowledge. This diagram provides avisual reference that identifies the position of the relocated debris material, alongwith its characteristics (e.g. morphology, elemental composition, chemical form)and conditions associated with its position (e.g. such as it being submerged inwater) that may affect worker safety and debris removal.

•

Examination measurements provide crucial information for decontamination anddecommissioning (D&D). Examples include characterising the mass, material
composition and morphology, and conditions associated with relocated material ateach location (in particular regarding the development and qualification of
appropriate debris removal equipment). Examination data also contribute to efforts
that address potential safety issues during clean-up, such as re-criticality,
pyrophoricity and radiation exposure/contamination.

•

Examination efforts also provide essential information that has the potential to
improve nuclear safety from a general point of view. Enhancements such as

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improvements to plant designs, operator guidance and improved models forpredicting severe accident phenomena were obtained that would not have beenpossible without the detailed post-accident examinations and evaluations
completed at TMI-2 and ChNPP4.
•

Prior clean-up efforts at 1F illustrate the need to prioritise future examination
requests, emphasising examinations required to minimise radiation releases or
hazards at the site, to ensure safe and efficient D&D, and, as resources allow,
provide knowledge related to accident progression and enhancement of reactor
safety (e.g. knowledge to improve severe accident system codes and accident
management measures). It is important to document the desired data and accuracy
and prioritise information requests based on the potential benefit for 1F D&D issues
and reactor safety enhancement.

These items have the potential to not only inform future D&D activities at 1F, but to also
provide an important perspective for any organisation having to complete post-accident
clean-up activities.
The objective of the characteristic table for debris in Task 1-1 (Table A.1) was to list thecharacteristics of various types of debris expected to be found at 1F (as indicated in the
figure of the debris location). Data for TMI-2, ChNPP4 and 1F debris were compiled. Table
A.1 and subsequent tables in this summary report only contain “example” portions of the
PreADES tables, considering the extensive amount of data reported in these tables. All of
the full tables are available in the PreADES project final report. Characteristics of debris,such as mechanical and thermal properties, were organised from microscopic/macroscopic
points of view. In most cases, properties were provided at temperatures expected during
debris removal operation (between 20 and 100°C) although local temperatures may bemuch higher if dry grinding or laser cutting is used. Also, the drainage may result inuncovered materials becoming hotter. In some cases, properties are provided for a range oftemperatures and extended to values expected during the accident (primarily for use in
severe accident analyses). Data shortages identified in Task 1-1 activities provide a basis
for analysis needs suggested in Task 2. These characteristics are important fordecommissioning, especially for completing the retrieval and storage of debris. In addition,such data may be used to reduce uncertainties in models for severe accident progression.
Items listed in both the micro and macro sections of the characteristic table for debris (Table
A.1) and their intended use are summarised below:•

Micro tableo Density: for design of debris retrieval tools, size and weight loading in storage.
o Hardness, elastic modulus, fracture toughness: for mechanical retrieval
methods such as grinding.
o Thermal conductivity, specific heat: for coolability evaluations to support
debris retrieval.
o Melting point: for evaluating debris retrieval tool operation, especially thermal
cutting.
o Behaviour at high temperature: for use in 1F severe accident analyses andstorage, etc.
o Stability: for evaluating debris ageing effects.

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Macro tableo Appearance (OM: optical microscopy and SEM: scanning electron
microscopy), composition, concentration, size, density: for design of debris
retrieval tools and storage containers.
o Concentration of fissile, fertile and neutron absorber elements, enrichment: as
reference values for criticality assessment models.
o Porosity/moisture: for assessment of criticality control (CC), hydrogengeneration, pre-treatment for storage (drying rate, remaining water in debris,permeability, etc.).
o G(H2) value: for assessments of radiolysis and hydrogen generation.
o Compressive strength: for evaluating proposed debris retrieval mechanicalmethods, such as grinding.
o Electric conductivity: for electrical ablation/removal methods, such as plasmaarc cutting.

In the outcomes of Task 1, it was found that periodical updating of debris end state figures
was effective for integrating information from post-accident examinations, plant-specificanalyses, plant instrumentation, and severe accident knowledge from the figure of thedebris’ location activity. This diagram provides visual references that identify positions ofrelocated debris material, along with the debris characteristics (e.g. morphology, elemental
composition, chemical form) and conditions (e.g. such as being submerged in water), which
may affect worker safety and debris retrieval. From the information, the characteristics ofdebris expected to exist were organised based on 1F decommissioning perspectives (e.g.
density of debris for design of debris retrieval tools, size and weight loading in storage)
along with TMI-2 and ChNPP4 debris characteristics. The organised characteristics in thetables indicate the range of possible 1F debris characteristics and combine past experienceand knowledge. As an example, densities in the ChNPP4 table are lower compared to the
TMI-2 and micro/macro 1F tables. The difference can be considered for the 1F
decommissioning perspective or for improving tools or systems used in TMI-2 and
ChNPP4. In addition, the density affects the water contents as discussed in Task 2. It may
also be helpful information for the selection of drill tools and CC systems because
boreholes were drilled for instrumentation (e.g. cameras, temperature sensors, γ- andneutron detectors) and a possibility of re-criticality was observed in ChNPP4. As seen in
the above example, understanding such differences could maximise the experience andknowledge gained from past decommissioning for 1F; the organised tables can make itpossible to clearly recognise these differences. Furthermore, analysis results from small
debris samples obtained inside the reactors in 1F may provide insights about bulk debris
characteristics such as hardness and thermal conductivity. For example, if samples
originated from a particular location in which the bulk debris consist mostly of U, Zr, andO, the Vickers hardness of the bulk debris may be expected to be 6-18 GPa from the 1F
tables. The Task 1 outcomes are also utilised in analysis needs in Task 2 discussion. Finally,
as experience and knowledge are gained, updated 1F tables are expected to identify where
there is a lack of information regarding debris characteristics.
Insights and lessons learnt from Task 1, and next steps for further projects•

To improve understanding of severe accidents and reactor safety assessments

There remains considerable uncertainty with respect to the events that occurred and theestimated location of debris at 1F. Safe and effective completion of D&D activities requires
reducing this uncertainty. In addition, insights gained from understanding events at 1F offer
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potential benefits to global nuclear safety. From this point of view, an integrated processof decommissioning, similar to the process used after the TMI-2 and ChNPP4 events,
seems to be effective. Plant instrumentation data, forensics examinations, and analyses
completed with severe accident systems analysis codes are also expected to be beneficial
tools.
•

To summarise the knowledge and expertise related to debris characterisationcollected from the different partners of the project

Debris characterisation in this task is indispensable for understanding the range ofcharacteristics required for D&D, especially debris retrieval, and for building a common
recognition among organisations related to D&D. Specifically, these results are reflectedin the design of debris retrieval tools, the methodology of debris retrieval, and thedevelopment of debris storage technologies.
Trial debris retrieval (possibly small grain size) will begin after 2024, and will advanceJapan’s efforts to the next stage where it is possible to start providing actual debris
characteristics by means of local facilities. At the same time, engineering issues considering
tools or systems will also be addressed to allow for scaled-up retrieval tasks. Therefore, it
is recommended that in future projects updates of the debris characteristics data be pursued
with the latest information on actual debris from various organisations.

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2. Task 2: Identifying needs and major issues for future debris sampling,
retrieval and analysis

2.1. Task 2-1: Analytical table for debris analysis needs
Important analysis items and debris analysis needs in the analytical table in Task 2-1 (Table
A.2) were organised from the viewpoint of four key requirements for the decommissioning
processes: “criticality control (CC)”, “establishing containment function”, “reducing
occupational radiation exposure”, and “maintaining cooling function”. Representative
example tables for each aspect are shown in Tables A.3 through A.6. In addition, importantdebris information needs were prioritised, considering their contribution to thedecommissioning, cost and timing of sampling. This activity provided dialogue
opportunities between PreADES participants and representatives from Japaneseorganisations; of particular importance were discussions regarding the significance of
current sampling work at 1F. The format of the analytical table for debris analysis needs
includes the following items:•

Access time (including priority for analysis)o The expected time when samples will become available is listed with analysis
prioritisation scores. In general, higher priorities were assigned to samples thatare easy to obtain.

•

Location of samples
o The position of samples (and likely access point) is described. The position is
described based on region (position in the primary containment vessel, or PCV)and zone (local/equipment position). For example, the region could be thedrywell outside the pedestal and the zone could be the control rod drive
exchange rail.

•

Potential issues concerning 1F safety in Japano Four high priorities are emphasised: Establishing containment function,
maintaining cooling function, CC, and reducing occupational radiation
exposure. Likewise, the analytical table emphasises topics that are underinvestigation in severe accident research programmes (e.g. EU-Japan severe
accident research program, SARP).

•

What was obtained and howo The type and shape of samples, and the analyses needed to obtain specificsample information are described (e.g. weight, volume, solid/liquid sediments,
particle, identification by OM, SEM-EDS: energy dispersive X-ray
spectroscopy, γ-spectroscopy).

•

Why (objective/motivation)o The purpose and the work required for the sample analyses are described (e.g.
selection of tools for removal).

•

Expected benefit/use
o Whether this case is applicable/valuable beyond the above objectives isdefined. These can include boiling water reactor (BWR) and possibly

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pressurised water reactor (PWR) accident management strategies or better
simulations to assist D&D efforts.
•

Priority in each stage in the overall decommissioning process: (a 5 designates ahigh priority and a 1 indicates a low priority).
o The score for each process requiring analysis/data of a sample is stated. The
processes are listed according to the expected chronological/logical order:
waiting period for retrieval; retrieval; transport, storage; conditioning, disposal;
severe accident research.

•

Statuso The current status of data acquisition and progression at 1F is mentioned.

•

Actual sampling dateo The date (where available) when the sample/data were taken at 1F is given.

