U.S. Perspectives Based onTMI-2 Defueling Activities
6th International Forum on theDecommissioning of the Fukushima Daiichi Nuclear Power Station
Fukushima Prefecture
August 28-29, 2022

;1Presented by: Joy RempeContributors: Joy Rempe, Rempe and Associates, LLC; Mitch Farmer, Argonne National Laboratory;
Damian Peko, U.S. Department of Energy; and Donald Marksberry, U.S. Nuclear Regulatory Commission

TMI-2 Background

Three Mile Island Unit 2 (TMI-2)
Accident on March 28, 1979
 Design and Other Considerations:• Plant contained two Babcock &Wilcox Pressurized Water
Reactors, each rated at 2,772 MWt• Two loop nuclear steam supply system housed in a large drycontainment• “Titanic” mentality about design safety existed

 Event Synopsis:• Loss of steam generator feedwater during maintenanceon condensate polishing demineralizer system• Pressurizer pilot operated relief valve failed to closeleading to undetected small break loss of coolant accident

Image Courtesy of EPRI (1979) Analysis of Three Mile Island– Unit 2 Accident. Electric Power Research Institute Nuclear
Safety Analyses Center Report NSAC-80-1, p. 28/531.

• Subsequent operator actions led to severe accident‒ Approximately 26% of core region voided; 20,000 kg of material relocated to reactor pressure vessel (RPV) lower head
‒ Hydrogen release led to ignition in containment• Limited radioactive material release: 15 Ci (560 E9 Bq) of Iodine-131; International Nuclear Event Scale (INES) Level 5[Accidents at Fukushima Daiichi rated as INES Level 7]• 144,000 persons within 15 miles voluntarily evacuated for ~1 week2

Insights Applicable to Daiichi

TMI-2 Approach to Identify and Prioritize
Cleanup Information Needs Process relies on instrumentation data,
post-accident examinations, existing
severe accident knowledge, andengineering analyses Efforts initially focused on stabilizing theplant before focusing on cleanup Key to prioritize activities, emphasizing those that:• Minimize future radiation releases and site hazards,• Ensure safe and efficient cleanup, and• As resources allow, reduce uncertainties related to accident progression and reactor safety enhancement.

• Most high priority information desired for reactor safety insights required for cleanup3

TMI-2 Background

Various Debris Retrieval Methods EmployedCore boremachine(adaptedmining drill) Six major regions: core cavity, lower core support
assembly (LCSA), behind and in the core baffle plate,
lower reactor vessel head, and ‘elsewhere’ in the plant Different location geometries and debris types required
different retrieval methods:
 Core Bore Machine (solidified molten core, LSCA cutting) Cutting (Plasma Arc, Hydraulic Shears and Saws)
 Bulk Removal ( Air Lift)
 Mini-submarine (in pressurizer)
 Manual Controlled Equipment (Grippers, Buckets) Difficulties (resolution) In-situ repairs (mockups and testing, spares and repair tools)
 Different debris/structure properties (prototypic testing) Water clarity (hydrogen peroxide) Heterogeneity (distribution and mass of samples)

Intact TMI-2 vessel and containment simplified cleanup
Graphics courtesy of EnergySolutions and INL

Manualscooper

Core grab
sample tool4

TMI-2 BackgroundExaminations Provided Insights for
Cleanup and Reactor Safety

K-9 Core Bore Image Showing
Material Interaction andCladding Dissolution
Graphics courtesy of INL and EnergySolutions

5

TMI-2 BackgroundCore Bore Machine Provided Insights for Reactor
Safety and Facilitated Subsequent Defueling
(a)

(b)

(c)

 Defueling paused for core bore
sample retrieval
 Acquired 9 full length coresamples of debris andremaining fuel structure Primary purpose: benchmark
severe accident codes[elemental composition,oxidation state, materialinteractions, peak temperatures,and retained fission products]
 Secondary purposes: support
defueling planning and provide
access to lower core supportstructure(d)Core bore machine(adapted mining drill)
Graphics courtesy of INL andEnergySolutions

 When defueling resumed, core
bore machine repurposed todestroy hard layer andeventually core supportstructure

Mosaic photographs of core bore samples for (a) peripheral core position D8,(b) central position G8, (c) peripheral position G12, and (d) central position K9.

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TMI-2 BackgroundTMI-2 Examination Perspectives
Desired Information
Debris location, topography,
fuel and structure damage(distortion, slumping, melting,fragmentation, oxidation,asymmetries, etc.)
Debris location andtopography

Graphics courtesy ofEnergySolutions and INL

Methods

Planned Use/Comments

On-Site (In-situ)
visual images (photos, Provided insights about core damage and debris
videos, etc.)
location, melt progression and insights for selection ofsubsequent samples. Documented conditions of samples
prior to removal.
Retrospective Comment: Most useful information fordefueling planning and design and model development /confirmation; required for cleanup
ultrasonic topography Backup to visual images, provided insights about debris
systemlocation; used throughout defueling efforts.
Retrospective Comment: Useful information for grossmodel development/confirmation. Confirmatory data fordesign and qualification of equipment for debris
removal, transport, and storage.

