Journal of Nuclear Energy最新文献

The ²³⁸U(n, γ) ²³⁹U and ¹¹⁵In(n, γ)^116mIn neutron capture cross-sections have been measured by the activation technique for several neutron energies between 150 and 630 keV. The neutron flux was measured with a calibrated long counter. The errors on the results at a confidence level of approximately 99 per cent were about ±7 per cent for U and ±4 per cent for In, with standard deviations of ±2 per cent.

Detailed fine-energy spectra in the energy range of resonance scattering effects are shown for an EBR-I1 type core radially surrounded by steel-rich or nickel-rich reflector and for steel-rich or nickel-rich subassembly-sized regions internal to an EBR-II type core. Results of use of fine-group-collapsed spatially dependent and spatially independent coarse-group cross sections for simulations of the resonance scattering effects are described.

The use of an extrapolated-endpoint boundary condition in criticality calculations is discussed from the point of view of two-group neutron-transport theory. The use of a single Milne-problem extrapolated endpoint is justified, from the lowest-order approximation to the transport-theory result, only for certain values of the material properties of a bare multiplying slab.

Fast neutron activation cross-sections have been measured at 14·78 MeV relative to the ⁵⁶Fe(n,p)⁵⁶Mn cross-section. The following cross-sections (in millibarns) have been determined: ¹⁴N(n, 2n)¹³N 7·45 ± 0·4, ¹⁹F(n, 2n)¹⁸F 47·9 ± 1·4, ⁶⁵Cu(n, 2n)⁶⁴Cu 1043 ± 37·7, ¹⁶⁰Gd(n, 2n)¹⁵⁹Gd 1456 ± 44, ¹⁹⁸Pt(n, 2n)¹⁹⁷Pt 2091 ±113, ²⁷Al(n, α)²⁴Na 115·5 ± 3, ³⁰Si(n, α)³⁰Mg 90 ± 11, ³⁴S(n, α) ³¹Si 194 ± 5·9, ⁵¹V(n, α)⁴⁸Sc 19·5 ± 1·2, ⁵⁹Co(n, α)⁵⁶Mn 32·3 ± 1·1, ²⁴Mg(n,p)²⁴Na 179·7 ± 5, ²⁷Al(n,p)²⁷Mg 68 ± 2·3, ²⁸Si(n,p)²⁸Al 265 ± 7·5, ²⁹Si(n,p)²⁹Al 131 ± 13·6, ³¹P(n,p)³¹Si 85·6 ± 2·6 ³²S(n, p)³²P 219 ± 7·1, ⁵¹V(n,p)⁵¹Ti 31 ± 1·3, ⁶⁵Cu(n,p)⁶⁵Ni 26 ± 1. The samples were in the form of thin foils and the activities produced after irradiation were determined using absolute 4πβ-γ coincidence counting techniques. The Value of the ⁵⁶Fe(n, p)⁵⁶Mn cross-section used as the standard was 109·8 ± 2·9 mb. The results are compared with those of other authors and show that by considering the ratio of individual cross-sections it is possible to determine if a single cross-section is grossly in error or not.

A formula that allows the exact determination of the non-trivial root $\text{ξ}{}^{\mathrm{\ast }}$ arising in the solution $\text{q(u) = ξ}{}^{\mathrm{\ast }}\text{(u)F(u)}$ to the slowing down equation is given. A general approximation scheme for obtaining approximations to $\text{ξ}{}^{\mathrm{\ast }}$ is derived. The classical approximations of Fermi and of Greuling and Goertzel are the two lowest order approximations of this scheme. A method of consistent approximation, as opposed to the usual inconsistent method of which the Fermi and GG are particular examples, is also introduced. It is shown that this consistent approach greatly improves on the inconsistent one. A new physical interpretation of $\text{ξ}{}^{\mathrm{\ast }}$ is discussed.

The decay of a neutron burst in pulsed experiments in lattices is computed in one-group diffusion with respect to the buckling, using a group theoretical theory based on a solid state-like formalism. A transport band structure is obtained and a generalized Nelkin's expansion for the neutron decay is computed for a square water-Beryllium lattices with respect to the radius of the central Beryllium rod. Moreover, the influence of the rod nuclear parameter in Nelkin's expansion is studied.

We report here the results of a series of calculations on the propagation of neutron waves in multiplying media using multigroup diffusion theory. For a heavy-water natural uranium subcritical assembly (taking into account the slowing down of fission neutrons), the variation of the attennuation constant α and phase lag ξ with frequency have been studied and compared with the available esperimental data. It is found that the multigroup theory gives satisfactory agreement with the experimental data. The striking feature of the multigroup theory is that it gives the correct frequency dependence of α and ξ not only at low frequencies but also at high frequencies. On the other hand, the two group theory results totally fail at high frequencies. The multigroup theory is also better than the Fermi-age theory because the latter, though gives a correct behaviour of the dispersion curve, shows a general disagreement with the experimental data for all frequencies.

Neutron-wave propagation in graphite assemblies of finite transverse dimensions has been studied in the diffusion theory approximation using multigroup (30 groups) approach and energy dependent boundary conditions. Frequency dependence of the thirty eigenvalues is discussed. It is shown that the critical frequency $\text{f}{}^{\mathrm{\ast }}$ depends upon transverse size. For infinite transverse size ${}^{\mathrm{\ast }}$ is nearly equal to 130 Hz whereas for some critical transverse dimensions (in the present case it lies between 80 × 80 cm² and 70 × 70 cm²) ^∗ becomes zero. The existence of a pseudo-discrete mode in the continuum is discussed. Strong interference between the cold neutron density and the ’epicold‘ neutron density is predicted at some specific distance from the source. The position and intensity of interference is found to depend upon the frequency of the wave, the transverse size of the assembly, the energy spectrum of the source neutrons and the temperature of moderator. Neutron wave propagation in finite assemblies of graphite at 200°K and 150°K is also discussed.

A method has been developed for the calculation of the channel wall dipole extrapolation lengths required in heterogeneous calculations of neutron flux in an Advanced Gas-Cooled Reactor. Considerable improvement over previous methods has been achieved by treating the geometry of the fuel cluster explicitly and by using a P1 angular flux distribution at the channel wall. The calculation is done in one energy group using a collision probability method.

The results are compared with other methods and with some measurements of cross-cluster flux tilts made at Berkeley Nuclear Laboratories.