SAFETY ENHANCEMENT TECHNOLOGY DEVELOPMENT WITH COLLABORATIVE INTERNATIONAL ACTIVITY

Transcription

1 SAFETY ENHANCEMENT TECHNOLOGY DEVELOPMENT WITH COLLABORATIVE INTERNATIONAL ACTIVITY KENJI ARAI Toshiba Corporation Yokohama, Japan FUMIHIKO ISHIBASHI Toshiba Corporation Yokohama, Japan Abstract IAEA report on reactor safety in the light of Fukushima accident highlighted the lessons learned in the key technical areas important for strengthening safety. These lessons are associated with severe accident management measures and, for the research and development (R&D) required to implement measures, it stresses the importance of an international coordinated approach. Since the accident, Toshiba has conducted R&Ds for the safety enhancement with the support from Japanese Government, which cover the key technical areas in the IAEA report. Some of the Toshiba R&Ds have been progressed with the international collaboration. For example, the project to develop the accident tolerant fuel (ATF) which is an application of SiC composite to fuel materials has had technical interaction with OECD/NEA expert group on ATF. The passive debris cooling technology development project leverages the expertise and the test facilities in Kazakhstan to obtain the property data of refractory materials and erosion behavior under the core-melt temperature condition. The model enhancements of a severe accident simulation code, MAAP and Fukushima accident progression analysis have been conducted in collaboration with US-EPRI. Those collaborations are effective to expedite the R&Ds with efficient use of the expertise available worldwide. The paper summarizes the achievements of these R&Ds focusing on the collaborative international activity. 1. INTRODUCTION Fukushima accident identified significance of the measures against the external hazards and highlighted the need to enhance the defence in depth. Since the accident, Toshiba has conducted the technology development for the safety enhancement of nuclear power plants with the support from the Japanese Government. Those technology developments covered a wide range of issues for the prevention and mitigation of severe accidents, which are selected and prioritized based on the Fukushima lessons learned. IAEA report [1] on reactor safety in the light of Fukushima accident highlighted the lessons learned in the following key technical areas important for strengthening safety: - Defence in Depth, - Extreme events/ external events, - Station blackout and loss of ultimate heat sink, - Hydrogen management, - Containment system and venting, - Severe accident management, - Instrumentation and control (I&C), - Spent fuel pools, - Research and development (R&D). These lessons are associated with safety measures for the prevention of accidents or the mitigation of accident consequences. For the research and development (R&D) required to implement measures, it stresses the importance of an international coordinated approach. The above mentioned Toshiba s R&D activity covers the key technical area identified in the IAEA report. Some of the Toshiba R&Ds have been progressed with the international collaborations which are effective to expedite the R&Ds with efficient use of the expertise, facilities and experiences available 1

2 worldwide. The paper summarizes the overview and achievements of these R&Ds focusing on the collaborative international activity. 2. OVERVIEW OF R&D ACTIVITY FOR SAFETY ENHANCEMENT TECHNOLOGY Figure 1 illustrates the overview of the R&D activities which Toshiba has conducted with the support from the Japanese Government after the Fukushima accident. The R&Ds covers the technologies regarding core/ debris cooling, containment integrity and hydrogen management in both pre- and post-core damage phases of an accident. Those for improving the severe accident simulation capability, instrumentation during a severe accident, DC batteries and seismic protection technology are included as well. The international collaborations play an important role in the R&Ds for passive debris cooling technology, accident tolerant fuel (ATF) and MAAP model enhancement which are described in the following section. (ATF: Accident tolerant fuel, SiC: Silicon Carbide) FIG.1 Overview of Toshiba s R&D Activity for Post-Fukushima Safety Enhancements Other major activities listed in the table are briefly summarized below. (a) In-Vessel Retention for Debris Cooling [2]: In-Vessel retention (IVR) technology has been studied in order to estimate the success probability of IVR for a large PWR with obtaining critical heat flux (CHF) data under a wide variety of the thermal-hydraulic conditions which can be encountered during hypothetical accident scenarios. The CHF enhancement effect of nanoparticles has been also experimentally studied. The Risk-Oriented Accident Analysis Methodology (ROAAM) was applied to estimate the success probability with using the CHF data and confirmed that it is very likely for a 4500MWth-class PWR to maintain the core debris inside of the reactor vessel by the external cooling. (b) Passive Containment Cooling for Containment Integrity: The passive containment cooling system (PCCS) with horizontal U-tube type condensers has been developed and the feasibility to conventional BWRs has been studied after the Fukushima accident. Thermal-hydraulic experiments have been conducted to clarify the transient behaviours of both PCCS and the containment considering SA scenarios in conventional BWRs by using the large scale containment test facility in Toshiba. The test results clarified the fundamental phenomena which determine the PCCS performance and demonstrated the effectiveness of the PCCS to suppress the containment pressure rise even in the conventional BWRs. 2

