Yoshihiko Kaneko

Evaluation of Delayed Neutron Data for Thermal Fission of U-235 Based on Integral Experiments at Semi-Homogeneous Experiment

Evaluation of the delayed neutron data for thermal fission of 235U is attempted through an indirect method based on the comparison between measured and calculated integral quantities such as kinetic parameters and reactivity worths of fuel rods and burnable poison rods for the critical assembly Semi-Homogeneous Experiment (SHE). The most probable values for both of the effective delayed neutron fraction βeff and the decay constants λs of delayed neutron pre-cursors are determined from the condition that the sum of the squared deviations of the ratios of calculated to measured values from unity is minimized for the integral quantities. The most probable value of βeff obtained in the present work is significantly large compared with that obtained with the Keepins delayed neutron data set, and is closer to the value obtained with the ENDF/8-V data file than that with the ENDF/8-IV. On the other hand, the most probable λ values obtained in the present work are not away from those for thermal fission filed i...

A Pulsed Neutron Source for Thermal Reactor Physics

A compact 200 kV Cockcraft type pulsed neutron source was designed and constructed as a versatile tool for various pulsed neutron experiments in thermal reactor physics. Special care was taken to obtain the maximum peak intensity of pulsed neutrons against the average background neutrons induced from accelerator operation. To this end, two particular design features have been adopted. One is pulsing of the R-F ion source probe voltage combined with pulsing of the deflector voltage of the post accelerated D+ ion beam. The other feature is the use of a beam analyzing magnet to the accelerated beam. With these devices, a peak to background ratio of 105 was obtained. Pulsed neutrons of 14 MeV can be generated at pulse widths from 0.13 μs 500 ms, with a neutron yield of 3.7×104–2×109 n/pulse. The repetition rate of the pulses can be changed independently of the pulse width. An arrangement for long focusing beam adjustment provides for selection of the target position in a wide range.

Effect of Reentrant Hole Perturbation on Angular Thermal Neutron Spectra in Time of Flight Measurement

The effect of the presence of a reentrant hole for extracting the neutron beam from within experimental systems of two different geometries is analyzed theoretically with use made of multi-group 2- dimensional discrete Sn method without resorting to bold assumptions for neutron transport nor drastic simplification of geometry. One of the two experimental systems is a rectangular light water prism 12 cm high of 40 × 40 cm2 cross section, poisoned with Cd and/or In, and provided with a 1, 2 or 3 cm diameter reentrant hole. The other system is a 1″ thick natural uranium plate sandwiched between two layers of pure light water, each 4.6 cm thick, which also is provided with a 1cm diameter reentrant hole. The following is concluded by comparing the angular neutron flux with and without the reentrant holes. With the first experimental system, perturbations of the order 10∼25% is caused, which is particularly strong below about 0.3 eV, except when the hole diameter is 1cm. The perturbation effect increases as the...

MEASUREMENT OF NEUTRON SLOWING DOWN TIME IN GRAPHITE.

A series of pulsed neutron experiments was performed to investigate the chemical binding effect on the neutron slowing down time. Bursts of D-T neutrons of 1 μsec width were generated in a hexagonal prism with 240 cm high with 120 cm flanks, made of reactor grade graphite with a density of 1.54. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF3 counters, and analyzed with a 256 channel time analyzer. Slowing down times in graphite were determined by interpreting the increment of the difference of events between the two counters to be due to the contribution made by the fraction of the slowing down neutrons at the time of measurement that was below the cut-off energy of the filter. The results of measurements showed good agreement with calculation based on the crystal model.

Reactor physics research activities related to the very high temperature reactor in Japan

Reactor physics research activities in Japan that are related to the very high temperature reactor (VHTR) for multipurpose use are briefly summarized. Emphasis is placed on critical experiments. Neutronic core design accuracy required for the experimental VHTR is made clear, and nuclear data compilation and neutronic calculation code development are described. For experimental work, after a review of the results of all reactor physics experiments performed on the Semi-Homogeneous Experiment at the Japan Atomic Energy Research Institute, its reconstruction program to the VHTR critical assembly is presented. The aim of this program is to perform a detailed mockup experiment of the experimental VHTR loaded with low-enriched uranium-coated particle fuels. Finally, improvement of the neutronic calculation accuracy attained through comparison between calculation and experiment is illustrated, and some future problems are pointed out.

