Fundamental Research Program for Removal of Fuel Debris

Japan Atomic Energy Agency

March 14, 2012

Tadahiro Washiya

International Symposium on the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Plant Unit 1-4

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Table of Contents 1. Introduction 2. R&D Schedule for Fuel Debris 3. Estimation of Fuel Debris Conditions in Fukushima

Daiichi NPS 4. Characterization on Fukushima’s Debris

Estimate scheme of the debris property Specific conditions in Fukushima Daiichi NPP Specific phenomena affected to debris character

5. Feasibility Study on Treatment of Removed Debris Aqueous processing Pyrochemical processing

6. Summary 7. Focus points for advice and proposal from experts

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Better understanding on the characterization of damaged fuel debris is important for the restoration work of Fukushima Daiichi NPS (1F). Especially for the following;

Debris Sampling / Removing Works Criticality Safety for the Debris Handling Material Accounting Evaluation of Accident Progression Screening of the Debris Treatment (which provide technical information to “judgment” of Debris

Treatment)

Previous knowledge of TMI-2 and severe accident (SA) research works at around the world are very valuable, and we should create countermeasures with international cooperation.

This work would devote to the accident management and nuclear safety in the future.

1. Introduction

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2. R&D Schedule for Fuel Debris

Item/Year Phase 1 Phase 2

2011 2012 2013

2014 2015 2016 2017 2018 2019 2020

(beginning) (mid) (end)

1. Manufacture of simulated debris

2. Estimation of

actual fuel debris characteristics with simulated debris

3. Comparison with

TMI-2 debris 4. Analysis of actual

debris properties 5. Development of

debris processing technologies

Debris sampling Tool design

Debris Removing

Tool design

Debris Sample

Feasibility Study Technical Evaluation Engineering Applicability

Evaluation on Long Term Storage or Disposal

Discussion start for the debris

treatment

Comparative Evaluation

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3. Estimation of Fuel Debris Conditions in Fukushima

Daiichi NPS

Weeping Cherry Blossom “Miharu-Taki-Sakura ”

(Miharu-cyo, Fukushima pref.)

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Damaged fuel core in TMI-2 reactor

Damaged reactor core in Fukushima is quite different from TMI-2 Reactor type (BWR) and structures ( materials and configuration ) Accident progression ( and various situations in each units ) Alternative coolant effect ( sea water effect )

Fukushima Daiichi NPP

Ref. TEPCO Home Page (2012.2.21) http://www.tepco.co.jp/nu/fukushima-np/series/index-j.html

Unit 1 (TEPCO estimation)

Unit 2,3 (TEPCO estimation)

3.1 Estimation of Damaged Reactor Core in Fukushima Daiichi NPS

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Analyzing of degradated core conditions by

MELCOR etc.

Thermodynamics Analyzing

Plant Operation Data

Reactor Core Information

Estimate cooling water level by neutron monitor

Thermodynamic analysis

TMI-2

TMI-2

Information to fabricate simulated

fuel debris Simulated fuel debris

Severe Accident Analyzing

1. Damaged core conditions will be estimated by SA code. 2. Material phase information of the debris will be obtained by using

thermodynamics analyzing.

cladding tube Upper plenum Damaged fuel pins

TMI-2

3.2 Estimation Scheme for the Damaged Fuel Debris

Temperatures[Hofmann, et al., NuclTech. Vol. 87, 1989]

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3.3 Results of reactor core analysis of TMI-2 Sample Temperature Analysis

Control Rod 393-982 ℃

Upper support pillar 510-732 ℃

Internal component

1227-1477 ℃ （stainless steel, inconel ）

Upper Debris Layer

> 2537℃ : （U,Zr）O2 (Av. < 1727℃)

(2827℃: Partially melted)

Core boring Sample

[ Upper Crust ] > 2537℃: Melted （U,Zr）O2

[ Remelted Layer ] > 2537℃: Melted （U,Zr）O2

[ Lower Crust ] 1127-1727 ℃: Melted Structural Material and Control Rod [ Stubbed Fuel ] < 647℃: No Crystallization of Cladding Material

Lower Debris Layer

> 2537℃: Melted （U,Zr）O2 ( 2827℃: Partially change of UO2)

Items TMI-2 1F Departure of 1F debris from TMI-2 Impacts on the core debris

Composition FP distribution

Storage container

Fuel Assem

bly

Structure Spacer grid Channel box Larger amount of zirconium. ○ ○ ×

Control rod Ag-ln-Cd/SS B4C/SS Eutectic interaction between boron and iron. Distribution of FPs through metals.

