Drawbacks of the Classical Theory of the α-Rossi Method and Its Alternative

Certain contradictions and inaccuracies in the classical theory of the -Rossi stochastic neutron method are pointed out. An alternative theory which is almost free of contradictions and deficiencies of the classical theory is presented.A new implementation regime for the method is proposed to eliminate any influence of the prestart dip on the correlated amplitude in the -Rossi method using modern time-delay analyzers – the scaled start regime, consisting in triggerings of the time-delay analyzer scaled by two, three, or more signals from the starting neutron detector. It is shown that the correlated amplitude contains the ratio of the first and second moments of the number of prompt neutrons in a chain.A new expression is obtained for taking account of the spatial-energy effect in the -Rossi method. The effect differs from the well-known one not only by the energy component but also by a large range of possible values.The possibilities of the -Rossi method for determining the physical parameters of multiplying and nonmultiplying media are considered: , k _p, k, _eff, F, F _s, and D.     

Study of the zirconium apex op the Zr-Ta-Nb phase diagram

Metallographical examination thermal analysis and electrical resistance measurements have been applied to a study of the zirconium apex, up to 82% zirconium and a temperature of 1200C, of the ternary system Zr-Ta-Nb, with limited solubility of tantalum and niobium in -zirconium ( phase), limited solubility and complete solubility of niobium in -zirconium, with eutectoid decomposition of the \ solid solution and three-phase eutectoid equilibrium + between - and-zirconium. In the investigated portion of the Zr-Ta-Nb phase diagram, the following phase regions were found: a) two one-phase regions and ; b) three two-phase regions + , + and + : c) one three-phase region + + ; the region contracts as the temperature falls below 1200 C.The solubility of tantalum and niobium in -zirconium in the system Zr—Ta—Nb is about 0.5%. On passing from Zr—Ta to Zr—Nb, the + and + regions are displaced toward lower temperature and high niobium concentrations; the boundaries of the + and + + regions are lowered from 790 for Zr—Ta to 612 C for Zr-Nb. Passing between the + and + regions is a binary eutectoid line which, from Zr-Ta to Zr-Nb is displaced toward lower temperatures and higher niobium concentrations. The solubility of niobium in ot zirconium in the Zr-Nb system is about 0.5%by weight. Eutectoid decomposition in the Zr-Ta system shifts the maximum of the martensitic-like transformation to the left and results in an increase in the stability of the phase at room temperature in quenched alloys.     

Synthesis of isotopes of fermium with mass numbers 247 and 246

The isotopes Fm²⁴⁷, Fm^247m and Fm²⁴⁶ were synthesized in the extracted beam of the 310-cm heavy-ion cyclotron in the Joint Institute for Nuclear Research by irradiating Pu²³⁹ with C¹² ions. ]he procedure of collecting the recoil atoms by means of gas jets is used, with subsequent recording of their ~- decay with Si(Au) detectors. For the isotope Fm²⁴⁷, -activities are obtained with two different half-lives - one with two obviously complex groups of -particles E=7.87±0.05MeV (70%) and EE=7.93±0.05 MeV (30%), whose half- life is T_1/2=35±4 sec; the other, with E=8.18±0.03 MeV has T_1/2=9.2±2.3 sec. Data are confirmed concerning the properties of the isotope Fm²⁴⁶(E=8.25±0.03MeV and T_1/2=1.6±0.4 sec), synthesized in 1966 at The Joint Institute for Nuclear Research.Translated from Atomnaya Énergiya, Vol. 22, No. 5, pp. 342–346, May, 1967.     

Characteristics of Liquid-Scintillation α Spectrometry

The energy calibration curve in the -particle energy range 4.01–8.77 MeV, suitable for solutions of a liquid scintillator with different light absorption, is constructed. The mechanisms of broadening and asymmetry of the peaks of various actinides are studied. Lists of isotopes, whose content can be measured quantitatively when they are present simultaneously in samples of different origin by liquid-scintillation spectrometry, are given.     

Liquid-Scintillation α Spectrometry

The laws of liquid-scintillation spectrometry are studied. Specially developed computer programs for processing and decomposing spectra substantially improve the resolution of liquid-scintillation spectrometer; this makes it possible to use this method for simultaneous determination of the most common main actinides. 5 figures, 1 table, 6 references.     

