International Symposium on the Decommissioning of TEPCO s Fukushima Daiichi Nuclear Power Plant Unit 1-4 1 Fundamental Research Program for Removal of Fuel Debris March 14, 2012 Tadahiro Washiya Japan Atomic Energy Agency
2 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
1. Introduction 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. 3
2. R&D Schedule for Fuel Debris Item/Year 1. Manufacture of simulated debris 2. Estimation of actual fuel debris characteristics with simulated debris Phase 1 Phase 2 2011 2012 201 3 2014 2015 2016 2017 2018 2019 2020 (beginning) (mid) (end) Tool design Debris sampling Debris Sample Tool design Debris Removing 3. Comparison with TMI-2 debris 4. Analysis of actual debris properties 5. Development of debris processing technologies Feasibility Study Discussion start for the debris treatment Technical Evaluation Engineering Applicability Evaluation on Long Term Storage or Disposal Comparative Evaluation 4
5 3. Estimation of Fuel Debris Conditions in Fukushima Daiichi NPS Weeping Cherry Blossom Miharu-Taki-Sakura (Miharu-cyo, Fukushima pref.)
3.1 Estimation of Damaged Reactor Core in Fukushima Daiichi NPS 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 ) Unit 1 (TEPCO estimation) Unit 2,3 (TEPCO estimation) Damaged fuel core in TMI-2 reactor Ref. TEPCO Home Page (2012.2.21) http://www.tepco.co.jp/nu/fukushima-np/series/index-j.html Fukushima Daiichi NPP 6
3.2 Estimation Scheme for the Damaged Fuel Debris 1. Damaged core conditions will be estimated by SA code. 2. Material phase information of the debris will be obtained by using thermodynamics analyzing. Severe Accident Analyzing Plant Operation Data Estimate cooling water level by neutron monitor Reactor Core Information TMI-2 TMI-2 Analyzing of degradated core conditions by MELCOR etc. Upper plenum Damaged fuel pins Thermodynamic analysis cladding tube Temperatures[Hofmann, et al., NuclTech. Vol. 87, 1989] Thermodynamics Analyzing Information to fabricate simulated fuel debris Simulated fuel debris TMI-2 7
8 3.3 Results of reactor core analysis of TMI-2 Sample Temperature Analysis Control Rod 393-982 Upper support pillar Internal component 510-732 1227-1477 (stainless steel, inconel ) Upper Debris Layer > 2537 : (U,Zr)O2 (Av. < 1727 ) (2827 : Partially melted) [ Upper Crust ] > 2537 : Melted (U,Zr)O2 Core boring Sample Lower Debris Layer [ 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 > 2537 : Melted (U,Zr)O2 ( 2827 : Partially change of UO2)
3.4 Possible impacts on the debris characteristics based on the comparison with TMI-2 Impacts on the core debris Items TMI-2 1F Departure of 1F debris from TMI-2 FP Storage Composition distribution container Structure Spacer grid Channel box Larger amount of zirconium. Fuel Assembly In-vessel structure Accident scheme Control rod Ag-ln-Cd/SS B4C/SS Fuel UO 2 UO 2 MOX Burn-up Lower part of RPV Melting period In-vessel pressure Cooling by sea water 3 months from commercial operation Guide tube only. Eutectic interaction between boron and iron. Distribution of FPs through metals. Change of the debris characteristics due to the difference between the O/M ratio of PuO 2 and UO 2. High burn-up Larger mass of FPs. Control rod drive shaft 1-2 h A few hours? Larger mass inclusions of noble metal FPs in the lower head debris possibly due to larger mass of iron. Possibly, larger amount of molten corium formed and larger amount of FPs was volatilized. Then, highly compacted debris formed. (Radiolysis) >5MPa 0.1-1MPa Pressure is expected to have little impact on the corium metallurgy. - 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) [Modified version of the original prepared by CRIEPI] 9
3.5 Arrangement and deliberation of plant information (e.g. Core Temperature, Amount of Material) <Example of deliberation> Presumption of debris creation process from the trend chart of core temperature (analytical result ) Maximum core temperature K 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. 2Core temperature rising rate: < 0.5 K/s In vapor rich condition, Zry oxidize to ZrO 2 below the melting temperature. 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/B 4 C Zry/SUS around control rod. Zr-Fe metallic compound, Fe-B compound fluidified around 1200, melting material moved core bottom and filled void. Basic specification of Fukushima Daiichi nuclear power station (construction permit application abr) Actual state Analogical inference of the composition of the fuel assembly from dimension data of public information such as construction permit application. e.g. UO 2 : Zry : SUS : B 4 C = 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. 10
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. Information from SA code Mass change Heating rate Cooling rate Aging temperature Maximum temperature Melting time At a specific location in RPV Temperature Thermodynamic equilibrium Temperature Selection of reactants UO 2 /Zry, UO 2 /ZrO 2, Zry/B 4 C, SUS/B 4 C, Zry/SUS etc Equilibrium reached? Determination of compositions Codes: ー FactSage ー GEMINI ー ThermoCalc Reference on database Minimizing ΔG Mass Quench Time Slow cooling Mass distribution of structural materials Estimation at each location in RPV. It s necessary to note the uncertainty of the SA analysis code. An example of the calculation result J.Nucl.Mat. 414, 23-31(2011) 11
3.7 Estimation of the Core status Prospect of the melting status type Debris character and chemical composition (expected) (Slightly damaged fuel) Clogging originated from Channel-Box and control rod materials Slightly damaged fuel Same as normal fuel Fuels debris (molten pool) Oxide (U, Zr, Fe)O 2-x, ZrO 2 Alloy of low melting point U-Zr-Fe alloy Clogging materials (core center~ lower head) MCCI debris Oxide (U, Zr, Fe)O 2-x, B, C compounds Fe 2 B, FeB, ZrB 2, ZrC Alloy of low melting point U-Zr-Fe alloy Phase of Oxide (mixture phase) (U, Zr)O 2 + SiO 2, Fe 2 O 3 Silicate compounds (U, Zr)SiO 4, Fuel debris (Molten pool: Layer relocation of oxide and metal) Reaction with container materials(mcci Debris) Debris include sea water composition Unknown 12
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) Clogging materials (core center~ lower head) MCCI debris Debris include sea water composition Oxide (U, Zr, Fe)O 2-x, ZrO 2 Alloy of low melting point U-Zr-Fe alloy Oxide (U, Zr, Fe)O 2-x, B, C compounds Fe 2 B, FeB, ZrB 2, ZrC Alloy of low melting point: U-Zr-Fe alloy Phase of Oxide(mixture phase) (U, Zr)O 2 + SiO 2, Fe 2 O 3 Silicate compounds (U, Zr)SiO 4, Unknown Original information for each type of debris 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 Necessary information for the planning of the removal work and its preparation 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 ) Estimate the data range for various type of debris 13
4. Characterization on the Debris of Fukushima Daiichi NPS Weeping Cherry Blossom Miharu-Taki-Sakura (Miharu-cyo, Fukushima pref.) 14
4.1 Specific phenomena affected to the debris characteristics 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 UO 2 /ZrO 2 by the partial melting conditions. 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. Fig. Chemical Interaction in core melting condition Ref. Current knowledge on core degradation phenomena [P.Hofman, J. Nucl.Materials,270,1999] < 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 15