1 Low-Activation Reinforced Concrete Design Methodology (11) -Preliminary FEM Analysis of Thermal Stress for Low-Activation Concrete - Yusuke Fujikura 1) , Hirokazu Nishida 1) , Norichika Katayose 1) , Ken-ichi Kimura 1) , Masaharu Kinno 1) , and Akira Hasegawa 2) 1) Technology Development Division, Fujita Corporation, Atsugi, Japan 2) Tohoku University, Sendai, Japan ABSTRACT In order to reduce long-lived residual radionuclide in a concrete shielding around a reactor, we newly developed two types of low-activation concrete, namely, “1/10-low-activation concrete” and “1/30-low-activation concrete”. The physical and mechanical properties of these concrete were tested by the experimental works. The thermal stress generated by cement hydration was calculated by the three dimension FEM analysis targeting massive concrete in the reactor. Upon this study, we evaluated the realization of two types of low-activation concrete, and assessed the utilization for applying the above low-activation concrete to the reactor shielding wall, by comparing the experimental data to the calculated data. The evaluation conducted the 1/10-low-activation concrete was effective for the use of the shielding wall. INTRODUCTION Concrete is very practical and inexpensive material for radiation shielding. While, after a long period of operation, the shielding concrete around a nuclear reactor change to low level radioactive waste because of the remaining long lived radionuclides. The disposal cost of this activated concrete is considered to be a hundred times expensive compared to that of non-activated concrete. This calls for the reduction of radioactive concrete to “clearance level” from a viewpoint of cost savings and reutilization of natural resources. Here, “clearance level” denotes the radioactive classification permissible for disposing of material as non-radioactive waste. In order to reduce the residual radionuclides in a concrete shield around a reactor, we developed several types of low-activation concrete [1]. Generally, a reactor shielding wall consists of a large section of member, and of massive concrete. The evaluation of crack generation in massive concrete due to cement hydration is, therefore, indispensable. However, no systematic study on crack generation of low-activation concrete regarding the shielding wall have been studied up to this time. The objective of this work is to establish an evaluation system of mix proportion design regarding avoiding crack generation of low-activation concrete by the experimental data and the FEM (finite element method) thermal stress calculation system. PROPERTIES OF LOW-ACTIVATION CONCRETE Low-activation materials and mix proportions of concrete Screening tests using the thermal reactor JRR-4 of the Japan Atomic Energy Agency were performed to identify low-activation raw materials for concrete that can be used for nuclear facilities. About 300 specimens of aggregate and cement were chosen. The dominant long-lived residual radionuclides induced in concrete are, in order of importance, 152 Eu (representative single value of the clearance level of IAEA-RS-G1.7 [2], C 152Eu = 0.1 Bq/g, half-life (T 1/2 ) = 13.54 yr), 60 Co ( C 60Co = 0.1 Bq/g, T 1/2 = 5.271 yr), and 154 Eu ( C 154Eu = 0.1 Bq/g, T 1/2 = 8.593 yr), produced by 151 Eu (n, γ), 60 Co (n, γ), and 153 Eu (n, γ) reactions, respectively. These radionuclides are known to occupy 99-100 % of the total residual radioactivity induced in ordinary concrete at the time of decommissioning[3]. The dominant target elements of concrete shield is, therefore, Eu and Co. Table 1 presents the dominant target elements of reference materials obtained by such screening tests. Here, we composed two types of concrete as shown in Table 2, namely, “1/10-low-activation concrete” and “1/30-low-activation concrete”. The reduction rate of the activation for the 1/10-low-activation concrete is designed to be 1/10 compared to the andesite concrete which is considered to be “ordinary concrete”. That for the 1/30-low-activation concrete is designed to be 1/30. The densities and the water absorption rates are presented in Table 3. The compressive strength, slump, and air content are designed as 33N/mm 2 , 15±2.5cm, and 4±1%, respectively. Test and inspection The items of the test and the criteria used in this study are listed in Table 4. We measured the properties of fresh concrete, mechanical properties of the hardened concrete and the adiabatic temperature rise as the cement hydration. Here, “JIS”, “JASS”, and “JCI” denote “Japanese Industrial Standard”, “Japanese Architectural Standard Specification”, and “Japanese Concrete Institute”, respectively. The bleeding water related to the physical properties SMiRT 19, Toronto, August 2007 Transactions, Paper # F02/5