Internal friction in uranium

Studies have been made of the temperature dependence of internal friction and the shear modulus in uranium. The internal friction in uranium depends on the heat treatment and is reduced after annealing in the ß and regions. During polymorphous transformations the internal friction changes its value isothermally. The transitions ß and ß are accompanied by a reduction in the internal friction and ß and ß by an increase in the internal friction. Each polymorphous modification of uranium in the temperature ranges for its existence has its own value of internal friction.     

Distribution of the ratio of radiation capture and fission cross sections for Pu239 with respect to height in the BR-5 reactor

A study was made of the distribution of the ratio of radiation capture and fission cross sections for Pu²³⁹ with respect to height in the BR-5 reactor. The distribution of neutron capture reactions was measured by determining the concentration of Pu²⁴⁰ from the rate of spontaneous fissions in plutonium samples irradiated in the reactor with an integral flux of 10²¹ to 10²² neutrons/cm². As initial material for irradiation, almost isotopically pure Pu²³⁹ (Pu²⁴⁰ content 5·10^–3% was used. The values of obtained rose from 0.1 to 0.8 on moving away from the center of the reactor. The data corresponding to equilibrium neutron spectra in the active zone and the outer region of the reflector agree with the results of measuring and _f in monoenergetic neutrons.Translated from Atomnaya Énergiya, Vol. 16, No. 6, pp. 497–500, June, 1964     

Calculating the transient temperature field in a reactor tube and the thermoelastic stresses in the fuel element cladding

Approximate analytic methods are given for calculating the transient temperature field in the fuel elements and the coolant temperatures at any point along the reactor tube, as well as the transient thermoelastic stresses in the cladding of a cylindrical fuel element. The coolant temperature at the input to the tube is constant, and the coolant undergoes no changes in state of aggregation. The approximate methods are illustrated by examples.Results are given, for comparison, of accurate calculations of the same examples made with a rapid calculating machine.List of symbols time - r; z coordinates (radius, distance along tube) - r₁; r₂ internal and external radii of fuel element cladding respectively - H total active length of fuel element - a₁; ₁;c _{1 1} coefficients of temperature conductivity, heat conductivity, specific heat capacity and specific gravity of fissionable material respectively - a₂; ₂; C_p2; ₂ cladding parameters - a; ; c_p; coolant parameters - mean cladding radius - f:f₂ cross-sectional area of tube for coolant and cladding respectively - w coolant velocity - coefficient of heat release to coolant - t (r, ); (); () fuel temperature, mean temperature over cross section of cladding, and coolant temperature at pointz. along tube respectively - qv() specific volume of coolant at pointz - values averaged overz - quantities at the initial instant of time - ₃ delay time - _n time required for coolant to go from z=0 to the point in question     

Liquid-Scintillation β–α Spectrometry

An approach permitting the use of liquid scintillation counters as full spectrometers capable of identifying the radionuclide composition of complex samples according to the measured – radiation spectra is presented. The approach is based on simulating the instrumental spectrum of the sample with library spectra of individual radionuclides.Two methods are presented for constructing the radionuclide library – direct measurement of – spectra of each radionuclide and simulation of the spectra using standard mathematical functions. It is shown that the influence of internal conversion on the simulation of the spectra of peaks by asymmetric Gaussian distributions must be taken into account.A comparative analysis is made of modern liquid scintillation counters and of examples of analysis of the spectra of control and real samples.     

Lawson concepts and criticality in DT fusion reactors

The original Lawson concepts (amplification factorR and parametern as well as their applications in DT reactors are discussed in two cases: the ignition regime and the subignition regime in a self-sufficient plant. The modified Lawson factor or internal amplification factorR (a function of alpha power) is proposed as a means to measure the ignition level reached by the plasma, in a more precise way than that given by the collective parameter (nkT). The self-sufficiency factor () is proposed as a means to measure the plant self-sufficiency, being more significant than the traditionalQ factor. It is stated that the ignition regime (R =1) is equivalent to a critical state (energy equilibrium); then, the corresponding critical mass concept is proposed. The analysis of theR relationship with temperature (kT), (n), and recirculating factor () gives the conditions for the reactor to reach ignition or for the plant to reach self-sufficiency; it also shows that an approach to ignition is not improved by heating from 50 to 100 KeV.     