2.1.1. Criticality control in analytical table for Task 2-1 activity (Table A.3)
The objective of this task activity was to summarise the important debris characteristics
according to their location in the reactor from the viewpoint of CC. In a severe accident,
the fuel and cladding heats up due to decay heat; at some point, the cladding is further
heated by the exothermic reaction between the cladding and high temperature steam, andthe fuel and cladding melt. These combine to rapidly lead to the formation of a liquid, UZr-O phase, which then relocates downward, dissolving reactor structural steels andresulting in the formation of so-called “in-vessel” molten debris containing principally Fe,
U, Zr and O. The debris, probably mixed with unmelted fuel accumulates in the lower head.
If the lower head of the reactor vessel fails, the molten debris can relocate further to thebottom of the PCV and react with the concrete floor and pedestal wall to form a silicacontaining “ex-vessel” debris bed. For such debris, coolant water is injected to remove
residual heat. Because the amount of fuel material originally loaded into a core is muchlarger than the minimum critical mass, the risk of a criticality incident in the damaged coreduring the accident progression and in relocated debris after the accident must be
quantified. Although no evidence of a criticality incident has yet been detected in 1F, lossof rod geometry and deformation of the debris and re-distribution of the significant
materials for the debris retrieval process can increase the risk of inserting positive
reactivity. For example, resuspension of machining dust in the water could insert positive
reactivity. In Units 1 and 3 of 1F, the water level has been observed above the bottom of
the drywell. In such cases, changes in the water depth might affect the potential for a recriticality. Moreover, early melting of the control devices and relocation to the lower headin the early phase of core disassembly may lead to uneven distribution of neutron absorbermaterial in the debris (i.e. a lower concentration of absorber in the higher portion of thedebris bed). Because of uncertainty in the density and distribution of nuclides affecting
criticality of the debris, a risk-based approach is needed instead of one based solely on
prevention. Among various parameters affecting criticality incidents, material properties
play the most important role.
Generally, the isotopic composition of the debris is the most important to determine theeffective multiplication factor (keff). Therefore, the fissile content in the debris would be
the most significant. Because the 235U and Pu enrichment varies with location in an intact
core, the focus is on the distribution of the enrichment and density of the debris distributed
in-vessel and ex-vessel. Oxidation conditions of the U and Pu are also important because
their density varies with stoichiometry and the number density of the fissile materials.
Compared to U and Pu, the significance of minor actinides, such as Am and Np, for rePREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  21

criticality is low as well. However, nuclides with larger thermal neutron capture crosssections (as represented by 157Gd and 10B), their affinity to the fuel material, and thehomogeneity of their mixing with fuel are also very important (absorbing isotopes include
155Gd, 151Eu, 153Eu and 155Eu). Considering the mean free path of the thermal neutron,a particle with 0.1 mm diameter of Gd should theoretically be treated as “heterogeneous”(e.g. appropriate size of samples).
Concerning the heterogeneity, or stratification of melt, such as molten oxide debris andmolten metals, differences in melting points and the density during the severe accident
progression are important inputs to severe accident analysis codes that are used to predict
such phenomena. The relocation of SS is also an important factor in evaluating the potential
for re-criticality because it is a main core component. Because the average cross section is
not large, homogeneity can be assumed even if there are SS particles with small diameters(a few cm).
Because neutron thermalisation is required to attain criticality with light water reactor(LWR) fuels, the volumetric ratio of the moderator to the debris, Vm/Vf is important. Also,
the volumetric ratio of the open porosity within the fuel material is important because water
ingress can occur. However, the internal porosity of large pieces can be blocked or pieces
could be non-permeable. Actual measurement (by immersion density and cross-sectional
microscopy) can give some indication of whether the internal porosity is connected orclosed. Crack propagation in the crust caused by debris removal activities can significantly
change Vm/Vf; thus, regular monitoring measurements during chiselling, cutting, andextracting of debris are necessary. Impervious ceramics with less pore formation are also
listed because they can contain water and may result in an over moderated condition, as
observed at ChNPP4. It is interesting to note that the decay heat supports the dehydrated
condition by evaporation of water. In the post severe accident phase, determination of thehazard area is important for both 1F and ChNPP4 by means such as temperature
measurements and optical photos for preliminary mapping. Setting neutron detectors neardebris, which was achieved by core-boring in ChNPP4, may also be an effective method at
1F. Monitoring leached fission products (FPs) can be used to identify an occurrence of recriticality. The 84,85,86Kr and 132,134,136Xe gas release behaviours may also be used todetect the approach to criticality. The FP gas release properties for I and Cs are also
important in a criticality evaluation due to their large neutron absorption cross section. For
terminating the criticality, the drainage properties of water from the debris should beconsidered.
Throughout the decommissioning steps, high prioritisation scores are given for invariant
parameters, such as homogeneity of 157Gd/U, the residual enrichment of 235U, the Pu
enrichment in U + Pu, and the presence of SS, because these quantities are essential for
determining keff. As for material compositions, the materials of larger “absolute” reactivity
are highly scored. The isotopic compositions of minor actinides and FPs are scored lower,
and the geometry and density of debris are scored even lower. However, these parameters
are still essential for determining keff and have a significant effect on the negative feedback
from temperature increases and the boiling of the water surrounding the debris. The shapeand nature of the debris may be changed by retrieval activities, such as chiselling, cutting,etc. (i.e. massive pieces to fine slurry suspensions). Thus, while results of the first samples
retrieved are important for assessing the criticality regime of the various debris forms, later
results from post irradiation analyses are also important as input for keff calculations.
Hence, continued post irradiation examination (PIE) data are essential for assessing howeasily the reactivity is inserted in fractured debris, how easily the criticality approach isobserved, and how much energy and FPs are additionally released by any anticipatedcriticality incident.

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In 1F, a risk-based CC approach should be adopted. This approach should be based on a“defence in depth” strategy in which “prevention” and “mitigation” phases are included.
For mitigation, analyses to assess the impact of the criticality incident are performed toensure that appropriate monitoring and termination measures are available. In the analytical
table, however, the mitigation phase analyses to assess and mitigate the impact of thecriticality incident are not emphasised more than prevention phase activities because
prevention measures are of a higher priority in this defence-in-depth strategy.

2.1.2. Establishing containment function in analytical table for Task 2-1 activity(Table A.4)
One of the motivations for this activity was to prioritise different processes for debris
retrieval to be implemented in the 1F damaged reactors. The safety assessment ofoperations for debris removal from the 1F reactors requires in particular detailed
investigations of the risks from dispersal of radioactive contaminants with sufficientaccuracy and confidence in the associated consequences to the 1F reactors and theirsurroundings.
To carry out this analysis, it is necessary to have data on the quantities and composition ofradioactive substances that can be released and transported inside the damaged facilities
and to the environment. In addition to the knowledge of the quantities of potentiallydispersible substances, the safety assessment relies on data to characterise the performanceof containment systems and devices used to purify gaseous releases and filter particulate
effluents. The performance of these systems and devices must be characterised for normal
operation and degraded situations (earthquakes, fires, etc.).
The assessment of potential releases of radionuclides (RNs) into the environment therefore
requires:•the characterisation of contaminants due to particle resuspension and particle
emission during cutting operations;•the characterisation of RN transport within the damaged facility;•

the characterisation of the performance of the static containment, RN
filtration/purification devices and mitigation means used to prevent dust scattering.

In general, the confinement of radioactive substances in nuclear facilities relies first andforemost on static confinement, provided by the interposition of physical barriers between
the radioactive substances and the environment (e.g. fuel cladding, reactor coolant system,
containment vessels/buildings). Mostly, this static confinement is supplemented bydynamic confinement, provided by a ventilation network, which induces a preferential
direction of air flow from the less contaminated areas to the most contaminated areas. In
addition to this dynamic containment function, ventilation is also used to provide sanitationand monitoring functions for work environments, which are necessary to protect workers
against ionising radiation, and to monitor and purify discharges to the public.
Therefore, operations are aimed, among other things, at the dismantling of equipment andthe remediation of contaminated surfaces. The increased risks of dispersion and fire should
be considered due to the techniques employed (cutting by tools that create pyrophoric andradioactive particles in suspension, contaminated materials or hot spots). These risks must
be the subject of a specific study to evaluate the containment function and the risks that
could lead to the degradation or even the total loss of this function.

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It is therefore necessary to focus on the mechanisms for airborne contamination as well as
the behaviour of protective equipment such as:•

resuspension of particles during debris and metal cutting operations or concrete
bush-hammering, when robots or operators are moved, during incidents or
accidents (falling objects, fire, etc.);•

the effectiveness of filter protection devices against particles;•

the behaviour of pre-filtration devices as well as those of High Efficiency
Particulate Air (HEPA) filters.

To prioritise each process, the concepts of nominal condition and critical condition are
introduced here:•

Nominal conditiono Debris removal operationso Nominal maintenance operations of systems (tools, containment equipment ofsuch filtration systems)•

Critical conditiono Strong degradation of confinement equipmento Loss of integrity of reactor containment and building

For the critical condition, the worst case may result from a loss of integrity of the reactor
containment and associated building, which can be induced by external events and bydebris removal operations.
•

Reactor building (RB) integrity - risk of total loss of confinement function due toexternal events:o earthquakeo typhoono tsunamio site explosion•

RB integrity - risk of loss of confinement function due to internal events:o structure degradation (corrosion, ageing)o fire, dust explosion, H2 combustion, deflagration due to high particle
concentrationo mechanical constraints on the structure of the RB due to mechanical cuttingtools

For the nominal condition, the confinement function can be addressed by implementing
basic containment principles, specific countermeasures and relevant maintenance forcontainment equipment:•

Containment function during nominal large removal process
o Negative pressure, ventilation network, HEPA filtration, etc.

•

Countermeasures during nominal large removal process
o Spray system, particles collection system, pre-filtration system, etc.

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Maintenance of confinement equipment during large removal process
o Filter clogging, filter replacement, etc.

Reliance on “large removal” systems and devices increases concerns about the risk
associated with their failure. Hence, the reliability of such equipment needs to be high. The
PreADES project prioritised scores for small and large removal processes, using a
conservative approach based on risk probability and potential impact assessments.

2.1.3. Reducing occupational radiation exposure in analytical table for Task 2-1
activity (Table A.5)
Based on the TMI-2 experience, model calculations, and JAEA rules for the handling ofradioactive material, the necessary debris characteristics and analyses for reducing
occupational radiation exposure were identified for each debris handling step.
Generally, the principle of reduction of radiation exposures consists of three parts:•

accurate monitoring, exposure assessment; investigation of the validity ofprotective actions, study of new protective actions;•principles of reduction of external exposure; distance, shielding and time;•principles of reduction of internal exposure (or incorporation); dilution, dispersion,
venting, removal, confinement and centralisation.

