EXAMINATION AND EVALUATION OF DEBRISFROM THREE-MILE ISLAND UNIT-2 (TMI-2)
presented by P. D. W. Bottomley (JAEA Invited Researcher)formerly Sector Head, European Commission DG JRC-Karlsruhe
Directorate G - Nuclear Safety & Security
Unit G.III.8 - Waste Management
PO Box 2340, Hermann-von-Helmholtz Pl. 1, 76125 Karlsruhe,
Germany
dboksb3@gmail.com

Outline of Presentation• Introduction to JRC nuclear work programme• TMI-2 Accident & Sample Investigation Project• Other fuel debris samples• a) Phébus pf bundle PIE• b) Chernobyl samples• Conclusions

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The European Commission’s
science and knowledge service
Joint Research Centre

JRC Karlsruhe Site =Unit G Nuclear Safety & Security
Unit G.III Nuclear
decommissioning = Ka site +units in Ispra (HoD VVR)
Unit G.III.8 Waste Management
ex-Hot Cells (HoU JS)
NDF Forum 01.07.2017

EURATOM Work Programme at the JRC(ie. Nuclear WP of JRC)
JRC-Petten

The mission of JRC-Karlsruhe
is to provide the scientificfoundation for the protection of
the European citizen against
risks associated with thehandling and storage of highly JRCradioactive material.
Headquarters- Brussels
JRC-Karlsruhe’s prime objectives
are• to serve as a reference centre
for basic actinide research,• to contribute to an effective
safety and safeguards system for
JRCthe nuclear fuel cycle, andSeville• to study technological andProspective
medical applications ofstudiestransuranium elements.

JRC-GeelStandards for
Nuclear Safety

Knowlege forNucl. Safety,
Nucl. Reactor
Safety &Emergency
Preparedness

JRCKarlsruhe,

Waste Management
Remediation, Nuclear
Fuel Safety, Nuclear
Safety, Safeguards &forensics, Advanced
Nucl. Knowledge

JRC- Ispra

Nucl. Sec.
&Safeguards
Nuc.
Dcommissioning,
Nucl. Safety-Ispra

JRC Sites

Non–nuclear
insitutes
1) Health &ConsumerProtection,2) Protection &Security of the
Citzen,3) Institute forEnvironment &Sustainibility

Unit G.III Nuclear Safety & Security
Section 8: Waste Management ( Hot cell laboratory)

Hot Cell facilities at JRC-Karlsruhe• 24 hot cells (licensed capacity 106 Ci = 3.7 1016 Bq)• ~ 160 kg of irradiated fuel (80 LWR fuel rods) and 3.5 kg Pu• shielded SEM, OM, EPMA, SIMS, XRD• infrastructure: supporting workshop incl. manipulators
maintenance• 3 hot labs for characterization of non-irradiated materials

Nuclear fuel studies at JRC- Karlsruhe
Topicsthermal transport
thermodynamics(specific heat, vapor
pressure, melting point)

Tools

LWR,
advanced reactors
HTR
high burnup UO2
MOX
MXinert matrices

matter transport (fgbehavior, actinides, fp)
radiation damage:mechanisms and effects

optical, electron microscopy, XRD
EPMA, SIMS, TEM-SEM

laserflash, POLARIS
high T laser-heating (melting,vaporization, conductivity, high-P)
Knudsen cell, hot cell annealingfuel rod (profilometry, radiography,
spectroscopy, puncture)

cladding

steady state
off-normal, accident
property changes
storagemicrostructure characterization

rim structure formation, evolution

corrosion, creep

densitycladding behaviour (creep)(hot) indentation
chemical analysis
leaching, electrochemistry
reprocessing/partitioning studies

multidisciplinary approachanalytical/modeling tools

Three Mile Island (TMI-2)

TMI-unit 2 (TMI-2) underwent a prolonged Loss of Coolant Accident(LOCA) on March 28th 1979. Subsequently the examination of TMI-2
samples was organised as OECD-NEA, CSNI project under the initiativeof the US Dept. of Energy (US-DOE) involving most major European
national research institutes

