Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Flux monitoring
Reexamination Certificate
1999-08-31
2002-09-24
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Flux monitoring
C376S259000, C250S382000, C250S385100, C250S390010, C250S391000, C250S392000
Reexamination Certificate
active
06456681
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a neutron flux measuring apparatus or instrument in a reactor pressure vessel in a boiling-water reactor (BWR, ABWR) of a nuclear power plant.
A neutron flux measuring apparatus is used to measure neutron flux, display the power of the reactor because the reactor power is in proportion to the neutron flux and to evaluate the burning degree of a fuel, and also used as a detecting element for protecting the nuclear reactor at a time of excess power output because of quick response to variation of the power.
This neutron flux measuring apparatus consists of a neutron flux detector and a measuring device amplifying and displaying the signal from the detector. Since the measuring range is quite wide, it is necessary to accurately measure the power output from rated power to about 10
−10
times as high as the rated power and it is, therefore, difficult to measure entire range using one type of a measuring apparatus. For that reason, start-up range neutron monitor (to be referred to as ‘SRNM’ hereinafter) detectors for measuring low power ranges and local power range monitor (to be referred to as ‘LPRM’ hereinafter) detectors for measuring high power ranges are used. A set of four LPRM detectors are vertically arranged in the reactor pressure vessel in axial direction and form a local power range monitor detector assembly as a whole.
Conventionally, eight or ten SRNM detectors are normally installed, whereas 52 LPRM detector assemblies (or 208 detectors) are installed in an ABWR. The SRNM detectors and LPRM detector assemblies are arranged separately in the reactor pressure vessel.
Now, description will be given to a case where ten SRNM detectors and 52 LPRM detector assemblies (208 detectors) are installed.
FIG. 12
shows the arrangement of SRNM detectors and LPRM detector assemblies at a reactor core in a conventional advanced boiling-water reactor (to be referred to as ‘ABWR’ hereinafter).
As shown in
FIG. 12
, ten SRNM detectors (A, B, C, D, E, F, G, H, J, L) are equally arranged at a reactor core
1
. Since one operation (arithmetic) unit is arranged for each SRNM detector, an SRNM consists of ten SRNM detectors and ten operation units.
The detectors are used as detecting elements for the protection of the reactor when the power output thereof is in excess and the detectors detect an abnormal transient change which takes place during operation, emit a reactor emergency shutdown (reactor scram) signal and shut down the reactor. To detect such an abnormal transient change, the detectors is divided into reactor protection system divisions (sections), respectively. The reactor protection system divisions are composed of double special logic structural circuits such as “1 out of 2” and “2 out of 4” and prevents unnecessary reactor shutdown due to erroneous operation and abnormal operation due to inactive operation.
The SRNM operation unit calculates a neutron flux level in the detector, calculates the increase degree of a neutron flux in the form of period (reactor period) and emits a control rod pull-out prohibiting signal or a scram signal when the calculated period is below a predetermined period, thus functioning as a safety protection system.
FIG. 13
shows that the SRNM detectors are divided into the reactor protection system divisions.
As shown in
FIG. 13
, the SRNM detectors (A, E, J) are in a division (section) I and the SRNM detectors (B, F) are in a division II. The SRNM detectors (C, G, L) are in a division III and the SRNM detectors (D, H) are in a division IV. The SRNM detectors are divided into the divisions I to IV each including tow or three ones.
Meanwhile, the LPRM consists of 52 detector assemblies (208 detectors) and 16 operation units, and the 52 detectors and the four operation units are partitioned into each of the four sections, as shown in FIG.
12
.
The LPRM operation units or average power range monitor (to be referred to as ‘APRM’ hereinafter) operation units are allotted with signals of the respective LPRM detectors in accordance with the reactor protection system divisions, standardize and calibrates the signals to local power level using signals from a TIP traversing incore probe (to be referred to as ‘TIP’ hereinafter) detector or a gamma thermometer. The signals are further fed to the APRON operation units in which signals from LPRM detectors belonging to the respective APRM channels are averaged thereby to create APRM signals. Each of the APRM operation units generates a trip signal such as a control rod pull-out prohibition and scram if the APRM signal exceeds a predetermined APRM signal level and activates scram by means of the double special logic structural circuits such as “1 out of 2” and “2 out of 4” described above.
In the meantime, it is required for the detectors such as the SRNM detectors as well as their respective operation units to be regularly inspected and maintained. While maintaining and adjusting these detectors and operation units, if it is detected that the data during adjustment is abnormal, a reactor scram signal is outputted and the reactor is shut down. For this reason, at the time of the maintenance and adjustment of the detectors or operation units themselves, the detectors or units are precluded from normal monitoring, which is referred to as bypassing. To execute bypassing, the detectors are divided into groups other than the reactor protection system divisions in the SRNM monitor.
This is because the range the SRNM detectors can monitor corresponds to the radius of the reactor core. The detector arrangement in the reactor as well as the bypass groups are set such that even if part of SRNM detectors or operation units are bypassed, two or more detectors in different reactor protection system divisions are present within a distance corresponding to the radius of the reactor from the arbitrary position of a control rod in consideration of monitoring an arbitrary range in the reactor without adversely influencing the reactor emergency shutdown function. With the ten detecors being provided, it is impossible to make the bypass groups coincident with the reactor protection system divisions.
Now, it is assumed that the bypass groups are made coincident with the reactor protection system divisions in the conventional start-up range monitor arrangement. In
FIG. 13
, if the SRNM detectors (A, F, L, D) are bypassed, there is no detector which can monitor the upper right range on the reactor core plane shown in FIG.
12
and the above conditions cannot be satisfied.
It is, therefore, necessary to set bypass groups different from the reactor protection system divisions.
FIG. 14
shows the bypass groups for the SRNM detectors.
As shown in
FIG. 14A
, the SRNM detectors (A, B, F, G) are sectioned in a bypass group {circle around (1)} and the SRNM detectors (C, E, H) are sectioned in a bypass group {circle around (2)}. The remaining SRNM detectors (D, J, L) are sectioned in a bypass group {circle around (3)}.
Further, as shown in
FIG. 14B
, since only one detector in the respective bypass groups from {circle around (1)} to {circle around (3)} is bypassed, the allowable number of bypassed detectors is up to three.
It is noted that since an operation unit is provided per detector in the SRNM monitor, detector bypassing and operation unit bypassing are carried out by the same operation.
Meanwhile, many LPRM detectors ace arranged and necessary number of detectors for monitoring average power output are arranged in the reactor for each reactor protection system division. For this reason, it is possible to make the bypass groups coincident with the reactor protection system divisions and to bypass a plurality of detectors to the extent that the number of detectors is not below the number required for APRM operation. Furthermore, even if all of the detectors belonging to an optional division (section) are bypassed, no problem occurs to the reactor emergency shutdown function as long as the remaining sections are in a state in which the average power output of the reactor
Goto Yasushi
Hirukawa Koji
Narawa Yuki
Oono Nobuaki
Tarumi Teruji
Carone Michael J.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Richardson John
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