Analysis of metallic sim-debris of the bundle
degradation tests
Anton Pshenichnikov

Presentation at the 2021 AESJ Fall meeting
pshenichnikov.anton@jaea.go.jp

Introduction
Source: TEPCO#1

Accident at Fukushima-1,
Photo of the Unit 3 taken on the17th of March 2011 from a helicopter

Source: TEPCO#2

Decommissioning preparation in Unit 2:
Pedestal debris inspection Feb. 2019

TEPCO decided to do a complete decommissioning of 1F damaged BWRs
CLADS is performing R&D to find new solutions for the decommissioning challenges
Main challenges:• Development of the decommissioning technologies for the damaged reactors of the 1F• Uncover the debris properties and distribution inside of the RPV and PCV of the 1F reactors#1 TOKYO ELECTRIC POWER COMPANY - Photos and Videos Library｜ Unit 3,Uniit4 of Fukushima Daiichi Nuclear Power Station (pictured from a helicopter) (tepco.co.jp)#2 TOKYO ELECTRIC POWER COMPANY - Photos and Videos Library｜ Fukushima Daiichi Nuclear Power Station Unit 2 Primary Containment Vessel Investigation Results
2- Preliminary Report on February 13 - (tepco.co.jp)

General understanding of the accident progression
Scheme of a BWR
during normal operation

Severe accident
progression

Decommissioning••
high dose rateCore
plateLower
plenumUncertainties
Accident progression in BWR• Final debris location
Debris chemical composition• Debris properties
In CLADS a test approach isdeveloped for studying simdebris after control blade
degradation tests to reduce theuncertainties

The goals• Understanding of BWR coreControl
degradation scenariosRod• Debris formation mechanisms
Bladesin BWRs• Possible properties of debris
under 1F-like conditions
Research results on debris distribution and properties would help
in choosing an effective decommissioning strategy and equipment3

10 years of R&D for the problem of 1F Severe Accident
ID DateSample features

Gas

CRFCB-1
20 Feb 2014

1 BladeAr
2 Channel boxes

CRFCB-2
20 Feb 2014
CRFCB-3
24 Feb 2014
CRFCB-4
26 Feb 2014
CR-1
3 Mar 2014

1 Blade2 Channel boxes
1 BladeAr
2 Channel boxes
1 BladeAr
2 Channel boxes

CRFCBF-01
16 Dec 2014
CRFCBF-02
18 Dec 2014
CRFCBF-03
2 Mar 2016
CRFCBF-04
7 Mar 2016
CLADS-MADE-01
22 Mar 2018
CLADS-MADE-02
18 Mar 2019
CLADS-MADE-03
12 Mar 2020
(post-test investigation)
CLADS-MADE-04
15 Sep 2021

1 Blade1 Blade2 Channel boxes
20 claddings
1 Blade2 Channel boxes
20 claddings
1 Blade2 Channel boxes
20 claddings
1 Blade2 Channel boxes
20 claddings
1 Blade2 Channel boxes
20 claddings
1 Blade2 Channel boxes
16 claddings
1 Blade2 channel boxes
16 claddings
1 Blade2 channel boxes
16 claddings

Initialrate, K/s

Maximum
T, K

2

1688

-Not tested

2

1807

21801

Ar

21713

Ar

21936

Ar

21818

Ar
Steam

21907

Ar
Steam

21910

Ar
Steam+Starvation

0.4

1750

Ar
Steam0.6

1880

ArSteam1

1872

Ar
Steam+Starvation

0.6

1900

CLADS-MADE-01 test (2018)**A. Pshenichnikov, S. Yamazaki, D. Bottomley, Y. Nagae, M. Kurata
Features of a control blade degradation observed in situ during severe
accidents in boiling water reactors, Journal of Nuclear Science andTechnology, Vol.56, No.5 (2019), P.440–453.
4

1F related severe accidents research using
Large-scale Equipment for Investigation LEISAN facilityof Severe Accidents in Nuclear reactors
LEISAN outer appearance

1.2 m-long

2016-present
2014-2015
Inside the furnace

CLADS-MADE-02 steam-rich conditions for 1F Unit 3 (2019)*

See next slide
2014-20152016-20192020-present*A. Pshenichnikov, M. Kurata and Y. Nagae A BWR control blade degradation observed insitu during a CLADS-MADE-02 test under Fukushima Dai-Ichi Unit 3 postulated
conditions, Journal of Nuclear Science and Technology,
5
https://doi.org/10.1080/00223131.2021.1906777