The above principles are taken into account to ensure activities meet the ALARA (as low
as reasonably achievable) principle, which considers the available time, specific facilities,
available manpower, etc. Discussions identified the surface dose rate (γ, n) of debris as animportant characteristic for assessing external exposure. For assessing internal exposure,the particle size of debris dust and RN concentration in the air inside the PCV wereidentified as indispensable. R&D on irradiated fuel analysis indicates that 148Nd is nonvolatile in UO2 fuels and proportional to burn-up. Thus, it is expected that 148Nd willremain in the fuel containing debris even though the debris may experience hightemperatures during the accident. If the burn-up of the fuel within the debris is known, anevaluation of the total amount (or upper bound) of Cs and other major FPs and total source
term is possible, and dose assessments by model calculations are improved. Therefore, ananalysis of 148Nd and other key FPs and irradiation products (e.g. Gd, Pu) in debris is vitalto improve and verify model calculations. From another perspective, the air dose rate (γ,n), FP source distribution and RN airborne concentrations in the working place, or fromlocal objects (such as wall, equipment), are also necessary for assessments of external
exposure and decontamination. Also, hand/feet checks, body surveys/badges and wholebody measurements are essential monitoring techniques for the control of radiation
exposure to personnel. However, such generalised personnel monitoring techniques are
already in place at the 1F site. This discussion is therefore looking principally at the
reduction of personnel dose rates resulting from closer contact to debris, reactor materials
and contaminated structures; and this is the main focus of the PreADES project.
Nevertheless, the links between decommissioning operations and possible contamination
to the external environment (as airborne aerosols and droplets or contaminated solutions)remain an important aspect of this project as well as a central aspect of the partner of the
ARC-F project.
In the analytical table, the scores of analysis items varied during decommissioning steps,
which included the following five sequential processes:(1) Waiting period for retrieval: This corresponds to the period after confirmation of
the cold shut down and before start of the debris retrieval procedure. With respect
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to reducing occupational radiation exposure, analyses of debris are not so importantbecause work may be performed remotely outside the PCV, and the debris does
not affect occupational radiation exposure directly (nevertheless, a raised
background dose and the effective robot lifetime need to be considered). To assess
the effects of the PCV leakage and prepare for debris retrieval efforts, the use ofquick look videos is highly effective for work planning in a manner that reduces
worker exposures.
(2) Retrieval: In this process, debris can be sucked, scooped, dug out, cut down, pickedup and stored in containers. Therefore, workers may interact more closely withdebris and encounter higher dose rates. It may be assumed the highest permitted
radiation will be reached at this point and the greatest benefits of dose reduction
measures will be achieved during this phase.
(3) Transport, storage: The external debris dose at the container’s surface is assessed
to ensure compliance with the standards of each country. The containers are stored
temporarily before they are transported to hot cells in other countries.
(4) Conditioning, disposal (including full analysis): For these processes, the full rangeof hot cells and decontamination facilities will be needed. In addition, a local“holding” laboratory/facility will be needed, adjacent to the entry/exit points of theactive zone, where the first spectroscopy measurements and examinations can be
made. From there, the debris can be transported to a full hot cell laboratory.
Firstly, samples for analysis are obtained. For the bulk of the material, there is the processof treating or conditioning the material to get it into a stable inactive form. The final stage
may be placing it into the inner container, before Non-Destructive Assay (NDA) analysis(e.g. by neutron counting techniques and segmented γ scanning) and verification of thefissile material and other contents, followed by tests to confirm the soundness of thecanister and the surface dose rate. This will probably be done in fully shielded hot cells
where the operators (outside the cells) only need general monitoring in the hot cell handlingzones. The experience and analyses carried out during the previous phases will be vital for:1) declarations of fissile material and 2) contact dose rate estimates for final or long-term
intermediate storage. There will also be a requirement to be able to repair anddecontaminate these hot cells. Therefore, a full range of decontamination and analyticalfacilities, using the previously gained experience of the operations and the fuel analyses,
will be needed. After analysis and conditioning, the samples are stored until final disposal.
1. Severe accident research: It is assumed that the dose rate itself is not important forthis process; however, the -, -, -spectroscopy, and monitoring data will be
useful for the prediction of long-term mobility and diffusion of longer-lived
nuclides such as 99Tc, 35Cl, and the actinides as well as for the verification of FP
leaching models. The data are also important for ensuring the adequacy of transportand storage containers. Such isotope tracking data (e.g. 241Am, 238–240 Pu[144Ce] and 90Sr) are still being monitored at ChNPP4 for verification of the watertable uptake and actinide and FP movements. Moreover, assessments of samples
to provide insight on the severe accident progression and phenomena are
indispensable.
Seventeen different cases have been assessed to address radiation exposure and concerns
about the integrity of the pedestal floor and the lower head of the RPV. Throughout thedecommissioning, a high score is given for the availability of using quick look videos
during the waiting period and retrieval process. The TMI-2 clean-up effort pioneered theuse of video camera technology for surveillance and inspection in nuclear power plants.
The cameras proved extremely valuable for task management and personnel safety; they
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT26  NEA/CSNI/R(2022)7
allowed supervisory work planning and guidance to be completed without the supervisor
being in the radiation-affected zone. This resulted in significant personnel dose savings in
the early years when radiation levels were at their highest. In the 1F decommissioning, theorder for debris removal locations and the tools to be used can be selected using theseremote observations along with data (e.g. temperature, dose rate from locally installed
monitors). This reduces the required manpower and radiation exposure and provides formore focused planning during the waiting period for retrieval. In the same way,observations from videos taken during cutting and retrieval leads to reduced working timeand radiation exposure, and improved tool design. However, observations may be notneeded in transportation or disposal. A high score of four is also given for surface dose rate(γ, n) measurement by γ- ray during retrieval where assessments of radiation exposure topersonnel are important, while a lower score of one is given for such measurements during
the waiting period for retrieval since there is no radiation exposure to the personnel.
Confirmation of the surface dose rate of debris-filled canisters will be required before
transportation or storage. In a second example, analysing a particle of dust in the air by
SEM and by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to assess the RN
concentrations is highly scored during the waiting period and during debris retrieval. Suchscores reflect the opinion that the sample can be obtained quickly if an area is being
currently accessed and the SEM analysis is performed away from the RB, without
interfering with other decommissioning operations. In conclusion, for the purpose of
reducing occupational radiation exposure, analyses needed before or during the retrieval
process are prioritised.

2.1.4. Maintaining cooling function in analytical table for Task 2-1 activity(Table A.6)
Due to the decay heat of the fuel, the debris from the accident of 1F should be cooled
properly during its removal, transport and storage; otherwise, re-melting or oxidation of
the debris may occur and consequently complicate the situation. It is therefore important tomaintain the cooling during the decommissioning process. Strong attention has been paid
to debris cooling capacity (coolability) in research on nuclear power safety because it plays
an important role in reactor stabilisation and termination of a severe accident. During asevere accident, for instance, the reactor core could melt down due to decay heat. The
molten debris will relocate downwards, and finally fall into the water pool in the lower
plenum. A debris bed is expected to form on the pool bottom due to fuel coolant interactions(FCI). If the debris bed is coolable in the long term, the integrity of the RPV will be secured,and the debris and FPs are thereby contained. However, if the emergency water injectionis unavailable or fails to cool the debris bed, the debris will re-melt. In that case, the vessel
will fail under the aggressive attack by the molten debris in the lower plenum, leading toejection of a melt jet into the cavity beneath the RPV. If the cavity is flooded (as a strategy
of severe accident management or a result of containment spray), the melt jet will fragmentin the water pool and the debris will settle on the floor. Proper cooling of this newly formed
debris bed is the last chance to arrest attack of relocated debris; otherwise, molten coreconcrete interaction (MCCI) will occur and eventually threaten the integrity of the PCV orcontainment, which is the last barrier to FP release.
Therefore, in a severe accident, debris beds may be formed in the RPV (in-vessel) and in
the PCV or containment (ex-vessel). Assessing the coolability of an in-vessel or ex-vessel
debris bed requires knowledge about: (i) the characteristics of the debris bed, including bed
configuration and porosity, as well as the debris particle size distribution; and (ii) thethermal-hydraulics of the debris bed, given the bed’s known characteristics, in particular todetermine its coolability. Numerous experimental and analytical studies have been
conducted to obtain this information. This knowledge base is instrumental in assessing
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debris coolability in the 1F accidents, and ongoing activities in the Reduction Of Severe
Accident Uncertainties (ROSAU) project are designed to provide additional information
about ex-vessel debris coolability issues.
Although characteristics of the debris fragments/chunks formed in the 1F accidents havebeen unavailable so far, results from water suspension tests indicate that they are in a
coolable state. Since the debris fragments/chunks in 1F Units 1 to 3 have been cooled inwater pools for nearly 10 years after the accidents, the decay heat should have decreased toa low level. If this debris fragments, it should have sufficient porosity that it can be cooledby air, as in the dry storage of spent fuels. The only concern is the large debris chunks(cake/crust/agglomerates) may not have enough cooling surfaces, so special care must be
paid to their coolability during removal. If they are cut into particulate debris during
removal, the cooling surfaces will be significantly increased, and the debris will be moreeasily cooled.
To estimate and maintain the coolable state of the debris during its retrieval in 1F, theanalytical table for cooling function identifies several important items:•

location;•decay power;•

object type (fragments, cake/agglomerates, molten pool/crust, MCCI debris);•geometries/dimensions;•porosity (especially open porosity);•debris particle morphology and size distribution.

In summary, there should be no difficulties to maintain cooling during debris retrieval, due
to the very low decay power of the debris. Moreover, existing data for characterising debris
bed coolability provide insights about particle size and possible closed porosity values.
Existing data, which will be confirmed by examinations during debris retrieval efforts,
indicate the smallest particles will be 1 to 5 mm and have a closed porosity volume of about
36%. This matches the maximum packing density possible of 64% (given by the packing
fraction of uniform spheres in a cubic volume). Existing closed porosity data also provide
a good basis for estimating the re-criticality risk of fissile material debris during retrieval.

2.2. Task 2-2: Major issues for safe handling and analysis of fuel debris
The objective of this task was to summarise the perceived major issues and methodologies
for safe handling of debris and performing analysis based on the proposal of Task 2-1. The
task covers international knowledge of the regulatory situation, transport, storage, legal
aspects, and criteria and models for safety analysis. The task feeds into Task 3 and longterm projects on the analysis of debris. The resulting table provides guidance and advice
regarding approaches used in the past and at 1F, including the proposed limits for criticality
and radioprotection to ensure safe handling and treatment during analysis, transport andstorage activities. To meet the analysis needs identified in Task 2-1, the major issues andthe associated debris analysis methods were organised in a table, with a brief descriptionof the various issues. Firstly, differences between debris and spent fuel considerations, suchas the shapes, compositions, MCCI or debris as opposed to fuel samples, are discussed. In
the columns, major criteria, experience and methodologies for the successive steps of“transport”, “handling in the facility”, and “storage”, are summarised. Finally, the safetymargins or degree of conservatism for the treatment of debris in the various steps comparedwith spent fuel were listed in the table.

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The format of the major issue and methodology table includes the following items:•

steps (e.g. transport, storage, handling for analysis);•

issues (e.g. hydrogen management, criticality safety, heat removal, containment,
radiation, legal issues);•

organisation, facility;•

knowledge, experience and methodology (from TMI-2, ChNPP4 and the latest
findings);•

suggestion/conclusion (methodologies for 1F)

As noted above, there are three steps: transport, storage, and handling for analysis. For thetransport step, experience and information from the Severe Accident Facilities for
European Safety Targets (SAFEST), Phébus FP, International Research Institute for
Nuclear Decommissioning (IRID) evaluation, TMI-2, and Japanese and European
regulation were collected and utilised. Casks, details of transport container specifications,and safety limits or boundary conditions regarding material to be transported weresummarised (e.g. design and geometry, containment specifications, limits in water content
related to hydrogen formation by radiolysis, limits in U/Pu and other actinides content
related to criticality safety, limits in radioactive content in relation to heat removal andradiological protection).
For the storage step, TMI-2 experience, evaluation examples from the JAEA Okuma
Analysis and Research Center, experience from the JAEA Chemical Processing Facility(CPF), evaluations from the Korea Institute of Nuclear Safety (KINS), and requirements inJapanese regulations were reviewed (e.g. for hydrogen management, criticality safety, heatremoval, radiological protection and safeguards).
Considering the handling for analysis step, evaluations by the Okuma Analysis andResearch Center and requirements in Japanese regulations for criticality safety, radiologicalprotection, and safeguards were summarised.
This table provides preliminary input for the future tasks based on the collected experienceof previous decommissioning sites and the 1F site as well as that of analytical laboratories
involved in transporting material to and from 1F and other sites.

