The radiative neutron capture cross section of ¹⁹⁹La has been measured in the energy region of 702, 540 eV using the 52 m neutron time-of-flight facility at the Japan Atomic Energy Research Institute electron linac. Neutron capture events were detected with a 3, 500l liquid scintillator tank. Individual resonances were analyzed using the area analysis method to obtain the capture widths. Capture data were corrected for the multiple scat-tering of neutrons in a sample by means of the Monte Carlo technique. Capture areas gΓ_nΓ_γ/Γ for 20 resonances and capture widths Γ_γ for 5 resonances were newly obtained. The present average capture cross section above 1.6 keV can be well reproduced by the statistical model calculation with the available average resonance parameters, but below 1.6 keV is found to deviate from its prediction.

View full abstract

For optimal design of a Sc filter to get 2 keV neutrons from a reactor beam port, neutron total cross section of Sc has been measured near the 2 keV cross section minimum using the standard time-of-flight method. The result is given by a polynomial σ_t(E)= 18.00-4.51E+56.810E²-44.351E³+20.674E⁴-5.2389E⁵+0.5572E⁶ b (E in keV, 0.25<E<3), least square-fitted to the experiment. The minimum value of the cross section obtained is 0.21±0.03 b. This value markedly differs from 0.71+0.03 b, which is the recently reported value measured by similar kind of experiment, and close to 0.27+0.07 b measured by a different kind of experiment. The lower value of present measurement indicates that one may design more thicker filter than usually used. Analysis with the Breit-Wigner multi-level formula shows that the lower value may not be any obstacle for the interpretation of dominant contribution of spin-antiparallel interaction with thermal neutrons.

View full abstract

Using the pile oscillator technique, the reactor transfer functions of the experimental fast reactor "JOYO" were examined to ascertain reactor stability and to elucidate the reactor feedback mechanism. Measurements were made in the frequency range of 0.001 to 3 Hz at various reactor power levels. Major results were as follows :
(1) Satisfactory agreement obtained between the zero-power transfer function measure-ments and calculations made by one point reactor model with six delayed neutron groups.
(2) Reactor stability confirmed from measurements of at-power frequency responses ; large negative feedback effect (•.85$/Δφ/φ at 50 MWt) observed.
(3) Feedback transfer function and transfer function between neutron flux and outlet temperature at the central fuel subassembly found expressible in approximation by first-order lag model with a time constant of about 3.5 s.

View full abstract

A parametric analysis was conducted to determine from experimental data the charac-teristics of pressure oscillations generated when steam fed through vent tubes is made to condense in a pool of subcooled water. The occurrence of the high-frequency oscillations (around 100200 Hz with the present experimental arrangement) was found to be intimately related to the extent of development of the low-frequency component of oscillations rep-resenting either the reciprocating movement of the steam-water interface in the vents-conducive to "chugging"-or under conditions where the interface had descended into the water pool, the alternating expansion and contraction of the steam bubble formed around the vent exit.
Four patterns of occurrence of the high-frequency pressure oscillations were distin-guished, each pattern corresponding to a specific range of parameters governing the experi-mental conditions, as defined by zones on a map drawn on the coordinate plane relating the pool-water temperature to the steam mass flux fed through the vent tubes.
It was noted, however, that the differences in the patterns of oscillation occurrence did not influence the basic characteristics of the wave shape presented by the pressure oscillations, which suggested a common mechanism governing the generation of high-frequency oscillation, whether occurring in pulsed form accompanied by "chugging" or in the continuous form of condensation oscillation. Associating the foregoing observation with the fact that the steam mass flux flowing through vent tubes varies in synchronization with the interface motion-which is intimately related to alternations in the degree of subcooling of the water in the vicinity of steam-water interface, a coherent explanation is presented on the patterns of occurrence of the high-frequency oscillations, under both conditions of chugging and of condensation oscillation.

View full abstract

The O/U ratios of UO_2+x (0 ?? x ?? 0.1) were determined at 1, 000 and 1, 200°C, using a thermobalance in atmospheres of controlled oxygen pressures set by CO/CO₂ gases. The oxygen potentials corresponding to the x values were measured in situ with a solid elec-trolyte oxygen sensor. For x ?? 0.015 the present ΔG_O2 data agree well with the previously published data. In the range of 0<x ?? 0.015, the increasing disagreement at smaller values of x was indicated ; the present ΔG_O2 values are 2 to 15 kcal/mol more negative than the prior thermogravimetric data. The results also showed that a reverse in ΔG_O2 at 1, 000 and 1, 200°C occurs in the range of 2.003<O/U<2.015. The maximum in the plot of the partial molar entropy ΔS_O2 vs. O/U ratio has consequently been found near ΔS_O2=11 e.u. for O/U=2.007. This ΔS_O2 value is considerably high in comparison with Markin & Bones' EMF data, suggesting a characteristic configuration of oxygen in the lattice near UO_2.007.

View full abstract

Corrosion of the steel cladding of mixed oxide fast breeder fuel elements is thought to be initiated by penetration of Cs (and oxygen) into the grain boundaries of the steel. Ex-periments were therefore performed to investigate the penetration of Cs into stainless steel (1.4970 and 1.4988) with the aid of radioactive tracers and with Rutherford backscattering techniques. The penetration distances were very limited and decreased with decreasing oxygen potential. Chromium diffusion was also measured in one of the steels (1.4970) and yielded D_Cr-values of the order of magnitude of reported D_Cs-values. Chromium diffusion along grain boundaries was enhanced in steel preloaded with Cs.

View full abstract

In order to study the effect of burst temperature on the coolant flow channel restriction, burst tests of fuel bundles were performed. Each bundle consisted of 49 rods (7×7 rods), and bursts were conducted in flowing steam. Burst temperature was changed by changing the internal gas pressure in rods. After the burst, the ballooning behavior of each rod and the degree of coolant flow area restriction in the bundle were measured.
Maximum swelling of rod occurs when the burst temperature is around α and α+β phase boundary, and this phenomenon is almost the same as that in single rod burst tests. Maximum coolant flow area restriction is also observed in this condition.

View full abstract

In order to study how the operating pressure and temperature have influence on the heavy water separation characteristics for a pair of bithermal trickle-bed type H₂/H₂O-exchange columns packed with the hydrophobic Pt catalyst, the authors utilized the analytical expressions derived from the differential equations expressing the material balance of the three components, hydrogen, water vapor and water in the exchange column.
The cases where the operating temperature in a hot column is 200°C, the deuterium enrichment factor is 3, the hydrogen superficial velocity is (0.2)×(Operating pressure (atm)) (Nm/s) and the column efficiency is high and low were studied. As the results, the optimum operating pressure is about 50 atm in both cold and hot column, the optimum operating temperature in a cold column is 100°C and other important parameters such as the optimum length of a cold and a hot column, the optimum hydrogen to feed water molar flow ratio and the optimum degradation ratio were also obtained.

View full abstract