[en] The results of various conventional tests with this 1000kW experimental reactor (KSTR) are shown in diagrams. The comparatively high negative temperature coefficient of the reactivity was found to be a great advantage. Some inexplicable temperature differences and temperature changes in the reactor vessel were found. (J.S.)

[en] The primary principle to be observed when handling fissionable material is to prevent uncontrolled criticality, as the reactions, once started, proceed so fast that there remains no chance at all to stop the process. This is why appropriate dimensioning, arrangement and material selection has to be guaranteed at any place of work and with any activity involving the handling of fissionable material, in order to prevent criticality under all circumstances. The contribution in hand reports about current knowledge in science and technology of the criticality-safe designs of installations, and explains the calculations and methods applied. (orig./HP)

[en] The Nuclear Weapons Effects Branch at White Sands Missile Range has proposed a storage vault for components of their fast pulse reactor. The reactor consists of eight stacked annular rings, a central safety block, and three rods of fully enriched uranium alloyed with ten weight percent molybdenum (rho = 17 gm/cc). The nominal mass per ring is 12.9 kg with inner and outer diameters of 10.16 and 20.32 cm. Rod and bolt holes penetrate through the eight ring stack. The design basis vault configuration consists of a 3 wide by 8 high planar array of reactor ring elements plus an assembled 8 ring stack on the reactor stand 45.7 cm in front of the array. The cubical unit cell dimension for the storage array is 30.48 cm. Vault materials are Portland concrete for the walls, ceiling and floor and Benelex 70 (Masonite) for the door

[en] Comparison was made among the various calculation methods of critical mass prediction. The geometrical system under consideration is composed of a sphere of mixed oxide-H₂O and a H₂O reflector of 40 cm thickness. Two cross section libraries, GGC-4 (99 groups for fast neutrons and 101 groups for thermal neutrons) and Hansen-Roach (16 groups) were used for the comparison. CITATION, ANISN, and KENO-2 codes were used for diffusion theory, for transport theory, and for Monte Carlo method for the calculation, respectively. The difference in the criticality prediction made by the combination of those libraries and codes was investigated with varying oxide concentration. As for the transport code, the effects of the order of Legendre polynomial expansion and the order of angular division were investigated. Comparisons with AHSB handbook data and with experimental data were also made. As a result of those investigations, the standard calculation procedure was established. It uses GGC-4 as the cross section library and CITATION for the range of low oxide concentration and ANISN for the high oxide concentration range. (Aoki, K.)

[en] Critical masses for spent fuel are larger than for green fuel and therefore use of the increased masses could result in improved handling, storage, and transport of such materials. To apply spent fuel critical masses requires an assessment of fuel exposure and the corresponding isotopic compositions. The paper discusses several approaches at the Hanford N Reactor in establishing fuel exposure, including a direct measurement of spent to green fuel critical masses. The benefits derived from the use of spent fuel critical masses are illustrated for cask designs at the Nuclear Assurance Corporation

No abstract available

[en] For criticality safety evaluation of burnup fuel, the general-purpose burnup calculation code, SWAT, was revised, and its precision was confirmed through comparison with other results from OECD/NEA's burnup credit benchmarks. Effect by replacing the evaluated nuclear data from JENDL-3.2 to ENDF/B-VI and JEF-2.2 was also studied. Correction factors were derived for conservative evaluation of nuclide concentrations obtained with the simplified burnup code ORIGEN2.1. The critical masses of curium were calculated and evaluated for nuclear criticality safety management of minor actinides. (author)

[en] Core 9 of the FR0 fast critical assembly was diluted with heavy water to 24 vol. per cent, contained in thin walled copper cans. The report describes measurements of the critical mass and the reactivity coefficient of heavy water in this core. The effect of the heterogeneous core composition on these items is also dealt with. The results are compared with theoretical predictions using several computer codes. Criticality is accurately predicted, but the measured reactivity coefficient of heavy water is about 20 % lower than the value obtained with the best available methods, involving the SPENG and DTF-4 programmes. The result of bunching measurements, in which the degree of heterogeneity of core composition was changed, is compared with theoretical estimates of the resonance shielding, flux advantage and leakage components of the heterogeneity effect

[en] First steps on building a look-up table for heat transfer of supercritical water in vertical, smooth tubes are presented. A look-up table is a pragmatic approach to predict heat transfer if, like with supercritical water, the fluid is characterized by highly nonlinear variations of properties in the vicinity of the critical point. As similarity laws are not applicable any longer then, correlations cannot predict heat transfer properly with reasonable effort. The main attention of the paper is to justify the approach taken, to outline the procedure and, as a first step, to focus on the smoothing of the experimental data to reduce the data scattering. Examples of heat transfer near the critical pressure highlight the technical problem of heat transfer in boiler tubes and indicate the challenge of data interpolation with changing flow regimes. (author)

[en] Criticality occurs when a sufficient quantity of fissionable material is accumulated, and it results in the liberation of nuclear energy. All process accidents have involved plutonium or highly enriched uranium, as have most of the critical experiment accidents. Slightly enriched uranium systems require much larger quantities of material to achieve criticality. An appreciation of criticality accidents should be based on an understanding of factors that influence criticality, which are discussed in this article. 11 references