Patent Number: 055442048
Section: summary

FIELD OF THE INVENTION The present invention relates to a process for automatically, reliably, and, in a fail-safe manner, influencing the reactivity of a neutron chain reaction in a nuclear reactor, to a process of providing stable control of the reactivity of such a chain reaction, and to an apparatus for carrying out the method or process. To the extent that the invention deals with controlling reactivity, it may also be considered to involve a method of operating a nuclear reactor. BACKGROUND OF THE INVENTION The reactivity of a neutron chain reaction in a nuclear reactor is a measure of the relative multiplication rate of neutron production. In a stationary state normal operation, the reactivity is zero. To increase the output, the reactivity may be increased and to reduce the output, e.g. in terms of the power generated, the reactivity may be reduced. Generally speaking in the nuclear reactor and in the region in which a fissionable fuel, also referred to as a nuclear fuel, provides the neutron flux, i.e. the reaction region of the nuclear reactor, the reactivity is primarily controlled by the effectiveness of neutron absorbers. For example, neutron-absorbing rods or elements can be introduced into the core of a nuclear reactor, i.e. into the neutron flux zone, to a greater or lesser extent, or the fuel rods may be shifted into a body of neutron-absorbing material to a greater or lesser extent. The introduction and retraction of the absorber rods, for example, reduce or increases the reactivity. Apart from the settings of the rods to control reactivity, the reactivity is influenced in normal operation by many other parameters. Of the greatest importance is the danger of uncontrolled liberation of neutrons in cases of uncontrolled reactivity with the consequence of potential danger of an explosion like increase of neutron production and loss of control of the reactor. This can be great enough to damage the reactor core and thus the most important barrier or containment preventing escape of radio-active substances. For this reason it is desirable to provide an automatic inherently reliable reduction in the reactivity of a neutron-chain reaction which can be built into the reactor and can in a fail-safe manner effect at least a limited reduction in the reactivity upon the development of an event necessitating that reduction, such as a loss of coolant flow. A system which can reduce reactivity intrinsically and automatically as the need arises, contributes to the stabilization or stable operation of the nuclear reactor and enables hot shutdown thereof. Some systems have been provided heretofore to generate a self-stabilizing effect in the operation of nuclear reactors. These can include the provision of a nuclear fuel having a negative reactivity temperature coefficient, or can utilize the negative reactivity void coefficient of the boiling water in a boiling-water reactor in which boiling water surrounds the fuel. In such cases a tendency toward a temperature increase will automatically reduce the reactivity and thereby lower the output in a self-stabilizing manner. These earlier self-stabilizing systems are, however, incompletely effective and it is desirable to be able to provide additional self-stabilizing automatic reactivity-controlling effects in a nuclear reactor. OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide a method of or process for and an apparatus for automatic stabilization of a neutron chain reaction in a nuclear rector. Another object of our invention is to provide a process for controlling reactivity of a nuclear reactor and, in particular for the reactivity automatically and in a self-stabilizing manner, utilizing techniques other than those known heretofore and which can be used effectively alone or in conjunction with other techniques. It is also an object of the invention to provide an improved process and apparatus for controlling the reactivity of a neutron chain reaction in a nuclear reactor which has advantages over earlier techniques. SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the invention, by providing a part of the nuclear fuel contributing to the neutron chain reaction so that it is subject to forces generated by a coolant flow and is maintained by these forces in the neutron-flux region, i.e. the active region of the nuclear reactor, but upon reduction of such forces, can be received in a space outside the reaction region to thereby withdraw this part of the nuclear fuel from contributing to the neutron flux and reduce the reactivity of the reactor. The fuel-suspension force is provided during normal operating flow of coolant through the reactor, i.e. by branching a portion of the main coolant flow through sieve-like inlets and outlet openings of a longitudinal compartment in a direction opposite the direction in which gravity acts upon the fuel particles, i.e. upwardly, so that in normal operation the particles are fluidized by the coolant flow and are suspended in this compartment over the height thereof. Since the upper part of this compartment or the compartment