INEL Idaho (as principal contractor) had extracted and examined thesamples from TMI-2. It then shipped samples to N. American &European institutes :AECL Canada,
PSI Würenlingen,
Argonne National Lab
Studsvik,
FZK-Karlsruhe,
CEA Saclay,
JRC-ITU Karlsruhe,
AEA Windscale
as well as JAERI Japan.
The principal aims of the examination were to establish:- what was corium (& other phases) composition- what temperatures were reached- what conditions (oxygen potentials/H2 production*) prevailed
hence what degradation reactions were likely* from Zircaloy steam oxidation

TMI-2 Examination: Positions of ITU samples

fuel rod
remnant
C7 3-35

Damaged core of the
TMI-unit 2 reactor in its
end-statedebrissamples
H8 7.2-7.9
upper
crust
D8-P2,3

lower
crust
N5-P1-E
O7-P4
core bore rocks
G12-P2-E, G12-P6-E,
G12-P9-B, G12-P10-A

1) molten core (yellow),2) fused crust/agglomerate(orange),3) loose debris above (red)– location of powder &loose fuel remnants.

green labels show samples
received by JRC-Karlsruhe
under the OECD TMI-2
sample analysis project.

TMI-2 Examination: Debris lying on top of melted core

Cross-section of cladding offuel rod remnant C7-3-35

ZrO2
Full &
ntergranular aZr(O) zone

C7-3-35 Fuel rod segment.
Note thin external ZrO2 oxide layer
present with a-Zr(O) containing
layers beneath.

Thin external oxide layers
indicate only slight exposure
to increased temperatures.
(~800C)a-Zr(O)containing
zone
ZrO2

TMI-2 Examination:
Core bore rock

Core bore rock wasprincipally oxidic
G12-P9-E Fracture Surface -SEMa) & b) show dense (U-rich) phases(white) and lighter (Zr-rich) oxide
phases (dark)a)b)

60x mag

60x mag

Ferrous-rich
phasec) Many fine metallic Ag precipitateson the surface.
Fe-rich oxide phases also present.

c)600x magTMI-2 Sample Examination
Core bore rock samples
TMI-2 - example of Molten core –bored–out rock samples
ferrous-richphaseU-rich phase

Zr-rich phase

Core bore rock G12-P9-B
Back-scattered image SEM image showing 3 phases in the fully molten rock:- dense or white (U-rich) phases;- less dense (grey) (Zr-rich)phases;- light element (dark) Fe ,Ni, Cr-containing phases
NDF-Forum, Iwaki, Japan 3July17

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TMI -2 Examination Core Bore Sample Analysis

Zr-richoxide

G12-P10-A fracture surface a)secondary & b) backscatteredimages. (2760x)
Note a) fine eutectic lamella approx.
0,5µm wide (to estimate cooling rate);b) small spheres are Ag spheres from
Ag-In-Cd absorber

U-richoxide
Fe-richoxide
inclusion
10µm

G12-P2-E (Mag. ~900x) 2-phase corium, lighter Urich oxide & darker, Zr-rich oxide +Fe inclusion.
matrix composition was approx. equimolar (U,Zr)O2

TMI -2 Sample Examination: Agglomerates

Agglomerate crust O7-P4-EA
Fe-Ni nodules with pure Fe metallic core (lighter
centres) surrounded by Fe-Ni oxide crust (darker
zones) due to preferential oxidation (BEI-1000x)
NDF-Forum, Iwaki, Japan 3July17

TMI-2 Sample Examination Agglomerates

TMI-2 Crust Samples (Agglomerates)
82Fe-8Ni

82Fe-18Ni

At. %U-20Zr-5Mo-3Sn-0,3 Cs

55Fe-44Ni
U-5Zr

54Fe-44Ni
U-22Zr-6Fe-3Pu

98Fe
At. %

SEM image of an agglomerate sample from
O7-P4-E zone of the lower crust.

- a Fe-Ni nodule is seen with a Ni-richoxide layer on the outside– example of preferential oxidation of
the Ni (intermediate oxygen levels)
NDF-Forum, Iwaki, Japan 3July17

Backscattered electron SEM imageof an upper crust (or agglomerate)
sample D8 P3-A.