2.3. Task 2-3: Radioactive material “hot” analysis capabilities
This task identified international radioactive material “hot” analysis facilities and theircapabilities for potential use in possible future projects to analyse debris samples from 1F.
The following viewpoints were considered and organised in the collection of the facility
information:•

facilities and techniques for debris analysis;•

experience related to severe accident research, including advantages andlimitations of techniques;•

technical requirements for sample preparation and analysis; and•feasible analysis and measurements to be performed on samples from 1F.

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The table of hot analysis capabilities (Table A.7) presents the information collected fromvarious international organisations for the following categories:(1) General description of hot cell facilities: Information on the number of hot cells orother hot analysis facilities and a general discussion of the various capabilities ofthe facility, along with a contact person who can provide further detailed
information.
(2) Material handling: Discussion of the material handling capabilities of the facility
including shipping and receiving of samples, handling of transport
containers/flasks, and capabilities for loading samples into hot cells.
(3) Sample preparation: Information on the capabilities of the facility to prepare
samples for analysis and identify laboratories that may be suitable for segmenting
larger samples for shipment to other facilities with limited handling capabilities.
(4) Non-destructive testing: Discussion of non-destructive examination capabilities forsamples (e.g. physical inspection, infrared measurements, profilometry, γ
spectroscopy, acoustic and eddy current measurements)(5) Destructive testing (mechanical analysis): Discussion of destructive examination
capabilities for samples (e.g. SEM, autoradiography, hardness testing, compression
testing, tensile and fatigue testing, mass spectrometry, furnace and oxidation test
facilities)(6) Chemical analysis: Discussion of chemical analysis capabilities (e.g. ICP-MS,
radiochemical analysis, isotopic analysis, thermal ionisation mass spectrometry)(7) Microscopy: Information on the microscopic examination capabilities such as OM
and related techniques (e.g. Raman spectroscopy) as well as electron microscopy
techniques on rough and prepared surfaces.
(8) Materials and surface science analysis: Information on capabilities for materials
analysis (e.g. metallography, ceramography, fractography, SEM, EDS, XRD: Xray Diffraction, XPS: X-ray Photoelectron Spectroscopy, SIMS: Secondary-Ion
Mass Spectrometry, TEM: Transmission Electron Microscopy, FIB: Focused Ion
Beam).
The PreADES project has managed to collect a significant amount of information on
several hot analysis facilities throughout the world. Although Table A.7 only presents
information from the Canadian Nuclear Laboratories (CNL), capabilities were collectedfrom all PreADES project member organisations:•

Canadian Nuclear Laboratories (CNL) – Canada•

European Commission Joint Research Centre (JRC), Karlsruhe – Germany/EU•

Commissariat à l’Energie Atomique et aux énergies alternatives (CEA) – France•

Japan Atomic Energy Agency (JAEA) – Japano JAEA Tokai, Nuclear Science Research Institute and Nuclear Fuel Cycle
Engineering Laboratories; JAEA Oarai Research and Development Institute;
JAEA Okuma Analysis and Research Center; Nippon Nuclear Fuel
Development Co. Ltd.; Nuclear Development Co. Ltd.

•

Korea Atomic Energy Research Institute (KAERI) – Korea•

Studsvik – Sweden

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Paul Scherrer Institute (PSI) – Switzerland•

United States Department of Energy (DOE) National Laboratories – United Stateso Argonne National Laboratory (ANL); Pacific Northwest Nuclear Laboratory(PNNL); Oak Ridge National Laboratory (ORNL); Savannah River National
Laboratory; Idaho National Laboratory (INL); Los Alamos National
Laboratory (LANL)

In parallel with collecting hot analysis capabilities, the PreADES project had reviewed
specific information that may be of interest from fuel/debris samples from 1F and has
developed some guidance on possible techniques that could be used to produce suchinformation. Information from the characteristic table from Task 1-1 and the analytical
table from Task 2-1 were used to produce a preliminary list of fuel sample characteristics
that may be of interest. The guidance sheet produced in Task 2-3 provides information onwhich hot analysis tests/capabilities are needed to generate the specific debris sample
characteristics of interest. The goal is to bridge the gap between the analytical table in Task
2-1 and the hot analysis capability table produced in Task 2-3.
The areas of interest currently identified in the preliminary hot analysis guidance sheet are
given below along with applicable techniques:•

Physical characteristics (shape, size, appearance, particle size distribution, density,
etc.)
o METHODS: OM, SEM, digital microscopy, sieving machine, X-ray Computed
Tomography (CT), Archimedes (mass immersion, density), density based on
chemical composition.

•

Composition (U, Pu concentrations, heavy metal ratios, U enrichment, also U, Pu
ratios with structural materials, e.g. Zr, Fe, Al, Sn, B, C)
o METHODS: EPMA, SEM, TIMS, ICP-MS, ICP-Atomic Emission
Spectroscopy (AES), ICP- Optical Emission Spectroscopy (OES), α
spectroscopy; Ion chromatography, oxygen titration.

•

Radiation analysis (α, β and γ nuclides, dose rates, etc.)
o METHODS: TIMS, ICP-MS, α spectroscopy, liquid scintillation, γ-ray spec,
gas flow counters, ICP-AES, Si- Low Energy Proton Spectrometer (LEPS), GeLEPS, γ scanning (X-ray CT), high range γ probe.

•

Elemental distribution and chemical state (cross section analysis, surface
observation, moisture content, oxidation state, etc.)
o METHODS: SEM/EDS/Wavelength Dispersive X-ray (WDX), XRD, XPS,
digital OM. Karl Fischer moisture titration, Thermogravimetry/Differential
Thermal Analysis (TG/DTA), Raman spectroscopy.

•

Mechanical characteristics (hardness, compression, elastic modulus, etc.)
o METHODS: hardness testing (Vickers, Rockwell, Brinnel, Knoop), fracture
toughness (IF method), pulse echo, stiffness measurement, compressive
strength machine.

•

Thermal characteristics (heat conduction/thermal diffusivity, thermal expansion,
melting point, heating value, calorimetric measurement, high temperature
characteristics/reactivity, specific heat capacity, etc.)

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o METHODS: Laser flash (thermal diffusivity and FP gas measurement),
TG/DTA (high temperature characteristics), Differential Scanning Calorimetry(DSC), and results from these measurements to estimate thermal conductivity.
•

FP release and hydrogen generation (FP release through leaching/elution/thermal
annealing and deposition testing, FP Aeration, drying and ageing releasecharacteristics, hydrogen generation, etc.)
o METHODS: Laser flash (FP gas measurement); ICP-AES, γ spectroscopy, αspectroscopy (leaching); TG/DTA (dry); gas chromatograph (hydrogen).

This preliminary list of debris characteristics of interest and guidance on techniques
available to produce such information were obtained based upon input from institutions
with hot analysis capabilities.

2.4. Task 2 outcomes
The analytical table identifies types of analysis data that should be obtained through 1F
decommissioning for each estimated 1F debris location. The analytical table shows priority
scores for analysis items in four key requirements for the decommissioning processes:“criticality control”, “establishing containment function”, “reducing occupational radiation
exposure”, and “maintaining cooling function”. These scores are based on various safety
viewpoints, and the highest priority of analysis items cannot be selected because all safety
viewpoints are important. In addition, "unique" analytical items were recognised regardlessof the scores. These unique analysis items, along with major points of discussion for eachgroup, are described below:•

Criticality control
o Most analysis items are scored high for all steps (e.g. waiting period for
retrieval; retrieval; transport, storage; conditioning, disposal)
o Mass ratios U + Pu, 157Gd / (U+Pu), homogeneity•

Containment functiono Retrieval process has many issues; differences in priority between small andlarge removal steps are summarised.
o Dust in cutting debris (particle size, etc.)o Ignition characteristics•

Radiation exposure
o Scores before transport storage step are higher; the following analysis items areprioritised: quick look video, airborne particle, aerosol in cutting.

•Cooling functiono The following analysis items are important in retrieval, transport and storage
steps: heating power by calorimeter and geometry, porosity, and fragment size
distribution by photography, SEM, etc.

The unique analytical items can be utilised in combination with the Task 1 discussion as atrial for this preliminary phase. For example, these unique analytical items could beallocated in the figure of debris’ location activity (Figure A.1). Descriptions related todebris (e.g. regions 1 to 8) were selected in the figure of the debris location. For regions 1,
3, 5 and 8, oxide debris, high density material, etc. are expected to exist. The four unique

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analytical perspectives are preferable because the target materials have unknown shapes,
compositions and origins.
Similarly, in regions 4, 6 and 7, particulate debris, sediment, etc. are expected to exist andthe following unique analytical perspectives are preferable:•

The criticality control perspective is required because the position and arrangementof existing water is fluid and affects criticality characteristics.

•

The containment function perspective is not required because particulate debris,
etc., do not need to be cut and may be almost oxidised.

•

The radiation exposure perspective is required because a reduction in working time
for searching is required.

•

The cooling function perspective is required because the decay heat associated withparticulate debris is unknown.

In addition, for region 2, intact fuel rods and pellets (without any resolidified moltenmaterials) are expected to exist:•

The criticality control and cooling function perspectives are not required because
the characteristics (maximum value of each characteristic) are already known.

•

The containment function perspective is not required because no cutting is required
for fuel rods and pellets.

•

The radiation exposure perspective is required because a reduction in working timeis required.

These discussions are still preliminary, but discussions are being held on the priorities ofdebris analysis and on getting common recognition in the future. For implementation of all
analyses in the analytical table, the major issue and methodology table and the hot testing
analysis capabilities table provide practical information considering major issues,
methodologies, and available techniques and facilities.
The PreADES project called attention to the representativeness of samples. Regarding thedegree of actual debris inhomogeneity, deposits in the pedestals of Units 1, 2 and 3 and Ubearing particles may vary widely in each location (based on results of investigations andsample analysis so far). However, performing quantitative analyses at many measurement
points is expected to reveal the overall tendency of debris characteristics. In other words,
insights regarding changes in debris characteristics can be gained by increasing the numberof quantitative analysis measurements within a reasonable range for each unit and region.

