For the aseismic design of a high temperature gas-cooled reactor (HTGR) with block-type fuel, it is necessary to predict the motion and force of core columns and blocks. To reveal column vibration characteristics in three-dimensional space and impact response•column vibration tests were carried out with a scale model of a one-region section (seven columns) of the HTGR core.
The results are as follows : (1) the column has a soft spring characteristic based on stacked blocks connected with loose pins, (2) the column has whirling phenomena, (3) the compression spring force simulating the gas pressure has the effect of raising the column resonance frequency, and (4) the vibration behavior of the stacked block column and impact response of the surrounding columns show agreement between experiment and analysis.

View full abstract

To obtain neutron and secondary r-ray albedos for 15-MeV neutrons, spectral measure-ments of backscattered neutrons and r-rays from Al and Pb slabs were carried out for monodirectional beams of D-T neutrons using the associated particle method. In the experiment, an NE-213 scintillator was used for simultaneous measurement of the neutron and photon spectra with the aid of the pulse-shape discrimination technique. The obtained pulse height distributions were unfolded to energy spectra by the FERDO code.
The accuracy of the measured spectra was examined using calculated results of the Monte Carlo code which was newly developed for the combined neutron and photon trans-port calculation. A fairly good agreement was obtained between the measured and cal-culated results, excluding some discrepancies which seemed to come from the inaccuracy in the cross section data used in the Monte Carlo calculation.
The obtained neutron and photon data in this paper are very meaningful for the ref-erence data of the neutron and secondary r-ray albedo calculation. The experimental technique in this work is shown to be useful in measuring the backscattered neutrons and r-rays simultaneously for the incidence of D-T neutrons.

View full abstract

In the Halden Boiling Water Reactor (HBWR), a resonant power oscillation with a period around 0.04 Hz is observed at power levels higher than about 9.5 MWt. Although this resonant oscillation is not so strong as to affect the normal reactor operation, it is significant, from the viewpoint of reactor diagnosis, to reveal the cause of this oscillation as well as to understand its characteristics.
Noise analysis based on the autoregressive (AR) modeling technique together with spectral and correlation analyses is performed to investigate the driving source, which indicates that it is attributed to the dynamic interference with the reactor of heat exchange process in two parallel-connected steam transformers.
The present study demonstrates the effectiveness of the technique applied here for determining the so-called noise source inducing variations of quantities in a system together with its applicability to various problems in the field of reactor noise analysis and diagnosis.

View full abstract

A new type ion source including sample inlet pipe of quadrupole type mass spectrom-eter for analysis of UF₆ has been made. Fundamental characteristics of the ion source, such as pressure characteristic, flow rate, sensitivity of the ion source for UF₆, inlet and evacuation characteristic were evaluated. The ion source showed good operational stability under low pressure and small flow rate, which were 1.02.0 Torr and 0.30.4 mg/h, re-spectively. Inlet characteristic was within 60 s, evacuation characteristic to remove residual UF₆ less than 0.1% level was 15 s. Sensitivity coefficient of the ion source was 0.51.0 Torr^-1. In consequence, it proved that the ion source had long life and memory effect was negligible small.
This ion source was also applied to measure ²³⁶U isotope ratio. Both precision and ac-curacy were within ±0.40% for UF₆ sample whose enrichment was more than 1.0%. It was clear that this ion source had good performances and it contributed to isotope ratio measurements with good accuracy.

View full abstract

Centrifugal extractor has the shortest contact time and it is to be used for FBR fuel reprocessing for the future. The two liquid phases are separated by centrifugal force in the settler and the light liquid flows out through the inside weir and the heavy liquid through the outside one. And the interface has to be hold to keep stable operation. How-ever, since the observation of the interface was very difficult, the behavior of the interface in the settler was not clearly understood before. In this study, in order to observe the correct diameter of the interface, the experimental equipment is made of transparent plastic and the diameter is determined by taking photos, lighting a flash. As a result, the stability of the interface is examined for various flow rates and rotation velocities. From the data, it is shown that the pressure drops at 1, 800 rpm is proportional to the flow rate at organic weir but is proportional to the power of 0.64 of the flow rate at aqueous weir.

View full abstract

In safety evaluation of nuclear waste repository systems, the analysis on migration of radionuclides in geologic media is important. In previous analytical models called Band-Release, Step-Release and Impulse-Release for decay chains, nuclide-specific release rate and time-dependence of release rate can not be taken into consideration. In this paper, two new release models are proposed. In Preferential-Release model, both of nuclide-specific release rate and time-dependence of leach are taken into account. Exponential-Release model is a special case of Preferential-Release model, in which no preferentiality in leach is as-sumed. The obtained analytical solutions for these new models are applicable even to four or more member decay chains and are as simple as those for the previous models.
As an example of the application of the new models, the discharge rates of the four nuclides in the decay chain of ²³⁸pu→²³⁴U→²³⁰Th→²²⁶Ra into a surface water body are cal-culated as a function of time after the start of leach and path length from the repository. By using the result, heavy hazard due to 238Pu at short distance and 226Ra at long distance are also discussed briefly.

View full abstract

The environmental dose evaluation around a nuclear facility is usually performed by means of calculational methods on the basis of effluent monitoring at a release point. It has been desired to develop practical techniques of direct monitoring in the environment, because the Japan Atomic Energy Commission set up 5 mrem/yr as an objective dose value for the whole-body exposure due to normal operation of LWRs on a single site.
In evaluating gaseous plume doses less than 5 mrem/yr in routine monitoring, there are many difficult problems to be solved. We have developed two techniques for separation of the plume exposures less than 1 mrem/yr from the background radiation, using a NaI(TI) scintillation counter having flat energy response.
One technique is based on the difference in energy spectrum distribution between plume gamma and natural gamma radiation. And, separation of the respective exposures is made by analysis of the two informations obtained by a two-channel monitoring system. Another technique is to reduce the natural gamma radiation components by using a special concave lead shield for the Nal (TI) scintillation probe. A combination of the techniques can much improve the efficiency of separation of 41Ar plume radiation from fluctuating background radiation.

View full abstract