Fissile materials detection in systems with pulsed neutron sources

Different approaches to detecting fissile materials which are being transported illegally across state borders or removed illegally from enterprises of the nuclear industry are examined. The content of fissile materials is determined using a pulsed neutron source, which is used in combination with two neutron moderators, separated by a screen with a large absorption cross section for thermal neutrons. The results of an experimental validation, the sensitivity limits, and the possibility of further development of these systems are presented. __________ Translated from Atomnaya énergiya, Vol. 101, No. 2, pp. 125–130, August, 2006.     

Analytical solution of the problem of neutron thermalization in a heavy moderator

A model of a kernel for neutron scattering in a heavy moderator is presented. The kernel has the correct asymptotic behavior at high energies and leads to a scattering operator which yields zero when applied to the Maxwell distribution. The model ineludes three arbitrary functions of neutron velocity which may be chosen so as to give the best approximation of true seattering kernels. Within the framework of the diffusion approximation, the neutron distribution function is found for this model in integral form for the general case of an arbitrary source, and the question of eigenvalues and eigenfunetions of the neutron transport equation is discussed briefly.Translated from Atomnaya Énergiya, Vol. 22, No. 2, pp. 100–107, February, 1967.     

Neutronics analysis of the target-moderator–reflector (TMR) configuration for compact long pulsed neutron sources

The compact pulsed neutron source can play an important role in the research, education, user training, and development of the advanced neutron scattering instruments. The materials for the target, moderator, reflector (TMR) and their configurations must be optimized to get the optimal yield of neutrons with energy in the range of 0.001–0.1 eV order. Several kinds of materials of the TMR, their configurations, and their dimensions are investigated by the Monte Carlo simulation and optimized for developing the compact pulsed neutron source. The results would contribute to the construction of the Compact Pulsed Hadron Source (CPHS) at Tsinghua University, China.     

Option for spallation neutron sources

Spallation reactions are a very important option for efficient neutron sources appropriate for fusion materials testing. An option of this option is the EURAC concept, which makes use of short-term accelerator technology in the cheapest way and is proved to provide the needed neutron flux to verify fast experiments on fusion materials performance. Its flexible conception allows an optimum combination of very high fluxes of about 10¹⁶ n/cm²/s, with decreasing fluxes along the testing zones in enough volume to perform the correct irradiations. With this assumption, the rate effect can be perfectly analyzed together with the end-of-life conditions assumed in the structural material of the future fusion environments. The possible negative effects of the high-energy neutrons in the Spallation spectrum have been taken into account, concluding their non-significance in the desired damage parameters. The EURAC concept can also be considered in light of other purposes like incineration processes, production, and, with the appropriate booster, high-flux cold neutron source.     

A pulsed neutron source for reactor measurements

A compact neutron tube capable of producing 10 μsec pulses of neutrons at a rate of 5 × 10¹³/second (5 × 10⁸/pulse) has been developed. The tube, which uses the D-T reaction consists of a P.I.G. ion source from which deuterons are accelerated onto a tritium-loaded titanium target. The design features which lead to these high output rates are discussed.The incorporation of the tube into a reactor probe operating at levels up to 10⁸/pulse is described. Pulse to pulse variations are small and a high degree of reliability has been achieved with the complete system.     

Collision probabilities for non-uniform neutron sources

Collision probabilities have been calculated for neutrons originating from sources with a spatial dependence which can be expressed by a symmetric, quadratic polynomial. The Monte Carlo method was used and applied to cylinders of various shapes and sizes and to cubes. The deviation from the results with a uniform source is at most about 5% for each finite dimension of the homogeneous bodies considered. Calculations on a typical BWR cell show that the change may be larger outside the source region. However, the flat-source approximation seems to be acceptable in most cases.     

Transmutation missions for fusion neutron sources

There are a number of potential neutron transmutation missions (destruction of long-lived radioisotopes in spent nuclear fuel, ‘disposal’ of surplus weapons-grade plutonium, ‘breeding’ of fissile nuclear fuel) that perhaps best can be performed in sub-critical nuclear reactors driven by a neutron source. The requirements on a tokamak fusion neutron source for such transmutation missions are significantly less demanding than for commercial electrical power production. A tokamak fusion neutron source based on the current physics and technology database (ITER design base) would meet the needs of the spent nuclear fuel transmutation mission; the technical issue would be achieving ≥50% availability, which would require advances in component reliability and in steady-state physics operation.     

