For the purpose of enhancing the efficiency of the burnup calculation of LWR lattice, two coarse time-step integration methods have been developed, both of which are to be used in combination with the ordinary Runge-Kutta-Gill method. It has been ensured through the numerical results of model problems simulating the depletion of ¹⁵⁷Gd in a gadolinium-poisoned rod that the maximum time-step size allowed by the proposed methods is roughly 4 or 5 times larger than that achieved by the Predictor-Corrector method known as an effective coarse time-step method, and consequently that the proposed methods would reduce the com-putation time to a half or less when applied to an LWR lattice burnup calculation. The factor of reduction of computation time is still more significant if compared with other con-ventional methods such as the Runge-Kutta-Gill method etc. In addition, it has been demon-strated through their application to the LWR lattice physics code TGBLA that no appreciable error is observed over the range of time-step size up to 1 GWd/t in the burnup calculation for a typical BWR lattice containing gadolinium-poisoned rods. Although the method develop-ment and verification presented here place emphasis on the cases of LWR lattice burnup, it is expected that the proposed methods would be applicable equally well to general problems dealing with the nuclide transmutation due to burnup.

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The "accretion" method by Leslie & Jonsson to calculate collision probabilities in a cluster-type fuel element is applied to evaluate the resonance absorption in the doubly hetero-geneous system in the fuel block of the Very High Temperature Gas-Cooled Reactor. With-out any homogenization process, the neutron flux in a fuel grain embedded in a fuel pellet is formulated in a slowing-down equation together with the flux in the surrounding graphite moderator. The collision probability relating to the fuel pellet is partitioned into that cor-responding to the grain and that of the binder in proportion to the fraction of neutron col-lision rate.
Several models are proposed to define the collision rate. The neutron spectrum in a multi-region cell is calculated by solving the above slowing-down equation in the dominant resonance energy range E<130 eV.
Numerical results show that all models give almost the same resonance absorptions for practical purpose. Satisfactory agreement is obtained between the calculated and the experi-mental values for the shielding of the resonance integral in media containing Au in the form of small particles.

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A lithium-fluoride pile was constructed by piling the lithium-fluoride ceramic blocks developed for the present study, and scalar neutron spectrum per source neutron in this pile was measured with a miniature NE213 spectrometer. The measured spectrum was unfolded with two window widths, a spectrometer resolution and one modified by one-time FORIST iteration. The measured spectrum was compared with ones calculated using the MORSE-GG Monte Carlo code with a modified point-detector estimator presented by Carter & Cashwell and the GICXFNS group cross-section set. The measured and calculated spectra agreed each other in their error bands, with the exception of the source energy peak.

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The power recovery characteristics of an in-line direct beam converter provided with electrostatic electron suppressor were studied numerically by tracing the orbits of fast pri-mary ions and secondary charged particles generated along their beam path by collision with background gas molecules. It is shown that, in reference to the electrostatic field potential at the point of impact, the energy distribution of secondary ions impinging on the suppressor has two peaks-one corresponding to a zone of high positive potential surrounding the col-lector and the other to one of slightly negative potential around the electron suppressor. Secondary electron emission from the suppressor is ascribed mainly to the latter peak, asso-ciated with impingement of slower secondary ions. Far much power consumed in secondary particle acceleration is spent for emitting electrons from the suppressor than for secondary ions generated by beam-gas collision. The upper limit of background pressure is discussed on the basis of criteria prescribed for restricting the power consumed in this secondary particle acceleration, as for practical convenience of electrode cooling. Numerical examples are given of calculations based on particle trajectory analysis of both primary ions and secondary particles, for the case of a 100 keV-proton sheet beam 10 cm thick of 35 mA/cm2 current density.

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Heat transfer coefficients and transient pressures in closed narrow channels were ob-tained experimentally, in order to assess the gap heat transfer models in the computer code WTRLGD which were devised to analyze the internal pressure behavior of waterlogged fuel rods. Gap widths of channels are 0.10.5 mm to simulate the gap region of waterlogged fuel rods, and test fluids are water (789.2°C) and Freon-113 (9.2°C).
The results show that the heater temperature and the pressure measured in the experi-ments without the DNB occurrence are simulated fairly well by the calculational model of WTRLGD where the heat transfer in a closed narrow channel is evaluated with one-dimen-sional transient thermal conduction equation and Jens & Lottes' correlation for nucleate boil-ing. Consequently, it is also suggested that the above equations are available for evaluation of heat flux from fuel to internal water of waterlogged fuel rods.
The film boiling heat transfer coefficient was in the same order of that evaluated by Bromley's correlation and the DNB heat flux was smaller than that obtained in quasi-steady experiments with ordinary systems, although the experimental data for them were not enough.

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Bowing characteristics were examined for IG-11 and H451 graphites sleeve specimens to which temperature gradients were given in the circumferential direction using a specially prepared rig. Measurements were also carried out on load or bending moment which was caused in the sleeve specimens under constraint as a result of temperature gradients. Ex-perimental data were well analyzed on the basis of the elastic theory on the deflection of beams. Bending tests of the sleeve specimens indicated that the bending moment generated in the sleeve specimens under constraint was less than 1/3 of that at fracture even when the maximum temperature difference along the circumferential direction was more than 200°C.

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