Elements of fuel, cladding & structural
material are seen.
There are wide grain boundaries(containing eg. Zr & Fe) in U-rich grains.
This shows gradual material interactions
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indication of lower temperatures than core

TMI-2 (Agglomerate material - interference macros)

2 phase metallic/oxidic zone

2 phasemetallic zone

D8-P3-A Agglomerate sample(190x)
interference micro showing
metallic and oxidic zones,
both with secondary phases.
Indicates incomplete
interactions - lower
temperatures or lower
durations at a temperature

Oxidic zone withsome secondary
precipitates
NDF-Forum, Iwaki, Japan 3July17

TMI-2 Agglomerate - metallic and oxidic phases

Example of partially oxidised material from agglomerate

Agglomerate N5-P1-E (1033x).
(CuO vapour coating)
Stainless steel control rod cladding with Ag spheres that contain Cr oxide nuclei.
A metallic-dominated phase from a control rod
r ~6.7 g/cm3 (compared to steel r= 7.93 g/cm3) implies it is steel dominated.

TMI-2 Debris samples
H8 7-3 from above the upper crust - BEI of a debris sample- Note nodule with a lamellar structure of Fe- Ni & Fe-Sn phases surrounded by a Cr-rich layer.
- Nodule itself is surrounded by Zr.
- Fe Fe-Ni & Fe-Sn phases have a lustrous metallic appearance.

light
Fe-Sn
dark
Fe>Ni
Cr-richZr

NDF-Forum, Iwaki, Japan 3July17

TMI -2 Sample Analysis -DebrisWhite
piecesare UO2Long grey pieceis zircaloy-UO2
mixtureMacroscopic photo ofvariousdebris pieces(approx. 5x)
NDF-Forum, Iwaki, Japan 3July17

bandedstructure iszircaloyinteracted withsteels or Nibased alloys

Debris H8-7-5-1 located on theupper agglomerate (40x)Note variety of debris particles: unchanged UO2
fuel particles & cladding, oxidised cladding
fused cladding, steel –cladding intractions.
Particles have different origins and histories,
and have suffered different temperatures

TMI-2 Ellingham Diagram of samples
OxygenPotential(kJ/mol O2
Oxidestability
zone

-400
Range of oxygen
potential observed incore bore rocks &agglomerate zones

This gives an indication of theconditions (T and PO2 or
PH2/PH2O) in the reactor for thecentral rocks and surrounding
agglomerate zones

-800
Metal
stability
zone

-1200

1200C

Temperature/ °C
PH2/PH2O (atm)
PO2 (atm)

2000C

TMI-2 Accident: Summary of conditions andtemperatures experienced-1
Conditions during accident
1) Max. Temperature- Edge of reactor
T < 800°C- Agglomerate T~1500°C (stainless st. mp)- fully molten core
T= 2000-2500C(some pure UO2 seen T=2850C?)
2) Cool-downcore- slow ( 2-54 h)
Agglomerate- more rapid & variable
Edge of core - transient rise in temp.;
only slight degradation
Phases formedCore - a UO2 fuel & Zry cladding melt that oxidised in steam to form H2 and:an U,Zr-containing oxide with a U-rich phase, a Zr-rich phase &smaller amounts of Fe,Ni,Cr oxides & Ag nodules
Agglomerate - mixed metallic and ceramic phasesfrom fuel/cladding/structure interactions (often incomplete)eg. (U,Zr)O2 phases, (Fe,Ni)-Zr-U oxides, Ni-Fe-Sn metal,
Ni,Fe partially oxidised nodules, & some Ag metal nodules

TMI-2 Accident: Summary of conditions andtemperatures experienced-21)Core bore rock were fully oxidic U,ZrO2 with Fe-rich (with Cr, Ni)phases.
2)Al2O3 also seen from burnable (Gd2O3) poison rods.
3)Slightly superstoichiometric
4)Oxygen potential during the accident were estimated at -150kJ/mol(pH2/pH2O = 1) at 2000C to -510kJ/mol O2 (pH2/pH2O = 106) for 1200C
temperature. Suggests high H2 presence could be possible at times.
5) Core centre probably cooled down in 4-50 hours (estd. from various
lamellae thickness in structure) .
6)Agglomerates were mixed cladding/fuel/structural materials debris
in partially oxidic & metallic form
(Fe-Ni-Sn metal, (Fe,Ni)-Zr-U oxides, partially oxidised Ni,Fe
nodules, metallic Ag nodules)Phébus FP project

lead by IRSN, Cadarache (with EC support) &participation of many EU national institutes)

PhébusPF
Facility

Phébus FP (fission product) with a driver core, test bundle and vertical line leading to the simulated
primary circuit. The circuit has a horizontal line with a simulated steam generator before the airborne
fission products pass into a containment tank & a sump to collect liquids.
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Phébus FP project

IRSN Cadarache
Tomography
Degraded
upperbundleComparative Bundle Post
irradiation Examination(PIE)c) FPT2

CentralCavityX-ray tomographic scans produced by
IRSN directly after testing:a) FPT0 (trace (8day) irradiatedfuel),b) FPT1 (2yr irradiated fuel) & steam
flowc) FPT2 (low steam flow- reducing
conditions).

