Toshio Sanda

Concept of hydride fuel target subassemblies in a fast reactor core for effective transmutation of MA

Abstract U–Th–Zr alloys with four different compositions were hydrogenated and examined for their hydrogen holding capacities, microstructural and hardness changes with irradiation and thermal diffusivities. Considerably high hydrogen capacity was confirmed up to about 1173 K. A certain degree of irradiation stability was observed, and relatively high thermal diffusivity was ascertained for these hydrogenated ternary alloys. Based on these results, a new concept for effective transmutations of MA was proposed, where target assemblies containing Np and Am in hydrogenated form are loaded in a fast reactor core. This concept has a great potential to achieve the best transmutation of MA with improvement of safety characteristics in a fast reactor core.

Theoretical Analysis of Two-Detector Coherence Functions in Large Fast Reactor Assemblies

The general expression of noise coherence function including the energy dependence of neutron detection cross section and the higher harmonic contributions in multi-dimensional model is developed on the basis of the modal expansion technique. Applicability of the method is demonstrated by numerical calculations carried out for a two-dimensional model of large fast reactor assemblies ZPPR-9 and −13C. The agreement between the theory and the measurement is satisfactory, which indicates the validity of the theory and the calculational model employed. In the assemblies, the coherence for a detector pair in a specific location can be approximately described by including the fundamental and the first harmonic modes.

Fast Reactor Core Concepts for Minor Actinide Transmutation Using Hydride Fuel Targets

Fast reactor core concepts are studied which reduce long-term radiotoxicity of nuclear waste by using minor actinides (MAs) in the form of zirconium-hydride fuel targets. A systematic parameter survey is carried out to investigate the fundamental characteristics of MA transmutation and the core safety parameters such as sodium void reactivity in a 1,000 MWe-class fast reactor core. Two core concepts are proposed, using 36 target assemblies, by adjusting the composition of hydride fuels. One is the MA burner core to transmute a large amount of MAs in a short time combined with Pu multi-recycling in fast reactors, whereby the MAs produced in about 13 LWRs can be transmuted every year with a 58% MA transmutation rate in discharged targets. The other is the MA once-through core to incinerate a small amount of MAs by fission, whereby the MAs produced in about 2 LWRs can be incinerated every year with a 64% MA incineration rate (a 93% transmutation rate) in discharged targets. This study shows these concepts have great potential to achieve good transmutation characteristics of MAs while providing the improved safety characteristics of a fast reactor core.

Neutronic Decoupling and Space-Dependent Nuclear Characteristics for Large Liquid-Metal Fast Breeder Reactor Cores

Eigenvalue separation, which is used as a criterion to determine the degree of neutronic decoupling of the core, is measured by a static flux-tilt method on Zero-Power Physics Reactor assemblies. Space-dependent nuclear characteristics, such as the radial distributions of the reaction rates and the control rod worths, are also measured for the same assemblies. The calculation/experiment (C/E) values very with core radius depending on the assemblies. The relationship between decoupling and C/E radial dependence is investigated, and a quantitative relation is found between the eigenvalue separation of the first radial mode and the C/E radial dependence.

Interpretation of noise coherence function measurements in liquid-metal fast breeder reactor critical assemblies

An improved technique for inferring the eigenvalue separation, which is important in spatial stability analysis, has been developed using the noise coherence function. It was applied to fast reactor critical assemblies of various sizes and compositions that exhibited a wide range of spatial decoupling. In each experiment, four lithium-glass detectors were used to measure noise coherence functions. Various ratios of the coherence functions were used to obtain the first two modes of separation considering higher odes and variations in detector efficiencies. The eigenvalue separation obtained by noise analysis agreed well with calculation.

Enhancement of Transmutation Characteristics of the Minor Actinide Burning Fast Reactor Core Concept Using Hydride Fuel Targets and Its Introduction Scenario

Transmutation characteristics of the minor actinide (MA) burning fast reactor core using hydride fuel targets are enhanced to reduce long-term radiotoxicity of nuclear waste. A scenario which introduces the concept is investigated. (1) The MA burner core with plutonium (Pu) multi-recycling can transmute a large amount of MAs; the amount is about that produced in 21 LWRs per year. The targets are shuffled after the 1-year irradiated in the core region and further irradiated for 2 years in the radial blanket region. (2) The MA once-through core can incinerate almost all of the MAs in targets by fission during a 6-year irradiation in the core region. (3) Introduction of the MA burner core for all fast reactors (FRs) after the year of 2020 allows the following scenario. The residual amount of MAs from LWR spent fuel can be held to zero within the 21st century and all FRs will be changed to the self-generated MA transmutation core without the targets. When the MA once-through cores are introduced with MA burner cores, the total number of targets to be reprocessed can be reduced by 50%. (4) Even in the low projection case while the MA burner cores are installed after conventional FRs, MA accumulation would also be held to zero within the 21st century.

