This application is related to U.S. patent application Ser. No. 12/986,217, entitled SELF-POWERED WIRELESS IN-CORE DETECTOR, filed Jan. 7, 2011, concurrently herewith, now U.S. Pat. No. 8,681,920.
1. Field of the Invention
The present invention pertains generally to apparatus for monitoring the radiation within the core of a nuclear reactor and, more particularly, to such apparatus that will not obstruct refueling of the reactor.
2. Related Art
In many state-of-the-art nuclear reactor systems in-core sensors are employed for measuring the radioactivity within the core at a number of axial elevations. These sensors are used to measure the radial and axial distribution of the power inside the reactor core. This power distribution measurement information is used to determine whether the reactor is operating within nuclear power distribution limits. The typical in-core sensor used to perform this function is a self-powered detector that produces an electric current that is proportional to the amount of fission occurring around it. This type of sensor does not require an outside source of electrical power to produce the current and is commonly referred to as a self-powered detector and is more fully described in U.S. Pat. No. 5,745,538, issued Apr. 20, 1998, and assigned to the Assignee of this invention.
In previous nuclear plant designs, the in-core detectors entered the reactor vessel from the lower hemispherical end and entered the fuel assemblies instrumentation thimble from the bottom fuel assembly nozzle. In at least some of the current generation of nuclear plant designs, such as the AP1000 nuclear plant, the in-core monitoring access is located at the top of the reactor vessel, which means that during refueling all in-core monitoring cabling will need to be removed before accessing the fuel. A wireless in-core monitor that is self-contained within the fuel assemblies and wirelessly transmits the monitored signals to a location remote from the reactor vessel would allow immediate access to the fuel without the time-consuming and expensive process of disconnecting, withdrawing and storing the in-core monitoring cables before the fuel assemblies could be accessed, and restoring those connections after the refueling process is complete. A wireless alternative would thus save days in the critical path of a refueling outage. A wireless system also allows every fuel assembly to be monitored, which significantly increases the amount of core power distribution information that is available.
However, a wireless system requires that electronic components be located at or very near the reactor core where gamma and neutron radiation and high temperatures would render semiconductor electronics inoperable within a very short time. Vacuum tubes are known to be radiation insensitive, but their size and current demands have made their use impractical until recently. Recent developments in micro-electromechanical devices have allowed vacuum tubes to shrink to microscopic sizes and significantly reduced power draw demands.
Accordingly, it is an object of this invention to improve the critical path for refueling a reactor by significantly reducing the number of cables attached to the reactor head that would have to be removed and reconnected in the course of the refueling process.
It is a further object of this invention to provide a fuel assembly with a self-contained instrument thimble assembly that can be inserted into the core of a nuclear reactor and placed in operation without the necessity of routing cabling and connectors through the reactor vessel to activate the instrumentation.
It is an additional object of this invention to increase the amount of in-core power distribution data that is communicated to the plant operator.
These and other objectives are achieved by the apparatus of this invention which avoids the necessity of running expensive electrical cables through the reactor head and reactor internals to connect with and energize the in-core instrumentation. In accordance with this invention, a nuclear reactor in-core detector system is provided, including an in-core nuclear instrumentation thimble assembly that is substantially wholly contained within an instrument thimble within a nuclear fuel assembly. The instrument thimble assembly includes a self-powered, fixed, in-core detector for monitoring a reactor core parameter indicative of a state of the reactor core and providing an electric output representative of the monitored parameter. The instrument thimble assembly also includes a wireless transmitter that is connected to receive the electrical output from the self-powered fixed in-core detector and wirelessly transmit that signal to a location outside the reactor. Desirably, the wireless transmitter comprises a number of electronic components at least one of which is a vacuum microelectronic device and, preferably, a vacuum diode placed in a grid circuit of an amplifier which is connected to the electrical output of the self-powered, fixed, in-core detector and responds substantially logarithmically, thus enabling the electronic components to follow the monitored neutron flux from start-up to full power of a nuclear reactor in which the in-core detector system is disposed.
In another embodiment, in addition to the amplifier, the electronics components include a current-to-voltage converter and a voltage controlled oscillator with an output of the amplifier connected to an input of the current-to-voltage converter whose output is connected to an input of the voltage controlled oscillator that provides a frequency output proportional to a voltage on the input of the voltage controlled oscillator. In that way, the current which is the electrical output representative of the monitored parameter, which is connected to the amplifier, is converted to a corresponding frequency signal that can be transmitted by a wireless transmitter. In still another embodiment, the voltage controlled oscillator comprises a micro-electronic reactance tube.
