Vol 121, No 2 (2016)

Computational estimates are made of the possibility of a self-sustaining nuclear chain reaction in beyond design basis accidents with core meltdown in a research reactor, where IRT-3M or IRT-4M fuel assemblies with high- and low-enrichment fuel are used. The certified MCU-PTR Monte Carlo code is used for the computational estimates. A conservative approach (estimaticphon from above) is used. It is shown that a selfsustaining nuclear chain reaction is impossible during core meltdown.

This article is devoted to the problems of reliability analysis of APCS. The advantages and disadvantages of different methods of qualitative reliability analysis of technical systems are examined, and the need for combined use of all methods studied is shown and validated. The most appropriate criterion of evaluation is found on the basis of an analysis of the primary reliability indices of the APCS means. This research shows that there is a need for developing a method of reliability and safety analysis that will make it possible to evaluate the risk of systems which are under development, raise the reliability indices to the level of economic calculations, and choose the APCS means taking account of economic factors, which would lower the cost of development work and delivery.

The kinematic viscosity of a eutectic salt mixture with the molar composition 0.855LiF–0.145AlF₃ at temperatures to 850°C is measured by means of damped torsional vibrations of a cylinder with the experimental liquid. This salt mixture is of interest for separating actinides and lanthanides during reprocessing of spent nuclear fuel. Below 800°C the behavior of the system is found to differ from that of a normal liquid. Such behavior is explained by precipitation on solid surfaces. The precipitated fraction of the material is evaluated as a function of temperature. An analytical expression is obtained for the kinematic viscosity at temperatures above 800°C. The standard deviation of the kinematic viscosity evaluated assuming the dispersion to be independent of temperature equals 0.006·10^–6 m²/sec. The total error of measurement does not exceed 3%.

The aim of this work is to compare different approaches to nuclear fuel cycle closure from the standpoint of plutonium utilization, fuel type, and reprocessing technology. It is shown that the choice of mixed uraniumplutonium nitride fuel for fast reactors will make it possible to effectively close the NFC and at the same time develope safe nuclear power; the combined (pyrochemical + hydro) technology will make it possible to reprocess any type of high-burnup short-decayed irradiated fuel from fast reactors and to obtain the final uranium-plutonium-neptunium product suitable for use in any fuel production technology.

A model is proposed for recovering the point cross sections in the region of unresolved resonances. The approach consists in using probability tables P_i(σ) to construct the effective cross sections σ_i^′(E) on some choice of energy intervals [E_i, E_i+1]. In constructing the effective cross sections the densities P_i(σ′) and P_i(σ) are preserved. This makes it possible to preserve on the intervals [E_i, E_i+1] characteristics of the neutron–matter interactions such as the average cross section, average free path, transmission function, and others. The use of pseudoresonances in the field of unresolved resonances is not distinguished from the use of ordinary point cross sections in the region of resolved resonances. The proposed approach does not require modifications of the library formats and modeling schemes for the neutron trajectories in Monte Carlo programs. The calculations of two problems showed good agreement between the calculations of the change ΔK_eff using probability tables and the MCNP code and calculations using the PRIZMA code, in which pseudoresonances were used.

Research has determined the actual ¹⁴C emissions from all types of reactors. The average emissions are (TBq/yr): VVER-440 (V-179, -230) 3.1 ± 1.5; VVER-440 (V-213) 1.3 ± 0.5; VVER-1000 2.0 ± 0.7; RBMK-1000 2.4 ± 1.6. The obtained data agree satisfactorily with the calculations, so that they can be used to predict the emissions from commissioned power reactors. The normalized ¹⁴C emissions from different types of foreign reactors, which approximately correspond to domestic reactors, are presented in UNSCEAR reports. The total atmospheric emissions of ¹⁴C by NPP reactors over the entire period of their operation (in 2015) equaled ~1.34 PBq. Over this same period about 54 PBq of native ¹⁴C entered the carbon cycle of the Earth’s biosphere; this is almost 50 times greater than the surveyed NPP.