Preparation and verification of mixed high-energy neutron crosssection library for ADS

An accelerator-driven subcritical system(ADS)is driven by an external spallation neutron source, which is generated from a heavy metal spallation target to maintain stable operation of the subcritical core, where the energy of the spallation neutrons can reach several hundred megaelectron volts. However, the upper neutron energy limit of nuclear cross-section databases, which are widely used in critical reactor physics calculations, is generally 20 MeV.This is not suitable for simulating the transport of highenergy spallation neutrons in the ADS. We combine the Japanese JENDL-4.0/HE high-energy evaluation database and the ADS-HE and ADS 2.0 libraries from the International Atomic Energy Agency and process all the data files for nuclides with energies greater than 20 MeV. We use the continuous pointwise cross-section program NJOY2016 to generate the ACE-formatted cross-section data library IMPC-ADS at multiple temperature points. Using the IMPC-ADS library, we calculate 10 critical benchmarks of the International Criticality Safety Benchmark Evaluation Project manual, the 14-MeV fixed-source problem of the Godiva sphere, and the neutron flux of the ADS subcritical core by MCNPX. To verify the correctness of the IMPCADS, the results were compared with those calculated using the ENDF/B-VII.0 library. The results showed thatthe IMPC-ADS is reliable in effective multiplication factor and neutron flux calculations, and it can be applied to physical analysis of the ADS subcritical reactor core.     

Analytical inversion formula for uniformly attenuated fan-beamprojections

A convolution backprojection algorithm was derived by Tretiak and Metz (198) to reconstruct two-dimensional (2-D) transaxial slices from uniformly attenuated parallel-beam projections. Using transformation of coordinates, this algorithm can be modified to obtain a formulation useful to reconstruct uniformly attenuated fan-beam projections. Unlike that for parallel-beam projections, this formulation does not produce a filtered backprojection reconstruction algorithm but instead has a formulation that is an inverse integral operator with a spatially varying kernel. This algorithm thus requires more computation time than does the filtered backprojection reconstruction algorithm for the uniformly attenuated parallel-beam case. However, the fan-beam reconstructions demonstrate the same image quality as that of parallel-beam reconstructions     

Pulsed neutron source measurements in the subcritical ADS experiment YALINA-Booster

A subcritical zero-power source-driven coupled core, the YALINA-Booster, has been constructed for experimental investigations of neutron kinetics of source-driven systems. In this study, the reactivity of two subcritical configurations has been determined by the area ratio method. The prompt neutron decay constants have been evaluated through slope fitting of the prompt neutron decay as well as through the pulsed Rossi-α method. It is shown that the slope fitting method and the pulsed Rossi-α method give stable results whereas the area ratio method results show spatial dependence. The reasons for the spatial spread are addressed.     

Fissile material detection by active neutron interrogation with radioactive neutron sources

Different radioactive neutron sources (Am-Li, Am-Be, Cf-252) were considered and tested for active fissile material detection. The low energy Am-Li neutron source has outstanding features. The low dose rate permits full access to the measuring site. The low energy (thermal) neutron flux at the source position is high compared to the high energy neutrons that reach the counter position. In a graphite matrix, of small size, 7 g U-235 led to a doubling of the neutron count rate whereas 200 g graphite changes this count rate by only 2%. The system is in addition operable in the presence of 10¹² Bq of fission products. The detection of fissile material down to 1 g and below has been shown to be possible with this instrument in small samples.     

Supermirror neutron guide system for neutron resonance spin echo spectrometers at a pulsed neutron source

A neutron guide system for neutron resonance spin echo spectrometers has been constructed at BL06 of the Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility. The spectrometers consist of two types of neutron spin echo instruments, a modulated intensity by zero effort instrument (MIEZE) and a neutron resonance spin echo instrument (NRSE), to cover a wide energy range for various sample environments. A neutron beam from the moderator is deflected by supermirror neutron guides, split, and separately guided into the MIEZE and NRSE. The characteristic wavelengths of the neutron guide tube for the MIEZE and NRSE are 2.9 and 4.9 Å, respectively. The cross sections of the exit of the MIEZE and NRSE guides are 15 mm × 50 mm and 30 mm × 50 mm, respectively. The neutronics and shielding design were optimized by using the heavy ion transport code system (PHITS), and the absolute average neutron fluxes at the exits of the MIEZE and NRSE guides are estimated to be 2.7 × 10⁸ and 6.9 × 10⁸ n/cm²/s/MW, respectively. The measured fluxes of the MIEZE and NRSE neutron guides are 0.56 and 0.95 times the calculated values, respectively.     

Contribution of external neutron sources in excess neutron generation for SCNES

Self-Consistent Nuclear Energy System simultaneously meets four requirements: energy generation, fuel breeding, burning of radionuclides and system safety. The key element of the system is excess neutron generation. Analysis of various neutron sources is done with respect to transmutation requirements. Impact of neutron source on energy system performance is analyzed in terms of excess neutron cost. Special emphasis is made on Fusion Neutron Source.     

An investigation of neutron streaming using the pulsed neutron source technique

The radial streaming of neutrons from H₂0 moderator assemblies pierced by a uniform array of cylindrical voids is investigated by the pulsed neutron source technique. A method of synthesis is developed by which the fundamental mode in the spatial distribution of the thermal neutron flux within a moderator assembly may be readily extracted from experimental data. The decay constants determined by the synthesis method were used to derive the magnitude of the radial diffusion constant D_rv. The value of D_rv agrees with that predicted by the theory of Leslie.