Spatial neutronic decoupling of large fast breeder reactor cores : Application to nuclear core design method

Space-dependent nuclear characteristics, measured by critical experiments on large-size fast breeder reactor (FBR) cores, were reviewed and interpreted. It was observed that radial neutron flux distributions were significantly distorted by perturbations, control rod reactivity interaction effects were large, and the point kinetics was not valid. These physical behaviors are enhanced as the spatial neutronic decoupling increases. To obtain stable and benign nuclear characteristics and to make the kinetics as close to the point kinetics as possible, it is necessary to reduce the spatial decoupling. This is an important issue that must be taken into account in the nuclear design for large FBR cores. A new nuclear core design method for large FBR cores is proposed in which neutronic stability is considered at the same time as performance and safety for the optimization of core design. The neutronic stability is improved by reducing the spatial decoupling and by taking into account the spatial higher harmonics.

Reactor noise analysis of the experimental fast reactor JOYO

Abstract Experimental fast reactor JOYO achieved its first criticality in 1977 spring. Since then a series of reactor noise analysis has been carried out. During zero power testing, the reactor kinetic parameters, i.e. β/l and subcriticality, were obtained, applying band-pass filter and polarity correlation methods. As the high power operation started from 1978, the reactor noise analysis based on neutron flux spectra has been proceeded. This paper presents study of noise analysis for rather lower frequency region at high power operation. At 50 Mwt power level operation, noises of neutron flux signals, reactor inlet temperatures and subassembly outlet temperatures were measured, and then auto power spectral densities and coherence functions were calculated. The frequency range for the noise analysis is for 10−3 − 10−1 Hz. In the APSD of neutron flux, a low peak was observed at 2.5×10−2 Hz. For neutron noise, it was estimated that the noises in the frequency region lower than 1.5 × 10−2 Hz is a core inherent temperature noises, while that in the frequency region higher than 1.5 × 10−2 Hz is a core inherent neutron noise. The coherence function of neutron fluxes, which are signals of ex-core detectors located at opposite positions against the core center, indicated that some space dependent phenomena of neutron noise might exist in 5×10−3 − 6×10−2 Hz. The space dependency in 5×10−3 − 1.5×10−2 Hz were estimated to be produced by the sodium temperature noise due to the insufficient mixing of two primary coolant loops. In 1.5×10−2 − 6×10−2 Hz, the peak existed at 2.5×10−2 Hz and reverse phases were observed in the coherence of two neutron signals at opposite side of the core. With the investigation of phase relations between neutron flux and subassembly outlet temperature in this frequency region, the phenomenon was estimated to be due to mechanical vibration of some reactor core components. When the static gain of reactivity-to-power transfer function increased, the changes of reactor noise characteristics were observed. Since the change of the gain indicates a change of the reactor core characteristics, the observation of reactor noise characteristics has a potential for the surveillance of the reactor core.

Feasibility study of large MOX fueled FBR core aimed at the Self-Consistent Nuclear Energy System

Abstract The potential of a MOX fueled fast breeder reactor (FBR) is evaluated with regard to its ability to transmute radioactive nuclides and its safety when incorporated in the so-called self-consistent nuclear energy system (SCNES). The FBRs annual production amounts of selected long-lived fission products (LLFPs), Se-79, Tc-99 Pd-107, I-129, Cs-135 and Sm-151, can be transmuted by using a radial blanket region and a part of a lower axial blanket region without any significant impact on its nuclear and safety characteristics. The other LLFPs are confined in the system. The hazard index level of the LLFPs per one ton of spent fuel from the system after 1000 years is as small as that of a typical uranium ore. To realize self-controllability (passive safety), the proposed FBR core concept employs gas expansion modules and sodium plenum above the core. To realize self-terminability, even if MOX fuel melting should cause a core compaction, recriticality of the core can be avoided by a fuel dilution and relocation module. The results show the MOX fueled FBR core has potential applicability to the SCNES. With the final goal of the ideal SCNES, fundamental applicability of various coolants and fuels is evaluated based on neutron balance. It is shown that the harder the core spectra is, the larger the potential for transmuting LLFPs would be.

Feasibility study of large MOX fueled FBR core and applicability of various coolants and fuels aimed at the self-consistent nuclear energy system

Abstract The potential of a large MOX fueled fast breeder reactor (FBR) is evaluated with regard to its ability to transmute radioactive nuclides and its safety when incorporated in the self-consistent nuclear energy system (SCNES). The FBRs annual production amounts of selected long -lived fission products (LLFPs), Se-79, Tc-99, Pd-107, I-129, Cs-135 and Sm-151, can be transmuted by using a radial blanket region and part of a lower axial blanket region without any significant impact on the reactors nuclear and safety characteristics. The other LLFPs are confined in the system. The hazard index level of the LLFPs per one ton of spent fuel from the system after 1000 years is as small as that of a typical uranium ore. To realize self-controllability (passive safety), the proposed FBR core concept employs gas expansion modules and a sodium plenum above the core. To realize self-terminability, even if MOX fuel melting should cause a core compaction, re-criticality of the core can be avoided by a fuel dilution and relocation module. The results show the MOX fueled FBR core has potential applicability to the SCNES. The fundamental applicability of various coolants and fuels is evaluated based on neutron balance toward the final goal of the ideal SCNES. The results show that gas coolant has a potential for increasing the transmutation efficiency of LLFPs. And an improved SCNES with several conventional FBRs and a FP transmutation reactor is also studied.