Nuclear reactor rod drop time testing method

Details Nuclear reactor rod drop time testing method Nuclear reactor rod drop time testing method

2000-03-22

2002-06-11

Jordan, Charles T. (Department: 3641)

Induced nuclear reactions: processes, systems, and elements

Testing, sensing, measuring, or detecting a fission reactor...

C376S228000, C376S258000, C376S259000

Reexamination Certificate

active

06404835

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for measuring the drop time for the control and shutdown rods of a nuclear reactor. Specifically, the present invention relates to a method for measuring the drop time for the control and shutdown rods of a nuclear reactor where the coils in the rod position indication system remain energized. In addition, the present invention provides test results, test traces and overlay capability and allows one to measure the drop time of all control and shutdown rods in one test thereby reducing test time which provides substantial economic benefits to the plant.
2. Description of the Related Art
In a nuclear power plant, electricity is produced from heat that is generated in a reactor core. The heat is thermally transferred from the reactor core to water flowing through a water cooling system. The heater water passes through a heat exchanger where it produces steam to drive a turbine which generates electricity.
FIGS. 1 and 2
illustrate the relationship between a nuclear reactor
10
and the control and shutdown rods
12
. The reactor core
14
is a chamber which contains the nuclear fuel. The operation of the nuclear reactor
10
is governed by a set of rods
12
which are inserted into and removed from the reactor core
14
. The rods
12
are formed from a material which absorbs the radiation emitted by the nuclear fuel. To start the reactor
10
, the rods
12
are removed from the reactor core
14
allowing the nuclear reaction to occur. While the reactor
10
is operating, the rods
12
are kept above the reactor core
14
, as illustrated in FIG.
1
.
In case of a nuclear mishap, the reactor
10
must be shut down as quickly as possible. To accomplish this, the rods
12
are inserted into the reactor core
14
to stop the nuclear reaction. The emergency insertion of the rods
12
is accomplished by releasing the rods
12
to gravity. The time required for each rod
12
to traverse the distance from the top of the reactor core
14
to the bottom of the reactor core
14
and come to rest in a dashpot
16
provided at the bottom of the reactor core
14
is referred to as the “drop time.” The drop time is critical and must be measured periodically, typically after refueling or maintenance of the reactor, to ensure that the rods fall within a specified amount of time.
FIG. 3
illustrates a conventional rod position indication system
20
. The rod position indication system
20
includes a number of circular coils
22
encircling a rod housing
24
which guides the drop path of the rod
12
. During normal operation of the reactor
10
, the circular coils
22
are energized. However, in the prior art, drop time measurements require a special procedure wherein the coil power is turned off. Once the power is off, the rods
12
are moved to the position above the reactor core
14
and released. As the rod
12
moves through the coils
22
, the coils
22
generate a signal representative of the speed at which the rod
12
falls. This signal is recorded and used to calculate the drop time for each rod
12
.
The prior art method is encumbered with a number of limitations. First, drop time measurements can not be taken automatically in the event of an actual mishap or a reactor trip. Second, taking drop time measurements using the prior art method could only occur after the nuclear reactor has been intentionally shut down for refueling or maintenance. Then, a lengthy procedure is employed to turn off the coils for testing purposes. This results in unnecessary wear and tear on the switchgears controlling the coil power. The reliability of these switchgears is of concern in nuclear power plants. Finally, test results are not available in real time with the prior art method nor are the test results calculated automatically and printed out with overlay capability. The operator does not have any indication of the rod position while the test is in progress.
Accordingly, there is a need for a method for measuring the drop time of the control and shutdown rods in a nuclear reactor with the coil power remaining on. The method need be capable of producing contemporaneous information of the rod position while the test is in progress and allow testing of all control and shutdown rods in one test as well as analyzing the test data automatically and producing results automatically in terms of rod drop traces that are plotted and rod drop time results that are automatically tabulated. Further, the method should reduce the wear and tear placed upon the switchgears controlling the coil power and reduces the time that is required to perform the rod drop tests. This reduction in test time is important to the plant from an economic standpoint.
Therefore, it is an object of the present invention to provide a method for measuring the drop time of control and shutdown rods in a nuclear reactor with the rod position indication system coils remaining energized.
It is another object of the present invention to provide a method for measuring the drop time of control and shutdown rods in a nuclear reactor which provides rod position information to the operator while the test is in progress.
It is a further object of the present invention to provide a method for measuring the drop time of control and shutdown rods in a nuclear reactor which does not place unnecessary wear and tear on the components of the nuclear reactor, does not require a lengthy procedure to turn off the power to rod position indication coils, and produces the test results automatically upon completion of the tests.
BRIEF SUMMARY OF THE INVENTION
A method for measuring the drop time of a control and/or a shutdown rod in a nuclear power plant is disclosed. The drop time of rods in nuclear reactors is measured with the coils remaining energized. The signal generated at the coils while the rod passes through the rod housing includes both the rod drop trace and the coil power. By applying a filter, the rod drop trace is separated from the coil power producing the same drop time results as would be obtained using the prior art method. Those skilled in the art will recognize that the filter can be either analog, digital, or a combination of both and be accomplished using either hardware or software. Once the rod drop trace has been isolated from the coil power, the drop time is calculated and a report is generated which may include the date and time of the event, the measured signal at the coils, the filtered rod drop trace, and the calculated drop time.
By taking the measurement while the coils encircling the rod housing remain energized, the method allows drop time measurements to be taken automatically in the event of any reactor trip. Additionally, unnecessary wear and tear on the switchgears controlling the coil power is avoided by eliminating the need to turn off the coil power. Finally, test results are available in real time while providing the operator with an indication of the rod position while the test is in progress.


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Hashemian, H. M, and Fain, R. E., “Experience With Measurement of Drop Time for Control and Shutdown Rods and Testing of CRDMs in Nuclear Power Plants,” pp. 259-271, 1995.*
Fain, R. E., Petersen, K.M., and Hashemian, H. M, “New Equipment for Rod Drop and Control Rod Drive Mechanism Timing Test in PWRs,” pp. 371-381, 1995.*
Westinghouse Electric Corporation Descriptive Bulletin Nuclear I&C No. DB21N-504 “Automatic Multiple Rod Drop Test System,” 1993.*
Analysis and Measurement Services Corporation Technical Bulletin No. RD9401 “Rod Drop and CRDM Timing Tests in PWRs

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