Insights and lessons learnt from Task 2, and next steps to further projects•

To identify the needs for debris analyses that contribute to the 1 F decommissioning

Task 2 outcomes provided expertise to help select the best methodologies for safe
decommissioning with respect to four major aspects: “criticality control”, “establishingcontainment function”, “reducing occupational radiation exposure”, and “maintaining
cooling function”. This assessment was detailed but needed to highlight some key issues
for 1F. The organised table is too extensive to easily interpret; therefore, a simplified table
was prepared based on provided comments. The prioritised analysis items organised in thistask support decisions for 1F decommissioning.
•

To create appropriate and optimal methodologies for future debris sampling,
retrieval and storage

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Major issues and methodologies for the safe handling of debris were proposed for the
performance of the prioritised analysis items (e.g. dose evaluation methods, criticality
safety, and regulation). The information is linked to the availability of all possible hot cell
facilities for pre-treatment and analysis of radioactive samples from 1F. Local hot cell
facilities to characterise and condition waste may be limited after scaled up retrieval,possibly causing delays in analysis. This task is expected to bridge the gap between theanalysis data needs and performing analysis work in hot cell facilities.
International organisations should review results obtained as 1F decommissioning
progresses.

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3. Task 3: Planning of future international R&D framework

Much uncertainty remains around debris retrieval operations as the obtained information is
not sufficient to appropriately assess the situation inside the reactors. The international
community should continue providing relevant insight to enhance the knowledge gained
regarding the debris distribution and characteristics in the three damaged reactors.
The Nuclear Accident Response Office of the Ministry of Economy, Trade and Industry(METI) of Japan shared Japan’s policy for 1F decommissioning in the PreADES meetingin July 2019. The policy had been defined at the meeting of the Team for Countermeasuresfor Decommissioning and Contaminated Water Treatment in Japan in 2019 (Appendix).
The policy acknowledged that analyses and investigations are necessary fordecommissioning work and contaminated water management at 1F, and that they will be
carried out to support 1F decommissioning safely and steadily. At the same meeting, Japanproposed that the future international research framework include joint evaluation and joint
analysis projects. The joint evaluation project will be proposed by Japan as the retrieval
works and analyses were found to be on track. The joint analysis project would be initiatedaround the mid-2020s at the earliest because a five-to-ten-year duration is predicted before
full‐scale debris retrieval can be initiated.
Since discussions on implementing an international collaborative research framework forthe actual debris analyses are important and should be conducted, the framework was
expected to be defined and commonly agreed upon prior to starting actual debris analyses,with the shared understanding of objectives for proposed debris analysis plans.
Thus, within the PreADES project, in addition to the knowledge collected in Tasks 1 and
2, test case studies (to define analysis plans focused on specific aspects such as analyses toaddress the re-criticality risk during debris retrieval operation in Unit 2) were proposed andinitiated with the aim to further the discussions on proposed analysis plans. An analysis of
the item “U and Pu isotopic analysis” was selected as a topic for a first test case study forthe following reasons:•In the discussion of debris analysis needs in Task 2-1, the analysis item was given
a high score in all steps of debris management (e.g. waiting period for retrieval,retrieval, transport, storage, conditioning, disposal) as well as for severe accident
research.

•

The analysis item was also given high priority in the analysis plan for 1F Unit 2(the first unit where debris retrieval will be started) by the Nuclear Damage
Compensation and Decommissioning Facilitation Corporation (NDF).

The test case study was further supported by a comprehensive technical analysis report
produced by the JAEA that was distributed to the project participants and made public andwhich described the needed evaluation items and related needed analyses for debris. U and
Pu isotopic analysis for evaluation items was shown to be relevant for:•criticality safety•cooling measures•storage management•

ageing changes

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The test case study considered each evaluation item (e.g. determining required debris
characteristics and analysis items). The detail is shown in the following sections. Other
required analysis items and evaluation viewpoints were expanded, and the resulting
analysis plan will be considered for future test case study exercises.

3.1. Criticality safety of fuel debris in test case study
A criticality safety assessment of debris is one of the most important items which requiresconsideration for debris retrieval. At present, potential criticality safety measures are understudy including the injection of borated water, the pre-injection of an insoluble neutron
absorber, and the monitoring of sub-criticality based on neutron measurement. The
potential risk for debris criticality, depending on the unit and location, was discussed.
Information is required on the heterogeneity of the composition and phases (metal, oxide
and other compounds) observed in the sample. Then, based on data obtained directly fromthese acquired samples, the re-criticality of the entire retrieval target region is to beevaluated. Items required for criticality assessment in the test case study are listed below.

Items required for re-criticality evaluation•

U concentration in debris = U /debris mass ratio•

U and Pu isotopes ratio•

155

Gd, 157Gd to U ratios (residual concentration of burnable poison) = 155Gd or
Gd /U mass ratio in debris, or Gd element to U mass ratio and average burn-up
of debris
157•

Structural materials (Fe, Zr) and neutron absorbers (B) to U ratio = Fe, Zr, B, etc./U
mass ratio in debris

Items required for burn-up evaluation•148

Nd (or alternative burn-up indicator) to U ratio (burn-up) = Mass of 148Nd, etc./U
mass ratio in debris

Items related to bulk density•

Bulk density = theoretical density (evaluated from average composition and phaseobservation) x (1 - porosity)

Important items for evaluating the mobility and coagulation of U and Pu due toenvironmental changes during retrieval (water environment, redox properties,
etc.)•

Chemical form of fissile material (U, Pu) and valence state for U (IV, V, VI) and
Pu (IV, V, VI).

3.2. Cooling measures in test case study
Cooling during retrieval may cause chemical changes in the debris. These changes and theireffects on the debris characteristics need to be included in this evaluation:•

analysis of samples obtained from the area where a local temperature rise wasobserved in the cooling water injection suspension test;

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confirmation of deterioration of the debris surface directly exposed to theatmosphere due to the cooling water shutdown;•

prediction of alterations when a new surface is exposed during debris retrieval.

In the case of debris retrieval in air (or in nitrogen), operations under negative pressure
control (or slight positive pressure control) are expected, and it is necessary to investigatechanges in the debris surface and sedimentation due to oxygen contamination in theatmosphere. If the temperature rises at a specific part, it is important to obtain information
related to the heat source, such as the U isotopic ratio and FP composition.
Since transuranic (TRU) nuclides may correlate with the U concentration and average burnup, the accuracy of the heat value can be improved by measuring the U and TRU
concentrations and measuring the burn-up on the same sample with high performance ICPMS (double-focusing, or multi-collector MS) and by using experienced operators, which
are needed for optimal heavy metal isotopic and burn-up analysis. Properties to be assessed
regarding the effects of cooling on debris include:•

mesoscale chemical properties of the debris main components;•

FP distribution and chemical state (solubility to water);•density, porosity;•

U isotopic ratio (evaluation of burn-up).

3.3. Storage management in test case study
The design and licensing of containers to store retrieved debris require basic data to assess
the criticality of the debris to be stored, including debris composition (isotope ratios). Basic
information such as the composition of materials (Gd as a burnable poison, Gd and 155Eu
as FP, B as a control rod material, Fe and other metallic elements as structural materials in
the reactor), density and water content of debris is required. In addition, information suchas 148Nd concentrations, which can be an index of burn-up, is also important as the input
for burn-up calculation. Since debris is a complicated, heterogeneous composition
originating from nuclear fuel, control rods and structural materials, monitoring its
properties, listed below, is important for criticality safety in the storage:•

actinide elements’ composition and isotopic ratio by ICP-MS or α spectrometer;•

isotopic ratio of Nd (by SEM-EDS/WDX, ICP-MS, etc.) and evaluation of the
behaviour of Nd with U and Pu;•

content in Gd, 155Eu, and B by SEM-EDS/WDX, ICP-MS, or other methods toevaluate their distribution with respect to that of U and Pu, and the content ofstructural materials in the core (SS, etc.);•

density (true density, bulk density, porosity, etc.).

3.4. Ageing change in test case study
Evaluation of the change of debris due to ageing is commonly performed using simulated
debris. When the prediction of the ageing process becomes possible by understanding themechanism, these predictions could be used for actual debris. Therefore, it is important toconfirm and verify the ageing mechanisms by using actual debris samples. For example,
for ageing assessments (especially the existence and chemical state of key elements suchas actinides), the verification of physical, chemical and biological mechanisms (elution by

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  37

contact with water and decomposition by microorganisms) is considered by completing thefollowing:•

Identification of major matrices, elemental composition of actinide, isotopic ratio
by destructive analysis such as SEM-EDS/WDX, α spectrometer, or ICP-AES/MS.

•

Measurement of density (true density, bulk density, porosity, etc.).

•

Measurement of mechanical properties with Vickers hardness tests.

•

Estimation of ageing mechanism using simulated debris. After that, as necessary,conducting various tests using debris samples for mechanism verification.

3.5. Test case study outcomes
The test case study led to the establishment of an approach to determining the required
debris characteristics and analysis items. Future test case studies are expected to expand
the number of required analysis items and evaluation viewpoints, and the process ofconstructing analysis plans will be applied in future international research. The discussions
on future activities to prepare for actual debris analyses, including feedback from the test
case studies, resulted in recommendations for future tasks related to debris characterisation,practical and safety aspects of debris retrieval operations, and analysis plans to contributeto debris retrieval operations and enhance severe accident knowledge. The
recommendations are summarised below.

3.6. Recommendations on fuel debris characterisation
Significant efforts were conducted in Task 1 in compiling and sharing knowledge to assess
expected debris characteristics at 1F. Though the knowledge compilation informs the
preparation of debris retrieval operation at 1F and enhances understanding of the accident
progression, it is, however, fully acknowledged that significant uncertainties remain
regarding the debris composition, physical properties, distribution and configuration ineach of the three damaged reactors. For instance, knowledge gained from the TMI-2 and
ChNPP4 accidents and core melt accident testing is only relevant for certain aspects of the
1F accident. The reactor type (BWR) and core material composition at 1F are different
from those at TMI-2 and ChNPP4. Furthermore, the three 1F reactors have different
damages and extensions, so various in and ex-vessel debris distributions and configurations
are estimated, making them not directly comparable to those at TMI-2 and ChNPP4.
Knowledge gained through preliminary analyses of actual samples collected at 1F will be
key to reduce these uncertainties. A preparatory discussion was started in the PreADES
project with the joint task force between the PreADES, the ARC-F (Analysis of Informationfrom Reactor Buildings and Containment Vessels of Fukushima Daiichi Nuclear Power
Station), and the TCOFF (Thermodynamic Characterisation of Fuel Debris and Fission
Products Based on Scenario Analysis of Severe Accident Progression at the Fukushima
Daiichi Nuclear Power Station) projects to evaluate the formation mechanisms of Ubearing particles and other components contained in actual deposit samples obtained from
1F. In addition to providing those data on actual debris characteristics, the joint task force
started to investigate the formation mechanisms of the U-bearing particles with theintention to provide additional knowledge on the accident progression.
Such co-operative efforts should continue after the PreADES project completion, using
data and information from samples collected during “trial” retrieval operations, as they are
expected to provide highly valuable data and information for implementing safe debris
retrieval operations and for enhancing severe accident analyses.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT38  NEA/CSNI/R(2022)7

3.7. Recommendations on practical aspects and safety issues related to debris
retrieval operations
Significant efforts were made in Task 2 to compile relevant knowledge and experience
regarding some practical aspects and safety issues related to debris retrieval operations.
The efforts addressed in particular the risk of re-criticality, the risk of dust emission andhow to appropriately manage the cooling of debris, confinement of radioactivity andradiological protection in debris sampling and retrieval operations. This information is ofvalue to support the development of guidance and recommendations regarding the designand safe management of debris retrieval operations but also to recommend necessary R&D
and analysis on actual 1F samples to further limit the identified risks. Views on practicalaspects and risks assessment could be refined in the future with knowledge gained throughanalyses of actual 1F samples.