above the space into which the fuel particles can fall for reactivity reduction, lies in the reaction region of the neutron chain reaction, the suspended fuel particles contribute to the chain reaction. Upon failure of the coolant flow, however, the fuel can settle into the lowest part of the compartment, i.e. into a space below the reaction region and hence out of the normal flux, thereby reducing the reactivity and leading, upon appropriate dimensioning of the reactivity reduction, to a hot shutdown of the reactor. Advantageously, the space and the fuel which can be contained therein are so dimensioned that the influence of this fuel upon the reactivity amounts to 0.5% to 1%. To vary the reactivity at this magnitude about 0.5% to 1% of the total fissionable fuel inventory must be contained in the upper part of the compartment when the coolant flow is normal and must be withdrawn from the reactive zone into space when the coolant flow fails. A reduction of the reactivity to values between -0.5% to -1% can result in hot shutdown of the reactor. According to a feature of the invention, a portion of the fuel at least in the compartment and space should be coated fuel particles, i.e. fuel particles in, for example, a carbon coating. The flow through the compartment and space can be a portion of the main coolant flow branched therefrom, i.e. a branched stream which can support these fuel particles against the gravitation forces and which can be returned to the main coolant flow through the upper sieve structure which can screen the fuel particles from the coolant flow. Coated fuel particles are commonly used in high-temperature reactors and in the case of the present invention, are carried upwardly by the branched flow of coolant entering the compartment through the lower sieve structure and are held in suspension by the flowing branched-coolant stream as long as the coolant flow is maintained. Should there be a defect in the coolant flow, these particles settle automatically out of the reactive region and reduce the reactivity in the manner described. Advantageously, the branched stream can be passed through regions of the reactor free from fuel. In fast reactors the branched structure can be guided through the breeder blanket and with thermal reactors through the reflector. According to yet another feature of the invention, the fuel in the compartment has a greater degree of enrichment than that in the remainder of the neutron flux or reactive zone. This increases the effectivity of the reactivity control system of the invention. The process for the automatic stabilization of the reactivity of a neutron-chain reaction sustained by a fissionable fuel in neutron-flux zones of a nuclear reactor can comprise the steps of: (a) supporting a part of the fissionable fuel in the zone by forces generated by a coolant flow into contact with the part of the fissionable fuel; PA1 (b) providing below the zone a space for accumulation of the part of the fissionable fuel out of the neutron-flux zone; and PA1 (c) automatically causing the part of the fissionable fuel to collect by gravity in the space upon reduction of the forces, thereby reducing the reactivity of the chain reaction. PA1 means forming a vertically elongated compartment in the zone within neutron flux therein; PA1 sieve structures at opposite ends of the compartment for confining fine fissionable fuel particles therein, the compartment communicating with a space below the compartment and generally outside the zone; PA1 means for connecting the compartment into a coolant stream for the nuclear reactor so that a coolant flow is branched from the main stream and flows through the compartment in a direction opposite the effect of gravity on the particles therein, whereby the coolant flow supports the particles in the zone and, upon reduction of the coolant flow, at least some of the fissionable fuel particles collect by gravity in the space upon reduction of supporting forces on the particles generated by the coolant flow, thereby reducing the reactivity of the chain reaction. In apparatus terms, the apparatus for the automatic stabilization of the reactivity can comprise: In this apparatus, an elongated compartment or hollow space is provided in the reactor core with its longitudinal axis parallel to the gravitational direction and with an upper part in the reaction region of the neutron-chain reactor. The sieve-like structures are provided at the ends of the compartment and the compartment is so connected to the main coolant flow that a branched or auxiliary coolant flow passes through the compartment in a direction opposite the action of gravity upon the fine-grained fuel particles in the compartment between the sieve structures. During proper flow of the coolant, the fuel particles are in the reaction region and upon failure of the coolant flow automatically settle in the compartment out of the reactive region to automatically reduce the reactivity of the reactor. In reactors with an annular or ring-shaped reactor core, the compartment or hollow space is advantageously within a central column of the core.