3 (c) Passive Hydrogen Removal for Hydrogen Management [3]: A massive amount of hydrogen can be generated by the metal-water reaction during a severe accident and becomes a threat of the containment overpressure and the hydrogen combustion in a secondary containment of a BWR. The hydrogen removal performance of metal oxides has been examined under a typical severe accident condition and the concept of the hydrogen removal system is developed to be used in inerted atmosphere of a BWR primary containment. (d) Instrumentation for Monitoring Severe Accident Condition [4]: Considering the severe condition during a severe accident, instrumentation has been developed for monitoring the reactor pressure vessel water level, hydrogen concentration and containment water level. The performance of the instrumentation was examined and confirmed under the severe accident condition. 3. R&D ACTIVITY WITH INTERNATIONAL COLLABORATION 3.1. Passive Debris Cooling: MCCI Prevention Technology [5-7] The R&D activity has progressed to develop a measure for conventional BWRs containments in order to prevent the MCCI which becomes a threat to the containment integrity. It has a refractory layer to prevent the direct contact between the core debris and the concrete containment by sustaining the debris on the layer. Series of experiments were conducted to obtain the thermal properties data of candidate refractory materials (Al 2 O 3, MgO, ZrO 2 ) under the severe accident temperature condition and to investigate the erosion behaviour of the materials due to the molten debris by using the dedicated test facilities on different scales in Japan and Kazakhstan. A large scale test using 60 kg UO 2 debris was carried out at the National Nuclear Center (NNC) of the Republic of Kazakhstan for the investigation of the interaction on the refractory material. In addition, a small scale apparatus was used as well for thermal erosion tests at NNC (Fig. 2). These test results will contribute to establishing the evaluation model for the thermal and chemical interaction between the debris and refractory materials. NNC has accumulated a lot of experience and expertise for the high temperature debris experiment using UO 2 with the dedicated test facilities. These are essential to obtain the data in an efficient and expedited manner. The collaboration was reported in the country report of the Republic of Kazakhstan at Forum for Nuclear Cooperation in Asia (FNCA) in 2016 [5]. One of major achievements was the establishment of the phase diagram for the composite of debris and refractory materials with different UO 2 -ZrO 2 ratio. Figure 3 is an example in which the eutectic tests results, liquidus and solidus temperatures for (UO 2 ) (ZrO 2 ) Al 2 O 3 composite are compared with the phase diagram analysis res ults which was obtained by using FactSage, showing that the analysis result agrees well with the test data. Liquidus and solidus temperature data were obtained for several UO 2 -ZrO 2 ratios in Japan and Kazakhstan which confirmed the validity of the phase diagram analysis. By leveraging these results, the refractory material will be selected for the MCCI prevention measure and the design will be established considering a wide range of accident scenarios. 3

4 FIG.2 Erosion Test Vessel at NNC [6] FIG.3 Comparison between Phase Diagram Analysis and Eutectic Test data [7] 3.2. Accident Tolerant Fuel: SiC Application [8,9] Fukushima accident highlighted the significance of the hydrogen management during severe accidents. The measures have been developed in two ways: One is to enhance the high temperature resistance of fuel material and suppress the hydrogen production caused by the metal-water reaction, that is the development of accident tolerant fuel (ATF). The other is to remove the excessive hydrogen passively in the inerted containment, which is briefly described in Section 2. For the ATF development, Toshiba identified a silicon carbide (SiC) ceramic as the most promising ATF material s ince it has less chemically active characteristics under the high-temperature-water steam environment and a smaller neutron absorption cross-section. Toshiba has been participating in two joint teams, involving Ibiden Co., Ltd, Nuclear Fuel Industries Ltd., the University of Tokyo, Tohoku University, Kyoto University and Hokkaido University, and continued the development. Figure 4 shows the roadmap for SiC/SiC composite application to BWR channel box and fuel cladding. One of the major challenges is to establish the fabrication technology. Toshiba and Ibiden have succeeded in the trial fabrication of the reduced-length channel box and cladding tube with SiC/SiC composite using CVD (Chemical Vapour Deposition) process (Fig. 5); the length of the channel box is 1000mm and the tube length is 800mm. R&D works are continued to achieve the full scale fabrication in parallel with the preparation for the irradiation tests. ATF development activities are progressing at international level. Multidisciplinary and long-term research works are needed before ATF is put into practical use, which are related to but not limited to fabrication, normal reactor operations, safety, fuel cycle and economy. It is therefore indispensable to pursue efficient and effective research approach by leveraging international collaborations, expertis e and facilities. Toshiba has shared the major progress of the development activity with the OECD/NEA Expert Group on ATF (EGATFL) and is participating in the development of the state-of-the-art report on ATF which summarizes the state of development, testing and/or development needs, and associated development risks in the NEA member countries. The report is expected to be a guide for efficient approaches for the development roadmap. 2012~ ~ ~ ~2030 Phase-1 (Feasibility Study) Channel box Development of Fabrication tech, Material design tech. mechanical tests environmental tests Fuel cladding Development of Fabrication tech, Material design tech. mechanical tests environmental tests Phase-2 Full length fabrication Irradiation test in test reactor Preparation of licensing for LTR Full length fabrication Irradiation test in test reactor Preparation of licensing for LTR Phase-3 Lead test for reload batch Start of commercial use Licensing for LTR LTR irradiation in commercial reactor Licensing for LTA LTA irradiation in commercial reactor LTR: Lead Test Rods LTA: Lead Test Assemblies Licensing for reload batch Start of commercial use FIG.4 Roadmap for Toshiba SiC Deployment [8] FIG.5 Trial of Channel Box and Cladding Tube [9] 3.3. SA Simulation Technology: MAAP Model Enhancement [10] The Modular Accident Analysis Program (MAAP), which is an Electric Power Research Institute (EPRI) owned and licensed computer software, has been extensively used to analyse the progression of severe accidents. MAAP version 4 analysis results for accidents at Fukushima NPPs reasonably explained the plant parameters measured at the NPPs before the core melts occurred, and however, the agreements were not satisfactory for the post-core-melt consequences. In order to identify the analysis models of MAAP which should be enhanced especially for analysing the molten-core and debris behaviour, sensitivity study on uncertain boundary conditions in the Fukushima accident analyses, e.g. fire engine injection rate, was conducted. Based on the study 4

