[en] Short communication

[en] Consistency of americium isotope fission factors in a wide range od excitation energy and observed cross sections of fission by neutrons with energies up to 20 MeV are analyzed. The behaviour of the first chance fission cross sections is determined according to the results obtained in coordinated analysis of cross section of (n, f) and (n, xn) reactions for ²³⁸U and ²³⁵U. The contradictions in the fissionability of ²⁴²Am compound nuclei at E ≤ 8 MeV, revealed during joint analysis of the observed cross sections of isotope chain fission, testify to the necessity of refining the energy dependence of cross sections σ_nf for ²⁴²Am^g target above the (n, nf) reaction threshold. Cross sections of the ²⁴¹Am (n, f), ²⁴²Am^g (n, f), ²⁴³Am (n, f) fission reactions for neutron energy E ≤ 20 MeV and those of ²⁴⁰Am (n, b) and ²³⁹Am (n, f) ones in the range of the first plato are evaluated

[en] The article presents a brief review of theoretical and experimental data on energy dependence of ²³⁸U fission cross sections by protons isomer ratio of ²³⁸U(p,3n) reaction, in which a short-lived ²³⁶Np^s (T_1/2=22.5 h) isotope and a long-lived ²³⁶Np^l (T₁/₂=1.55x10⁵ year) are produced, as well as on the ²³⁸U(p,3n)²³⁶Np^s reaction cross section

[en] The fission cross sections of the symmetric SL-mode and the asymmetric lumped (S1+S2)-mode of the ²³⁵U(n, F), ²³⁷Np(n, F) and ²³⁸U(p, F) reactions are calculated up to E_n=200 MeV within a statistical model. For each fissioning nuclide, emerging in (n, xnf) reactions, a separate triaxial outer fission barrier is assumed for the SL-mode. To reproduce the measured branching ratio of symmetric and asymmetric fission events, strong contribution of fission from neutron-deficient nuclei is assumed. Damping of the contribution of triaxial collective modes to the level density at the SL-mode outer saddle seems to be essential for the description of the branching ratio. The sensitivity of the calculated branching ratio to the fissility of the target nuclide and the incident particle is investigated

[en] Fission level densities (or fissioning nucleus level densities at fission saddle deformations) are required for statistical model calculations of actinide fission cross sections. Back-shifted Fermi-Gas Model, Constant Temperature Model and Generalized Superfluid Model (GSM) are widely used for the description of level densities at stable deformations. These models provide approximately identical level density description at excitations close to the neutron binding energy. It is at low excitation energies that they are discrepant, while this energy region is crucial for fission cross section calculations. A drawback of back-shifted Fermi gas model and traditional constant temperature model approaches is that it is difficult to include in a consistent way pair correlations, collective effects and shell effects. Pair, shell and collective properties of nucleus do not reduce just to the renormalization of level density parameter a, but influence the energy dependence of level densities. These effects turn out to be important because they seem to depend upon deformation of either equilibrium or saddle-point. These effects are easily introduced within GSM approach. Fission barriers are another key ingredients involved in the fission cross section calculations. Fission level density and barrier parameters are strongly interdependent. This is the reason for including fission barrier parameters along with the fission level densities in the Starter File. The recommended file is maslov.dat - fission barrier parameters. Recent version of actinide fission barrier data obtained in Obninsk (obninsk.dat) should only be considered as a guide for selection of initial parameters. These data are included in the Starter File, together with the fission barrier parameters recommended by CNDC (beijing.dat), for completeness. (author)

[en] Experimental and evaluated cross-section data on fission, (n,3n) and (n2n) reactions leading to the short-lived state of the ²³⁶Np nucleus is analyzed using a self-consistent statistical approach. Integral and differential (n,2n) reaction data are compared. Earlier evaluations appear to be inconsistent with recent experimental data. (author). 38 refs, 5 figs

[en] The observed fission cross sections of transuranium isotopes are analyzed with use of systematics of the statistical characteristics of the equilibrium and transition states of the nuclei, as well as parameterization of the spectra of inelastically scattered neutrons, obtained in a consistent description of neutron cross sections for the ²³⁸U nucleus. This makes it possible to describe the observed cross sections for fission of U (A = 232-238), Pu (A = 238-244), and Am (A = 241-243) isotopes by neutrons of energy 1-20 MeV. It is shown that a detailed study of the energy dependence of the fission cross sections of transplutonium elements requires improved experimental data that may make it possible to refine the energy dependence of the fission cross section above the threshold of the (n,nf) reaction for Am and Cm isotopes

[en] The knowledge of neutron cross-sections for Curium isotopes is necessary for solving the problems of the external fuel cycle. Experimental information on the cross-sections is very meager and does not satisfy requirements and existing evaluations in different libraries differ substantially for fission and (n,2n) reaction cross-sections. This situation requires a critical review of the entire set of evaluations of the neutron cross-sections for Curium. 17 refs, 3 figs

[en] It is shown that the correct estimation of prefissionable neutron spectra, emitted in (n, xnf) reaction as well a simple simulation of neutron spectra from the fission fragments permit to reproduce the experimental spectra of prompt fission neutrons and predict the variation of averaged fission neutron energy for reactions of ²³⁷Np(n, F) and ²⁴¹Am(n, F) under the neutron flux at energy range 20 MeV

[en] In January 2007, the National Nuclear Data Center (NNDC) produced a set of preliminary neutron covariance data for the international project 'Nuclear Data Needs for Advanced Reactor Systems'. The project was sponsored by the OECD Nuclear Energy Agency (NEA), Paris, under the Subgroup 26 of the International Working Party on Evaluation Cooperation (WPEC). These preliminary covariances are described in two recent BNL reports. The NNDC used a simplified version of the method developed by BNL and LANL that combines the recent Atlas of Neutron Resonances, the nuclear reaction model code EMPIRE and the Bayesian code KALMAN with the experimental data used as guidance. There are numerous issues involved in these estimates of covariances and it was decided to perform an independent review and assessment of these results so that better covariances can be produced for the revised version in future. Reviewed and assessed are uncertainties for fission, capture, elastic scattering, inelastic scattering and (n,2n) cross sections as well as prompt nubars for 15 minor actinides (^233,234,236U, ²³⁷Np, ^{238,240,241,242}Pu, ^241,242m,243Am and ^{242,243,244,245}Cm) and 4 major actinides (²³²Th, ^235,238U and ²³⁹Pu). We examined available evaluations, performed comparison with experimental data, taken into account uncertainties in model parameterization and made use state-of-the-art nuclear reaction theory to produce the uncertainty assessment