3.8. Recommendations on analysis plan to contribute to debris retrieval operations
and to enhance severe accident knowledge
The project proposed a first approach to define an analysis plan intended to adequately
support safe debris retrieval operations and to provide information and data of importancefor severe accident analyses. It is recognised that much remains to do to achieve a fully
practical analysis plan that will adequately prioritise the needs and interests for debris
retrieval on one side and severe accident analyses on the other side. Analysis plans willcertainly be revised with the progress and practicalities of debris retrieval operations, as
was the case at TMI-2.
In Task 3, a first test case study was conducted to identify evaluation items for debris with
U and Pu isotopic analysis to address criticality safety, cooling measures, storage
management and ageing changes. Required analysis items and evaluation viewpoints wereexpanded. Other test case studies are expected to be discussed in the future; where
warranted, a specific analysis plan may be developed.
In addition, the development of an international round robin analysis exercise using
samples from past severe accident experiments on representative corium mixtures was
discussed during the project. It was recognised that the activity would have several possible
benefits such as:•

developing and sharing experience in the analysis and transport of debris samples;•

establishing optimised procedures for samples preparation and analysis for future
work on 1F debris sample analysis;•contributing to the enhancement of debris analysis capabilities;•

analysing the variability in analysis results and qualifying uncertainties;•

establishing facility and analytical tools capability for debris characterisation(micro and macro-analyses, chemical analyses, dose and radiation analyses);•establishing a frame for a future international research implementation.

It is recommended that efforts for the development of such an international round robinanalysis exercise be continued after the project completion.
The trial debris retrieval from 1F is currently planned to start after 2024. A futureinternational project based on actual debris analyses would then be carried out on a“routine” basis. This project is foreseen to start around the middle of the 2020s at theearliest because a five- to ten-year period of preparation is expected before full‐scale debris
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  39

retrieval can be initiated. New on-site facilities being built for analysing, characterising andconditioning debris should also be operational by that time. However, it is strongly
recommended that the sharing of expertise among PreADES participants, relevant Japaneseorganisations and other 1F relevant projects under the CSNI be maintained as newimportant preliminary data and information on actual 1F material may become available inthe meantime.
It was discussed whether continuing international co-operation before full-scale debris
retrieval starts should be required as significant knowledge gaps remain regarding debris
characteristics and distribution in the three damaged units, and as new significant
knowledge can be obtained through analyses of actual samples collected in exploratory
investigations in the damaged units. All the participating organisations agreed that there
was value in maintaining the existing information exchange channel between organisationsin Japan and NEA member country organisations that are currently involved in post-1F
joint undertakings under the CSNI. Therefore, it was proposed that the Fukushima Daiichi
NPS Accident Information Collection and Evaluation (FACE) project be established.
The project participants recognise that an important ingredient in the success of a futurecollaborative initiative will be the implementation of conditions for fluid dialogue andexchanges of information between the involved organisations. As these organisations may
have different priorities (e.g. some may have a deeper interest in severe accident analyses
while others may have a deeper interest in decommissioning under severe constraints), it isimportant that informed exchanges be established. Japanese organisations in charge of thedecommissioning should inform participants about the relevance of some analyses fordebris retrieval operations and for improving the understanding of severe accident analyses(in general and for the 1F accident progression in particular). Research organisations incharge of severe accident analyses should demonstrate the significance of the analyses,
which they propose either for severe accident analyses or for decommissioning purposes.
Japanese organisations in charge of collecting and distributing information to the partnerorganisations should inform these partners of practical constraints and limitations and alsoof specific issues which they would like the partners’ organisations to address. Such a
constant open dialogue is necessary with more focused guidance and recommendations forthe completion of debris retrieval.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT40  NEA/CSNI/R(2022)7

4. Conclusion

The PreADES project, which was launched as a “near-term project” by the SEG on Safety
Research Opportunities post-Fukushima (SAREF), has been an active international cooperation initiative in which expertise and experience to support future debris sampling andretrieval was accumulated and shared.
A first task was completed to provide expected debris characteristics in the three damaged
reactors at 1F, based on knowledge gained from the TMI-2 and ChNPP4 severe core melt
accidents, relevant severe accident testing and preparatory testing at 1F.
A second task was conducted to identify the major challenges in terms of practicalities andsafety for future debris sampling and retrieval, and to share experience on approaches andmeasures to limit and control the risks related to these operations, in particular the risk of
re-criticality, the risk of losing cooling efficiency, the risk of radioactive release and therisk of occupational radiation exposure. A prioritised set of relevant sampling and analysis
plans was proposed to inform these risks and support the design of adequate measures fordebris sampling and retrieval. The project had proposed a first approach to define ananalysis plan intended to adequately support safe debris retrieval operations and provide
information and data of high significance for severe accident analyses. In this second task,
participating organisations also reviewed international facilities, including their capabilities
and experiences, for potential use in future projects to analyse debris samples from 1F. The
participating organisations also initiated discussions on the implementation of aninternational round robin analysis exercise using samples from past severe accident
experiments on realistic corium mixtures to enhance the preparation for future debris
sample analyses. This exercise may also help establish procedures for the preparation andanalysis of 1F samples in the future.
In the third and final task of the project, participating organisations discussed and proposed
future research that would entail timely and collaborative support during full-scale debris
retrieval operations. While a large knowledge basis was established and experience wasshared, significant knowledge gaps remain on the characteristics, composition anddistribution of in- and ex-vessel debris at 1F. This is primarily due to the uniqueness of the
1F accident, resulting in three severely damaged BWRs with different in and ex-vessel
degraded core configurations and lingering uncertainties regarding the accident
progression. Obviously, analyses of actual samples from the damaged reactors, including
samples obtained during prior explorations in the damaged reactor units and future
explorations, should bring information and data that will reduce these knowledge gaps. It
is recommended that, after the PreADES project, Japanese organisations in charge ofpreparing the debris retrieval operations continue to share data and information withparticipating organisations. The continued collaborative effort would seek to refine theknowledge of the debris characteristics and to assess how this new knowledge would affectdebris sampling and retrieval, as well as to enhance the understanding of the accident
development in the three damaged units. It is also recommended that new knowledge of
proposed analysis plans be used to contribute to refining debris sampling and retrieval
operations. Prioritised analyses to address the practicalities and safety of debris sampling
and retrieval would need to be proposed. Refined analysis plans to enhance understandingof the accident’s development would also need to be considered. The proposals are notexpected to be detailed operational analysis plans, as many practical factors should be
integrated, such as accessibility, the possibility to retrieve a sample, and the availability of
hot cell facilities for pre-treatment and analysis of radioactive samples from 1F. However,

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  41

they should provide useful guidance to prepare and conduct debris sampling and retrieval.
As occurred with TMI-2, it is expected that such plans will need to be often reviewed andoptimised as debris sampling and retrieval progresses. It is also recommended that aninternational round robin analysis exercise be organised using samples from past corium
experiments to further establish capabilities and experience related to debris analysis. This
exercise could also help sort out which analytical means are best suited to provide reliable
and highly significant data for debris sampling and retrieval as well as the understandingof severe accidents.
In addition to the three main tasks, the participating organisations recognised theimportance of considering analysis results in the PreADES project, and established the jointtask force from the PreADES, ARC-F, and TCOFF projects. The joint task force evaluatedthe formation mechanisms of U-bearing particles and other components based on thesample analysis and proposed some scenarios to possibly explain the formation
mechanisms of the U-bearing particles.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT42  NEA/CSNI/R(2022)7

Annex A. Figures and Tables
Table A.1. Characteristic table: macro TMI (part of table)

*1
*3
*4
*5
*6
*8
*9

Takano, M. et al. (2017), “Revisiting the TMI-2 core melt specimens to verify the simulated corium for Fukushima Daiichi NPS”, HOTLAB 2017, Mito, Japan, 17-22 Sept.
Nagase, F. et al. (2012), “Thermal properties of Three Mile Island Unit 2 core debris and simulated debris”, Journal of Nuclear Science and Technology, 49, 1, 96–102.
Olsen, C.S. et al. (1988), “Examination of Debris from the Lower Head of the TMI-2 Reactor”, GEND-INF-084.
Olson, C.S. et al. (1989), “Materials interactions and Temperature in the Tree Mile Island Unit 2 Core”, Nuclear Technology, 87, 57-94.
Marchetti, M. et al. (2020), “Elastic properties of severely degraded fuels”, Journal of Nuclear Materials, 529, 151918.
Akers, D.W. et al. (1990), “TMI-2 Core Materials and Fission Product Inventory”, Nuclear Engineering and Design, 118, 451-461.
Russell, M.L. et al. (1987), “TMI-2 Accident Evaluation Program Sample Acquisition and Examination Plan for FY 1987 and Beyond”, EGG-TMI-7521.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  43

Table A.2. Characteristic table: macro ChNPP4 (part of table)(1)Analysis at 1990, and converted to the value at the time of 26 April 1986*1
Arutyunyan, R.V. et al. (2010), Nuclear Fuel in the “Shelter” Encasement of the Chernobyl NPP, Nauka Publishing. (in Russian)*2
Krasnov, V. et al. (2015), “Monitoring of radioactive dust and LFCM state in destroyed Unit 4 ChNPP”, CLADS Decommissioning Workshop -International Collaboration
toward Advanced Decommissioning of Fukushima Daiichi Nuclear Power Plant -, Japan, November.
*3
Odintsov, O. (2012), “Study of solubility of radionuclides from fuel containing materials of object ‘UKRYTTYA’”, Problems of nuclear power plants' safety and of Chornobyl,
19, 70-80, 2012. (in Russian)*4
JSME (2013), “Study group on optimization of nuclear regulation” [Translated from Japanese.], A-TS 08-08, Jun.
*6
Krasnov, V. et al. (2015), “Current state of destroyed ChNPP Unit 4 and fuel containing materials”, CLADS Decommissioning Workshop -International Collaboration toward
Advanced Decommissioning of Fukushima Daiichi Nuclear Power Plant -, Japan, November.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT44  NEA/CSNI/R(2022)7
Table A.3. Characteristic table: macro 1F (part of table)
Image ofappearance(Shape)*1

