Journal of Nuclear Energy最新文献

A series of Monte Carlo calculations have been performed in order to evaluate the effect of separated vs merged spin statistics on the analysis of spherical shell neutron transmission experiments for gold. It is shown that the use of separated spin statistics consistently results in larger average capture cross sections of gold at 24 keV. This effect is explained by stronger windows in the total cross section caused by the interference between potential and J₊ resonances and by J₊ and J₋ resonance overlap allowed by the use of separated spin statistics. Using the latest published separated spin statistics, the value of the average capture cross section for gold at 24 keV is 710 ± 25 mbarn.

A new method for reactivity determination is presented. This method involves two measurements of the reactor response to a neutron source: (a) when subcritical, the source—multiplication, M, is measured and (b) when critical, the linear power rise, R, is measured. The reactivity is then determined from the relation $\text{p/β=}\text{R}\text{M}\text{1}\text{(β/α}\text{+}\text{∑}\text{i}\text{(βdβ)}\text{γ}{}_{\mathrm{i}}$, or from a good approximation to it $\text{ϱ}\text{β}\text{≈}\text{T}{}_{\mathrm{d}}\text{[R/M]}$. This method is compared with the asymptotic—period method; expected accuracy is discussed, and possible measurement procedures are described. The feasibility of combining both methods for the determination of $\text{β}$Λ is discussed.

The ⁵⁶Fe(n, p)⁵⁶Mn and ²⁷Al(n, α)²⁴Na cross sections have been measured at the average energy of 14·78 MeV with an energy width of 200 keV. Aluminium and iron samples in the form of foils were irradiated in an accurately known neutron flux and the resulting ⁵⁶Mn and ²⁴Na activities determined using absolute 4πβ-γ counting techniques. The flux was determined using a recoil proton monitor to an accuracy of ±2·1 per cent. The values obtained were 109·8 ± 2·9 mb and 115·5 ± 3 mb where the errors have been obtained by adding the systematic error in determining the neutron flux linearly to the random error calculated at the 99 per cent confidence level in determining the ⁵⁶Mn and ²⁴Na activities.

The collision term in the quantum-mechanical transport equation of neutrons derived by Osborn et al. (1965) is reformulated with the aid of the S-matrix theory of scattering, and the usual transport equation is generalized to include the effect of multiple scattering of neutrons. It is shown that the random phase approximation is no longer necessary by virtue of the adiabatic switching off of interactions.

A semi-quantitative analysis of the effect of multiple scattering is made on the basis of the free gas model, and it is concluded that the validity of the usual transport equation depends on the spatial resolution, the energy of the neutron under consideration and the energy spectrum of all neutrons. The usual transport equation is not valid for a very cold neutron (of energy below 00001 cV), and not generally valid for a cold neutron when the energy spectrum of the neutrons is softer than a Maxwellian. It is also inapplicable to a neutron of any energy when the spatial resolution is not much less than the mean free path of the neutron.

An approximate treatment of the collision integrals for inelastic scattering and (n, 2n) reaction is studied in order to take the energy distribution of neutrons emitted in these reactions into the slowing down calculations. In this paper the (n, 2n) reaction is treated by cluster model. An approximate synthetic kernel for inelastic scattering is proposed in order to calculate the inelastic collision integral by a simple recursion relation, which allows its rapid computation. The application of the present approximate method to calculations of fast neutron spectra and the fast effects proved that the present synthetic kernel was very useful for treatment of inelastic scattering and (n, 2n) reaction.

In the neutron energy range below 20 MeV about 20 neutron reactions are energetically possible. These reactions are analyzed using a statistical model. Since direct reaction effects can be important the calculations are normalized to experiment so as to partly compensate for deficiencies in the model. The empirical model is similar to previous work used to describe (n, 2n) reactions but is extended to included charged particle emission. Relatively good agreement is obtained with measured excitation curves and 14 MeV cross-sections using a single set of fitting constants. The method of analysis has been incorporated into a computer code THRESH. For crude calculations the code requires only Z and A as input but improved fits in individual cases can be attained through simple parameter variation. Cross-sections and fission spectrum averages are calculated for nuclides in the range Z = 21−41.

Existence and uniqueness of the solution of the initial value problem for the Boltzmann transport equation in an indefinite homogeneous multiplicative medium are proved. Moreover, the existence of at least one real eigenvalue of the transport operator is displayed. Finally, the asymptotic behavior of the neutron density is indicated as t → + ∞.