Subcritical Reactivity Monitor Utilizing Prompt Self-Powered Incore Detectors

Abstract

A subcritical reactivity monitor that utilizes one or more primarily gamma sensitive (prompt responding) self-powered detector style radiation measurement devices located within the core of a nuclear reactor to determine the amount that the reactor multiplication factor (K.sub.eff) is below the reactivity required to achieve or maintain a self-sustaining nuclear chain reaction. This invention utilizes measured changes in the self-powered detectors' current(s) to allow a reactor operator to measure the value of K.sub.eff at essentially any desired interval while the reactor is shutdown with a K.sub.eff value less than the critical value of 1.0. This invention will enable integration of the output of the value of K.sub.eff directly into the Reactor Protection System, which will enable the elimination of the operational and core design analysis constraint costs associated with the current Boron Dilution Accident prevention methodology and enable automatic control of the Chemical Volume Control System.

Claims

1. A nuclear reactor system comprising: plurality of fuel assemblies housed within the nuclear core with at least some of the fuel assemblies having one or more primarily gamma sensitive, prompt responding self-powered neutron detectors located therein, responsive to the fission reaction activity within the core to provide an output indicative thereof; a subcritical reactivity monitor that receives the output from the one or more of the primarily gamma sensitive, self-powered neutron detectors and is responsive thereto to generate a core average value of a gamma radiation distribution within the core; a processing system configured to calculate a value of K.sub.eff in accordance with the following equation 1; $\begin{array}{cc}{K}_{\mathrm{eff}}\left(t\right)=1-\frac{f\left(t_{\mathrm{Ref}}\right).Math.I_{\mathrm{Ref}}}{f\left(t\right).Math.I\left(t\right)}.Math.\left(1-K_{\mathrm{Ref}}\right)& \left(1\right)\end{array}$ and output the value of K.sub.eff; and a Chemical Volume Control System configured to receive a triggering signal to add Boron to the coolant in the event a predetermined, undesirable change in K.sub.eff is detected.

2. The nuclear reactor system of claim 1 wherein the value of K.sub.eff is monitored substantially continuously during a shutdown period of the nuclear reactor system.

3. The nuclear reactor system of claim 1 wherein the subcritical reactivity monitor is configured to determine the fraction of the output from the primarily gamma sensitive, self-powered neutron detectors caused by gamma radiation released by fission byproducts in the vicinity of the primarily gamma sensitive, self-powered neutron detectors and, the fraction of the measured output directly caused by subcritical fission events.

5. The nuclear reactor system of claim 1 wherein the fraction of the output of the gamma sensitive, self-powered neutron detectors resulting from fission byproducts is subtracted from the output of the primarily gamma sensitive, self-powered neutron detectors to determine a real-time value of K.sub.eff.

6. The nuclear reactor system of claim 1, including a source range detector, for monitoring fission events within the core and providing an output from which the fission events can be determined, the sourced range detectors being positioned outside of the pressure vessel, wherein the value of K.sub.eff is calculated without input from the source range detectors.

7. The nuclear reactor system of claim 1 wherein the Chemical Volume Control System automatically adds Boron to the coolant in the event a predetermined, undesirable change in K.sub.eff is detected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] 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:

[0022] FIG. 1 is a simplified schematic of a nuclear reactor system to which this invention can be applied;

[0023] FIG. 2 is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which this invention can be applied;

[0024] FIG. 3 is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; and

[0025] FIG. 4 is a block diagram of an exemplary system for implementing the steps of the process of one embodiment of the system of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Commercial nuclear reactor operators are required to ensure that the reactor remains shut down by a minimum margin as defined by plant technical specifications (Tech Specs). The amount of reactor shutdown is determined via the calculated value of K.sub.eff; specifically by the amount that K.sub.eff is less than 1.0, defined as the Shutdown Margin. One current methodology for the calculation of K.sub.eff requires a number of conservative measures be included in the calculations to ensure the amount of boric acid added to the reactor coolant system bounds potential shutdown accident scenarios such as a control rod ejection, rapid reactor coolant system cool down, or unintentional dilution of the reactor coolant system boron concentration. The conservative amount of boric acid added to the reactor coolant system to ensure that K.sub.eff remains less than the limits imposed by plant Tech Spec requirements must be removed again from the reactor coolant system when it is time to restart the reactor after the shutdown. If the shutdown occurs during the period at the end of an operating cycle, it can take the addition of hundreds of thousands of gallons of pure demineralized water to remove the boron added to ensure a conservative shutdown condition. This places a huge burden on the Chemical Volume Control System 74 (FIG. 4) needed to remove and store the borated water being replaced by the pure water. It also significantly adds to the operating costs of the reactor. Implementation of the preferred embodiment of this invention will allow the Shutdown Margin to be continuously measured so the calculational uncertainties necessitating the conservative operation are eliminated so that the boric acid levels are optimized to minimize the needed Chemical Volume Control System 74 processing and storage requirements.