RegionNo.1

U, Pu
RemarkSUS

Composition*3 Concentration*2*3 Concentration*2*3
(wt%)

(wt%)

B4C
Concentration*2*3
(wt%)

Gd

Cl

Concentration*2*3 Concentration*2*3
(wt%)

(wt%)

U
enrichment*2*3

Size*3Porosity

Bulk

Moisture

G Value*2*3*2*3

density*2*3(Molecule/100
eV)（vol％）(wt%)

3（g/cm ）

(wt%)

Compressive
strength*2*3(MPa)● Arround Upper
Plenum8
Molten and/or Main compound
failure structure : Fe

1300

(Microtable)● Unmolten failure fueland/or structures● Powder / Pebble

[H26]

[H26]

Unmolten failure
Main compound
fuel pin and/or: UO2, Zry-2
structure

90

0.05

0

0.005

0.0002

1.87

several(cm)toseveral(m)

10

1

0.5

280

Materials of
quenched andturned into
small pieces
from Molten
core structure

90

0.05

0

0.005

0.003

1.87

several(µm)
toseveral(cm)

88

14

0.5

230

Main compound: (U,Zr)O2-C ,(Zr,U)O2-T*1
Eidam: Eidam, G.R. (1986), Core Damage (1986), “The Three Mile Island Accident”, Chapter 5, ACS Symposium Series, 293, 87–106.
EPRI: Holton, W.C., C.A. Negin and S.L. Owrutsky (1990), “The Cleanup of Three Mile Island Unit 2 A Technical History: 1979 to 1990”, EPRI NP-6931.
H26: International Research Institute for Nuclear Decommissioning, et al. (2015), Research Report on the Development of Technologies for Characterization and Processing of Fuel Debris”, Subsidy programsfor the Project of Decommissioning and Contaminated Water Management.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  45
H29: International Research Institute for Nuclear Decommissioning, et al. (2018), “Interim Research Report on the Development of Technologies for Grasping and Analyzing Properties of Fuel Debris”,
Subsidy programs for the Project of Decommissioning and Contaminated Water Management.
*2 Value shown in red: Use reference values, Value shown in green: Engineering judgement value*3 Size (volume): Used for selecting the removal method, removal tools, for designing collecting cans, etc.
The form prior to removal is indicated. Note that this is subject to change, depending on the removal process.
Porosity (porosity ratio/void ratio): Used as referential values and/or for calculating moisture ratio while examining critical-state assessment models.
Bulks are expressed in terms of the porosity ratio. Fragments and powder are expressed in terms of the void ratio. Expressed as integers in consideration of uncertainty.
Moisture (moisture content): Used as referential values to assess the amount of hydrogen-generating sources for storage, and for input conditions in examining the drying processes.
Expressed as integers in consideration of uncertainty.
G (H2) value (hydrogen-generation G-value): Used to assess hydrogen generation during the storage and transport periods. Expressed with one significant figure in consideration of uncertainty.
Compressive strength: Used as referential values when examining and/or selecting machining tools, such as those for boring. Expressed with two significant figures in consideration of uncertainty.
Concentration: (U+Pu, SUS, B, Gd) Used as referential values when examining critical-state assessment mode.
(Cl-) Used to assess for wet storage, soundness (corrosion) of containers, etc. Expressed with one significant figure in consideration of uncertainty.
Enrichment of U: Used as referential values for examining critical-state assessment models.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT46  NEA/CSNI/R(2022)7

Table A.4. Characteristic table: micro 1F (part of table)
Category

Density

Materials/Phases(g/cm3)Vickers
hardness
(GPa)

UO2

11

6

ZrO2-T

6

11

(GPa)

Fracture
toughness(MPa･m1/2)

190-230

2

200

10

Elastic modulus

Thermal conductivity

Specific heat

Melting point(W/mK)(J/g･K)(°C )10(at 100 °C )

0.3

2850

0.6

(tr.)

Oxide

230(polycrystalline)

1

ー

3

Stability

UO2 should be oxidised
under oxygen existing,and it causes volume
change, makes
powders(at 1400 to 100 °C )(U,Zr)O2-C

6

ー

11

6

ー

18

140

ー

220

3

1

ー

10

0.3

ー

0.6

150

ー

200

8

1

ー

3

0.5

ー

0.6

2500

ー

2850

(at 1400 to RT °C )(Zr,U)O2-T

6

6

ー

18

(tr.)

(at 1400 to RT °C )

SiO2

2

ー

3

4

ー

17

Al-Ca-Si-O

2

ー

3

4

ー

12

100
40

ー

1(at <600 °C )1(at 200 °C )1.3

11(at 20 °C )

0.8

80

1710
1600

ー

1700

(at 20 °C )
Cr2O3

5

Fe3O4

5

FeO

6

Fe2O3

5

(Zr,U)SiO44

UO4･4H2O

ー

22

ー

29

7

100

ー

240

1

10

20

ー

110

2

20(at 25°C )

ー

33

0.8

2400

0.8

1597
1377
1566

9

4

8

ー

11

20

ー

110

2

6(at 100 °C )

0

ー

1

30

ー

50

1

1

0.5

0.8

(d.)

0.4

(d.)

23(at 675 °C )

0.3

1850

ー

10

ー

(at 100 °C )
Metal
Zry-2

7

1

ー

3

60

ー

110

α-Zr(O)

7

2

ー

11

120

ー

210

23(at 675 °C )0.3

SUS/Fe

8

1

ー

10

190

ー

200

200

80(27 °C )0.4(at 25°C )

7

ー

9

160

ー

200

3

80(at 27 °C )

0.4

1500

450

5

29(at RT)

2.3

2450

440

10

24(at RT)

0.7

3040

200

10

24(at RT)

0.6

1389

Fe2Zr

7

ー

8

15(ar 25 oC)
3ー

5There are explosion
cases of Zr fine

1850

ー

2130

1075

ー

1535

Others
B4C

3

ZrB2

6

Fe2B

7

24
19

ー

16

22

Legend: tr. : phase transition, d. : decomposition, RT : room temperature
value show in red: Use reference values
value show in green: Engineering judgement value

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  47

Table A.5. Analytical table①
Analytical Table
No.

③
Location of Sample②
Access time(including priority for analysis)
Region

5.56

Zone

④
Potential issues
concerning 1F
safety in Japan⑧
Priority in each process
High: 5 to Low: 1⑤
What/ How Obtained

What(Target Object)(Form)

How(Analysis items)

Local sampling
ICP-MS, SEM-EDS, EPMA, XRD,NDA (ex. NIGS) for canister (Bulk,
during retrieval)⑥
Why(Objective/ Motivation)⑦
Expected Benefit/Use(Larger frame)

For
For
For
For
Preparation
Transport Conditioning, For SA
Retrievalof retrieval
storage,disposalresearch

Drywell (Pedestal)

5.57

Bottom- Criticality
control

MCCI material

5.58

inside / outside pedestal- Criticality
control

MCCI material

5.59

inside / outside pedestal- Criticality
control

MCCI material

5.60

inside / outside pedestal- Criticality
control

MCCI material

Average density of (U+Pu) in MCCI material

Base information for calculate keff.

4

4

3

3

4

Local sampling
OM, X-ray CT, SEM-EDS, EPMA, αAutoradiography etc.

Distribution of U+Pu in MCCI material, homo /hetero,fuel particle larger than several mm should be treated as"hetero".

4

4

3

3

4

Local sampling
OM, X-ray CT, SEM-EDS, EPMA, αAutoradiography etc.

Particle size of U+Pu and its distribution (ifhetero)In MCCI materials, U+Pu are expected to form
“particle/island” in media. Criticality is attained if
neutrons from a particle induces fission in anotherparticle. These information is used for the calculation ofkeff, at first.

4

4

3

3

4

Crack and opened pore in MCCI material

Water path to regions around U+Pu

4

4

1

1

4

Local sampling
OM, X-ray CT, SEM-EDS, EPMA, αAutoradiography etc.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT⑨
Status

⑩
M
e
t
h
o
dU-1
s
/

⑪
Actual sampling date

U-2

U-3

48  NEA/CSNI/R(2022)7
Table A.6. Criticality control in analytical table (part of table)④
Potential①
issuesAnalytical Table
concerningNo.
1F safety +CC issues in
SA
Cr-4

Cr-5

③
Location of Sample

RegionZone⑤
What/ How Obtained
What(Target Object)(Form)
Damaged FA, stub,fuel fragment, crust,loose-debris, MCCImaterial, sediments,
sludge, slurry, water

How(Analysis items)⑥
Why(Objective/ Motivation)⑦
Expected Benefit/Use(Larger frame)⑧
Priority in each process
High: 5 to Low: 1For
Waiting
period for
retrieval

For
Retrieval

For
For
For SA progressTransport Conditioning(e.g. recriticality
storage,, disposal
during/post SA)

Local sampling
ICP-MS, TIMSNDA (ex. NIGS) for
canister (Bulk, during
retrieval)

Mass ratios U + Pu isotopic ratio
Isotope / (U + Pu) andIsotope / fuel debris

The maximum fissile content isused for the “minimum boron
concentration CBmin” toprevent/terminate any criticality
incident and to determine the max.
size of intermediate storage
container.

5

5

5

5

5

fuel fragment, crust, Local samplingloose-debris, MCCI- ICP-MS, TIMS etc.
material, sediments,
Initial location, lower sludge, slurry,
Criticality
vessel head, belowevaluation
RPV+PCV lower vessel head,focusing on U+Pu
floor of PCV inside /content
outside pedestal

Homogeneity of isotopic
composition of once-molten fuel.

Conservative CBmin by the max IE
assumption might be mitigated bymeasured homogeneous / uniform
distribution of the residualenrichment. Island of high-enrichedvolume of a pellet size should betreated as "hetero'

5

5

5

5

5

Bulk sampling + NDA (ex. Uniformity of distribution of nonNIGS)
melt fuel pellets and fragments

Frequency distribution of those canbe used for uncertainty evaluationof current state of keff.

3

3

3

3

5

Neutron absorber reducing keff.

3

3

3

3

3

Cr-6

Damaged FA, stubCr-7

Damaged FA, stub, Local sampling
fuel fragment, crust, ICP-MS, TIMS
loose-debris, MCCImaterial, sediments,
sludge, slurry, water

Mass ratio of Np,Am isotopes in/(U, Pu)

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Table A.7. Establishing containment function in analytical table (part of table)③
Location of Sample①
Analytical Table
No.

Region④Potential issues
concerning 1F
safety in Japan

Zone

What
(Target Object)(Form)

Pedestal floor- Establishingcontainment function

Black material,
Fragments

Ct-13

Pedestal floor- Establishingcontainment function

Black material,
Fragments

Ct-14

Pedestal floor- Establishingcontainment function

Black material,
Fragments

Ct-12

How
(Analysis items)

・Chemical forms by XRD,
XPS, Raman spectroscopy

Ct-16

Pedestal floor

Pedestal floor⑥
Why(Objective/ Motivation)

・Cl concentration
by ion electrodeParticles size (aerodynamic
diameter) by In situ dust
monitoring with size
- Establishing
Dust in cutting MCCI spectrometer or opticalcontainment function
debri
counter. If in situ
measurements are notpossible, post analysis withTEM.

