It is well known that adding a moderator in the blanket or strongly absorbing reflector in a fast reactor increases the decay time of the neutron density from the neutron pulse. Measurements during the international experiment BERENICE [1] on determining the effective delayed-neutron fraction Beff of the temporal distribution of prompt neutrons by the ~-Rossi method gave an unusual result. The computed lifetime of the prompt neutrons in the core and blanket of the MASURCA-R2 critical assembly (Cadarache, France) was equal to -0.35 and 1/~see, respectively, and it was expected that the decay time ~-of the main component of the neutron density under suberiticality conditions p = 0.15/3eft will be close to 40-45 tzsec.In reality, the measured temporal distribution was described by two exponentials with decay times r I = 22.5 + 1.5 and r 2 = 127 + 6 psee. In [2] it was hypothesized that the appearance of these two exponentials is due to two small polyethylene blocks in the blanket. Numerical calculations using the model of two groups of prompt neutrons in a reactor with a reflector [3] confirmed the possibility of such an explanation. A weak point was the lack of measurements in an assembly with no polyethylene blocks.To check the hypothesis experimentally, measurements were performed on the KBR-22 critical assemblies [4]. Its main characteristics are: 235U thorium (approximately 20% enrichment) core, depleted uranium dioxide blanket, 61-cm high and -80 cm in diameter core, 40-cm thick radial blanket, 20 cm thick axial blanket. A section of the assembly along the central plane is displayed in Fig. 1. The same neutron detectors (3He counters) as in the experiment on the MASURCA-R2 critical assembly were placed inside the empty channels.The a-Rossi temporal distribution was measured for four compositions. The first is the assembly described above. The second composition was obtained by introducing two polyethylene blocks into the blanket. Each block was obtained by replacing the uranium dioxide by polyethylene in three cylindrical tubes. The centers of the 25-cm high polyethylene cylinders in these tubes were located in the central plane. The third composition contains four blocks (two more blocks in the right-hand side of the blanke0. The fourth composition was obtained by separate placement of 12 polyethylene cylinders in the second row of the tubes in the blanket. The measurements were performed with 0.15/3el f subcriticality.The measured c~-Rossi temporal distribution is displayed in Fig. 2. The figure also shows the short-lived correlated part, obtained by subtracting out the long-lived part. Table 1 contains r 1 and r2 and the ratio of the "areas" under them $2/S r The characteristic neutron lifetime r r in the polyethylene blocks and cylinders was measured in an additional experiment. It turned out to be 65 and 50 ~,sec, respectively, in the uranium dioxide environment.Therefore it has been established experimentally that introducing into the blanket of a fast reactor polyethylene blocks with neutron lifetime l...
Hydrogen as an energy carrier will play a considerable role in the future structure of energy production when nuclear reactors will replace fossil fuels. Investigations performed in recent years have shown that the production of large quantities of energy with high efficiency can be accomplished on the basis of thermochemical cycles using reactors with coolant temperature at the exit from the core of about 900°C. A variant of such a fast reactor with sodium as the coolant is proposed. The main physical characteristics and the main problems which must be solved to build such a reactor are presented. According to its properties, this reactor will meet the modern requirements for nuclear and radiation safety. It can also be used in other promising high-temperature technologies, for example, high-efficiency production of electricity.According to modern ideas, reactors with coolant temperature ~900°C at the exit from the core will play a large role in the future structure of nuclear structure. They will make it possible to solve important problems such as the production of hydrogen in large quantities, making electricity production more efficient, and expanding the use of high-temperature technology in industry.High-temperature helium-cooled reactors are now being considered for this purpose. For example, designs of a modular thermal helium reactor GT-MHR [1] with gas-turbine energy conversion and a fast GFR [2] for use in hydrogen production using an iodine-sulfur water separation cycle without the formation of greenhouse gases are being developed in the USA and some other foreign countries. In our country, a group of enterprises at the forefront with the Russian Science Center Kurchatov Institute [3] has developed a technical proposal for developing an electrotechnical facility on the basis of a thermal high-temperature helium HTGR for a chemical-industrial process for steam conversion of methane or products of gasification of coal with a low calorific value.
Statistical methods of measuring the power of a reactor are based on the point model of kinetics, which introduces methodological errors into the final result. There exist three main sources of error which are associated with the value of neutrons, the finite dimensions and coordinate of the detector, and the finite time required to establish the neutron flux. Expressions for the corrections as well as the results of Monte Carlo calculations performed for the FS-1M critical stand are presented. It is shown that the resulting correction factor can range from 0.969 to 1.131 depending on the location of the neutron detector.Statistical methods for investigating the physical characteristics of nuclear reactors appeared at the beginning of the 1950s, but already in the 1970s they were replaced by more effective methods using pulsed neutron generators. Nonetheless, statistical methods for measuring the power of a reactor are still needed, since they are more easily implemented than, for example, the absolute counting chamber method, and their statistical accuracy is high and the measurement time is acceptable (3-5% over 30-60 min). Many variants of these methods have been developed, but they all have the same drawbackthey are based on the point kinetics model. This introduces methodological errors into the final result.The present article examines the sources of error and methods for computing the corresponding corrections for one of the most widely used methods -the frequency-noise method. The computational estimates of the corrections are made for the FS-1M critical stand. Sources of Errors and Expression for Corrections.In the frequency-noise method [1], the fission rate -the number of fissions N in a reactor in 1 sec -can be expressed in terms of the measured parameters (detector current I and the spectral density S k ) as follows:
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