MATERIALS SELECTION AND ACTIVITY EVALUATION SYSTEM TO REDUCE RADIOACTIVE WASTE FROM STEEL REINFORCED CONCRETE OF NUCLEAR PLANTS

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1 MATERIALS SELECTION AND ACTIVITY EVALUATION SYSTEM TO REDUCE RADIOACTIVE WASTE FROM STEEL REINFORCED CONCRETE OF NUCLEAR PLANTS Hasegawa A. 1, Kinno M. 2, Satou M. 1, Kakinuma N. 1, Abe K. 1, Kimura K. 2, Uematsu M. 3, Hayashi K. 4, Nakata M. 5, Tanosaki T. 6, Yoshino R. 7, Sato M. 8, Ito S. 9 1 Graduate School of Engineering, Tohoku University, Sendai, Japan 2 Fujita Corporations, Tokyo, Japan 3 Toshiba Corporations, Yokohama, Japan 4 Hitachi, Ltd., Hitachi, Japan 5 Mitsubishi Heavy Industries, Ltd., Yokohama, Japan 6 Taiheiyo Cement Corporation, Sakura, Japan 7 Dennki Kagaku Kougyo K.K. Omi, Japan 8 Nippon Steel Technoresearch Corporation, Kawasaki, Japan 9 Tohoku Electric Power Co., Inc. Sendai, Japan 1. Introduction Most of the concrete shielding walls and pipes around a reactor pressure vessel of a light water reactor become low level radioactive waste at decommission phase because they contain radioactive nuclide by thermal-neutron irradiation during its operation. The radioactivity of some low level radioactive wastes is close to the clearance level. It is very desirable in terms of life cycle cost reduction that the radioactivity of those low level radioactive wastes is decreased below clearance level. Recently, low activation concretes are partly used for some of the small proton accelerators to produce radiopharmaceutical. In case of light water reactors, however, methodology of low activation design of a nuclear plant has not been established yet because the reactor is a large-scale facility and has various structural materials. Database for low activation material production, tools of radioactivity estimation in a reactor and large-scale production technique of low activation structural materials such as cement and reinforced steel bars are required. The objectives of this project are (1) to develop the database of the content of the target elements, which transform long half-life nuclides, in raw materials for judging the activation classification (high beta and gamma emitter low level waste (L1), low level radioactive waste (L2), very low level radioactive waste (L3), clearance level (CL)), (2) to clarify the component of the plant which can become lower level radioactive waste by using low activation materials, and (3) to develop the low activation cement and reinforcing steel bars for structural components. To realize the objectives, raw material sampling and chemical analysis, development of tools for calculation, optimization of production process of low activation cements and steel bars are conducting under the collaboration works. Finally, we can supply the new design methodology of nuclear plants, that is Low Activation Plant Design Methodology (LAPDM), for reduction of the low level radioactive wastes in the decommission process of nuclear power reactor. This project is mainly aiming for reducing activation level of shielding wall of a new reactor such as reactor shielding wall (RSW) and a biological shielding wall (BSW) of a boiling water reactor (BWR) and a primary shielding wall (PSW) outside of the pressure vessel of a pressurized water reactor (PWR) by selecting low activation materials before construction. Additionally, to apply the newly developed calculation tools to