⑦
Expected Benefit/Use(Larger frame)

For selection of chemical spray to to Soundness of containment function, judgement
prevent powdering and roiling in air for necessity of countermeasures is confirmed

Neededor notneeded

For
For
For
For
Waiting
Retrieval Transport Conditioning For SA
period forprocessstorage,, disposal research
retrieval

1

4SR / 2LR

1

1

NeededSoundness of containment function, judgement
for necessity of countermeasures is confirmed

1

1SR / 5LR

5

5

NeededEvaluation of corrosive environment
by elements leached from debris for Soundness of containment function, judgement
confirmation of containment
for necessity of countermeasures is confirmed
integrity and judgment

5

2SR / 5LR

5

5

Not

Assessment of transport, deposition,
resuspension coefficient, filtration
efficiency, mitigation means
efficiency (such as spray systems),
Soundness of containment function, judgement
radionuclide contamination.
for necessity of countermeasures is confirmed
For resuspension coefficient,thermal-hydraulics conditions andventilation & convection flows havealso to be known

1

5SR / 3LR

1

1

Not

1

5SR / 3LR

1

1

Not

・Residual water and organic
materials by thermal
Evaluation of hydrogen generation
analysisamount/volume concentration incanister・RN concentration by ICP-MS

PCV

Ct-15

⑧
Priority in each process
High: 5 to Low: 1⑤
What/ How Obtained

Particles mass or number
concentration by in situ dust
monitoring with size
spectrometer or opticalRadionuclide contamination, HEPA
- Establishing
Dust in cutting MCCI
counter. If in situ
filter clogging, dust explosion,containment functiondebrimeasurements are notagglomeration phenomena
possible, post analysis ofparticle mass deposit on
sampling filter

SR: Small removal/ LR: Large Removal(LR) Sample may be analysed before large removal.
PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTSoundness of containment function, judgement
for necessity of countermeasures is confirmed

50  NEA/CSNI/R(2022)7

Table A.8. Reducing occupational radiation exposure in analytical table (part of table)①
Analytical Table
No.

③
Location of Sample

RegionZone④
Potential issues
concerning 1F
safety in Japan⑧
Priority in each process
High: 5 to Low: 1⑤
What/ How Obtained

What(Target Object)(Form)

How(Analysis items)

Quick Look Video

R-13- Reducing
Lower head ofoccupational
RPVradiation exposure

Molten pool,
Crust
Rock,
Particle

R-14- Reducing
Lower head ofoccupational
RPVradiation exposure

Molten pool,
Crust
Rock,
Particle⑥
Why(Objective/ Motivation)⑦
Expected Benefit/Use(Larger frame)

Needed ornotneededFor
For
For
Waiting
For Retrieval Transport Conditioning,
period forstorage,disposal
retrieval

For SA
research

General safety concerns that were typically addressed in safety Improvement of calculation forevaluation report (SERs) of TMI-2 cleanup activities
dose evaluation

5

5

1

1

Needed・Handling of retrieved samples is assumed.
・Evaluation of effective dose and equivalent dose for large
removal.

Improvement of calculation for
dose evaluation

1

4

3

1

Not

Improvement of calculation for
dose evaluation

1

4

3

1

NeededPrediction for distribution of radiation sources in reactor, andto make countermeasure

Improvement of calculation for
dose evaluation

5

5

3

3

Needed・Particle size of dust in air by
SEM
Improvement of working efficiency and shortening exposure・Radionuclide concentration time by easier breathing is expected.
by ICP-MS

Improvement of calculation for
dose evaluation

5

5

1

1

Not

Surface dose rate (γ, n)by Gamma ray detector
Neutron detector

148- Reducing
Lower head ofoccupational
RPVradiation exposure

Molten pool,
Crust
Rock,
Particle

R-16- Reducing
Lower head ofoccupational
RPVradiation exposure

Molten pool,
Crust
Rock,
Particle

R-17

Atmosphere at- Reducing
lower head of occupational
RPVradiation exposure

R-15

RPV

Airborne
Particle, Aerosol148

Nd concentration using
ICP-MS

Properties related tomobility, such as vapor
pressure, leaching rate,
viscosity etc.

Nd is proportional to burnup. Because element is nonvolatile, it should remain in debris even though debris
experienced accident.
→If the burnup is known, evaluation of Cs total amount ispossible.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  51

Table A.9. Maintaining cooling function in analytical table (part of table)①
Analytical Table
No.

③
Location of Sample

RegionZone⑤
What/ How Obtained④
Potential issues
concerning 1F
Whatsafety in Japan (Target Object)(Form)

How(Analysis items)⑧
Priority in each process⑥
Why(Objective/ Motivation)⑦
Expected Benefit/Use(Larger frame)

High: 5 to Low: 1

For
For
For
For Retrieval
Conditioni
Waiting
Transport
processng,
period forstorage,disposal
retrievall

Needed or notneededFor SA
research

C-10

Pedestal floor- Maintaining
cooling function

・Heating power by
・Benchmarking relevant models and simulation
calorimeter・To identify ex-vessel corium state
tools, and improving our knowledge on debris
Debris bed (packed
・Geometry, porosity and・To assess cooling condition, adjusting it in removal formation
with fragments)
fragment size distribution by if necessary
・Determining debris retrieval/transport/
photography, SEM, etc.
storage strategy without heat-up risk

C-11

Pedestal floor- Maintaining
cooling function

・Benchmarking relevant models and simulation・Heating power by
・To identify ex-vessel corium state
tools
Cakes/agglomerates calorimeter・To assess cooling condition, adjusting it in removal
・Determining debris retrieval/transport/
・Dimensions by photography if necessarystorage strategy without heat-up risk

C-12

Pedestal floor- Maintaining
cooling function

・Composition by SEM
Crust of MCCI if any ・Location and mass byphotography

C-13

Pedestal floor- Maintaining
cooling function

MCCI debris if any

・Composition by SEM
・Location and mass byphotography

・Benchmarking relevant models (e.g. volcanoes
・To identify ex-vessel corium statein MCCI) and simulation tools
・To assess cooling condition, adjusting it in removal
・Determining debris retrieval strategy/method
if necessary
without heat-up risk

1

5

4

1

NeededC-14

Inner wall ofPedestal- Maintaining
cooling function

Attachment

・Composition by SEM
・Location and mass byphotography

・To identify ex-vessel corium state, in particular the ・Benchmarking relevant models (e.g. volcanoes
characteristics of debris/crust on the surface
in MCCI) and simulation tools
・To assess cooling condition, adjusting it in removal ・Determining debris retrieval/transport/
if necessarystorage strategy without heat-up risk

1

5

4

1

NeededC-15

Structures
under RPV

- Maintaining
cooling function

Penetration/
sediment

・Visual identification byphotography

・To identify ex-vessel corium state・Improving t model(s) for corium ejection
・To assess cooling condition, adjusting it in removal ・Determining debris retrieval/transport/
if necessarystorage strategy without heat-up risk

1

5

5

1

NeededLower
drywell

1

5

5

1

Not needed

1

5

5

1

Not needed

・Benchmarking relevant models (e.g. crack
・To identify ex-vessel corium state
formation) and simulation tools
・To assess cooling condition, adjusting it in removal
・Determining debris retrieval/transport/
if necessarystorage strategy without heat-up risk

1

5

4

1

NeededPREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT52  NEA/CSNI/R(2022)7

Table A.10. Hot analysis capabilities table (part of table)

Hot Cell Capailities Chart
Organization CNL - Canada
Capability
Section 1: Hot Cell Facilities General Description
Description

15 cells to perform post-irradiation examination of reactor components and fuel materials. Universal Cells
are flexible non-destructive testing facilities providing a range of capabilities in general purpose andmechanical testing along with facilitating Co-60 isotope extraction. Facility specs: 3 Cells UC1 and UC2(2.7x2.4x4.6 m), UC3 (4.9x1.8x4.0 m) all with 1.1m concrete shielding surrounding. Fuel and Materials
Cells provide a wide range of capabilities for destructive post irradiation examinations. The FMCs consist of
10 hot cells with various capabilities including: Gas Puncture and Fission Gas Capture, Fuel Sectioning andLeak Testing, Metallographic and Ceramography, Sample preparation, Optical Microscopy, DSC andprecision weighing, and Temporary sample storage. CNL maintains the certifications for Environmental
Management (ISO 14001:2004); Quality Management (ISO 9001:2008) and Accreditation of the Analytical
Chemistry Laboratories (ISO 17025).

Facilities

Universal Cells (UC), and the Fuels and Materials Cells (FMC)

Contact Person

Andrew Morreale (andrew.morreale@cnl.ca)

Section 2: Material Handling
Capability Description

Facilities

NRU Bays accommodate unloading of flasks, provide capability for shielded Fuel and component interim
storage (underwater) before and after examination; allow for underwater visual inspection of fuel andcomponents via portable telescope and camera setup.

NRU bays (underwater) to receive, store, and perform preliminary examination on fuel and reactor
components

Section 3: Sample Preparation
Capability Description

FMC Facility capabilities include fuel sectioning and sample preparation for various PIE analysis techniques.
High and low-speed saws allow fuel to be sectioned at precise locations to defined lengths in preparation
for future destructive examinations. The low-magnification microscope can image sectioned fuel to identify
locations of interest on the fuel sample.
Preparation for Optical Microscopy: Sample preparation cells allow for the cold-mounting, grinding,polishing and etching of samples to bring out the various microstructural features in the fuel and sheathing.

Facilities

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORTNEA/CSNI/R(2022)7  53

Figure A.1. “Unique” analysis items allocated in the Figure of debris’ location

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT54  NEA/CSNI/R(2022)7

References
Barrachin, M., A. Nakayoshi, M. Kurata, A. Morreale and N. Onder (2022), “Preliminary Investigationof Samples Taken from the PCV Inside of Fukushima Daiichi Nuclear Power Plant andImplications on Severe Accident Analysis”, Proc. of 19th International Topical Meeting on Nuclear
Reactor Thermal Hydraulics (NURETH-19), Belgium, 6-11 March.
Nakayoshi, A., J. Rempe, M. Barrachin, D. Bottomley, D. Jacquemain, C. Journeaue, V. Krasnov, T.
Lind, R. Lee, D. Marksberry, S. Mizokami, A. Morreale, D. Peko, M. Pellegrini and J.H. Song
(2020), “Review of Fukushima Daiichi Nuclear Power Station debris endstate location in
OECD/NEA preparatory study on analysis of fuel debris (PreADES) project”, Nuclear
Engineering and Design, 369, 110857.

PREPARATORY STUDY ON ANALYSIS OF FUEL DEBRIS (PREADES PROJECT): SUMMARY REPORT