For additional information regarding TMI-2 examination requests, see EGG-TMI-6169 at:https://inldigitallibrary.inl.gov/TMI/EGG-TMI-6169-r1.pdf#search=EGG%2DTMI%2D6169
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TMI-2 BackgroundTMI-2 Examination Perspectives (Cont’d)Desired Information

Methods

Planned Use/Comments

Laboratory

Graphics
courtesy ofINL andEnergySolutions

Reactor coolant system chemical,
Data to benchmark code
structures andradiochemical,
predictions for temperaturecomponent information and metallurgical and fission product transport[peak temperatures,exams, andand deposition. New models
temperature history,laboratory
developed, as needed.
and inventory,techniques forRetrospective Comment:
distribution, form, and measuring
Fission product distribution
size (if particulates) of physicaland temperature information
deposited fission
properties ofused for gross model
calibration.
products)]
deposits

8TMI-2 BackgroundTMI-2 Examination Perspectives (Cont’d)Desired Information

Corium,vessel,andnozzle
samples
Broken Fuel Rods

Lower Head Look
Graphics courtesy ofEnergySolutions

LaboratoryLoose Debris – composition, fission product retention/release, physicalform (size, porosity, permeability), peak temperature, liquefaction
temperature, cooling rate
Fuel rod segments – characterize fuel rods at boundary or transitionzone between the melted and rubble debris; fission productretention/release in intact pellets, oxidized fuel, liquefied fuel,fuel/cladding interactions, residual cladding integrity, peak fuel andcladding temperatures.
Core stratification samples (bore samples containing several fuel andone control rod) – composition (ratio of fuel to non-fuel), materialsinteractions (damage, materials interactions), fission productretention/release, coolability information (porosity, permeability),
fission product retention/release)
Components (spacer grids, in-core instrumentation, end fittings, vesselsteel) - characterize peak temperatures and materials interactions

Methods

chemical, isotopic,
elemental,radiochemical, gammascanning,metallographic exams
and various laboratory
methods (pushrod
dilatometry, laser flash
diffusivity, etc.) to
obtain mechanical(hardness, tensile
strength, compressivestrength) and thermal
properties (density,thermal conductivity,
thermal expansion
Debris relocated to the vessel lower head ( grab samples from hard layer
coefficient, specificof debris after breakup) – composition, fission productheat capacity, liquidus
retention/release, fission product retention/releasetemperature)

Planned Use/Comments

Data to develop and benchmark code predictions
for temperature and fission product transport anddeposition. New models developed, as needed.
Insights regrading reactor pressure vessel integrityand potential failure modes.
Retrospective Comment: Location-dependent
composition data and fission product deposition
data useful for gross model calibration butproperties vary with oxidation and porosity. Easier
to obtain with unirradiated materials (confirmingirradiation effects, as needed). Peak temperature
information only useful for gross calibration ofmodel results. Confirmatory data for design andqualification of equipment for removal, transport,and storage. Additional data/photos tocharacterize debris/structure interface on lower
head would have been useful.

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TMI-2 Background

International Participation Important Aspect ofDeveloping Accident Progression Consensus Insights
 International programs included sample
examinations, stand-alone testing, and systems
analysis code calculations.
 Exams completed by laboratories in European Union
Joint Research Centre, Canada, France, Federal
Republic of Germany, Sweden, Japan, Switzerland,and United Kingdom
 Included debris ‘grab’ samples and fuel rod segments
 Focused on peak temperatures, elemental composition,
chemical form, oxidation state, materials interaction,morphology, fission product retention, and cooling rates /atmospheres

 Examination data, stand-alone test information,
analysis code results, and discussions led toimportant consensus accident progression insights:

Graphics courtesy of JRC and INL

 Mass and composition of relocated material within each
RPV region
 Fission product retention
 Coolability
 Oxidation state Hydrogen generation potential
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TMI-2 BackgroundExaminations Supported Defueling,Transport, and Storage Activities
1986 to 2001
Wet Storage inINL TAN Facility
Spent FuelStorage Pool

Fuel & Debris Storage
Idaho National Laboratory (INL)

2000 – 2001
Removed from pool,dewatered, dried, and placedin dry storage at INL INTEC
Facility (25 miles/40 km)~2200 miles/ 3450 km

7 canisters per
Rail Transport
Cask up to 3
casks pershipmentThree Mile Island
Middletown, Pennsylvania

Graphics courtesy ofINL and EnergySolutions

1986 to 1990
342 canisters of fuel and debris in 49
shipments by Rail Transport Cask to INL
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Insights Applicable to Daiichi

Types of TMI-2 and Daiichi
Defueling Challenges Similar Incomplete knowledge:• Location of fuel-containing materials• Material properties to design removal equipment and shipping cannisters• Locations of highest contamination

 Possible defueling safety concerns:• Decay heat
• Recriticality• Combustion/pyrophoric reactions• Radiation release (dust generation)• Containment or vessel structural failures (load drop, seismic events)• Other (industrial occupational safety, fire protection, etc.)

 Stakeholder communication
Graphics courtesy ofTEPCO Holdings andEnergySolutions

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Closing Remarks
 At the time, TMI-2 accident and cleanup effort
challenges unprecedented. Challenges met using
flexible ‘step-by-step” approach:• Broad stakeholder input for information requests• Specialized technology development (adapting existing methods)• Endstate diagrams and 3D models useful for communication

Image courtesy of NDF (2021), Technical Strategic Plan 2021
for Decommissioning of the Fukushima Daiichi Nuclear Power
Station of Tokyo Electric Power Company Holdings, Inc.

•••

(updated as new data obtained)
Severe accident systems analysis code development
Domestic and international programs
Transparent public communications

 Japan applying systematic ‘step-by-step’ approach to address more complex Daiichi D&D:

• Broad stakeholder input for information request identification and prioritization (Japan MEXT /US DOE MOU)
• Advanced technology development and testing with mockups (some technologies have normal operation and maintenanceapplications)• 2D and 3D visualization methods for communication (updated as new data obtained)• Severe accident systems analysis code enhancements (facilitate subsequent D&D)
• Domestic and international programs (promotes common understanding of accident progression)• Transparent public communications (e.g., websites facilitating stakeholder interactions, public meetings, etc.)13