Recent Findings from Fukushima Daiichi Unit 1
Primary Containment Vessel Investigations

M. Cibula1, M. Pellegrini2, T. Honda1, S. Shinzawa1, S. Mizokami1
1Tokyo Electric Power Company Holdings, Inc.
2The University of Tokyo
FDR2022, Fukushima J-Village
October 16 th, 2022

General information◼ Plant type: BWR-3◼ Pedestal radius: 2.5 m◼ Containment type: Mark I◼ Pedestal wall thickness: 1.2 m◼ Investigation tool: submersible ROVs

◼ Pedestal axis to shell (floor lvl.): ~6.5 m◼ Investigated area: PCV D/W floor◼ Pedestal floor area: ~20 m2◼ Water level: about 2 m◼ D/W floor area: ~90 m2

Estimated state based on previous analyses and investigations
RPVPedestal

PCV
Worker accessairlock

Pedestal opening(Worker access)Spread of debris
outside of pedestal

Fallen parts of CRD systemDeposits

1Investigation results up to 2022
◼ High contamination of reactor building closed cooling water (RCW) system (2011 R/B
investigation)◼ Water leakage from sand cushion pipe (2013) and vacuum breaker (2015) in torus room
◼ No large fuel mass detected in the core region (cosmic muon radiography in 2015)◼ 1st floor of PCV investigated (2015, 2017) and deposits found below on D/W floor◼ Corrosion products largely found in deposit samples (2017)2015, 2017 investigation scope
Basementfloor
pedestalopening

Sump pitwith RCWpiping

Torusroom leakpoints

Sampling

D/W 1st floor

D/W basement floor

2

State of the PCV basement before accident
Final aim of the investigation
RCW

Scope of theinvestigation

JD-A

JD-D

PLR(B)
PLR(A)
Eq. sumppumpRCW3

Image of state in front of the pedestal opening◼ Deteriorated pedestal concrete exposing rebar◼ Shelves attached to the pedestal concrete

PLR(A)◼ 2 layers of shelves attached to the D/W liner◼ Collapsed shelves in front of openingBC

PLR (A)

ADDPedestalopening

HE

EG

F

E: Jet deflector connecting D/W with torus room water leakage*Rendering created based on the past investigation
results and might differ from the actual state4

Results of the deposits height distribution investigation◼ Generally decreasing height with increase of the distance from the pedestal opening◼ Maximum evaluated height of ～1.1 m (outside of pedestal)
Contrast with estimate of core and reactor internals piled only in the pedestal: ～ 1.1 m**rough estimate not considering equipment inside pedestal, sump pits and downwards progression of MCCINA

D

Not investigated due torisk of cable tangling

X-2
penetration

Opening180°0°Upper shelf
PLR(A)
pipingCollapseddeposits

⑱

H

⑰

E
State in front of the pedestal opening⑭
Height from the D/W floor⑬
Not investigated due to presence ofinterfering objects near the water G
surface and risk of cable tangling

F90°0.8～1.1m
0.4～0.8m
0.2～0.4m

Measurement: Ultrasonic waves transmission/reflection while moving on the water surface
Evaluation: height of deposits from the D/W floor based on design data and water level of 2.0 m
5Distribution of deposits◼ Deposits height increasing with distance from the pedestal opening in some areas◼ Could some of them come from upper floors?

→FUnder PLR lead wool shielding→ :direction towards the pedestal opening
6

Deposits above water level◼ Deposits hanging on pipes and supports◼ Traces of solidified metal found on RCW and other piping

Upperopening

CRD exchange rail

→ :direction towards the pedestal opening
7

What could contribute to the amount of deposits?
◼ Lead shielding and Zn galvanized grating from all D/W area (relatively small amount)◼ Piping insulation materials?
◼ Materials coming from the pedestal area through the 1st floor opening?
Has to be considered to correctly distinguish and evaluate the spread of molten core
Solidified metal droplets?

2015 investigation of
1st floor grating area

Gratings of Unit 4 of Fukushima Daini(experienced temperature ~150°C)

ExposedpipeInsulation

D/W 1st floor
RCW piping near equipment drainsump pump (under water)

RCW piping above water8

State of the pedestal opening◼ No significant damage to the upper part of concrete◼ Deposits shelves on the right side of the wall increasing in height towards pedestal◼ “lavacicles” on bottom surface of the shelves
Free space existed below the shelves when “lavacicles” formed
ConduitConduitPhoto of the middle parttaken before 2011 notavailable

9Pedestal concrete degradation below shelves◼ Concrete degraded up to ~1 m from the floor◼ No significant deformation of inner skirt◼ Minor deformation of rebar (embossed pattern still visible)◼ Height of the deposits in the pedestal ~1 m
Possibility of heat being transferred by inner skirt/rebar
Deposits in pedestal(Ref.) State at the time ofconstruction

Remaining
concrete

Inner skirt

Rebar
ROV frame
Rebar

Bulky deposits in pedestal opening

Lower layer of deposits
10Pedestal concrete degradation below shelves◼ Many metallic structures found to be relatively
without deformations under the shelf◼ RCW piping not found◼ Rebar found without significant deformation
Low temperature concrete degradation?
Long term effects (coolant flow, etc.)?

Shelf

④ ③

②①RCW(Ref.) State before 2011④③(Ref.)①Cable relay box (A)②Cable relay box (B)③Support④Reactor Vent line
Flow glass②Lower deposits①Rebar11

Formation of cavities and shelves◼ Cavities found under the deposits◼ Collapsed shelves found in higher amount near Jet Deflector E and PLR piping12Formation of cavities and shelves◼ Collapsed shelves seemingly forming one layer before◼ Upper shelf coupled with the lower near liner◼ No visible deformation of jet deflectors, PLR piping or support structures
DPedestal opening
PLREDeposits

Liner13

Formation of cavities and shelves◼ Only one shelf with deposits piled from above◼ Collapsed shelves not found in big amounts
Different in nature than shelves near PLR and Jet Deflector E or buried in other materials?

Pedestal opening→ :direction towards the pedestal opening

←
EF

14

Summary
Several currently unexplained observations need to be investigated in more detail for
improved understanding of SA phenomena:◼ the origin and nature (fuel content, physicochemical properties, etc.)of the materials
comprised in D/W deposits (material balance)◼ formation mechanism of deposit shelves and cavities◼ relatively low temperature pedestal concrete erosion mechanism◼ soundness of PCV shell and other metallic structures in the vicinity and under the shelves◼ degree of pedestal wall concrete damage above/below the deposit shelves◼ effect of events (cooling water flow, etc.) in long term after the accident and possible
contribution of time-related deterioration to currently observed state15

Thank you for your attention（＃）