The slowing down of fast neutrons in thin hydrogeneous moderators

The slowing down of neutrons in thin hydrogeneous moderators of different thickness and finite lateral dimensions is studied using two experimental methods, a) the direct method of measuring the time distribution of neutrons slowed down to a specified energy and leaving the moderator, b) an indirect method which consists of determining the influence of such a moderator on the resonance width of a known neutron resonance at low energy measured by the time-of-flight neutron spectrometer. The experimental results are compared with Monte Carlo calculations and with theoretical expressions derived elsewhere. Fairly good agreement is obtained between experiment and the Monte Carlo calculations. The theoretical predictions are seen to be of limited range of validity only and especially a great difference between the theoretical and experimental time variance of the slowing-down time is observed.The author would like to thank Dr. R. Joly and Dr. A. Michaudon for their suggestion that this work be performed in CEN Saclay as well as for their support and interest during the measurements in Saclay. He would also like to acknowledge the help of Dr. P. Ribon, Mr. Paya, Mr. Derrien, his wife Jarmila and Mr. Girard and the accelerator group in CEN Cadarache under the direction of Mr. Leroy during the measurements on the Van de Graaff accelerator. He is very obliged to the International Atomic Energy Agency in Vienna for having offered a stipend in France without which the present work could not have been done.     

墙壁反射中子对脉冲堆波形的影响

墙壁的反射中子会对快脉冲堆的波形产生明显的影响.堆芯中子泄漏后,经过墙壁的反射有一定的概率返回堆芯,由于能量的差异,泄漏中子的返回时间是一个连续的分布.传统的双区模型只考虑了相互作用概率,而没有时间信息,尽管可以很好地解决稳态问题,而无法解决瞬态问题.本文采用等效的方法,把泄漏中子等效为时间相关的堆芯本征源,建立了含有反射效应的时间关联双区模型.求解得到的脉冲波形与CFBR-Ⅱ的实验结果一致,从而合理解释了脉冲波形后沿衰减变慢和坪功率提高的实验现象.     

Possibilities of using synthetic polymers in nuclear emulsions for the dosimetry of fast neutrons

The study contains the results obtained in the process of the preparation and study of the properties of nuclear emulsion of a new type made especially for fast neutron dosimetry.Synthetic hydrophilic polymers were used for the preparation of the emulsion and the sensitivity to fast neutrons — by the detection of recoil protons — was increased by enriching the emulsion with elementary hydrogen and by the use of additive recoil proton radiators.The dosimetric results obtained with this emulsion are compared with the results achieved by various authors with the gelatine emulsions used up to now.     

Operating strategy generators for nuclear reactors

Operating strategy generators, i.e., the software intended for increasing the efficiency of work of nuclear power plant operators, are discussed. The possibilities provided by the domestic and foreign operating-strategy generators are analyzed.     

Emission of neutrons following muon capture in heavy nuclei

The energy and multiplicity distribution of neutrons following muon capture in heavy nuclei is investigated. High energy neutrons (E > 10 MeV) are shown to be emitted directly by the fundamental capture process.     

Energetic neutrons from muon capture by a heavy nucleus

The spectrum of energetic neutrons observed in a recent experiment on $\text{μ}{}^{\mathrm{?}}\text{+}{}^{165}\text{H}{}_0\text{→ n + X + ν}{}_{\mathrm{\mu }}$ is discussed using a simple model. It is found that the neutron spectrum can be reproduced reasonably well in terms of the nuclear Fermi motion deduced from other sources of information such as high-energy heavy-ion collisions.     

Experimental investigations of the nuclear level density by using heavy ion reactions

The transition of the level density parameter a _off from the low excitation energy value a _off=A/8 MeV⁻¹ to the Fermi gas value a _FG ∼ A/15 MeV⁻¹ was discovered a few years ago studying particle spectra evaporated from hot compound systems of A∼ 160. A number of experiments have been recently performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Furthermore, precision coincidence experiments have been done in the lead region in which evaporation residues are tagged by low energy gammarays. Those experiments open the possibility of a detailed study of the level densities in nuclei where the shell effects are important.     