2 predict radioactive level of current reactors, reduction of decommission cost and radioactive waist is also expected by improving the accuracy of radioactivity classification. In this paper, a framework of the project and some preliminary results will be shown. Prediction of the activation level of the current plant in service (D/C) map Judging the activation classification when the plant shut-down Improvement of the judgment accuracy of the activation classification Reduction of the low level radioactive wastes Database of target elements in >1000 kinds of raw materials Development of the selection system of low activation material Reduction of the low level radioactive wastes of the future plant by low activation design Selection of the low activation raw materials Selection of the manufacturing process condition using test plants Manufacturing the mock-up of low activation concrete Proving its low activity by radioactivation analysis Figure 1 The work structure and flow in this project to establish the LAPDM 2. Overall scheme of the project The work structure and flow in this project to establish the LAPDM is shown in figure 1. To achieve the target of this project, it is necessary to develop (1) the database of the chemical composition of raw materials, (2) the tool for calculation of the neutron flux and the neutron spectrum distribution of nuclear plants, (3) the trial-manufacturing of the low activation concretes and the low activation reinforcing steels, and (4) the selection system of low activation material for the LAPDM. The target in each step and the future practical contribution of this project, which should be achieved by integrating the above-mentioned from (1) to (4), is shown in figure 2. The first step of this project includes (1) investigation of content rate of target elements in materials of the typical component, (2) radioactivation calculation for the typical LWR conditions, (3) material design using the low activation raw materials, and (4) study of production technique of the low activation concrete materials. The second step includes (1) development for practical use of low activation concrete materials in the nuclear plants, (2) development of design method of nuclear plants using the low activation materials, and (3) development of the material database for judging the activation classification (L1, L2, L3, CL). This project aims for achieving the first and second step. The low activation materials, which can meet a requirement for the general construction material, will be manufactured after the first and second step. We are planning to perform some tests at the final stage of this project to confirm whether those low activation materials can meet a requirement for the

3 structural materials of nuclear plant. The feasibility study has been finished using the limited data of materials at the first step. The numeral target of the reduction of the target elements was clarified to get the materials radioactivity below the CL for the RSW, BSW, and PSW. In consequence of this feasibility, it was indicated that the low activation concretes and reinforcing steels would be successfully manufactured using the currently existing raw materials. Development of the database of the major cement materials in Japan and optimization of the manufacturing process of the low activation steel reinforced concrete has proceeded as the second step of this project. Step 1 (1) Investigation of target elements in materials of the typical component (2) Radioactivation calculation for the typical region of LWR (3) Material design using low activation raw materials (4) Study of production technique of low activation concrete materials Step 2 (1) Development for practical use of low activation concrete materials in the nuclear plants (2) Development of design method of nuclear plants using the low activation materials (3) Development of the material database for judging the activation classification (L1, L2, L3, CL) Future Practical Use This projects can contribute the improvement of 1) Economic issue 2) Safety issue 3) Public acceptance issue of nuclear plants by reduction of the low level radioactive wastes when the plant shut-down. Figure 2 The target in each step and the future practical contribution of this project 3. Progress of the project 3.1 Quantitative analysis of target elements in the raw materials for cements and reinforcing steels The conventional chemical analysis was carried out to evaluate the content of target elements by using the inductively coupled plasma mass spectrometry (ICP-MS). The radioactivation analysis was also carried out for the raw materials to detect tracer level radioactive elements after thermal-neutron-irradiation at the KUR (Kyoto University Research Reactor) at Kyoto University and JRR-3 (Japan Research Reactor No.3) and JMTR (Japan Materials Testing Reactor) at Japan Atomic Energy Agency (JAEA). The samples of the raw materials mainly gathered in Japan. The detail of the samples is as follows: 53 kinds of cement products, 80 kinds of limestone, 22 kinds of gypsums, 7 kinds of clays, 48 kinds of fly ash, 20 kinds of iron source materials, 24 kinds of silica, 7 kinds of sands, and 30 kinds of reinforcing steel bars. 3.2 Development of the tool for the radioactivity evaluation The libraries of the neutron transport cross section and the neutron activation cross section, and the radioactivation calculation system using these libraries have been developed for the radioactivity evaluation. These new libraries were calculated based on the JENDL3.3 using NJOY The number of neutoron energy groups is 183 and number of nuclides proceessed is 105, which is important in decommissionin phase. The accuracy of calculation by these tool was evaluated using JPDR (Japanese Power Demonstration