THERMAL NEUTRON SPECTRA IN LIGHT WATER POISONED WITH CADMIUM AND/OR INDIUM.

The thermal neutron spectra in light water of slab geometry poisoned with Cd and/or In were measured by the time of flight method, using a 20-MeV electron linear accelerator. The thermal spectra were simulated to those in the Pu built-up core of a commercial light water reactor corresponding to a fuel burnup of about 15,000 MWD/T. The results of measurements were compared with calculations based on the S 4 method using the Haywood scattering law. Fairly good agreement was obtained between the calculated and measured results except in a limited range of energy above the 0.176 eV resonance of Cd. It is concluded that the P 1 components of the source neutrons as well as the neutron scattering kernel play a significant role in the calculation of the thermal neutron spectra with large flux gradients, and that the scattering kernel of light water based on the Haywood model will be accurate enough to evaluate the infinite multiplication constant k∞ of light water reactor cores with high fuel burnup within an err...

Determination of Neutron Multiplication, (II): Experiments and Analysis

The purpose of the present work is to confirm experimentally that the prompt neutron decay constant of fundamental mode, α000, satisfies the following two requirements from the concept proposed in Part (I): To be uniquely measurable free from space dependence and detector specification, and to be calculable from the Boltzmann equation. To prove this, a series of pulsed experiments was made at various subcritical states in a reflected graphite-moderated 20% enriched fuel system (SHE). The experimental results confirmed that α000 was obtained as a common and unique value at any point of both core and reflector, determined by separation of spatial harmonics, using bare and Cd-covered BF3 counters. The state of SHE was changed by attenuating the core, with or without central control rod, and by adding distributed poison to the core and/or to the reflector. Direct comparison was made between the experimental results and the calculated values from multigroup treatment using P1 and S4 approximation. Calculated v...

Measurements of Multiple Control Rods Effect in Graphite Moderated Reactors

Measurements on the effects of multiple control rods arranged in ring geometry were undertaken with application of the pulsed neutron technique in SHE, a heavily reflected graphite-moderated 20% enriched uranium critical assembly. The aggregate number of experimental cases exceeded 100, which systematically covered the four fundamental factors to determine the control rod effects: 1. The atomic ratio of 235U to C in the core region (C/235U = 2,226 and 5,378) 2. The neutron absorbing substances in the control rods and their densities (B4C powder and Cd plate. Density of B4C: 0.1 and 1.0 g/cm3) 3. Geometrical factors of the control rods (Cylindrical shape, 20 or 44 mm in diameter) 4. Number of control rods (One to six control rods inserted in circular geometry) The experimental results are compared with calculations based essentially on a system of simple two group Nordheim-Scalettar method used as tool to solve the three-dimensional diffusion equation. Agreement within about 10% was obtained between measur...

Measurement of Anisotropic Diffusion Coefficients in H2O-Al Slab

Diffusion coefficients were measured for an H2)O-Al slab lattice (H2O 8 mm, Al 8 mm) in parallel (D∥) and perpendicular (D⊥) directions, applying the pulsed neutron technique. The measured results were compared with those of theoretical calculations based on the collision probability method, taking into account anisotropic scattering, and assuming a source neutron distribution of Maxwellian configuration. Fairly good agreement between theory and experiment was obtained for D ⊥ but a considerable discrepancy, of the order of 19%, resulted for D ∥. The relation between measured and theoretical quantities in a heterogeneous system are also discussed.

Measurement of Neutron Slowing Down Time in Light Water, Ice, Paraffin and Santowax

Measurements of the neutron slowing down times in light water, ice, paraffin and santowax have been made by pulsed neutron technique. Bursts of D-T neutrons of 0.1μsee width were generated in moderator cubes of 40×40×40 cm3. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF3 counters, and analyzed with a 256-channel time analyzer. Slowing down times in the moderator were determined by interpreting the increment of the difference of events between the two counters as attributable to the fraction of the neutrons slowing down at the time of measurement below the cut-off energy of the filter. The measured slowing down times below 0.63 and 0.43 eV agreed well with theoretical values on the 0°K free gas model. On the other hand, the measured values below 0.20 eV were found to be appreciably greater than the theoretical, which would appear to indicate that he effect of thermal agitation and chemical binding come into play at this range of energy.