Y. Fujii-e

Self-consistent nuclear energy systems

Abstract A concept of self-consistent nuclear energy system (SCNES) has been proposed as an ultimate goal of the nuclear energy system in the coming centuries. SCNES should realize a stable and unlimited energy supply without endangering the human race and the global environment. It is defined as a system that realizes at least the following four objectives simultaneously: 1. a) Energy generation — attain high efficiency in the utilization of fission energy, 2. b) Fuel production — secure inexhaustible energy source: breeding of fissile material with the breeding ratio greater than one and complete burning of tansuranium through recycling, 3. c) Burning of radionuclides — zero release of radionuclides from the system: complete burning of transuranium and elimination of radioactive fission products by neutron capture reactions through recycling, 4. d) System safety — achieve system safety both for the public and experts: eliminate criticality-related safety issues by using natural laws and simple logic. This paper describes the concept of SCNES and discusses the feasibility of the system. Both “neutron balance” and “energy balance” of the system are introduced as the necessary conditions to be satisfied at least by SCNES. Evaluations made so far indicate that both the neutron balance and the energy balance can be realized by fast reactors but not by thermal reactors. Concerning the system safety, two safety concepts: “self controllability” and “self-term inability” are introduced to eliminate the criticality-related safety issues in fast reactors.

Scientific feasibility of incineration in scnes

Abstract We confirm the simultaneous realization of burning or transmutation of radioactive nuclides and of net energy generation. An investigation of the neutron balance in a reactor core is carried out. It is numerically shown that the neutrons can burn all the transuranium elements (TRUs) produced in the core as fuel in the SCNES reactor. It is numerically found that the fission products (FPs) whose half-lives are longer than one year can be contained and transmuted into harmless nuclides in the core without losing the neutron balance. It is shown that isotope separation of the FPs is required to realize the SCNES. As an example, we investigate the required energy for a scheme of the atomic vapor laser isotope separation (AVLIS) of FPs. It is shown that, in principle, the energy required for the isotope separation is much lower than the generated fission energy. The SCNES is scientifically realized in principle.

Safety features of self-consistent nuclear energy system

Abstract The basic condition and concept to eliminate recriticality-related problems has been investigated from the viewpoint of the safety features of the SCNES. The passive shutdown capability of the intact core was achieved by self-controllability due to the flat core geometry of high thermal conductivity fuel. Recriticality during severe transients was found to be eliminated by the self-terminability due to the controlled material relocation of the leading channel, where the incoherent and localized fuel motion was generated in the degraded core.

A core concept for the self-consistent nuclear energy system based on the promising future technology

Abstract Feasibility of FP burning while maintaining fuel breeding capability for the Self-Consistent Nuclear Energy System is evaluated through neutron balance and a fast reactor core. It is shown that all radioactive FPs produced by itself can be burnt by a fast reactor while maintaining breeding capability, assuming separation of radioactive FP and stable FP isotopes. Assuming that the recovery system of fuel and FPs to be burnt is based on a pyro-chemical process, the major long-lived FPs of I, Pd, Tc, Sn, Se can be burnt with keeping breeding capability by suitably arranging materials in the fast reactor core.

A fuel cycle concept for self-consistent nuclear energy system

Abstract A fuel cycle concept for Self-Consistent Nuclear Energy System which releases no fission products to the environment is proposed. Material and energy balances are shown for this system.

Development of general methodology of safety analysis and evaluation for fusion energy systems (GEM-SAFE)☆

Abstract A synthesized methodology of safety analysis and evaluation for fusion systems has been developed to concretely assess the adaptability of fusion systems to the environment from the earliest stages of system development. The methodology objective was to summarize both the safety design requirements and achieve rational safety in fusion systems. The framework of the methodology was constructed to clarify its logical consistency. The safety characteristics of fusion systems were then investigated in detail paying attention primarily to potential hazards, so that a fusion system was identified as a distributed system in regard to energy sources and radioactive materials. Based on this recognition, a General Descriptive Model (GDM) of a fusion system has been constructed which is a highly generalized and integrated expression. The safety ensuring principle, on the other hand, set up items to be protected and categorized events for a fusion system. The development of the safety ensuring principle was a key to the practical performance of safety analysis and its evaluation in a general fusion system. Finally, by using the Function-Based Safety Analysis (FBSA) on the GDM, abnormal events were summarized into 16 typical events, according to the safety ensuring principle. Consequently, 20 design based events for the general fusion system were selected to envelope all credible abnormal events.

