The density wave instability in a once-through boiling flow system is analyzed using a simplified model characterized by lumped parameters and moving boundaries. An an-alytical solution of the stability boundary is derived in generalized form, which is then verified by comparison with experimental data obtained in a joule-heated system. Further comparison with published data obtained from experiments with sodium-heated steam generator indicates that the proposed model can be validly used for preliminary estima-tion of the stability boundary. Several dimensionless parameters governing the stability are also derived from the proposed model.

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Two correlated Monte Carlo methods, the similar flight path and the identical flight path methods, have been improved to evaluate up to the second order change of the reactivity perturbation. Secondary fission neutrons produced by neutrons having passed through perturbed regions in both unperturbed and perturbed systems are followed in a way to have a strong correlation between secondary neutrons in both the systems. These techniques are incorporated into the general purpose Monte Carlo code MORSE, so as to be able to estimate also the statistical error of the calculated reactivity change.
The control rod worths measured in the FCA V-3 assembly are analyzed with the present techniques, which are shown to predict the measured values within the standard deviations. The identical flight path method has revealed itself more useful than the similar flight path method for the analysis of the control rod worth.

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Membrane gas separation cascades are analyzed at steady state. The methods of calculating the flow rate and concentration profiles in the cascade are examined, using formulas expressing the various membrane separation cell characteristics. The method adopted is applicable to relatively high concentrations and separation factors. Consider-ations are further given on the steady state performance of four theoretical forms of cascade : (a) with common value of cut for all stages, (b) with symmetric separation cells, (c) with no mixing at the junction at each stage, and (d) ideal cascade.
The analysis showed that, with membrane cells, the ideal cascade would have a pressure ratio varying from stage to stage. The symmetric separation cascade would provide a separation performance lower than the ideal cascade on account of the mixing at the junctions of streams possessing different concentrations, whereas the cut and separation factor of the no-mixing cascade requiring minimum membrane area exhibits zig-zag curves when plotted against stage number. Both these circumstances contribute to the lower separation performance obtained with these two forms as compared with the ideal cascade, and results in larger total membrane area ; but these semi-ideal forms retain the advantage of easy practical treatment with their pressure ratio common to all stages.

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The extraction of Nd(III) with di-isodecylphosphoric acid was studied and compared with that by di(2-ethylhexyl)phosphoric acid. DIDPA possesses higher D_f values in a strong acid medium but poorer separation factors against lanthanoids(III) than that of DEHPA. Mono(2-ethylhexyl)phosphoric acid increased extraction of Nd(III) with DIDPA in a similar manner as in the case of DEHPA, by a factor of D_f of 10³.
The extraction of Sr(II) was also investigated with DIDPA, which showed a maxi-mum of D_f at pH 5.0. Results showed that DIDPA is a promising extractant for the partitioning of the high level liquid waste.

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Concerning ideal cascades with separation factor varying from stage to stage, inter-stage flows are exactly determined by solution of the difference equation representing the conservation of the desired material flows. It is shown that, in such a cascade, the separative powers relevant to the desired and undesired materials are additive. This additive property is proved for the desired and undesired materials respectively by calcu-lation establishing that the summation of separative powers of all stages is equal to the total separative power of the ideal cascade.

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Radiolysis of N₂O adsorbed on silica gel degassed at 200, 450 and 600°C has been studied to investigate the behavior of electrons on the gel surface. The G-value of nitrogen as the major radiolytic product increases with an increase of N₂O concentration adsorbed approaching a plateau value which depends on degassing temperature of the gel prior to irradiation. By the competitive electron scavenging of N₂O with several electron scav-engers, such as CCl₄, SF₆ and nitrobenzene, initial yield of electrons G(c), and relative rate constant for reaction of electrons with scavenger to that with N₂O, ks/k_N2O, have been obtained. The G(e-) is 3.8 and 2.0 for the gel degassed at 200 and 600°C respec-tively. In the former gel, the value of k_s/k_N2O is comparable with that in aqueous system, while in the latter, with that in non-polar hydrocarbon such as neopentane. The observed difference may be attributed to the presence of silanol groups and residual water which facilitate some form of hydration of electrons on the gel degassed at low temperature. Despite of the complexity of the heterogeneous system, Hammett's rela-tionship is observed among the reactions of electrons with monosubstitutcd derivatives of benzene, providing the reaction constant p as 2.8.

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A new type lung monitor capable of estimating the distribution of activity in the lungs was developed in order to make correction for the variation in the counting effi-ciency resulting from the difference of the activity distribution and from the transfer of activity to other organs. The deposition area of activity is roughly estimated by means of image divide counting method.
The designed detector consists of a single NaI(T1)-crystal of 9.5 in. diameter by 0.5 in. thick and seven 3 in. diameter phototubes. The minimum distinguishable amount, defined for localized distribution, is of order of 0.16 nCi ²⁴¹Am for a 60 min counting period.

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The NSRR programme is in progress in JAERI using a pulsed reactor to evaluate the behavior of reactor fuels under reactivity accident conditions. This report describes briefly the experimental results and preliminary analysis of two cluster tests.
In the cluster configuration of five fuel rods, the power distribution in outer fuel rods are not symmetric due to neutron absorption in central fuel rod. The cladding temperature on the exterior boundaries of the cluster is higher than that in interior. Good agreement was obtained between the calculated and measured cladding temperature histories. In the 3.8$ excess reactivity test, cluster averaged energy deposition of 237 cal/g UO₂, cladding melting and deformation were limited to the portions of the fuel rods that were on the exterior boundaries of the cluster.

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The rupture of UF₆ gas line connected to hot UF₆ cylinder, being one of various accidents in UF₆ vapor leak-out, is considered as a postulated accident for uranium enrichment plants. For this type of rupture, we will estimate the amount of UF₆ vapor release based on a simplified calculation model and then make an evaluation of UF₆ vapor release through a ventilation system of feed vaporization facility. Assuming an instantaneous steady state for the change of UF₆ states, an unsteady state thermodynamics process is solved. Numerical examples show that about 52% of the initial UF₆ quantity are vaporized at 80°C (the temperature of the liquid UF₆ in the cylinder). Furthermore, by using the amount of released UF₆ vapor and the collection capacity of HEPA filter for HF gas, the amount of gaseous UO₂F₂, HF which may be dissipated to the environment are conservatively estimated.

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An evaluation is made of the reactivity control capability of the fuel processing sys-tem (FPS) in a molten-salt breeder reactor. The principal functions required of the FPS are : (a) Isolation of 233Pa from regions of high neutron flux during its decay to 233U, and (b) the removal of fission products from the system. The FPS can very use-fully serve also to control the primary system reactivity by appropriately utilizing its function of extracting uranium and reconstituting the fuel contained in the salt. The principles of operation are quite similar to the chemical shim control system currently installed in PWR's whereby the core reactivity, affected by changes in the moderator temperature, fuel burnup and transient Xe, is adjusted by regulating the concentration of boric acid introducted in the moderator as neutron absorber. The present study ex-amines the capability of the FPS to follow transient Xe as in PWR's, and proves that the FPS should effectively serve as a system for adjusting not only long-term changes in reactivity but also short-term transient variations without any accompanying difficulties foreseen in operation.

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