○ ○ ×

Fuel UO2 UO2 MOX

Change of the debris characteristics due to the difference between the O/M ratio of PuO2 and UO2.

○ × ×

Burn-up 3 months from

commercial operation

High burn-up Larger mass of FPs. ○ ○ ○ (Radiolysis)

In-vessel structure

Lower part of RPV Guide tube only. Control rod drive

shaft

Larger mass inclusions of noble metal FPs in the lower head debris possibly due to larger mass of iron.

○ ○ ×

Accident schem

e

Melting period 1-2 h A few hours ?

Possibly, larger amount of molten corium formed and larger amount of FPs was volatilized. Then, highly compacted debris formed.

○ ○ ×

In-vessel pressure ＞5MPa 0.1-1MPa Pressure is expected to have little impact on the corium metallurgy.

Cooling by sea water - After the melt

down

The effect of sea water is still unknown. (e.g. Leaching behavior of FPs through the mid to long term storage.)

○ ○ ○ (Corrosion)

3.4 Possible impacts on the debris characteristics based on the comparison with TMI-2

9 ［Modified version of the original prepared by CRIEPI］

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3.5 Arrangement and deliberation of plant information （e.g. Core Temperature, Amount of Material）

Actual state Analogical inference of the composition of the fuel assembly from dimension data of public information such as construction permit application. e.g. UO2 : Zry : SUS : B4C = 61 : 32 : 6 : 0.8 wt% The material composition data of each core part is needed since the properties of the plant materials affect the composition of generated debris.

<Example of deliberation> Presumption of debris creation process from the trend chart of core temperature (analytical result )

Actual time

③ Around 2000℃ : dissolution of UO2 by metallic melting material (Zr, Fe, B)

→In core part, generation of (U, Zr, Fe) oxide and alloy, boride, carbide.

→Moving of the melting materials to core bottom part.

→ stratification of oxide layer and metallic layer.

Max

imum

cor

e te

mpe

ratu

re K

Basic specification of Fukushima Daiichi nuclear power station (construction permit application abr)

①IC stopped, ②PCV leak (assumption), ③W/W vent open, ④W/W vent close, ⑤injection of seawater, ⑥ expanding PCV leak (assumption)

① Around 1200℃ : Surpassing reaction of SUS/B4C、Zry/SUS around control rod.

→Zr-Fe metallic compound, Fe-B compound fluidified around 1200℃, melting material moved core bottom and filled void.

②Core temperature rising rate: < 0.5 K/s →In vapor rich condition, Zry oxidize to ZrO2 below the melting temperature.

Temperature

3.6 Estimation of in-vessel phase Target：Successive support for the debris removal, storage, treatment and disposal. >> Debris characterization will be referred to the results of severe accident

code and estimated as some fluctuations range.

An example of the calculation result J.Nucl.Mat. 414, 23-31(2011)

At a specific location in RPV…

Time

Mass distribution of structural

materials

Information from SA code

・Selection of reactants

Reference on database

・Equilibrium reached?

Thermodynamic equilibrium Codes: ーFactSage ーGEMINI ーThermoCalc …

Determination of compositions

Mass

・Mass change ・Heating rate ・Cooling rate ・Aging temperature ・Maximum temperature ・Melting time

UO2/Zry, UO2/ZrO2, Zry/B4C, SUS/B4C, Zry/SUS etc…

・Temperature

Minimizing ΔG

Quench

Slow cooling

Estimation at each location in RPV.

