Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Flux monitoring
Reexamination Certificate
2002-11-21
2004-10-05
Kam, Jack (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Flux monitoring
C376S245000, C376S255000
Reexamination Certificate
active
06801593
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for measuring the subcritical neutron multiplication factor, K
eff
of a nuclear reaction, and more particularly, to a method for determining all reactivity changes that occur while a core of a nuclear reactor is subcritical.
2. Background Information
In a pressurized water reactor power generating system, heat is generated within the core of a pressure vessel by a fission chain reaction occurring in a plurality of fuel rods supported within the core. The fuel rods are maintained in space relationship within fuel assemblies with the space between fuel rods forming coolant channels through which borated water flows. The hydrogen within the coolant water moderates the neutrons emitted from enriched uranium within the fuel to increase the number of nuclear reactions and thus increase the efficiency of the process. Control rod guide thimbles are interspersed within the fuel assemblies in place of fuel rod locations and serve to guide control rods, which are operable to be inserted or withdrawn from the core. When inserted, the control rods absorb neutrons and thus reduce the number of nuclear reactions and the amount of heat generated within the core. Coolant flows through the assemblies out of the reactor to the tube side of steam generators where heat is transferred to water in the shell side of the steam generator at a lower pressure, which results in the generation of steam used to drive a turbine. The coolant exiting the tube side of the steam generator is driven by a main coolant pump back to the reactor in a closed loop cycle to renew the process.
The power level of a nuclear reactor is generally divided into three ranges: The source or startup range, the intermediate range, and the power range. The power level of the reactor is continuously monitored to assure safe operation. Such monitoring is typically conducted by means of neutron detectors placed outside and inside the reactor core for measuring the neutron flux of the reactor. Since the neutron flux in the reactor at any point is proportional to the fission rate, the neutron flux is also proportional to the power level.
Fission and ionization chambers have been used to measure flux in the source, intermediate and power range of a reactor. Typical fission and ionization chambers are capable of operating at all normal power levels, however, they are generally not sensitive enough to accurately detect low level neutron flux emitted in the source range. Thus, separate low level source range detectors are typically used to monitor neutron flux when the power level of the reactor is in the source range.
The fission reactions within the core occur when free neutrons at the proper energy level strike the atoms of the fissionable material contained within the fuel rods. The reactions result in the release of a large amount of heat energy which is extracted from the core in the reactor coolant and in the release of additional free neutrons which are available to produce more fission reactions. Some of these released neutrons escape the core or are absorbed by neutron absorbers, e.g., control rods, and therefore do not cause additional fission reactions. By controlling the amount of neutron-absorbent material present in the core, the rate of fission can be controlled. There are always random fission reactions occurring in the fissionable material, but when the core is shut down, the released neutrons are absorbed at such a high rate that a sustained series of reactions do not occur. By reducing the neutron-absorbent material until the number of neutrons in a given generation equals the number of neutrons in the previous generation, the process becomes a self-sustaining chain reaction and the reactor is said to be “critical”. When the reactor is critical, the neutron flux is six or so orders of magnitude higher than when the reactor is shut down. In some reactors, in order to accelerate the increase in neutron flux in the shutdown core to achieve practical transition intervals, an artificial neutron source is implanted in the reactor core among the fuel rods containing the fissionable material. This artificial neutron source creates a localized increase in the neutron flux to aid in bringing the reactor up to power.
In the absence of a neutron source, the ratio of the number of free neutrons in one generation to those in the previous generation is referred to as the “Neutron Multiplication Factor” (K
eff
) and is used as a measure of the reactivity of the reactor. In other words, the measure of criticality for a nuclear core is K
eff
, that is, the ratio of neutron production to total neutron loss contributable to both destruction and loss. When K
eff
is greater than 1, more neutrons are being produced than are being destroyed. Similarly, when K
eff
is less than 1, more neutrons are being destroyed than are being produced. When K
eff
is less than 1, the reactor is referred to as being “subcritical”. Currently, there is no direct method for measuring when criticality will occur from the source range excore detectors. Presently, plant operators estimate when criticality will occur through a number of methods. One method for estimating when criticality will occur is made by plotting the inverse ratio of the count rate obtained from the source range detector as a function of the change in the condition being used to bring the plant critical, e.g., withdrawal of the control rods. When the plant goes critical, the source range count rate approaches infinity and hence, the Inverse Count Rate Ratio (ICRR) goes to zero. Due to the physics of the reactions occurring within the core-of the reactor, the ICRR curve is almost always convex, and sometimes concave. Therefore, estimating the conditions under which the plant will go critical from the ICRR curve is subject to much uncertainty, but also subject to considerable scrutiny by the Nuclear Regulatory Commission and International Nuclear Power Organization.
U.S. Pat. No. 4,588,547 discloses a method and apparatus for determining the nearness to criticality of a nuclear core. The invention takes advantage of the fact that when the reactor is subcritical, the neutron flux generated by an artificial neutron source, and the direct progeny by fission, is higher than that generated by neutrons from natural neutron sources in the reactor fuel and progeny of those neutrons. However, that method does not appear applicable to reactors that do not use artificial neutron sources and does not address the approach to criticality when a reactor approaches criticality due to withdrawal of control rods.
U.S. Pat. No. 6,181,759 discloses another method of estimating the Neutron Multiplication Factor K
eff
that involves control rod withdrawal and the measurement of the source range detectors at a number of discrete spaced-time intervals during a transient portion of the source range output. While this method appears applicable for a wider range of startup conditions, it still only provides an estimate rather than a direct measure, which requires that a conservative margin be designed into the estimate to satisfy regulatory concerns.
Accordingly, it is an object of this invention to provide a means of more accurately measuring when the core of the reactor approaches criticality.
It is an additional object of this invention to provide a method for directly measuring reactivity changes when the reactor is subcritical.
Additionally, it is an object of this invention to provide a linear measure of reactivity changes over time from the source range detector outputs.
SUMMARY OF THE INVENTION
This invention provides a direct measure of the Subcritical Neutron Multiplication Factor K
eff
by applying a correction factor to the ICRR curve data that results in the corrected data being linear in K
eff
. The correction factor is derived by analytically determining the impact of the dimensional nature of a nuclear core on the response of excore detectors. From the application of the correction factor, changes in K
eff
, known as
Chao Yung-An
Heibel Michael David
Hill Donald James
Secker Jeffrey Robert
Kam Jack
Richardson John
Westinghouse Electric Company LLC
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