Constraints on very-high-β, edge-stabilized tandem-mirror reactors

There are several tandem-mirror schemes which propose a very high and edge stabilization for the center-cell plasma ( being the ratio of the plasma pressure to the vacuum magnetic-field pressure). While the exact criteria for the edge stabilization are uncertain, it is possible to analyze the option space in which a very-high- mirror reactor would operate. The primary physics constraints on such a reactor are the energy balance at ignition, the buildup of He⁴ ash and the hot-particle( ^hot ), and the need for adiabatic conservation of the hot-particle gyro-orbits in the axial field gradients at the center-cell ends. There are also engineering constraints on the allowable wall loading and plant size. In this paper, a wall-stabilized tandem-mirror reactor is analyzed and is found to be an attractive device requiring low center-cell vacuum fields (of the order of 2 to 3 tesla). A primary requirement is that the plasma edge have a thermal conductivity near classical values.     

Separation of the stable isotopes of boron

This article describes methods for separating the stable isotopes of boron. Three of them can be used to prepare concentrates of B¹⁰ isotope in industrial quantities. The method of chemical exchange has a comparatively high coefficient of separation ( = 1.03); however, the production rate of the apparatus is small due to the high molecular weight of the complex. The preparation of B¹⁰ by fractionating BF₃ offers possibilities ( = 1.0075); however this process must be carried out at a temperature of –l00 °C, and requires the use of large quantities of liquid air. The method of separation by fractionation of BCl₃ has a low value of (1.003), but can be carried out at atmospheric pressure and room temperature.     

Mechanical properties of uranium

The paper describes the results of mechanical tests on uranium at room temperature and at elevated temperatures. Data are given on the hardness of uranium in the temperature range 20–600 C, flow pressure on extrusion in the ^– and ^– phase ranges, tensile properties and impact strength at temperatures of the ^–, ^–, and ^– phases. The anisotropic behavior of the individual grains of coarsegrained uranium during mechanical tests has been elucidated. It is shown that the existence of allotropic transformations and the difference in the crystal structure of the modifications of uranium influence its mechanical properties to a marked degree. It is also shown that the mechanical properties depend upon the carbon content of the uranium.     

Influence of the structure and properties of uranium on its behavior during irradiation

Data are presented on the variations in dimensions and form of uranium specimens during irradiation. It is shown that by regulating the composition of the uranium and treatment conditions (degree of deformation in the-region and heat-treatment conditions), in consequence of variation in grain size and texture, it is possible to vary within wide limits the magnitude of surface distortion due to irradiation and the value of G_i.A study has been made of the dependence of the variation in grain size of quenched uranium, as well as hardness, tensile strength, and yield strength, on the iron, silicon, and aluminum content of uranium. The cooling rate and content of these impurities influence the critical point of the transformation on quenching; for example, for a cooling rate of 400C/ sec and a silicon content of 0.05%, the critical point of the transformation drops to 530C.Experimental results show a creep acceleration during irradiation (nv = 6·10¹² neutr/cm²·sec) of 50–100 times, i.e., by 1.5–2 orders for textured uranium and uranium with disoriented structure. The rate of creep of uranium with a disoriented structure is connected to the burnup rate.The results are given of tensile tests made on uranium directly in the reactor. Even after remaining a short time in the neutron field (up to 1 hour), the percentage elongation is diminished somewhat and the tensile strength is increased.The following assisted in the experimental work: A. G. Lanin, V. M. Teplinskaia, V. K. Zakharova, L. N. Protsenko, V. N. Golovanova and K. A. Borisov.     

Radiation Conditions during Decontamination of an Experimental Facility for Reprocessing Nuclear Fuel

The basic characteristics of the changes occuring in the radiation conditions in an experimental facility for reprocessing spent nuclear fuel during decontamination of the facility after reprocessing irradiated uranium and uranium–plutonium BOR-60 reactor fuel and plutonium tetrafluoride into dioxide by the pyroelectrochemical method are examined. An expression is obtained experimentally for calculating the decrease in the power of photon radiation from contaminated surfaces as a function of the number of decontamination cycles. It is shown that for one-time processing of the surfaces of stainless steel equipment by the two-bath method the decontamination coefficient varies over the range 2.5–25 for emitters and 5–30 for and , emitters.     

