Masahiro Ishihara

Graphite core structures and their structural design criteria in the HTTR

Abstract The Japan Atomic Energy Research Institute (JAERI) is now proceeding with the construction design of the High Temperature Engineering Test Reactor (HTTR) to achieve first criticality in FY 1995. The reactor internal structures of the HTTR are made up of mainly graphite components. The characteristics of graphite are quite different in stress-strain behavior from metals, since the ductility of graphite is significantly less than metals. Therefore, the design codes provided for metal components cannot be applied directly to graphite components. The design criteria for the graphite core and core support components in the HTTR have been developed by JAERI and will be authorized by the Japanese licensing authorities. The design criteria were developed by partially modifying ASME Sec.III, Div.2, Subsection CE Code (draft) in the items of bi-axes failure theory, buckling limit and oxidation effects on the basis of test data. This paper describes the graphite core structures of the HTTR and the design criteria developed by JAERI and details the limits different from the ASME CE Code. A brief explanation is also made in this paper for quality control specified in the design criteria.

Development of structural analysis program for non-linear elasticity by continuum damage mechanics

The brittle damage constitutive equation developed by Chow and Yang is used to simulate the non-linear elastic deformation behavior of graphite using finite element method (FEM). This model is achieved by introducing a damage surface that is similar to the yield function in the conventional theory of plasticity. A special form of damage surfaces is constructed to illustrate the application of the model. For verifying the FEM program including the Chow and Yang model, the predicted deformations by this model are compared with both the experimental ones in the graphite structural model and the calculated ones without the continuum damage mechanics.

Irradiation effects on thermal expansion of SiC/SiC composite materials

Abstract Irradiation-induced dimensional change and thermal expansion of two kinds of composites, self-particle reinforced SiC p /SiC composites and a Hi-Nicalon™ SiC fiber reinforced SiC f /SiC composite, and monolithic α-SiC were measured after irradiation at 0.2 dpa with irradiation temperatures of 573, 673 and 843 K using the JMTR. From the measurement, swelling was observed for the SiC p /SiC composites and the monolithic α-SiC, on the contrary, the SiC f /SiC composites showed a shrinkage. The measured thermal expansion increased with increasing the specimen temperature below the irradiation temperature, and then rapidly decreased over the irradiation temperature. The so-called ‘temperature monitor effect’ of the silicon carbide was clearly observed for all specimens, the monolithic α-SiC and both composites.

Development of Thermal/Irradiation Stress Analytical Code “VIENUS” for HTTR Graphite Block

A new thermal/irradiation stress analysis code “VIENUS” has been developed for the graphite block in the High-Temperature Engineering Test Reactor (HTTR). The VIENUS is a two- dimensional finite element visco-elastic analysis code to take account of graphite behavior under irradiation in detail. In the analysis, the effects of both fast neutron fluence and temperature on material properties are considered. The code has been evaluated by the irradiation test results of the Peach Bottom fuel elements to confirm the thermal/irradiation stresses in the graphite block. It is clarified that the calculated results are able to estimate a tendency of the test results, and that both the irradiation- induced creep and dimensional change are the most important parameters in the thermal/irradiation stress analysis. From the present study, it is suggested that the VIENUS code is a useful tool to evaluate the thermal/irradiation stresses in the HTTR graphite blocks.

Fundamental thermomechanical properties of SiC-based structural ceramics subjected to high energy particle irradiations

SiC composites have a potential to be used as in-vessel structural components in a future fusion reactor concept. To investigate the fundamental process for deformation and fracture of polycrystalline ceramics, α-SiC monolith mini-specimens were irradiated with heavy ions, 180 MeV Au and 90 MeV Ni. The specimens were tested in bending after different degrees of exposure to elucidate the strength change due to artificial near-surface defects. A microindentation experiment was also carried out to characterize the irradiation damage effects resulting from near-surface defects. Moreover, an FEM analysis was carried out by taking account of the irradiation damage at near-surface region, and the result was compared with the experimental results. This paper presents the particle irradiation effect on the bending strength on the basis of both experimental and analytical results.

