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A two-section reactor is a system with two degrees of freedom. Controlling a two-section reactor and, first and foremost, the transfer into a state corresponding to a definite fission intensity in the sections or a pulse with a given total number of fissions are much more difficult than controlling of a single-section reactor. In a two-section reactor a change in state of any section entails a change in the intensity or total number of fissions in both sections. Finding the required starting state by trial and error or by sorting through all possible positions of the reactivity control units of both sections is, under given conditions, a long and inaccurate procedure because of the two-dimensionality of the problem. The operation of finding the required starting state of a two-section reactor becomes successful if the two-dimensional problem of searching for the required starting state can be reduced to a sequence of one-dimensional problems [1] that are typical for a one-section reactor or special, complicated methods for controlling objects with many degrees of freedom, such as the uncoupling method [2, 3], can be used.Solutions of this problem based on the kinetic relations characteristic for two-section reactors signifying invariance of the intensity ratio or the total number of fissions in the sections to changes in the state of one section were found in [1]. Relations of this kind greatly simplify the problem of determining the required starting state, reducing it, for example, to fixing a prescribed ratio 12111 of the total number of fissions in the sections over a pulse using displacement of one of the reactivity regulating units and thereby establishing the absolute values of I 1 and 12 by using displacements of the other unit.Several sequences of operations for determining the starting state of a two-section reactor corresponding to a pulse with prescribed fissions in the sections were formulated for using this approach. These sequences of events are based on calibrated measurements of the total number of fissions (or intensities) in sections with different positions of a particular reactivity control rod and using an auxiliary (reference) stationary or pulsed neutron source placed in one of the sections. The sequences of operations do not require measurements of the reactivity of the sections and the reactor as a whole, and they also do not require introducing these concepts explicitly.The sequences of operations formulated in [1] are intended predominantly for fast reactors and boosters. The kinetic relations on which they are based are exact for transient processes with participation of only prompt neutrons or both prompt and delayed neutrons. These conditions hold well in fast pulsed reactors, where short pulses are produced without the participation of delayed neutrons. In this case, to study short pulses the kinetic relations contain /r -the multiplication factor on prompt neutrons, and for long pulses they contain the total multiplication factor k.Unfortunately, for short pulses in thermal react...
The concept of coupled reactors could be of considerable interest for transmutation of radioactive wastes on the basis of a system consisting of a heavy-particle accelerator, a neutron target, and a blanket in the form of a subcritical assembly [1]. The theory of such systems was first developed by R. Avery at the Argonne National Laboratory in the USA and was reported in 1958 at the 2nd Geneva Conference on peaceful uses of atomic energy [2].Coupled reactors are under study by many specialists. Considerable attention is often devoted to reactor safety with breeding of fuel. Usually, a reactor in which fission in the active zone is produced only by fast neutrons and fission in the blanket reflector, which is the breeding zone, is produced by both fast and thermal neutrons. This is a combined system consisting of a fast reactor and a thermal subcritical assembly, which is safer than standard fast reactors with respect to positive-reactivity accidents.Multiple-section coupled systems in the form of a combination of a high-power reactor and serially positioned subcritical assemblies with neutron valve coupling between sections have been discussed [3, 4]. Layers of 235U, cadmium, and a moderator, placed at the boundaries between sections, have been proposed for use as valves. It has been observed that practical implementation of a system of this type provides the opportunity to increase substantially the degree of burnup of the fissioning substances, increase by several factors, due to the higher dilution of fuel in the subcritical assembly, the neutron flux density in the active zone, and guarantee safer conditions of power production because the valves prevent the accident from spreading from the subcritical assembly to the reactor. Two-section reactor systems are also under intensive discussion in the context of aperiodic pulsed reactors, in application to which they open up the possibility of decreasing the duration of the radiation pulse or increasing the volume of experimentally accessible zones with a high radiation level [5, 6]. Devices of the latter type have been realized in practice [7, 8] or are incorporated in important projects, for example, high-power nuclear-pumped lasers [9].It is shown in [10] that the creation of neutron valve coupling between sections is the most effective means for decreasing pulse duration in coupled reactors or subcritical assemblies. An effective method of valve coupling based on the use of a threshold fissioning substance in one of the sections is also examined in [10]. In the two-section BR-K booster reactor, developed at VNIII~F, using 237Np as the material in one of the sections the radiation pulse duration is almost ten times shorter than that obtained with the standard single-section arrangement [11].Returning to transmutation and a system consisting of an accelerator, a target, and a blanket, developed as part of the ABC/ATW program, it can be conjectured that coupled subcritical assemblies will be useful for achieving deeper subcriticality of the blanket in the workin...
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