The collection efficiency f-value of a parallel-plate ionization chamber exposed to a pulsed X-ray field of high exposure rate has been discussed. The dependence of the f-value upon various parameters was clarified by numerical calculations of the rate equations for positive and negative ions. The saturation curve was also obtained with the normalized variables.
A method was proposed for determining the f-value in an unknown pulsed X-ray field. It was found that the true f-value could be evaluated by fitting a normalized saturation curve to several ionic charges at different applied voltages. Experiments were carried out in a pulsed X-ray field generated by an electron linear accelerator, and the effectiveness of the method was confirmed.

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A benchmark calculation of full fission product was performed for thermal reactor application using an isotope transmutation code DCHAIN based on 185 nuclides with revised nuclear data library. The fission product model for BWR lattice calculation was studied and tested with the benchmark results, and a model containing 45 explicit nuclides and one pseudo nuclide was selected as a reasonably best model to predict the burnup reactivity with high precision for practically all types of fuel and reactor operating conditions. The evaluated thermal cross section and resonance integral for the pseudo nuclide are σ_{2, 200}=2.6 b and RI=10.6 b, combined with the pseudo fission yield values of 1.3898, 1.3233, 1.3675 and 1.2773 for fissions from ²³⁵U, ²³⁸U, ²³⁹PU and ²⁴¹Pu, respectively. The present results are believed as equally applicable to PWR lattice calculation.

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Two-point coupled reactor equations are derived for the analysis of stability of xenon oscillation of coupled thermal reactor by the nodal method with use of Green's function. A simple formula for the stability criteria is obtained as a function of coupling coefficient. As coupling coefficient becomes smaller, xenon oscillation becomes more unstable as expected. Some numerical examples for a coupled slab reactor are given. The present method may be used also for usual thermal reactors by regarding them as coupled reactors by dividing them into appropriate two regions.

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An analytical model of iodine stress corrosion cracking (SCC) of hexagonal close packed metals is developed. The SCC susceptibility is represented in the model by the elastic strain energy in the single crystals with tensile strain toward the <C> axis of the crystal. The model incorporates the effect of crystallographic textures and multi-axial stress states.
The SCC susceptibilities of the Zr and Zr alloy fuel claddings with three different textures are calculated based on the model. It is found that the stress for the tangential tensile loading of cladding is about 40% of that for longitudinal tensile loading with the same SCC susceptibility.
Measurements are made on the number of microcracks on the inner surface of the Zircaloy-2 fuel claddings. These cracks are produced during the iodine SCC tests performed at 350°C under the biaxial stress state with a constant Mises' effective stress. The SCC susceptibility obtained by the proposed model was found to agree with the experimental results. The SCC susceptibility for the a-zirconium single crystal, as calculated by the model, is also shown to be in good agreement with experimental results.

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Tensile test, load relaxation test and low stress abrupt load change test were performed at 473 K to study the nonelastic behavior of annealed Zircaloy-2 fuel cladding tube. Experimental results were analyzed by using a unified phenomenological constitutive model originally proposed by Hart. Parameters in the model were determined by using the nonlinear least square fitting method. Using the parameters, the model was numerically integrated along the loading conditions under which experiments were performed. The model simulated the observed behavior fairly well. It shows the applicability of the model for the nonelastic deformation of annealed Zircaloy-2 at 473 K.

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The chemical forms of uranium in an artificial seawater are determined by use of the ion-association model. The data on the stoichiometric association constants of the ion pairs are used in calculation to evaluate the distribution of the major ionic species. Also, the more accurate values of the thermodynamic stability constants are used for the uranyl complexes. The calculation indicates that a dominant species of uranium is UO₂(CO₃)⁴₃ in artificial seawater at 25°C.

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Behavior of aerosol resulting from sodium fires in a closed vessel is investigated and the changes in the particle size distribution of the aerosol due to coagulation and humidity have been studied. The initial mass concentration is in the range of 80500 mg/ms and the relative humidity is varied between 50 to 98%. The initial size of the released aerosol is found to be 0.9 pm. Equilibrium diameters of particles growing in humid air have been computed for various humidity levels using water activity of sodium hydroxide. Both theoretical and experimental results have yielded growth ratios of about 3 at about 95% relative humidity. It is recommended that the computer codes dealing with aerosol coagulation behavior in reactor containment should include an appropriate humidity-growth function.

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With a view to examining the embrittlement behavior of Zircaloy due to inner surface oxidation occurring in an LWR loss-of-coolant accident, ring-like Zircaloy-4 cladding specimens were heated at the isothermal oxidation temperature ranging 8901, 194°C in an environment of stagnant steam, which simulated the atmospheric condition inside the ruptured cladding.
The embrittlement of the specimen due to oxidation in an environment of stagnant steam is influenced primarily by the amount of hydrogen absorbed by the Zircaloy-4. Ring compression tests conducted at 100°C on oxidized ring-like cladding specimen showed that Zircaloy containing more than about 500 wt. ppm of hydrogen had become brittle.
The results of the present experiment support the idea supposed in the previous paper that the hydrogen absorption on the inner surface of the ruptured cladding, causing severe embrittlement, is strongly related with the atmospheric condition inside the ruptured cladding.

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