Preferably, the electronic components comprise—an input of a first amplifier connected to the electrical output of the self-powered, fixed, in-core detector; the input of the current-to-voltage converter connected to an output of the amplifier; the input of the voltage controlled oscillator connected to the output of the current-to-voltage converter; an input of a second amplifier connected to the output of the voltage controlled oscillator; and a wireless transmission circuit connected to an output of the second amplifier for wirelessly transmitting the output of the second amplifier. Desirably, the nuclear reactor in-core detector system includes a wireless receiver circuit and signal conditioning component designed to be situated outside the highly radioactive environment of the nuclear reactor containment, and preferably, including conventional solid state components.
In still another embodiment, the nuclear reactor in-core detector system includes a wireless receiver positioned outside and within the vicinity of the reactor vessel for receiving signals from the wireless transmitter and a retransmitter for transmitting outside the containment the signals received from the wireless transmitter. Desirably, the retransmitter is a second wireless transmission circuit that transmits the signals received from the wireless transmitter to a second wireless receiver that communicates the signals received from the wireless transmitter, by way of the wireless receiver and the retransmitter, to processing circuitry outside the containment. Desirably, the second wireless receiver is positioned within the vicinity of a containment wall that shields the primary circuit of a nuclear power generation facility in which the in-core detector system is placed.
In a further embodiment, the invention comprises a nuclear fuel assembly having a top nozzle and a bottom nozzle and a plurality of thimble tubes extending between and substantially connected to the top nozzle and the bottom nozzle. At least one of the thimble tubes comprises an instrumentation thimble that houses and substantially completely contains the fixed in-core monitoring component of the detector system of this invention.
A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
The primary side of nuclear power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume a pressurizer, pumps and pipes for circulating pressurized water, the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration,
An exemplary reactor design incorporating this invention is shown in
The upper internals 54 can be supported from the vessel 40 or the vessel head 42 and includes an upper support assembly 72. Loads are transmitted between the upper support assembly 72 and the upper core plate 66 primarily by a plurality of support columns 74. Each support column is aligned above a selected fuel assembly 80 and perforation 68 in the upper core plate 66.
The rectilinearly movable control rods 56 typically include a drive shaft 76 and a spider assembly 78 of neutron poison rods that are guided through the upper internals 54 and into aligned fuel assemblies 80 by control rod guide tubes 79.
The fuel assembly 80 further includes a plurality of transverse grids 88 axially spaced along and mounted to the guide thimbles 86 (also referred to as guide tubes) and an organized array of elongated fuel rods 90 transversely spaced and supported by the grids 88. Although it cannot be seen in
As mentioned above, the fuel rods 90 in the array thereof in the assembly 80 are held in spaced relationship with one another by the grids 88 spaced along the fuel assembly length. Each fuel rod 90 includes a plurality of nuclear fuel pellets 94 and is closed at its opposite ends by upper and lower end plugs 96 and 98. The fuel pellets 94 are maintained in a stack by a plenum spring 100 disposed between the upper end plug 96 and the top of the pellet stack. The fuel pellets 94, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding, which surrounds the pellets, functions as a barrier to prevent the fission byproducts from entering the coolant and further contaminating the reactor systems.
To control the fission process, a number of control rods 56 are reciprocably movable in the guide thimbles 86 located at predetermined positions in the fuel assembly 80. Specifically, a rod cluster control mechanism (also referred to as the spider assembly) 78 positioned above the top nozzle 84 supports the control rods 56. The control mechanism has an internally threaded cylindrical hub member 102 with a plurality of radially extending flukes or arms 104 that with the control rods 56 form the spider assembly 78 that was previously mentioned with respect to
As mentioned above, in the AP1000 nuclear plant design, the in-core monitoring access is through the top of the reactor vessel, which is a significant departure from previous designs which fed the fixed in-core detector cables through the bottom of the vessel and into the fuel assembly instrument thimbles through the lower fuel assembly nozzle. The change in design means that during refueling all conventional in-core monitoring cabling will need to be removed before accessing the fuel. This invention provides a wireless in-core monitor that is wholly contained within the instrument thimble within the fuel assemblies without any tether that extends outside the core and would permit access to the fuel without going through the costly and time-consuming steps of removing and reconnecting the cabling. In accordance with this invention, the in-core instrument thimble assembly is illustrated as a block diagram in
The primary electrical power source for the signal transmitting electrical hardware is the rechargeable battery 132 shown as part of the exemplary power supply illustrated in
As can also be appreciated from
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
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