Evaluation of aseismic integrity in HTTR core-bottom structure. I: Aseismic test for core-bottom structure

Abstract The aseismic tests were carried out using 1 5 - scale and 1 3 - scale models of the core-bottom structure of the HTTR to quantitatively evaluate the response of acceleration, strain, impact load etc. The following conclusions are obtained. 1. (i) The frequency response of the keyway strain is correlative with that of the impact acceleration on the hot plenum block. 2. (ii) It was confirmed through 1 5 - scale and 1 3 - scale model tests that the applied similarity law is valid to evaluate the seismic response characteristics of the core-bottom structure. 3. (ii) The stress of graphite components estimated from the scale model test using S2-earthquake excitation was sufficiently lower than the allowable stress used as the design criterion.

Amorphization with ion irradiation and recrystallization by annealing of SiC crystals

Abstract The effects of reactive atoms on microstructural changes in polycrystalline sintered α-SiC thin film specimens irradiated with 30 keV N2+ or 20 keV Ne+ at RT (room temperature) and isochronically annealed (400–900∘C) after irradiation were studied by TEM. Irradiation-induced amorphization occurred during irradiation at RT with both N2+ and Ne+ irradiation. Bubbles were also observed to form, but nitrogen bubbles were more difficult to form than Ne bubbles. To investigate the effects of nitrogen on recrystallization behavior, the specimens irradiated with high dose and low dose of N2+ were annealed, respectively. High nitrogen content depressed the epitaxial growth from the crystalline region to amorphous region, while the dependence of the recrystallization behavior on Ne dose was not clearly observed.

Ultrasonic signal characteristics by pulse-echo technique and mechanical strength of graphite materials with porous structure

Ultrasonic testing (UT) is an important non-destructive method to detect internal flaws and is widely applied to product control in industrial fields. In an investigation on ultrasonic signal characteristics in porous ceramics, the present authors developed an ultrasonic wave propagation model for the pulse-echo technique by improving an existing one for the transmission technique. A wave-pore reflection process was taken into account in the improvement. In the developed model, both diffusion and scattering losses can be treated as important factors of ultrasonic wave attenuation. The model was demonstrated by experimental data on ultrasonic signal characteristics of nuclear grade graphite. As an application of the model, the authors proposed a new approach combined UT signal with fracture mechanics to evaluate the mechanical strength of porous ceramics from UT signal. The combined approach was tried to apply to the acceptance test and the in-service inspection conditions of graphite components in the High Temperature Engineering Test Reactor (HTTR) as an example. This paper presents the developed propagation model for the pulse-echo technique as well as the combined approach. Moreover, both acceptance test and in-service inspection techniques of graphite components in high temperature gas-cooled reactors (HTGRs) using the combined approach was also proposed in this paper.

Evaluation of aseismic integrity in HTTR core-bottom structure II. Vibrational characteristics of keyed graphite components

Abstract The assembly system consisting of keyed graphite components, which is employed in the core bottom structure of a high temperature engineering test reactor (HTTR), has a nonlinear vibrational characteristic. The vibrational test was carried out to grasp the vibrational characteristics using the element model of the key-keyway structure. The analytical code was developed based on the experimental results. The main conclusions are summarized as follows: 1. (i) the stress distribution around the keyway is independent of whether it is induced under a dynamic or a static state; 2. (ii) the stiffness of the key-keyway structure has the nonlinear characteristics due to contact behavior. The stiffness can be evaluated by contact analysis, taking account of the relative slip and the deformation on the contact surface between key and keyway; 3. (iii) the analytical code employing a nonlinear spring for the key-keyway structure is available to predict the vibrational characteristic of the keyed graphite components.

Bubble formation with electron irradiation in SiC implanted with hydrogen or deuterium

Abstract The bubbles were formed and grew when SiC implanted with high fluence of hydrogen ion was irradiated with electron. In this study we tried to inspect the supposition that the energy deposition of the electron beam to hydrogen caused the migration of hydrogen and gave rise to bubble formation and growth. We used hydrogen (H) or deuterium (D) as implanted ion to change the cross section of the reaction that D or H is given energy by electron beam. The energy deposition cross section in the case of H is 2–3 times as large as that in the case of D. Bubbles were less likely to be formed and grow in the case of D than in the case of H. This result can be explained in terms of the difference of the concentration of the mobile gas atom caused by the difference of the energy deposition cross section, and does not contradict the supposition.