Toward Fuel Debris Analysis~ The Path from PCV Internal Investigation
to Fuel Debris Retrieval ~August 29, 2022

Shinya Mizokami
Fuel Debris Retrieval Program Dept.
Fukushima Daiichi Decontamination and Decommissioning
Engineering Company,TEPCO Holdings
無断複製・転載禁止 東京電力ホールディングス株式会社

1

1. Assessing conditions inside reactors based on accident progression analysis(before internal surveys were introduced)Time
BeforeEstimation⑩

Stable condition⑨

CCV failure⑧

FP release⑦

Reactor failure⑥

Reactor behavior⑤

System initiation④Operation⓷Component failure

Design & Ops InfoTsunamiEarthquake➀⓶

Based on the analysis of accident progression conducted in 2013,
it was assumed that the proportions of fuel debris that fell weregreater in the order of

Unit 1 > Unit 3 > Unit 2
無断複製・転載禁止 東京電力ホールディングス株式会社

FuturePastCore
SpraySystem

FeedWaterSystem2
2. Internal surveys conducted so far and the results

Muon
Measurement
Mar. 2015

Unit 2 PCV internalsurvey (Jan. 2017)
Unit 1 PCVinternal survey(Mar. 2017)
Unit 3 PCV
internal survey(Jul. 2017)
Unit 2 fuel debris
touching survey(Feb. 2019)• Starting with Muon measurement conductedin 2015, surveys to assess conditions inside
reactors have been conducted.
• Information on deposits inside PCVs has
been obtained. In addition, through atouching investigation, fuel debris has beenfound to be movable at Unit 2.

Unit 1 PCVinternal survey(from Feb. 2022)

Unit 2 trial retrieval offuel debris
to start in 2022
Retrieval via step by step approach

Further expand
retrieval scale
Partial submersion - Side entry method
Partial submersion - Top entry method
Submersion - Top entry method

Information from actual investigation results
has been added to assessment using analysis
code

無断複製・転載禁止 東京電力ホールディングス株式会社

３．Assessment of fuel debris distribution inside the reactor at Unit 1
using Muon measurement method

無断複製・転載禁止 東京電力ホールディングス株式会社

3

４． Reconstruction of 3D distribution image based on the resultsfrom PCV internal investigation at Unit 3

無断複製・転載禁止 東京電力ホールディングス株式会社

4

５． Touching investigation into fuel debris deposits inside PCV at Unit 2

無断複製・転載禁止 東京電力ホールディングス株式会社

5

6

6. Assessing conditions inside reactors by combining internal investigation results& accident progression analysis
Optimization of analytical conditions
TimeBefore

Estimation⑩

Stable condition⑨CCV failure⑧FP release⑦

Reactor failure⑥

Reactor behavior⑤

System initiation④Operation⓷

Component failure

TsunamiEarthquake➀⓶Future

PastCoreSpraySystemFeedWater
SystemInformation obtained at the Fukushima Daiichi decommissioning site
Upper tie plate
PCV floor
LV.
T.P. 4044

4
Accuracy of the past estimation proven & Info on concrete distribution added
無断複製・転載禁止 東京電力ホールディングス株式会社

７．

Results from analysis of particles sampled from Unit 2 refueling floor
Crystal structure analysis shows
existence of BCC (pure Fe) andspinel (Fe3O4)7

Coexistence of Fe and Fe3O4Fe(Liquid phase)

FeFeFeO
Fe3O4Cooling
Fe2O3Fe

Pure Fe precipitation Fe distribution

Fe,Fe3O4 Coexistence region- It was confirmed that U is contained up to 10% in a particle with a uniquemicrostructure that mainly consists of pure Fe and Fe3O4.
- Spherical shape is assumed to be formed from evaporation & condensation process- This particle was generated as an FeO particle under high temperature and Fe andFe304 phases were separated during the cooling process.
- Reflects the situation where the fuel (U) melted, involving in-core structures (Fe).
- The samples may be identical to the fuel debris inside the pedestal at Unit 2that is assumed to contain a good amount of Fe specific to fuel debris.
無断複製・転載禁止 東京電力ホールディングス株式会社

8. Assessing conditions inside reactors through synergetic effect between
internal investigation results & accident progression analysis

8

Optimization of analytical conditions
TimeBefore

0.5μm

SE (Secondary electron) image

Estimation⑩

Stable condition

Cubic -UO2

Identifying
Formation Processand Condition⑨

CCV failure

Amorphous -SiO2⑧FP release

Actual plant info.

⑦

Reactor failure

Compare withnormal situation⑥Reactor behavior⑤

System initiation④Operation⓷

Component failure

Design & Ops Info

TsunamiEarthquake➀⓶

FuturePastCoreSpray
SystemFormation
Mechanism
Modeling

Upper tie plate
Gapanalysis

Indirect Extra/Interpolation ofinfo. consideringformation process

Direct Extra-/Interpolation of info.
無断複製・転載禁止 東京電力ホールディングス株式会社

FeedWaterSystem９．

9Summary⚫ The analysis results for the fuel debris have enabled us tounderstand its properties and distribution, providing more
information on its composition and mechanical characteristicsetc.
⚫ If analysis can allow us to ascertain information as to how thefuel debris was formed at the time of the accident, anestimation of what has not been observed so far will become
possible.
⚫ Considering the limited number of analyses as compared tothe vast amount of fuel debris, assessment of the fuel debris
formation mechanism is essential.

無断複製・転載禁止 東京電力ホールディングス株式会社