5 results, following physical models are extracted as significant models for molten core and debris behaviour analysis: - Core melt progression model; modelling of additional relocation paths including fuel support piece and shroud wall failure. - Lower plenum (LP) model; increased nodalization of LP, CRD tubes and stratified debris bed in LP. - Melt spreading and MCCI model; modelling of melt spreading, convective heat transfer in debris, and ablation front evolution, and update of corium-concrete properties. These model enhancements have been implemented into MAAP in collaboration with EPRI. The updated MAAP, MAAP 5.03, has been applied to Fukushima accident analyses to support the decommissioning of Fukushima NPPs and has been benchmarked in the OECD/NEA/BSAF project (Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant). Member countries and organizations in the MAAP Users Group can access the MAAP with the model enhancements. In Japan, MAAP 5.03 is implemented into the SA simulator for the Nuclear Regulatory Authority. From the viewpoints of dissemination of the achievements in severe accident simulator model enhancements, the MAAP Us ers Group organized by EPRI is effective. OECD/BSAF is an excellent opportunity to share the state-of-the art severe accident modelling and obtain experts feedback on the further model improvements. The collaborative network plays an important role for advancing the severe accident modelling capability. 4. SUMMARY A wide range of the Post-Fukushima R&D activities has been conducted for the safety enhancement technology. The overview of the activities which Toshiba is leading is provided in the paper. Some of the Toshiba R&Ds have been progressed with the collaborative international activities which play important role from following viewpoints: - Efficient use of the expertise, facilities and experiences available worldwide. - Pursuing efficient approach for multidisciplinary and long-term research works. - Dissemination of R&D achievements to the international community. ACKNOWLEDGMENTS The R&D activities mentioned in the paper were financially supported by Ministry of Economy, Trade and Industry (METI) of Japan. REFERENCES [1] IAEA report on reactor and spent fuel safety in the light of the accident at the Fukushima Daiichi nuclear power plant, 2012, [2] TSUDA, S., et al., Study on In-Vessel Retention Analysis of In-Vessel Retention using Risk Oriented Accident Analysis Methodology, Proc. ICAPP 2017, Fukui and Kyoto. [3] IWAKI, C., et al., Development of Hydrogen Treatment System in Severe Accident (2) Study on Reaction Characteristic of a hydrogen proceeding unit, Proc. ICONE , [4] TAKEMURA, M., et al., Development of Instrumentation Systems for Severe Accidents, Proc. ICONE , [5] Forum for Nuclear Cooperation in Asia (FNCA) 2016, Country Report of the Republic of Kazakhstan, [6] KURITA, T., et al., Evaluation Plan for Passive Debris Cooling System and Refractory Layer, Proc. ICAPP, Paper 14179, [7] TAKAHASHI, Y., et al., Development of Passive Debris Cooling System, Fall Meeting of Atomic Energy Society of Japan, I12, 2014 (in Japanese). [8] Kakiuchi, K., et al., Progress on ATF Development of SiC for LWR, Proc. Topfuel2016,

6 [9] Uchihashi, M., et al., Development of SiC/SiC Composite for Nuclear Reactor Core with Enhanced Safety, Proc. ICONE , [10] KOJIMA, Y., et al., MAAP Enhancements for Ascertaining and Analyzing Reactor Core Status in Fukushima Daiichi NPP, Proc. ICAPP