Search for neutrons from ‘cold nuclear fusion’

We have searched for neutrons from a Pd-electrode loaded with deuterium in electrolysis setups similar to those of Fleischmann and Pons [1] and Jones et al. [2]. Within the sensitivity of our neutron detection system corresponding to a neutron source strength of 5 x 10^{? 2} neutrons/s we did not find any neutrons due to ‘cold nuclear fusion’. We emphasize the necessity of using several independent neutron detectors.     

System size effects on probing nuclear dissipation with neutrons

Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 19~Os, 2~~Hg, and 21~po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size.     

System size effects on probing nuclear dissipation with neutrons

Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 190Os, 200Hg, and 210Po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size.     

Personnel dosimetry of slow neutrons by solid-state nuclear track detector

A fission track solid-state detector, based on aluminophosphate or sodium-calcium glasses and foil of natural uranium, was investigated. The etching conditions and evaluation of detectors are discussed. In the given arrangement a sensitivity of 1 rem for aluminophosphate glasses and 5 rem for sodium-calcium glasses was attained with slow neutrons.     

Pulsed chaos and its control in nuclear reactors

New properties and mechanisms of pulsed chaos in reactors are studied. Novel methods of controlling pulsed stochastic modes are considered.     

Some methods for calculation of perturbations in nuclear reactors

Some methods for calculation of local perturbations of neutron fields and reactivity effects accompanying them are considered. Existence, uniqueness, properties and methods for finding solutions to the considered problems are discussed.     

Single-sheet identification method of heavy charged particles using solid state nuclear track detectors

The theoretical and experimental investigations of the penetration of charged particles in matter played a very important role in the development of modern physics. Solid state nuclear track detectors have become one of the most important tools for many branches of science and technology. An attempt has been made to examine the suitability of the single-sheet particle identification technique in CR-39 and CN-85 polycarbonate by plotting track cone length vs. residual range for different heavy ions in these detectors. So, the maximum etchable ranges of heavy ions such as ⁹³Nb, ⁸⁶Kr and ⁴He in CR-39 and ⁴He and ¹³²Xe in CN-85 polycarbonate have been determined. The ranges of these ions in these detectors have also been computed theoretically using the Henke-Benton program. A reasonably good agreement has been observed between the experimentally and theoretically computed values.      

Holographic imaging of atoms using thermal neutrons

In a recent paper [L. Cser, G. Krexner, and Gy. T?r?k, Europhys. Lett. 54, 747 (2001)]] the use of thermal neutrons with wavelengths close to interatomic distances in condensed matter was proposed to obtain holographic images on an atomic scale. Two experimental methods were considered which either put the radiation source inside and the detector outside the object or vice versa. The second approach, called the inside-detector concept, requires strongly neutron-absorbing isotopes acting as pointlike detectors in the sample. In the present work, we demonstrate the feasibility of this technique by recording a holographic image of a lead nuclei in a Pb(Cd) single crystal.     

Prospects for and problems of using light-water supercritical-pressure coolant in nuclear reactors in order to increase the efficiency of the nuclear fuel cycle

Trends in the development of the power sector of the Russian and world power industries both at present time and in the near future are analyzed. Trends in the rise of prices for reserves of fossil and nuclear fuels used for electricity production are compared. An analysis of the competitiveness of electricity production at nuclear power plants as compared to the competitiveness of electricity produced at coal-fired and natural-gas-fired thermal power plants is performed. The efficiency of the open nuclear fuel cycle and various versions of the closed nuclear fuel cycle is discussed. The requirements on light-water reactors under the scenario of dynamic development of the nuclear power industry in Russia are determined. Results of analyzing the efficiency of fuel utilization for various versions of vessel-type light-water reactors with supercritical coolant are given. Advantages and problems of reactors with supercritical-pressure water are listed.