A reactivity control method was proposed for a boiling water reactor (BWR) fuel bundle, which has a potential for higher burnup with an increase in fuel enrichment. The new method optimized the distribution and amount of nonboiling water area in a fuel bundle in order to enhance the reactivity control capacity.
Using the method, a 9×9 lattice fuel bundle with a small-sized channel box, large-sized water rods and a reduced fuel rod diameter was proposed for the discharged burnup of 70 GWd/t and the operational cycle length of 18 months. The core, which consists of the proposed fuel bundles with the bundle-averaged enrichment of 5.8% and includes other modifications concerning a neutron low leakage loading pattern, natural uranium axial blankets, and spectral shift with recirculation flow control, has a cold shutdown margin greater than the design limit (1%Δk) with minimum fuel bundle shuffling. Further, it has potentials for natural uranium savings of about 20% per unit power and reduction in the amount of reprocessing of about 60% per unit power, compared with current BWR designs.

View full abstract

The shielding experiments using iron-polyethylene slab shields with a ^2:2Cf neutron source were carried out to find out an optimum arrangement to minimize the total dose rate ; that is composed of neutron and secondary γ-ray dose rates. The total thickness of the iron slabs was fixed at 32 cm, while a variety of thickness and location of a polyethylene slab in the iron slabs were employed as a parameter.
The minimum dose point (i. e. optimum shielding arrangement) was observed when the polyethylene slab was located at approximately 20 cm depth from the source side in the arrangements. The ratio of the minimum dose rate obtained for the optimum arrangement to the maximum for the worst arrangement became 1/1.3 for the polyethylene slab of 1-cm-thick, 1/2.0 for 3-cm-thick, 1/2.9 for 6-cm-thick, 1/3.9 for 10-cm-thick and 1/3.6 for 14-cm-thick. The appearance of the optimum arrangement for the total dose rates, and changing profiles of the secondary γ-ray as well as the neutron dose rates were reproduced for the typical two cases of the polyethylene slab thickness by the Monte Carlo calculations with the splitting technique.

View full abstract

The neutron energy dependent response of the α-track detector, cellulose nitrate film (Kodak LR-115 type I) in contact with a lithium radiator was calculated. The "mean effective thickness L_eff" of the radiator is obtained as a function of the neutron energy and incident angle. The distribution of the etched-through-track density is evaluated from the calculated L_eff and compared with the measured results in the fusion blanket assembly.

View full abstract

Experimental studies using Slab Core Test Facility (SCTF) and Cylindrical Core Test Facility (CCTF) indicated that the degree of heat transfer enhancement due to the radial power distribution during the reflood phase of a PWR-LOCA was governed mainly by the radial power ratio itself and less dependent on the shape of radial power distribution within the maximum power ratio of 1.36. The experimental condition covering the wide ranges of the reflood phase and the scale of core radius from 1/4.6 to 1/1 had little effect on the two-dimensional heat transfer behavior. The heat transfer coefficient under nonuniform radial power distribution was expressed as a sum of the heat transfer coefficient obtained under a complete mixing condition and an additional value given by an empirical correlation based on the SCTF results. The temperature rise at the peak power rod calculated with this expression tended to be lower than that calculated with the complete mixing model used in a reflood analysis code REFLA. That is, the complete mixing model was proved to give a conservative result under a nonuniform radial power distribution condition in a full size core.

View full abstract

Accelerated short-term leach tests in a laboratory are neccessary in order to estimate, with reasonable accuracy, the long-term leaching behavior of high-level waste glass. In the present study, static leach tests of an SRL-165 high-level waste glass were carried out in deionized water at two different glass-surface-area to solution-volume ratios (SA/V-ratio), namely 0.85 and 0.079 cm^-1 at 90°C, and 0.85 cm^-1 at 40°C.
First, an equation was examined which related Si-concentrations with time, temperature and SA/V-ratio under the present static conditions. The parameter determined at 90°C, 0.85 cm^-1 can be used to calculate the Si-concentration at 40°C, 0.85 cm^-1. Second, at the low SA/V-ratio of 0.079 cm^-1, the concentrations of Ca and Mg in the leachates peaked and then decreased a little. The equation used above does not explain the variation of the concentrations of Ca and Mg at a low SA/V-ratio. The precipitation of Ca and Mg onto the glass surface is probably caused by the adsorption efficiency of the surface layer or the formation of crystalline materials at the low SA/V-ratio of 0.079 cm^-1. Third, the in-depth profiles of some elements obtained by secondary ion mass spectroscopy (SIMS) were qualitatively in agreement with the results of solution analyses. This indicates the particular usefulness of SIMS for analyzing leaching behavior of the glass in in-situ burial studies where solution analyses are often impractical.

View full abstract

The decontamination technique is required to have a high decontamination rate and decontamination factor, to be applied to irregularly formed metallic wastes, and to minimize secondary waste volume for component replacement and decomissioning. The authors have developed a new decontamination technique using Ce⁴⁺ for decomissioning, namely "REDOX decontamination technique". In previous paper, the REDOX decontamination conditions and stainless steel and carbon steel dissolution conditions were established in fundamental tests. This paper describes about the establishment of REDOX decontamination system in the engineering scale equipment based on the fundamental test results. In comparison with the fundamental test results, Ce⁴⁺ generation rate and current efficiency, stainless steel and carbon steel dissolution rate, and Ce⁴⁺ regeneration rate and current efficiency were not effected by scaling up. The system which consists of Ce⁴⁺ generation-stainless steel and carbon steel decontamination (dissolution)-Ce⁴⁺ regeneration-reduction from Ce⁴⁺ ions to Ce³⁺ ions-drying-plasticization has been established in engineering scale equipment as the REDOX docontamination system.

View full abstract

A three-dimensional numerical atmospheric model (PHYSIC) was developed to apply it to atmospheric transport and diffusion evaluation The main frame of PHYSIC is made up of momentum equations with the hydrostatic and Boussinesq approximation, the second-order turbulence closure model level 2.5 and other basic equations of physical process in the atmosphere. A terrain following z_*, coordinate system is used.
The calculation results at a coastal plain were examined using meteorological data observed during atmospheric diffusion experiments at Tokai in 1983. The present model successfully predicts the temporal change of wind field within 20 h from evening to next noon and the occurrence and structure of sea breeze are simulated satisfactorily. The model performance concerning the structure of sea breeze and the distribution of eddy diffusivities is thought to be reasonable, although the direct comparison between calculation and observation is restricted to wind and temperature profiles by limited observation data. Further examination of model performance relating to above items, and over mountainous region, will be needed.

View full abstract