[en] Highlights: • A sufficient condition for real k-eff in heterogeneous B_N equations is established. • Consequences on symmetry properties of fluxes and currents are given. • A simple numerical model supports these results. • Buckling dependent rules to define a minimum-size sub-cell for calculation are given. - Abstract: In heterogeneous B_N models, the interplay between symmetries leaving the material distribution invariant and the direction of the buckling vector is of key importance. A sufficient condition for the reality of the buckling-dependent multiplication factor is established, and its consequences on the symmetry properties of integrated fluxes and projected currents are outlined. A simple numerical model supports the results so established. Rules for definition of a minimum-size sub-cell for calculation are given, together with a proposal of application to a method of characteristics model

[en] Some of the criteria and margins that are applied in Sweden, differ from those applied in some other countries. Two unlikely, concurrent events must not lead to criticality. The small, sometimes negative bias corrections, established by comparison of computer calculations to experiments that are applied in other countries, are in most cases not judged as sufficient. Statistical uncertainties in the estimated multiplication factor from calculations (Monte Carlo), mechanical variations and other sources are restricted in that the standard deviation normally must be less than 0.01. Mixed arrays of dissimilar units must be evaluated in combination, not separately as this is not generally considered safe. Unrealistic (conservative) assumptions concerning some parameters in the safety analysis should not be used to cover errors and uncertainties in other parameters

[en] The American National Standard ''Guide for Nuclear Criticality Safety in the Storage of Fissile Materials,'' ANSI/N16.5-1975 is the subject of this paper. The Guide was reaffirmed in 1982. The technical bases for the conditions and requirements are discussed. Suggestions for applications and several general problems addressed by the Guide are presented. The development of information needed for future extensions of the area of applicability is given

[en] An equation of material number density sensitivity coefficient is derived using first-order perturbation theory. The beginning of cycle of Super-Phenix I is taken as the reference system for this study. Effective multiplication factor of the reference system is defined as system response function and fuel enrichment and fuel effective density are chosen for the variation of reference input data since they are described by material number density which is a component of Boltzmann operator. The nuclear computational code system (KAERI-26 group cross section library/1DX/2DB/PERT-V) is employed for this calculation. Sensitivity coefficient of fuel enrichment on effective multiplication factor is 4.576 and sensitivity coefficient of effective fuel density on effective multiplication factor is 0.0756. This work shows that sensitivity methodology is lesser timeconsuming and gives more informations on important design parameters in comparison with the direct iterative calculation through large computer codes. (Author)

No abstract available

[en] The single-sided lower tolerance factors frequently used for non-trending assessment of the validation bias and bias uncertainty are sensitive to departures from normality. When used properly, the tolerance limits ensure that an appropriate fraction of the true population of applicable critical experiments lies above the calculated lower tolerance limit with the required statistical confidence level. One condition necessary to ensure that the appropriate proportion of the true population of k_eff values in the validation suite lies above the lower tolerance limit is that the assumption that the normality of the underlying population of critical experiments is valid or conservative. This paper discusses various methods used to assess whether the assumption that the validation suite may be treated as a random sample drawn from a normal distribution is acceptable. Techniques for assessing the validity of this underlying assumption include common omnibus hypothesis tests for normality, assessment of sample skewness and kurtosis of the validation suite, and graphical techniques. These techniques are used to assess the nature and potential conservatism/non-conservatism imparted by various departures from normality where possible. A review of hypothesis testing is also presented to frame the discussion of omnibus normality tests. Additionally, two cases are analysed with these techniques to provide an example of how they should be implemented. Both of the example data sets are taken from the SCALE 6.2.2 validation report, they deal with a U²³³ solution and with a HEU solution and the data sets were used to create histograms and Q-Q plots

No abstract available

[en] Nuclear Safety Guide, TID-7016 Rev 2 was issued as NUREG/CR-0095 in 1978. Table 2.8 of this report has been found to contain errors. The table was designed to indicate configurations with effective multipliction factors of 0.95. Because of an error in configuration descriptions, some of the configurations have multiplication factors as high as 1.09. A corrected table is available from the undersigned, and Revision 3 of the report is being prepared. Norman L. Pruvost, LANL HSE-6, P. O. Box 1663, M/S-F691, Los Alamos, NM 87545

[en] Theoretical computation of the Los Alamos National Laboratory's critical assembly THOR (a thorium-reflected plutonium sphere) yields a high eigenvalue when compared to the experimentally measured eigenvalue. Several calculational improvements are investigated in an effort to reduce the discrepancy. Finally, the experimental procedure of reducing the raw configuration to clean specifications is reviewed

[en] The subcriticality of two interacting solution tanks was determined using ²⁵²Cf-source-driven neutron noise analysis methods. These experiments were the first test of this method for an interacting system with materials (in this case, uranyl nitrate) typical of nuclear materials in processing plants. The experiments were performed to test the conclusions from previous interaction experiments with uranium metal discs for a fissile system with moderation, and to provide data to test theoretical models for coupled systems. The uranium metal experiments showed that the subcritical neutron multiplication factor, k/sub eff/, could be determined using point kinetics without any correction for spatial effects from measurements with the source and detectors located adjacent to the same cylinder, whereas for source-detector configurations with either the source and/or detectors adjacent to different cylinders, a model which incorporates the coupling is required to obtain subcriticality. In the previous experiments with two coupled uranium metal cylinders, the measurements were performed at frequencies (<50 kHz) much lower than the break frequencies (>1 MHz) for the metal discs. Thus, many aspects of the theory relating to the relationships between various spatial models could not be verified in previous experiments. These measurements and their interpretation have shown that (1) point kinetics interpretation of the measured ratios of spectral densities yields the subcritical neutron multiplication factor when the source and detectors are located in or adjacent to one of the cylinders of uranyl nitrate, and (2) the coupling reactivity can be obtained by incorporating a third neutron detector adjacent to the cylinder without the source using the kinetics model of Difilippo, which includes one additional spatial mode in the direction of the coupling