A probabilistic safety assessment (PSA) is made on the boiling accident of a highly active liquid waste tank, which may result in significant consequences, in accordance with the procedure for PSA developed for nuclear power plants. Obtained as results are the frequency of boiling accident of a certain tank of 2.0×10−8/y (frequency of boiling accident of any tank of 4.1×10−8/y), its error factor of approx. 6, and information on the relative risk importance based on the FV index and RAW for various components, systems and activities of personnel and on the sensitivity of key parameters. Furthermore, the effect of the time required for repairing failed instruments on the frequency of accident, how to deal with the common cause of failure of the duplicated dynamic components, one of which is at least in operation, and conservative exposure dose in the event of an accident are examined. The database for the Rokkasho reprocessing plant has not been established yet, but the PSA results utilizing available failure rate databases of existing nuclear power plants and reprocessing plants in Japan and abroad can be used effectively to optimize operations and maintenance, if they are interpreted properly and some uncertainties are taken into account.
The seismic probabilistic safety assessment (PSA) is an available method for evaluating the residual risk of nuclear plants, which are designed under definitive seismic design conditions. Our preliminary seismic PSA analysis has indicated that the malfunction of electric panels would have a significant influence on the core damage frequency (CDF). In recent years, there has been a growing desire to raise the reliability of the seismic PSA; thus, a higher seismic capacity of the electric panels is necessary. The Japan Nuclear Energy Safety Organization (JNES) tested eight kinds of main electric panels and thirty kinds of main electric parts at high acceleration, which considerably exceeds the design level. From the test results, JNES obtained precise seismic capacity data of the electric panels that have a significant effect on CDF. JNES also proposed a method of evaluating seismic capacity. The method can use the seismic capacity data of electric parts and the acceleration amplification ratios calculated from the FEM analysis. JNES expects that the seismic capacity data obtained from this study will increase the reliability of the seismic PSA.
The radionuclide migration forecasting system in the off Shimokita region has been developed to predict the routine and accidental releases of liquid radioactive wastes during the operations of a spent nuclear fuel reprocessing plant in Aomori Prefecture, Japan. The results obtained from the case studies are as follows. (1) The calculation results following the ocean general circulation model by using the techniques of data assimilation and nesting reproduce well the flow pattern of a coastal area. (2) The results of the hypothetical release of 3H show that the concentration of 3H is about 800 times higher at maximum than the background level at the off Hiranuma region (8 km south from the release point). (3) The 137Cs concentration of the hypothetical release from the off Shimokita region is the same or less than that of the background level measured at the same area.
In this paper, the burn-up characteristics of a 200 MWe molten-salt reactor are studied. This reactor has a three-region core in order to reduce the peak in fast neutron flux distribution. The reactor is operated for 30 years with the load factor of 0.75. The fuel is fed to the reactor every 33 days. The chemical processing of the fuel salt is performed every 7.5 years. Based on calculations using the nuclear analysis code SRAC2006 and the burn-up analysis code ORIGEN2, the following results have been obtained. (1) The graphite moderators can be used throughout the reactor lifetime without replacement. (2) The reactor is self-sustainable having an average fuel conversion ratio of 1.01. (3) The initial inventory of 233U is 1.13 t, net feed in 30 years is 0.34 t; thus, the necessary amount of 233U is 1.48 t. (4) Pu isotopes are produced at 1.5 kg and minor actinides at 27 kg per 1 GWe output in 30 years, which are absolutely small compared with those produced by BWR.
The Japan Atomic Energy Agency (JAEA) has been conducting R&D on hydrogen production by the thermochemical Iodine-Sulfur (IS) process to meet the massive hydrogen demand in the future hydrogen economy. In the IS process, sulfuric acid (H2SO4) is evaporated and decomposed into H2O and SO3 in a H2SO4 decomposer operated under high-temperature condition up to 500°C. We have proposed a new decomposer concept of a block-type heat exchanger made of SiC ceramic, which has a salient corrosion-resistant performance under severe corrosion condition. To verify the mechanical feasibility of the concept, the temperature distributions of sulfuric acid and helium gas were analyzed using a newly coded analytical system applicable to binary mixtures such as H2SO4 whose boiling point depends on the concentration. Thermomechanical simulations with a general-purpose FEM code, which used the heat exchange simulation results, showed that the maximum stress was generated around the dryout region, which was about half of the allowable tensile stress of SiC.
The objectives of this study are to develop a new inspection probe for complex structures using eddy current testing method (ECT), to establish the recognition technique of real flaw shape such as stress corrosion cracking (SCC), and to develop a rapid nondestructive inspection system. An ECT system consists of measurement and computational modules that are connected via a network circuit. The measurement module includes a flexible ECT sensor, which could fit on the curvature surface of radius larger than 25 mm with a mechanical manipulator. The computational module is used for SCC sizing. It includes a database of ECT signals that are used to execute numerical calculations. SCC sizing can be evaluated by comparing the measured ECT signals with the calculated ECT signals within the database. We have demonstrated that the system could be used to estimate the SCC length and depth sizing in 140 s. The accuracy of length sizing showed an RMS error of 3.7 mm and the depth sizing was 0.63 mm in this work.
An onset condition of gas entrainment (GE) due to free surface vortex has been studied to establish a design of sodium-cooled fast reactor with a higher coolant velocity than conventional designs. Numerous investigations have been conducted experimentally and theoretically; however, the universal onset condition of the GE has not been determined yet due to the nonlinear characteristics of the GE. Recently, we have been studying numerical simulation methods as a promising method to evaluate GE, instead of the reliable but costly real-scale tests. In this paper, the applicability of the numerical simulation methods to the evaluation of the GE is discussed. For that purpose, a quasi-steady vortex in a cylindrical tank and a wake vortex (unsteady vortex) in a rectangular channel were numerically simulated using the volume-of-fluid type two-phase flow calculation method. The simulated velocity distributions and free surface shapes of the quasi-steady vortex showed good (not perfect, however) agreements with experimental results when a fine mesh subdivision and a high-order discretization scheme were employed. The unsteady behavior of the wake vortex was also simulated with high accuracy. Although the onset condition of the GE was slightly underestimated in the simulation results, the applicability of the numerical simulation methods to the GE evaluation was confirmed.
To clarify the dissolution mechanism of clay minerals, the basal and edge surface structures of dioctahedral 2:1 phyllosilicate were optimized using the semiempirical molecular orbital method. The stability of the surface structure was estimated from the bond length, bond strength and bond energy of bonds of interest along the basal and edge surfaces. The basal surface structure of montmorillonite was optimized using the periodic boundary condition. The optimized basal surface structure resides at a local minimum on the potential energy surface, because each normal mode for the optimized structure has a real vibrational frequency. The ideal (010)-type edge surface of pyrophyllite was initialized using crystal chemical methods. The edge surface structure that was protonated allowing for pH dependence was optimized. On the optimized edge surfaces, structural relaxation occurred under all pH conditions, then it was found that the Si-O bonds in the silanol groups were stronger and the Al-O bonds in the aluminol groups were weaker than the corresponding bonds in the bulk structures. The rate-determining step is thought to be governed by the hydrolysis of the outer Si-O bonds on edge surfaces during the alkaline dissolution of pyrophyllite and by the hydrolysis of Al-O bonds in bridging the Si-O-Al bonds on edge surfaces during the acid dissolution.