Formation of porosity in uranium under the action of thermal cycling

In accelerated thermal cycling with a cycle of 50 sec period, considerable changes appear in uranium after 50–1000 cycles, depending upon the temperature range of the cycle. Cycling in the temperature range of the -phase (with heating up to between 550 and 600 °C) produces in texturcd uranium (containing about 0.1% carbon) a directional deformation and porosity, accompanied by a drop in density. After 5000 cycles, the drop in density amounts to 8% Thermal cycling with = ß = -transformations produces a pronounced distortion of the original shape of uranium speclmens and intense porosity formation, with a considerable drop in density, which attains 30% after 1000 cycles.     

Scintillating glass for the detection of slow neutrons

A scintillating glass was prepared and investigated. The glass, with the composition Li₂O. 2SiO₂(Ce) may be used for the detection of slow neutrons. The scintillation efficiency of the glass relative to NaI(Tl) under electron excitement, was 1.4%. The ratio of the scintillation yields for electrons and -particles was 3.8–4. The de-excitation time constant was 0.15 sec. The efficiency of glass 0.1 cm thick, containing lithium with an Li⁶ concentration of 90.5%, for thermal neutrons was 82%. For neutrons with an energy of 10 ev, the efficiency of glass 0.5 cm thick was 40%.The work was reported at the Second All-Union Conference on Scintillators, November 1957, Kharkov.The authors would like to thank Z. M. Karpova for help in preparing the glasses.     

Investigation of the structural changes taking place in a uranium-molybdenum alloy under the action of neutron irradiation

An alloy of uranium with 9 wt.%molybdenum was subjected to neutron irratiation, after which the electrical resistivity was measured and the structure studied by x-ray and microposcopic analysis. Preliminary heat treatment allowed specimens to be obtained with an + eutectoid structure having various grain sizes. It was established that the rate of diffusion leading to homogenization under the action of irradiation in annealed specimens is inversely proportional to the square of the grain size (the period of the lamellar eutectoid). In a homogeneous specimen (-phase) irradiation causes a change in properties and structure which rapidly (in 2–4 hours) attains a limiting value, which is explained as being due to radiation annealing. Disordering was found in the -phase, with a transition to the cubic lattice which took place under the influence of irradiation during the first hours of exposure.All these phenomena are satisfactorily accounted for on the basis of the theory previously developed [2], although the magnitudes of the thermal-spike region and of the energy liberated in it, equal respectively to 2.5 · 10^–17 cc and approximately 2 Mev, are less than were determined in [2].     

Some characteristics ofγ-ray scattering at the boundary of two media

The characteristics of -ray scattering (0.33 E 1,25 MeV) at the boundary between two media were experimentally investigated. It was established that the scattered radiation was produced mainly by a narrow beam of primary radiation. The effective scattering area was contained within a region with a radius r 10h (for h z). The contribution to the total energy from radiation scattered by the medium increased for angles of incidence of the primary narrow beam up to 85–88 °, and then decreased monotonically.An estimate was made of the effective dimensions of the area from which scattered quanta were emitted into the backward half-space when an isotropie source was located at the surface of the scatterer.The energy flux of the scattered radiation was measured with a gas counter which had a practically constant sensitivity for -rays of different energies.The authors are deeply grateful to Prof. O. I. Leipunskii and to A. S. Strelkov for assistance and for discussions of the results.     

Radiotoxicity and Residual Energy Release of Actinides and Fission Products of Spent VVÉR Fuel in Long-Term Storage

The time dependence of the residue energy release and radiotoxicity of spent VVÉR-1000 nuclear fuel with long-term storage or uniform accumulation in long-term storage is investigated. The calculations of the energy release take account of the contribution of , , and radiation, and the calculations of the radiotoxicity take account of the maximum admissable activity of nuclides in water and air. The data presented can be used for developing a strategy for long-term storage of spent nuclear fuel from power reactors. 4 figures, 4 tables, 4 references.