Mock-up of BWR assembly after CLADS-MADE-02 testLower part Slight oxidation
T < 1174 °C

Only oxidized
T > 1500 °C

beforeafterSample
SupportT < 1000 °C

Damagedcontrolblade partsRemaining control rod
1400 < T < 1500 °C

More details in Proceedings of the ICONE 28,
4-6 Aug 2021, Paper No. ICONE28-65129

Solidified melt in the≈ 1200 < T < 1400 °C

A 10 mm-thick pile of thecontrol blade melt 6

Three types of debris
at the beginning of an accident in BWRs
Type II Oxidic debris(ZrO2 or UO2 or mixture of them + minor oxides + FPs)
Type III Degraded rests of a control blade or assembly parts(may contain original chemical composition
but changed microstructure)
Type I Molten and solidified metallic debris blockage(chemical composition?)
Type II Relocated oxidised debris
Type I Relocated and solidified metallic debris
Type III Relocated parts of a control blade
A. Pshenichnikov, Y. Nagae and M. Kurata, On the degradation progression of a BWR control
blade under high-temperature steam-starved conditions, Mechanical Engineering Journal, Vol.7,
No.3 (2020), P.1-10.

Post-test schematic 3D view7

Post-test control blade debris characterization
Optical microscopy
CLADS-MADE-02
section 6, 510 mm
Tmax= 1508 K(1235 °C)

ReactedchannelboxCrackingafter cool-down

Raman spectroscopyRemaining B4C

Melt blockage cross-section
New-formed graphite
Channel box(Zircaloy-4)

B4C
granulesEx-tubes
with B4C
now melted
Ferrous melt

control blade covered by melt

Area reacted with ferrous melt
B4C turned into graphite
8

Control blade debris characterization by Raman spectroscopy*
762 mm, Tmax = 1663 K (1390 °C)
C map

B4C
map

metallicmap
Liquid Fe melt
penetrated
everywhere

Patrial
B4C+C
Completedegradation
only C

H3BO3
map
Debrissurface*In Proceedings of the TopFuel 2021, 24-28 Oct 2021 (online). Paper accepted. To be published.
Raman investigation of the CLADS-MADE-02 test debris to confirm the mechanism of the volatile and non-volatileboron compounds formation
Anton Pshenichnikov, Yuji Nagae, Masaki Kurata

9Outcome: Comparison with the real debris
Type I. Solidified melt (metallic)

Type I. Stone-like melt agglomerates (metallic)

LEISAN facility
CLADS-MADE tests
Type II. Zr oxidized debris (oxidic)

Tokyo Electric Power Company Holdings, Inc.
2019/02/14Fukushima Daiichi Nuclear Power
Station Unit 2 Primary Containment VesselInternal Investigation
Available from:https://www.tepco.co.jp/en/news/library/archivee.html?video_uuid=vy9uep38&catid=61785

Type III. Partially melted original parts (metallic)

A. Pshenichnikov, Y. Nagae and M. Kurata,
Comparison of Fukushima Dai-Ichi Unit 2
observed debris with simulated debris fromCLADS-MADE-01 control blade degradation
test, Journal of Nuclear Science andTechnology, 58:4, P.416-425 (2021)

10

Conclusion• Sim-tests look promising for grasping the mechanisms of initial core melting on alarge scale, in particular, for understanding of a BWR control blade degradation• Over the last 10 years much of new data and insights were accumulated on thefeatures of the high-temperature degradation of a preoxidized BWR bundle with acontrol blade under steam-starved and steam-rich conditions• Post-test investigations revealed mostly three types of sim-debris• Post-test characterization suggested that sim-debris need to be further
investigated regarding their properties because same kind of metallic debris with B
is possible also in the damaged 1F units• Comparison of the sim-debris and the TEPCO HD 1F Unit 2 PCV investigation data
showed many similarities between the outer appearance of the sim and the realdebris, which is promising for study of the real debris formation mechanisms byusing sim-tests
pshenichnikov.anton@jaea.go.jp

Thank you for your attention
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