Coriumpool

Degradation results common to all 3
tests:a) an heavily-oxidised upper bundleb) melting of hot central zone & its
relocation to produce :c) a corium pool at quarter-height .
FPT1- irradiated
FPT0-trace
IPSN Cadarachefuel –full steam
irradiated fuel –flowfull steam flow

FPT2- irradiated23
fuel –low steamflow

Phébus FP project
Upper Phébus bundle(FPT1)
fuel adhering
to cladding

large columnar
grains of ZrO2

cladding
fuel

200 µm

60 µm

Micrograph of fully oxidised cladding
from a fuel rod at +607 mm height of
FPT1 bundle.

Micrograph of degrading irradiated fuel at the+607mm height of the FPT1 bundle- lenticular porosity due to fission gas bubble
movement in overheated fuel.

Note heavily oxidised cladding anddegraded fuel, in semi-liquified condition24

Phébus FP project
Fuel rod
remnant

Corium Pool
PIE
Comparison of the corium pool of
FPT1 bundle at +228mm height(from bundle bottom) with thetomography for the same position.

outer
crust
Corium pools compositions of FPT0, FPT1
& FPT2: U/Zr atomic ratio= 1.06 to 1.44
TMI-2 core (G12-P 9-B) U/Zr estd. at 1.18
- Non-destructive tomography is very accurate- Corium pool composition reasonably
consistent between Phébus tests and TMI-2
data

Central pool25

Lava samples from Chernobyl reactor unit 2 at ITUPart of a collaboration of JRC-Ka with
Khlopin Institute, St. Petersburg(P. Pöml/B. Burakov)

Sample of brown lava and SEM micrograph of crystal structures found in
this lava sample. The highly dense (white) phases are U-rich fuelparticles in a poly-phase matrix.

Lava samples from Chernobyl reactor unit 2 at ITUa) secondary electron imageb) Si X-ray image

Backscatteredimage of U
zircon crystal
with inclusions.

c) U X-ray image••d) Zr X-ray image

X-ray image mapping shows the layered compositions during slow cooling to form a
U-rich centre and Zr & Si-rich outer edge.
This can be used to determine its temperature as it solidified. This data can be used tovalidate lava flow models and to understand the conditions of the accident

Conclusions•

Examination of samples in TMI-2 investigation have enabled majoradvances in understanding of reactor degradation mechanisms.

•

Techniques include simple density & porosity measurements forpreliminary characterization/sorting of samples, detailed compositional
analysis (by SEM/TEM/EDS/WDX) and crystallographic determination
(XRD)

•

Assessing the compounds metals/oxides formed has enabled the possible
interactions, and mechanisms of degradation to be proposed; theconditions of formation (eg. temperatures reached; O2 potential/steam
content); also estimates of H2 formation can be made.

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Conclusions (contd.)/ Outlook•

Later research (eg. Phébus PF project & Chernobyl but also elsewhere) has
improved bundle degradation & PF release knowledge.
 Reproducible corium pool geometry was demonstrated
 Zry cladding interactions with reactor steels etc. (giving low-melting
mixtures) that attack fuel & cause relocation at low temps.(~1200C)
was confirmed.
 Detailed geometry effects (eg BWR/VVER design) & other materials(eg B4C moderator) still need research.

•

X-ray tomographic techniques have been very accurate and are powerful
non-destructive techniques (other NDT techniques eg. g-spectroscopy
continue to advance)•

Modelling and simulation techniques are also a very important tounderstand reactor accidents and materials behaviour modelling.

•

Current research is more oriented on understanding fission productbehaviour, Corium interactions with reactor vessel and its retention &Corium interactions with concrete containment.
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