Akinao Shimizu

Calculation of Gamma-Ray Buildup Factors up to Depths of 100 mfp by the Method of Invariant Embedding, (I): Analysis of Accuracy and Comparison with Other Data

The method of invariant embedding has been applied to calculations of gamma-ray buildup factors for point isotropic sources in infinite homogeneous media up to depths of 100 mean free paths (mfp) without bremsstrahlung. A comprehensive survey of buildup factors was performed to estimate errors due to energy, angle and space meshes adopted in the transport calculations by the present method. It is confirmed numerically that the exposure buildup factors can be calculated up to depths of 100 mfp with an error less than 10% by using the present method. The exposure buildup factors for water, iron and lead for typical source energies of 10 MeV, 1.0 MeV and 0.1 MeV are provided up to depths of 100 mfp. Those buildup factors are found to agree well with other data including the moments method calculations and the Monte Calro calculations by EGS4. These results indicate that the method of invariant embedding is able to provide gamma-ray buildup factor up to depths of 100 mfp with an accuracy enough to be used in shielding calculations.

Calculation of gamma-ray buildup factors up to depths of 100 mfp by the method of invariant embedding, (III): Generation of an improved data set

An improved data set of gamma-ray buildup factors for point isotropic sources in infinite homogeneous media has been generated by the method of invariant embedding. The points of improvement compared with the standard data set ANSI/ANS-6.4.3 include (1) extension of the buildup factors up to depths of 100 mean free paths, (2) improved treatment of bremsstrahlung, (3) addition of the effective dose buildup factors, (4) consistent use of the cross-section PHOTX to all materials and (5) a quantitative evaluation about the accuracy in transport calculation. The data set obtained are compared precisely with the standard data set ANSI/ANS-6.4.3.

Calculation of Gamma-Ray Buildup Factors up to Depths of 100 mfp by the Method of Invariant Embedding, (II) : Improved Treatment of Bremsstrahlung

An improved method to calculate the gamma-ray buildup factors including bremsstrahlung has been developed. The exposure buildup factors with bremsstrahlung were computed by the present method for lead, iron and water at the source energy of 10.0 MeV up to depths of 100 mfp. The accuracy of the present method was checked by comparison with the calculations by use of EGS4. Excellent agreement was obtained between the calculations by both methods about the exposure buildup factors per energy (energy spectrum of transmitted photons) for lead up to depths of 10 mfp and the ratio of the exposure buildup factor with bremsstrahlung to that without bremsstrahlung for lead, iron and water up to depths of 40 mfp. It is confirmed that the present method has an accuracy sufficient to be used to the generation of an improved set of gamma-ray buildup factors including bremsstrahlung.

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.

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.

Application of Invariant Imbedding to the Reflection and Transmission Problem of Gamma Rays, (I)

Equations were derived for the reflection and transmission functions of γ-rays through slabs of finite thickness, making use of the invariant imbedding principle. Numerical solutions were obtained for the reflection function (albedo) of semi-infinite medium. The results agreed well with the solutions by Monte Carlo method. Comparisons were also made with experimental results. Fair agreement was obtained except for the angular distribution of the reflected photons from light elements.

Application of the Response Matrix Method to Criticality Calculations of Two-Dimensional Reactors

Response matrix method was applied lo criticality calculations of two-dimensional reactors. The response matrices of component blocks in a reactor were first calculated analytically with group-diff...

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.

Calculation of a materials relocation experiment simulating a core discruptive accident condition in fast breeder reactors

This paper describes an interpretation of the SIMBATH (Simulationsexperimente in Brennelementattrapen mit Thermit) experiments that use the Simmer-II code. A series of SIMBATH experiments has aimed at simulating fuel pin disintegration and following materials relocation in the test sections of a single pin to 37-pin bundles. In the calculation, three modifications were incorporated into the Simmer-II code. With these modifications, the calculation showed good agreement with the experimental measurements with respect to the void region propagation in sodium flow and the molten materials relocation leading to flow blockage. A set of parametric calculations has clarified the range of applicability of parameters for materials relocation and flow blockage formation. The particle radius rp in blockage regions and the multiplier for particle viscosity (PARVIS) are recommended to be rp ⪆ built12Dh and 0.001 Pa s ⪅ PARVII ⪅ Pa s respectively.

On self-controllability and self-terminability of fast reactors

The paper gives an outline of simple evaluation models first for passive and inherent power stabilization of the intact fast reactor cores under the conditions of an anticipated transient without scram, and then for termination of the accident progression into further disruption of the degraded core and into a possible recriticality leading to a power excursion under postulated core damaged conditions. Based on the simplified models, general guidelines are given which enhance the passive and inherent power stabilization capability for intact cores, and those which minimize the amount of fuel removals necessary to avoid recriticality events. Also consistency between the self-controllability and self-terminability requirements is discusse.