Safety characteristics of the SCNES core

Abstract The core concept of the Self-Consistent Nuclear Energy System (SCNES) and its safety characteristics have been investigated from the view point of the elimination of recriticality. The recriticality potential can be eliminated based on characteristics of self-controllability to prevent the core damage and self-terminability to limit the propagation of core disruption. These two characteristics are simultaneously achieved by the radial heterogeneous two region core with different height. This core consists of leading and driver zones where hybrid metallic fuels with different melting point are installed. The self-controllability can be achieved by decreased coolant density effect due to the above core sodium plenum at the leading zone. The self-terminability is achieved by the Controlled Material Relocation (CMR), which is essentially the preceding downward in-pin fuel relocation selectively generated at the leading zone. U-Pu-1Zr alloy is used to the leading zone fuel due to lower melting point (900°C) than the driver fuel of U-Pu-10Zr(1100°C). Based on the quantitative investigations, it was emphasized that the recriticality potential can be eliminated by the in-pin fuel CMR even for severe unscrammed events such as a total pump stick for the primary coolant system and a total control rods withdrawal.

Long-lived FP burning based on the actinide recycle metal fuel core

Abstract Feasibility of burning of the major long-lived FPs (I, Pd, Tc, Sn, Se, Zr, Cs) while maintaining fuel breeding capability for the Self-Consistent Nuclear Energy System is evaluated based on the actinide recycle metal fuel core of a fast reactor. It is shown that I, Pd, Tc, Sn, Se, and Zr can be burnt simultaneously by an aid of the isotope separation of Pd-107, Zr-93 and Se-79. Cs, which is difficult to burn with the other FPs, should be utilized as an in-reactor shielding material to confine in the system. The selection of the target FPs to be burnt are also validated by using the Burden Index. The overall assessment based on those results indicates that the developed system has the great potential to achieve the goal of the Self-Consistent Nuclear Energy System.

Application of GEMSAFE to ITER CDA and its comparison with FER

A safety analysis for the design of International Thermonuclear Experimental Reactor (ITER) in the Conceptual Design Activity stage was performed by the GEMSAFE methodology, and its results were compared with those of Fusion Experimental Reactor (FER), a Japans facility planned next to JT-60. The objectives of this study are to confirm the applicability of GEMSAFE to ITER and to select design basis events of ITER and identify R&D items with comparison to FER. Function-Based Safety Analyses (FBSA) were carred out to select 19 and 25 design basis events for FER and ITER, respectively. The major reason for the difference is that ITER has a class-2 RI source, e.g., tritium of 7.5 × 105 Ci in mobile form, in the coolant for the first wall and blankets as well as a class-3 RI source, e.g., the immobile tritium of 2.2×107 Ci absorbed in first wall and dust.

244Cm Transmutation in Accelerator-Driven System

Mixed oxide (MOX) fueled LWRs are characterized by enhanced accumulation of curium isotopes compared to uranium fueled LWRs. Toxicity of relatively short-lived isotope 244Cm (T 1/2=18 yr) appears to be dominant in transuranics discharge. The present paper deals with analysis of the scientific feasibility to transmute 244Cm. Liquid bismuth-curium core operated in a subcritical mode with neutron support from accelerator is proposed to provide transmutation rate through fissioning that exceeds the rate of natural decay by one order of magnitude. Beam current in the range 30–40 mA is required to drive the core. One accelerator-driven transmutor could solve the problem of curium accumulation in a large-scale nuclear energy system based on MOX fueled LWRs.