Effects of a reduced seed concentration on performances of a nonequilibrium MHD generator are investigated by calculating power densities and electrical efficiencies for both Faraday and Hall generators.
It is shown that high electrical efficiencies and high power densities are obtained simultaneously by suppressing the ionization instability with reduced seed fractions (10^-4 10^-5) when values of the effective electrical conductivity and the effective Hall param-eter recover to above 50% of their ideal ones. Results also indicate that argon is more suitable for a working fluid than helium in the case of a reduced seed concentration.

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A framework of boiling channel stability theory is analyzed. The fundamental equations are those of STABLE code : Three conservation laws of mass, energy and momentum applied to one-dimensional channel, together with Bankoff' slip and Marinelli-Nelson's pressure drop correlation. These equations are analyzed to yield "Void Equation", "Linear-ized Void Equation", "Volume Conservation Law" and the "Flow Impedance" R(s), defined by the dynamic response of pressure drop to the inlet flow. The impedance contains all the information such a stability index, dominant frequency and damping ratio. It is shown that R is a sum of the form R_IA + N_F^-1R_D + N_RR_R + N_OR, where N's are non-dimensional parameters and R's characteristic impedances determined by three kinds of parameters, N_X, N_s, and the power distribution parameter. Systematic edition of the characteristic imoedances according to the non-dimensional parameters will reduce the need for case-by-case STABLE calculations. Hydraulic stability of BWR channels under constant system pressure, is a phenomenon with three parameters in view of complexity. Furthermore an analysis is conducted to confirm the above stability structure and three typical instabilities are identified.

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Parameter identification and adaptive control schemes are presented for a point reactor with internal feedbacks which lead to the nonlinearity of the overall system. Both are shown stable with new representation of the system, which corresponds to the nonminimal system representation, in the vein of the Model Reference Adaptive System (MRAS) via the Lyapunov's method. For the sake of the parameter identification, model parameters can be adjusted adaptively as soon as measurements start, while plant parameters can also adaptively be compensated through control input to reduce the output error between the model and the plant for the case of the adaptive control.
In the case of the adaptive control, control schemes are presented for two cases, the case of the unknown decay constant of the delayed neutron and the case of the known constant. The adaptive control scheme for the latter case is shown extremely simpler than that for the former. Furthermore, when plant parameters vary slowly with time, computer simulations show that the proposed adaptive control scheme works satisfactorily enough to stabilize an unstable reactor and that it does even in the noise with small variance.

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This paper reports the experimental results of vibrations of a fuel pin model -herein meaning the essential form of a fuel pin from the standpoint of vibration- in a parallel air-and-water two-phase flow.
The essential part of the experimental apparatus consisted of a flat elastic strip made of stainless steel, both ends of which were firmly supported in a circular channel con-veying the two-phase fluid. Vibrational strain of the fuel pin model, pressure fluctuation of the two-phase flow and two-phase-flow void signals were measured. Statistical measures such as power spectral density, variance and correlation function were calculated.
The authors obtained (1) the relation between variance of vibrational strain and two-phase-flow velocity, (2) the relation between variance of vibrational strain and two-phase-flow pressure fluctuation, (3) frequency characteristics of variance of vibrational strain against the dominant frequency of the two-phase-flow pressure fluctuation, and (4) fre-quency characteristics of variance of vibrational strain against the dominant frequency of two-phase-flow void signals.
The authors conclude that there exist two kinds of excitation mechanisms in vibrations of a fuel pin model inserted in a parallel air-and-water two-phase flow ; namely, (1) parametric excitation, which occurs when the fundamental natural frequency of the fuel pin model is related to the dominant travelling frequency of water slugs in the two-phase flow by the ratio 1/2, 1/1, 3/2 and so on ; and (2) vibrational resonance, which occurs when the fundamental frequency coincides with the dominant frequency of the two-phase-flow pressure fluctuation.

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Density wave instability in a once-through boiling flow system was systematically ob-served using a uniformly joule-heated water loop operated at system pressures ranging from 20 to 43 ata. The test section consisted of two heated tubes connected in parallel and further paralleled by an unheated by-pass. The oscillatory behavior is analyzed as well as the effects brought on the threshold of instability by changes in the operating variables-mass velocity, system pressure, inlet resistance, inlet subcooling, by-pass ratio and exit quality. The experimental data are also presented in form convenient for verify-ing various analytical methods devised to predict the stability boundary.

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A correction to the numerical solution of the diffusion equation solved using the finite mesh spacing method has been studied for a fast breeder reactor. It has been found that the correction to the effective multiplication factor k_eff calculated with the finite mesh spacing method can be expressed approximately as
-δk(Δr, Δz) ≡ k - k_D(Δr, Δz) ?? -[(Δr)² (c_1ρ +c₂) + (Δz)² (c_3ρ +c₄)],
where k and k_D (Δr, Δz) are, respectively, the eigenvalues of the diffusion equation and its approximate finite difference equation with mesh spacings ΔrJr in the x-y plane and Δz along the z axis. The quantity ρ is the total reactivity worth of control rods inserted into the core. The constants c₁, c₂, c₃ and c₄, are determined from the geometrical shape and atomic densities of constituent materials of a reactor core.
For a prototype LMFBR, δk with Δr = Δz = 11.5 cm (hexagonal-z mesh model) amounts to 0.8%Δk for the state with all control rods fully withdrawn or 1.9%Δk for the state with control rods fully inserted. The δk's with Δr = Δz = 3.83 cm (triangular-z mesh model) are less than 0.23%Δk for various patterns of the control rods. As for the power distribution, the hexagonal-z mesh model overestimates the power near the midplane of the core with all control rods fully withdrawn (inserted), by 01.6% (03.9%) in the inner core and by 03.7% (04. 8%) in the radial blanket, but underestimates it by 01.4% (01.6%) in the outer core, compared with that from the triangular mesh model.

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The adoption of two-unit separation cells in radioactive rare gas membrane separa-tion equipment enhances the separation factor, but increases the required membrane area and compressive power. An analytical economic evaluation was undertaken to compare the conventional separation cell with the two-unit separation cells, adopting as parameters the number of cascade stages, the membrane area and the operating power requirements.
This paper describes the models used for evaluating the separation performance and the economics of cascade embodying these different concepts of separation cell taken up for study, and the results obtained for the individual concepts are mutually compared.
It proved that, in respect of the number required of cascade stages, of operating power requirements and of the annual expenditure, better performance could always be expected of the two-unit separation cells as compared with the conventional separation cell, at least in the range of parameters adopted in this study. As regards the minimum membrane area, the conventional separation cell and the series-type separation cell yielded almost the same values, with the parallel-type separation cell falling somewhat behind.

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