[0027] The K.sub.eff calculation methodology described in U.S. Pat. No. 6,181,759 utilizes an excore Source Range detector to calculate K.sub.eff following periodic control rod withdrawals during the approach to achieving a critical condition in the reactor (K.sub.eff=1). Use of the Source Range detectors requires pauses between control rod withdrawals to acquire statistically consistent count rate data sets to determine a value of K.sub.eff with a defined target accuracy. The methodology described in this invention uses the signal output from one or more prompt responding Self-Powered Detectors 70, such as a Platinum-based design, to continuously monitor the value of K.sub.eff with the reactor in a shutdown condition, up to and including achieving a critical condition. The use of a prompt responding Self-Powered Detector will allow the processed Self-Powered Detector's output signals to be used as the basis for control of a Boron-Dilution Protection System 76 that will be unaffected by changes in reactor coolant system temperature; conditions that commonly affect ex-core detector responses.

[0028] The key to the application of this approach is the ability to determine the fraction of the measured Self-Powered Detector's signal caused by the gamma radiation released by fission products in the vicinity of the Self-Powered Detector's element, and the fraction of the measured signal caused by subcritical fission events. Following a reactor shutdown the fission product concentration changes with time in a well understood manner. The impact of the gamma radiation emitted by the fission products on the measured Self-Powered Detector's signal will, therefore, change in a corresponding way. By adjusting the measured Self-Powered Detector's signal to remove the influences of the fission product gamma radiation using a simple time dependent analytical correction factor, the signal due only to the gamma radiation produced by the fission process can be calculated from the measured Self-Powered Detectors signal(s). This fission gamma signal is directly proportional to the thermal neutron population. Once the thermal neutron population on at least a relative basis is known, the value of K.sub.eff relative to a reference condition of known K.sub.eff can be determined from simple subcritical multiplication formulation using the expression:

[00002]$K_{\mathrm{eff}}\left(t\right)=1-\frac{f\left(t_{\mathrm{Ref}}\right).Math.I_{\mathrm{Ref}}}{f\left(t\right).Math.I\left(t\right)}.Math.\left(1-K_{\mathrm{Ref}}\right)$

Where f(t) is the fraction of the measured Self-Powered Detectors current at time t (I(t)) due to fission gamma radiation. The value of K.sub.Ref can be established using either the rod withdrawal method described in U.S. Pat. No. 6,181,759, where the Source Range count rate measurement data from the source range detectors 72 is replaced by the value of the fission product fraction adjusted Self-Powered Detector's current, or by calculation using a tool like the Westinghouse BEACON System to calculate the value of K.sub.eff at the reference shutdown condition from a nuclear model that captures the reactor operating history. The fact that the fission product adjusted currents are used in a ratio removes the need to perform any explicit conversions of the Self-Powered Detector's signal measurements to neutron flux units, and allows the fission product correction factor to be represented by a relative shape function. The fission product correction shape function will be well represented by the time dependent fission product gamma decay relation used in various well documented reactor shutdown heating calculations, where t.sub.Ref is the time after shutdown, and t is the time after t.sub.Ref. Since the Self-Powered Detectors detector current is inherently less statistically variable than the measurement of Source Range detector pulse rate data, the use the Self-Powered Detector's current signal data can be used on a continuous basis rather than the periodic nature resulting from pulse data statistics.

[0029] Thus, this invention utilizes one or more primarily gamma sensitive (prompt responding) Self-Powered Detector style radiation measurement devices 70 located within the core of a nuclear reactor to determine the amount that the reactor multiplication factor (K.sub.eff) is below the reactivity required to achieve or maintain a self-sustaining nuclear chain reaction. This capability provides the reactor operator with vital information on the operating state of the reactor. This invention utilizes measured changes in the Self-Powered Detector's current(s) to allow the reactor operator to measure the value of K.sub.eff at essentially any desired interval while the reactor is shut down with a K.sub.eff value less than the critical value of 1.0. Since this invention uses one or more prompt responding Self-Powered Detector's instruments contained within the reactor core, and the outputs from the Self-Powered Detectors do not require external power supply, the accuracy and reliability of the K.sub.eff information derived from this invention far exceeds the implementation described in U.S. Pat. No. 6,181,759. Moreover, a preferred embodiment of this invention will allow integration of this capability directly into the Reactor Protection System, which will allow the elimination of the operational and core design analysis constraint costs associated with the current Boron Dilution Accident Prevention methodology.

[0030] This invention provides a novel use of prompt responding in-core Self-Powered Detectors signals to provide continuous measurement of K.sub.eff in a subcritical reactor core. The primary novelty in the processing of the Self-Powered Detector's signals is the correction of the measured Self-Powered Detector's signal to remove the fission product gamma contribution to the measured signal based, well documented, decay heating functions.

[0031] 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.