11 It’s necessary to note the uncertainty of the SA analysis code.

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3.7 Estimation of the Core status

Reaction with container

materials（MCCI Debris）

（Slightly damaged fuel) Prospect of the melting status

Clogging originated from Channel-Box

and control rod materials

Fuel debris （Molten pool： Layer relocation of oxide and metal）

type

Debris character and chemical composition (expected）

Slightly damaged fuel

●Same as normal fuel

Fuels debris （molten pool）

●Oxide (U, Zr, Fe)O2-x , ZrO2

●Alloy of low melting point U-Zr-Fe alloy

Clogging materials （core center～

lower head）

●Oxide (U, Zr, Fe)O2-x , ●B , C compounds Fe2B, FeB, ZrB2, ZrC ●Alloy of low melting point U-Zr-Fe alloy

MCCI debris

●Phase of Oxide（mixture phase） (U, Zr)O2 + SiO2 , Fe2O3 ●Silicate compounds (U, Zr)SiO4 ,

Debris include sea water

composition Unknown

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3.8 R&D items according to the core status

type Debris characteristics and

chemical composition (expected）

R&D Items （Physical property

measurements etc.） Refraction of R&D results

Slightly damaged fuel

●Same as normal fuel Mass distribution in core Separate heavily damaged fuel（after this, possible as normal fuel)

Fuels debris （molten pool）

●Oxide (U, Zr, Fe)O2-x , ZrO2

●Alloy of low melting point U-Zr-Fe alloy

Clogging materials

（core center～lower head）

●Oxide (U, Zr, Fe)O2-x , ●B , C compounds Fe2B, FeB, ZrB2, ZrC ●Alloy of low melting point: U-Zr-Fe alloy

MCCI debris

●Phase of Oxide（mixture phase） (U, Zr)O2 + SiO2 , Fe2O3 ●Silicate compounds (U, Zr)SiO4 ,

Debris include sea water

composition

Unknown

Decision of retrieve method（tool、access path）（ ① ② ③ ④ ）

Criticality design of container （ ① ⑤ ）

Anti-corrosion methods of the container during the wet storage （① ⑥ ）

Disposal form and criticality management（ ① ③ ④ ⑤ ⑦ ）

Judgment of the necessity of stabilization for the disposal（volume reduction、lowering the dose, chemical inactivation etc.）（ ① ④ ⑥ ⑦ ）

① Analysis of composition ② Measurement of

Mechanical properties （hardness etc.）

③ Estimation of the mass distribution in the core

④ Evaluation of dose ⑤ Criticality calculation

（include porosity measurement and evaluation of fissile amounts）

⑥ Evaluation of permeability（measurement of leaching rate and dissolution rate）

⑦ Metallurgic measurement

⑧ Mass evaluation

Estimate the data range for various type of debris

Original information for each type of debris

Necessary information for the planning of the removal work

and its preparation

4. Characterization on the Debris of Fukushima Daiichi NPS

Weeping Cherry Blossom “Miharu-Taki-Sakura ”

(Miharu-cyo, Fukushima pref.)

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Fig. Chemical Interaction in core melting condition

Ref. Current knowledge on core degradation phenomena [P.Hofman, J. Nucl.Materials,270,1999]

・Insufficient hard crust debris might be formed by low Fe and Ni conditions.

・Relocation of the molten debris to the bottom of reactor head, might be occurred.

< Steps of the debris estimation > ① Evaluate progression ② Pick up specific phenomena ③ Thermodynamic Analysis ④ Study on typical events and reactions

(fuel & concrete reaction, etc.) ⑤ Evaluate the properties of generated

products

・Long term uncontrolled conditions might generate powder-state debris.

・Fuel elements might be reached out to the water phase (including sea water contents)

・Heterogeneous debris would be induced by lower reactor temperature than TMI-2

・Particulate and colloid would be generated from heterogeneous UO2/ZrO2 by the partial melting conditions.

4.1 Specific phenomena affected to the debris characteristics

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