In the nuclear power calibration process of JOYO, the first experimental fast breeder reactor in Japan, the indication of the Intermediate Range Monitoring System (IRMS), employing Average Magnitude type Campbelling System (AMCS), was calibrated with the reactor power at 45.82 kW. The reactor power was then decreased and the nonlinear indication lowering of the IRMS was observed.
In this paper, we present a derivation of an equation representing the AMCS characteristic and show that the indication lowering occurs when the product of mean arrival rate of pulses and their width is small. The computed values based on the derived equation agreed very well with the observed ones in the JOY() IRMS below 45.82 kW, and it was confirmed that the evaluation method was applicable. Furthermore, it was evident from the evaluation that the indication lowering of the JOYO IRMS above 45.82 kW was negligibly small, and did not affect the reactor power ascension testing. Also, it was proved in a following thermal calibration test above 50 kW that the indication lowering was allowably small indeed in this power region.

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Ampoule irradiations using ⁶⁰Co γ-rays were undertaken on CCl₂F₂, C₂Cl₄F₂ and C₂Cl₃F₃ to gain information concerning the radiation damage of fluorocarbon, used as solvent in the recovery of ⁸⁵Kr from the off-gas of fuel reprocessing plants operating on the solvent absorpdon process. The dose rate was varied in the range of 2.6 x 10⁴8.0 x 10⁵ rad/h and the irradiation temperature in the range of -10070°C. Identified among the numerous degradation products were six compounds deriving from CCl₂F₂, eleven from C₂Cl₄F₂, and eleven also from C₂Cl₃F₃. The decomposition yield of each of these fluorocarbons increased with irradiation temperature, and decreased with dose rate. It rose roughly in proportion to the absorbed dose. Comparison of decomposition behavior between the four fluorocarbons (including CCl₃F covered in the preceding report) was made in reference to the irradiation temperatures corresponding to those at which these fluorocarbons would respectively be applied in practice as solvent for ⁸⁵Kr absorption. At these temperatures, the decomposition yields of the fluorocarbons decreased in the order C₂Cl₄F₂>C₂Cl₃F₃>CCl₂F₂>CCl₃F. At these same temperatures, the yields of Cl^- decreased in order CCI₃F>CCl₂F₂>C₂C₁₄F₂=C₂Cl₃F₃, while conversely those of F^- decreased in order C₂Cl₃E₃>C₂ClLF₂>CCl₂F₂>CCl₃F.

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Post-irradiation examinations were performed on three types of uranium carbide UC, UC₂ and U₂C₃, that were irradiated with a relatively high thermal neutron flux (7 x 10¹³ n/cm²•s or 9 x 10¹² fission/cm³•s for UC) and in a dose range between 2.7 x 10¹⁵ and 3.3 x 10¹⁸ fission/ cm³ (i. e. 2.1 x 10¹⁶ and 2.4 x 10¹⁹ nvt) in the JMTR (Japan Material Testing Reactor). On UC and UC₂, trends similar to previous works were obtained in the irradiation effects. New preliminary results, however, were obtained on U₂C₃ for changes in the electrical resistivity and the lattice parameter which showed reduced values after attaining a maximum at 10¹⁷ fission/cm³.
Successive annealing effects on the resistivity and the lattice parameter following the reactor irradiations were examined by pulse heating. Two major steps were observed, at around 400 and 600°C, in the recovery processes. A low temperature step, which was re-vealed previously at about 200°C in UC and UC₂, was missing in this study, because of higher irradiation temperatures (about 300°C) in the JMTR. Considering the results of quenching works on UC and partly on UC₂, it was suggested that rather simple defects played roles for the recovery processes below 1, 000°C in all three carbides.

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Ring tension testing methods were applied for studying the iodine stress corrosion cracking (SCC) behavior of unirradiated cold-worked (CW) and recrystallized (RX) Zircaloy-2 tubing at 300°C. Two kinds of tests-constant load and constant cross-head speed-were conducted on the grooved ring tension specimen simulating a plane-strain stress state under uniaxial loading, as well as on the simple ring tension specimen. The results showed that SCC failure threshold stresses were approximately 450 and 300 MPa, respectively, for the grooved CW and RX specimens. The ratio between the threshold stress of the grooved specimen and the yield stress derived from the simple specimen is equal to 1.4 for both CW and RX materials. The threshold value of the grooved RX specimen is 1.3 times larger than found on the same material of simple shape, which corresponds to the yield stress ratio between the grooved and simple specimens. Inside surface observations of specimens stressed in iodine environment suggested that corrosion lines, presumably corresponding to corrosion pits, and chemical attacks on grain boundaries would be incipient phases of SCC for CW and RX materials, respectively. They may appear at stresses lower than those for SCC failure.

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The long term variation of the mean dose due to natural radiation in Japan received by an individual person was investigated. The mean exposure rate in each prefecture obtained by in-situ measurements was adopted as a basis of the radiation level. Population data were taken from both the resident registration and the census taking into account the essential difference of their meanings. It was revealed that the per capita dose due to outdoor natural radiaiton in Japan has been decreasing in the last approximate 30 yr. It will be due to the population movement from countrysides towards urban areas. That is because countrysides generally consist of stable mountainlands where granitic rocks distribute though urban areas generally consist of alluvial plains where volcanic ash or humus dominates. Although the contribution of low radiation level (below 9.0 μR/h) areas to the nation-wide collective dose still remains under 50%, its relative importance has certainly been increasing in the last three decades. It is expected that human population will be the dominating factor which eventually controls the mean dose due to natural radiation in Japan.

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The validity of the ¹⁰⁷Ag (n, n')^107mAg reaction for reactor neutron dosimetry has been examined through an irradiation experiment using the standard neutron field in the "YAYOI", a fast neutron source reactor. The test foil used was 99.0% ¹⁰⁷Ag enriched silver, which was irradiated along with five reference foils already well evaluated. The reaction rate of ¹⁰⁷Ag was determined by K X-ray counting with a high-purity Ge detector. The cross section for the ¹⁰⁷Ag (n, n')^107mAg reaction was compiled from the excitation cross section data of ¹⁰⁷Ag published in ENDF/B-IV, in combination with the decay scheme of ^107mAg listed in the "Table of Isotopes". Using the neutron spectrum which had previously been precisely measured, it was found that the compiled activity of ^107mAg was underestimated by 12.8±5%. This error is attributed to significant underestimation involved in the cross section of the 107Ag(n, n')107mAg reaction.

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In order to evaluate void fraction in a bundle geometry during the reflood phase, reflooding experiment with a 4 x 4 simulated fuel array was conducted.
As the result, it was found that the effects of the clad temperature and the power of the heater rods are small and the effects of the pressure and the inlet flow rate are large on the relationship between the superficial steam velocity and the void fraction in a bundle geometry during the reflood phase. It was, also, found that there is no distinct difference of the void fractions caused by the different flow patterns in the wet clad region and in the dry clad region in a bundle geometry during the reflood phase, when compared at the same superficial steam velocities.
Furthermore, the applicability of Cunningham-Yeh's void fraction correlation was investigated under a wide range of conditions anticipated during the reflood phase. The range of conditions under which Cunningham-Yeh's correlation predicts the void fraction within an error band of ±20% were made clear.

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