Kenji Sumita

Status of OKTAVIAN I and Proposal for OKTAVIAN II

OKTAVIAN I was built at Osaka University and has been operated since 1981 as an intense deuterium-tritium (D-T) neutron source at 1.5-ns pulses of 103 D-T neutron/pulse or as a continuous neutron s...

Pulse Propagation Experiments of Thermal Neutrons in Graphite

Pulse propagation experiments of thermal neutrons were carried out in a graphite system to in-vestigate the diffusion and the thermalization processes of neutrons in graphite.The principle of the thermal pulse source used was based on the distinct difference in the slowing down and the thermal-ization times between paraffin and graphite.From the Fourier transformed data of the measured propagating thermal pulses were derived the inverse attenuation length and the phase lag per unit length as functions of frequency.Peaks and caves distinctly observed in the gain and the phase lag curves at various distances from the source surface in the range of frequency above 2, 000rad/sec were attributed to the existence of a continuum eigen value region, due to cold neutron penetration into the frequency dependent at-tenuation.The diffusion and diffusion cooling coefficients were derived into the range below this frequeny.The results(reduced to 20°C, 1.6g/cm3 density)are:D0=(2.16±0.02)×105cm.sec-1, C0=(2.8±0.3)×106cm4•sec-1.The results of die away experiment using the same graphite under critical buckl-ing are:D0=(2.21±0.08)×105cm2•sec-1, =(2.6±1.2)×106cm4•sec-1.This agreement is obtained by rejecting the data in the continuum region, and the present values are closer to the more recent theoretical values than derived from experiments by other authors.

Fusion neutron damage on optical fibers and optoelectronic devices

Radiation-resistant optical fibers fabricated with purified silica glass were irradiated at room temperature with 14-MeV neutrons from the RTNS-II. The induced loss in light transmission was measured in situ during irradiation. It was found that the fusion neutron response of the optical fibers can be approximately classed in three types as follows: (1) a large induced loss with little recovery; (2) a large induced loss with a quick recovery; and (3) a small induced loss with a saturated tendency. The induced-loss rate has been calculated on the basis of damage components that depend mainly on neutron fluence and show little recovery. Fusion neutron irradiation effects on related optoelectronic devices and their hardness levels are shown. These are the neutron fluences at which important performance parameters of the devices begin to degrade. The data are useful for fiber-optics design for fusion diagnostic systems. >

Tritium solid targets for intense D-T neutron production and the related problems

Abstract The first section of this paper discusses possible designs for higher tritium-containing surfaces, and describes an effective cooling system for the production of intense neutron sources used in rotating target systems. The maximum intensity available from tritium targets will be estimated using data obtained during the planning of future D-T sources. Both, the absolute intensity and the angular-dependent neutron energy spectrum of tritium solid targets are essential parameters in the analysis of experimental results. Sometimes, there are significant space-dependent distortions in the neutron spectrum and the neutron flux near the targets and irradiation samples. The second part of this paper presents real data of these parameters and examples of solid tritium targets with complex geometries such as RTNS-II of LLNL at Livermore and OKTAVIAN at Osaka. In the last part, tritium contamination problems due to the unavoidable release of tritium from targets will be discussed. In some cases, tritium contaminations can be caused by scattered flakes and powder from tritiated target films. Tritium hazard control measures and the performance and effectiveness of a tritium collection system will be discussed, taking OKTAVIAN as an example of the modestly intense D-T neutron source.

OSAKA UNIVERSITY 14MEV INTENSE NEUTRON SOURCE AND ITS UTILIZATIONS FOR FUSION STUDIES (OKTAVIAN PROGRAM)

A high beam current 300 kV Cockroft-Walton type D-T neutron source, OKTAVIAN, was built and has been operated at Osaka University for fusion studies. One of particular performances of this source is the generation of intense 1.5 nano second pulsed neutrons with 10 4 D-T neutrons/pulse. In the D-D neutron production experiment, 20 mA D + beam current produced 3 × 10 10 neutrons/sec with a rotating self-loaded target. This source is expected to produce continuously 3 × 10 12 D-T neutrons/sec with a rotating TiT target. Several fusion neutronics experiments have been carried out for fundamental understanding of the characteristics of D-T fusion neutrons; integral neutronics experiments and nuclear data measurements to assess blanket nuclear design, calculation, mainly by pulsed neutrons. OKTAVIAN is available for neutron irradiation service using continuous D-T neutrons and for light ion irradiation service using H + and He + ion beams.

Fast response Faraday cup for low‐energy, nanosecond‐pulse ion beams

A fast response Faraday cup has been developed for measuring low‐energy, nanosecond‐pulse ion beams. It is made of a coaxial Faraday cup before which a thin gold foil has been set, and it measures the waveform of low‐energy, nanosecond‐pulse ion beams by using the secondary electrons emitted in the forward direction from the foil.

Integral benchmark experiments on BeLi-graphite systems for tritium breeding blanket design

Abstract To evaluate the possibility of tritium breeding in a fusion reactor system, integral benchmark experiments for the evaluated nuclear data files and several calculational methods have been realized for BeLi-graphite sphere systems as Japan-USA-PRC collaboration works. By using OKTAVIAN of Osaka University, as a point D-T neutron source, time-dependent reaction rates were measured for natural Li in the form of a large metal sphere with/without an inner neutron multiplier, Be, and with/without an outer graphite reflector. From the measured results, it was confirmed that a TBR, the production number of tritium per injected D-T neutron, could be obtained over 1.4 by using a Be multiplier with a graphite reflector. By comparison with calculated data, the present status of the data files and calculational methods are generally acceptable except for the following point. A discrepancy between integral experimental results and calculated values is caused by uneven evaluation of the available nuclear data libraries for Be-9 in the neutron energy range under several MeV.

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.

Simple profile monitor for low intensity, low‐duty cycle pulse ion beams

A simple beam profile monitor useful for rapid optimization of low intensity, low duty cycle pulse ion beams is described. It is based on a fixed multiwire beam detector.

Reactivity effects and flux perturbations due to spatial random dispersal in the number densities of core materials

An analysis was made on the reactivity and the flux profile modulation caused by spatial random perturbations in the number densities of core materials of a bare, uniform, critical, reference reactor. Using an orthonormal function set of the Helmholtz modes, a one-group diffusion equation with the multiplication factor k is transformed into an infinite dimensional vector equation with the reactivity ϱ = 1 − 1k. The problem is then analyzed as the perturbations in the smallest eigenvalue in magnitude and the corresponding eigenvector of an infinite dimensional matrix. This approach leads to the conclusion that spatial random perturbations cause the positive reactivity effect and a flattened flux profile. The results are exact up to the second order of perturbations when zero ensemble average of perturbations is assumed, and are exact up to the third order of perturbations when the Gaussian distribution is assumed in addition.