4 Reactor) benchmark experiment, and compared with the ordinary calculation results and experimental results. Combined with the material database and the radiaoctivity evaluation tools, a material selection system for a new reactor component is developed. Figure 3 shows a flow chart of the selection system for the LAPDM. Using the material selection system, we can select the right raw materials combination for a component of nuclear reactor. Raw material database Activation cross section database Chemical composition of the plant component materials Radiation environment of the plant component Radioactivation calculation of the plant component (E,I) x (E) x t Calculation of radionuclide yield and its attenuation in each plant component Judging the activation classification L1,,CL Unacceptable Acceptable Warranty of the activation classification Re-evaluation Figure 3 The flow chart of the selection system of low activation material for the LAPDM 3.3 Development of the low activation steel reinforced concrete The low activation cement The optimization of the manufacturing process was performed to obtain the low activation cement and mortar containing lower level target element by evaluating the relation among the mixing ratio of raw materials, the amount of moisture, and shrinkage and heat value of materials The low activation reinforcing steels The behavior of target elements during manufacturing process of the reinforcing steels and steel plate was invastigated by the analysis of the raw materials and intermediate products of steel making process. The properties of the low activation reinforcing steel using electrolytic irons were also investigated. 4. Results

5 4.1 Material analysis and low activation material design Radioactive analysis shows that Cobalt (Co) and Europium (Eu) were claryfied to be the major target elements which decide the radioactivity level of reinforced concreate. Material database for the content of Co and Eu was developed based on the chemical analysis and radioactivation analysis. The correlation between the Co and Eu content was obtained for cement materials. Therefore it is claryfied that the low activation cement would be successfully manufactured by adequate selection of raw materials. The prospect of manufacturing the 1/3 activation level low-heat Portland cement was obtained by using the limestone and natural gypsum. The prospect of manufacturing the low activation non-portland cement was also obtained by selecting the a place of origin of silica materials. 4.2 Development of some tools for the LAPDM It was claryfied that the accurcy of calculation results by our tool for the radioactivity evaluation was very high and available by the benchmark calculation for the JPDR. The specification of the mapping system for judging the activation classification was also developed by using the general-purpose radioactivation calculation tool. The outline of the selection system of low activation material for the LAPDM was developed to confirm the consistency and bugs of this system. The result of the radioactivation analysis can be directly incorporated into this system. The prospect of development of the low activation selection system was obtained. 4.3 Development of the low activation materials The low activation heavy-mortar, by which almost all the heavy-mortor component of the RSW of the BWR can be below the CL, has been trial-manufactured. Material property tests of this heavy-mortar has also been carried out. Preliminary results showed that the resistance to the shrinkage should be improved for this material. The 1/30 and 1/10 radioactive concrete materials, by which all the concrete component of the BSW of the BWR can be below the CL, has been trial-manufactured. Material property tests has also been carried out. Preliminary result showed that those materials can be successfully manufactured by controling the heat of hydration of the white cement and by using the low activation low-heat-portland and non-portland cement. The 1/300 radioactive concrete materials was studied for the PSW of the PWR. The low activation mortor made of the same raw materials as 1/300 radioactive concrete has been trial-manufactured. Material property test has also been carried out. As a result, it showed that the resistance to the degradation of long-term strength by phase-transition and the resistance to the shrinkage should be improved for this material. For the development of the low activation reinforcing steel, the lower limit of the target elements in the blast furnace iron has been studied by chemical analysis of the iron ores, lime stones, cokes, and anthracites used as the blast furnace feed, and the blast furnace pig irons. As a result, the content of Co in the blast furnace pig irons was about 20 ppm. The prospect of development for the 1/3 radioactive reinforcing steel by steelmaking without scrap irons was obtained.

6 The evaluation of mechanical properties has also been carried out for the steel rods manufactured by small-scale melting using high purity iron materials. The results showed that the mechanical properties of the high pruty steel bars was almost the same as the SD345. Therefore, the prospect of development of the low activation reinforcing steel was obtained. 5. Future plans The following studies and developments are planed to be performed, (1) The expansion of the material database for judging the activation classification by radioactive and chemical analysis should be performed. (2) The low activation cement of about 100kg made of the low activation raw materials should be trial-manufactured at the relatively small-scale plant. The optimization of the installation condition of the concrete material available for the structural strength component should be also performed. (3) The behavior and content of the target elements during manufacturing process of the reinforcing steels and cements should be clarified. (4) The prediction of the radioactivity for the actual nuclear plant should perform. Ackowledgment This program is supported by a grant-in-aid of Innovative and Viable Nuclear Energy Techonogy (IVNET) development projet of Ministry of Economy, Trade and Industry, Japan.