The cross sections for the formation of the metastable states of ⁹⁰Zr, (σ_Zr), for neutron energy of 14.8 MeV, and that of ²⁰⁷Pb, (σ_Pb), for neutron energies from 4.1 to 5.2 MeV have been measured and the results are reported. The experimentally deter-mined value of the cross section for the former is 700±94 mb, while for the latter, the cross section increases from 316±44 mb to 788±110 mb. The cross section ratio, σ_Pb/σ_Zr, is also studied, which is found to vary from 1.87 to 2.18 for neutron energies from 14.8 to 18.2 MeV respectively.
The equality in the half-lives of the metastable states of ⁹⁰Zr and ²⁰⁷Pb is exploited in the present measurements, which enabled comparative evaluation of the cross sections.
Natural samples of zirconium and lead were irradiated simultaneously and the γ-rays were measured off-line by a 55 cm³ Ge(Li) detector. Cyclic activation process was adopted to improve the counting statistics. The overall accuracy achieved in the measurements is about 10%.

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A method has been developed that effectively estimates the detailed distribution of power generation in the fuel or blanket assemblies in nuclear reactors. A two-dimen-sional, one-group diffusion model is applied to a region of homogeneous composition enclosed in a contour devoid of concavity viewed from outside. The diffusion equation is reduced to the form of Helmholtz equation, and a non-homogeneous boundary condition of Dirichlet or Neumann type is given on the contour, using neutron fluxes previously obtained in coarse mesh diffusion criticality calculations covering the whole reactor. This boundary value problem in two-dimensional space is made to yield a solution in the form of a potential due to a single or double layer. The method is applied to a hexagonal cell of a fast reactor. The results of calculation are amply accurate in comparison with the corresponding values from the usual fine-mesh diffusion scheme and with much shorter computing time.

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The diffusion of radioactive manganese, zinc and cesium into stainless steel tubes exposed during 7001, 500 hr to liquid sodium containing the radioisotopes was studied by measuring the residual radioactivity on samples with their surface layers successively removed by electropolishing.
Plots of the logarithm of the residual radioactivity in the tube wall drawn against the square of the thickness polished off from the surface were found to fall along straight lines, which changed their slopes in three steps. That of outermost step was most steep, with the residual radioactivity indicating a drop of about one order of magnitude within a layer of a few microns from the surface. The ensuing step presented a much gentler slope, from which the diffusion coefficients were determined. The ratio obtained between the diffusion coefficient and the approximate jump frequency of the diffusing radioisotope was correlated with the reciprocal of absolute temperature, which indicated that the diffusion mechanism of radioactive manganese and zinc differs from that of radioactive cesium.

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To gain basic information for the utilization of fission fragments in radiation chem-istry, measurements were made on the escape fraction of the fission fragments from a fuel of U-Pd alloy foil, and on the distribution of the fission products in a stainless steel capsule containing the fuel and ethylene together with their γ-activities, for the case of a gaseous chemical reaction system of ethylene. The escape fraction measured on ¹⁴⁰Ba -¹⁴⁰La was 69%, which is in good agreement with the value of 70% given from theory for a heavier fission fragment. The only fission products detected in the gas phase were ¹³¹I, ¹³³I, ¹³³Xe and ¹³⁵Xe. The sum of the measured γ-activities covering all the fission products was approximately equal to the amount calculated by the empirical formula proposed by Way & Wigner. The behavior of the fission fragments is also discussed.
This study suggests that consideration for decontamination associated with this fissiochemical reaction system could well be limited to I, Xe and Kr.

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For analyzing the mixing of particles in a spouted bed, a model is proposed, which assumes that : (a) the spouted bed consists of a spout part characterized by perfect mixing and an annulus part occupied by piston flow, (b) the ingress of particles from annulus into spout takes place within a zone below a depth h below the top of the annulus, and (c) the rate of ingress of particles into the spout is constant throughout the bed.
The time-dependence of the concentration C*(t) of marked particles within the spout can be calculated with this model assuming suitable values for unmeasurable parameters.
The value of C*(t) was measured from mixing experiments undertaken using alumina particles, to which a small batch of marked alumina particles was dropped transiently onto the bed in stable operation.
Comparing the experimental data with the calculated results, it was found that : (a) the mixing of particles in the bed can be satisfactorily described with this model, pro-vided that a suitable correction is added to the quantity of particles present in the spout to account for the existence of mixing in the annulus, whereas the model assumed piston flow ; (b) the upper edge of the ingress zone of particles from the spout into the annulus is located in the conical part of the bed. Increasing the flow rate of spouting air short-ened the cycle time of particles, while the mixing rate within one cycle was not affected to any significant extent by changes in flow rate.

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The release of fission products associated with the combustion of sodium was ex-perimentally compared with that of sodium. About 0.5 g of sodium sample was loaded onto a quartz crucible, together with trace amounts of ¹³¹I, ¹³⁷Cs and ⁸⁹Sr, and set in a combustion tube, where it was burned at a temperature ranging from 400° to 600°C, and in an atmosphere containing 2 to 21v/0 oxygen, flowing at 0.1 to 1.5 Nl/min.
The release fractions of radioisotopes-defined as the ratio between the radioactivity carried by the sodium released upon combustion and the total radioactivity including that of combustion products remaining in the crucible-increased almost linearly in relation to the release fraction of sodium similarly defined. The release fractions reached 20% for ¹³¹I, 80% for ¹³⁷Cs and 0.5% for ⁸⁹Sr, in association with a 30% sodium release fraction. A decrease was observed in the release fraction of sodium as well as of the radioisotopes with increase of the oxygen concentration and of the humidity in the combustion gas. This was ascribed to the formation of sodium oxide and sodium hydroxide scum on the burning surface of the sodium samples, judged from the fact that gas phase reaction was dominant in the sodium combustion.

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A solid methane cold neutron source has beeninstalled in a 45 MeV electron linear accelerator pulsed neutron source. A cold neutron gain factor of 40 has been obtained, and the device has shown a record of safe for extended periods. A description is given of the facility and the performance obtained therewith.

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