Nuclear Instrumentation

The following descriptions are an example of the detail which the Exitech Nuclear Instrumentation models achieve.

Local Power Range Monitor

The Limerick Local Power Range Monitor (LPRM) consists of forty-three LPRM neutron detector string assemblies located in various core positions, with each detector string having four LPRM neutron detectors. Each of the 172 detectors is grouped in one of eight channels: LPRM groups A or B, or APRM channels A through F. Each channel is composed of either twenty-one or twenty-two LPRM in-core neutron detectors. The plant LPRM system receives inputs from the neutron detectors and provides outputs to the computer, the Rod Block Monitor (RBM) selection matrix, to the Average Power Range Monitor (APRM) system, and to upscale and downscale alarm trip circuits. The simulated LPRM system (see Figure 3.1) outputs to the APRM and alarm trip circuits; however the selection matrix interface is handled by the APRM system. The following discussion concerns the processing of the individual LPRM neutron detector signals and the processing of outputs within the LPRM system. Reference C51-1080-E-011, 13, 15, 17, 19, 21, 27, and 28.

Each LPRM in-core neutron detector (an ionization chamber) in the plant is connected to a signal string containing an ion chamber power supply and flux amplifier, all in series. The neutron flux will ionize the gas in the ionization chamber, changing the resistance between two electrodes. The current between the electrodes will affect the voltage output from the flux amplifier. This voltage output (which will actually be a normalized variable in the simulated system) will then be sent to the computer, RBM, upscale and downscale trip circuits, and APRM system.

The upscale trip circuit is modeled in the LPRM system and compares the output of the flux amplifier to a reference voltage. When the reference voltage is exceeded the following will happen : the trip circuit is actuated; a manually reset trip circuit will light an upscale light on backpanel P607 (for APRM F); an auto reset trip which goes to the annunciator, and an auto reset trip which lights an upscale light on panel 649. The manual reset trip can be reset from backpanel P607, for APRM F. The operation of the downscale trip circuit is the same as the upscale trip circuit, except that the circuit is tripped when the flux amplifier output becomes less than the downscale reference voltage.

Each LPRM card in the plant has an BYPASS/CALIBRATE/OPERATE switch for that particular LPRM. The OPERATE position will connect the LPRM to its normal configuration. The CALIBRATE and BYPASS positions will connect the flux amplifier input to a calibration current source; 0 volts will be connected to the RBM and computer outputs, and -15 volts will be connected to the meter function switch S3. Meter function switches S2 and S3 will select a particular LPRM to be displayed on the LPRM meters. If the selected LPRM is in BYPASS or CALIBRATE mode, then -15 volts is provided to the calibrator and the calibrator provides a calibration current to the selected LPRM. A summary of the plant switches and potentiometers for each LPRM is as follows :

The above explanation of the LPRM card and associated outputs was given in reference to plant operation. In the simulator, the LPRM cards are not mounted in backpanel P607. The bypass function of the LPRM cards are simulated with remote functions 12, 13, and 14.

 

Average Power Range Monitor

The APRM system module will include simulation for two primary subsystems : the APRM subsystem and the Flow Unit subsystem. Each is explained separately.

APRM Subsystem

The plant APRM subsystem consists of APRM channels A through F. Each APRM channel takes the signals (twenty-one or twenty-two) from the associated LPRM flux amplifier and processes those signals, along with test signals from backpanel P607. The simulated APRM system includes not only APRM channels A through F, but also LPRM groups A and B after the flux amplifier. All 172 flux amplifier voltage outputs are normalized and are simulated in the LPRM module. The LPRM module also includes computer outputs, RBM outputs, and alarm outputs which come from the LPRM cards. The APRM module receives the flux amplifier output from the LPRM module, and simulates the following :

Each LPRM in the operate mode will be connected to the APRM averaging circuits, card Z29 and Z30 in the plant. A minimum of fourteen LPRMs are required for the APRM averaging circuit to be functional. The averaged power output from the averaging circuit goes, through aux unit Z31, to the APRM trip circuits (quad trip unit Z35), to the Rod Block Monitor channels, and to the computer, recorders, and meters. Trip circuits in turn will go to control relays which will provide signals to the Reactor Protection System (RPS), to Reactor Redundant Control Systym (RRCS), and to alarm circuits. The count circuit is a part of aux unit Z31 and is also simulated. The total count of all LPRMs in an APRM are put together and output to (1) APRM channel mode selector switch to APRM meter, and (2) INOP trip circuit on Z35. The APRM meter will read the total number of LPRMs in OPERATE position when the P607 selector switch S3 is selected to COUNT. Each operating LPRM gives 5% of total indication. The output provided to the INOP trip circuit will be reduced below the acceptable limit (non-adjustable) if less than fourteen LPRMs are being averaged in the APRM. See Figure 3.2 for the averaging circuit.

Each of six APRM channel (and not the two LPRM channels) receives an input signal of 0 to 125% flow from the flow channels of the Flow Unit subsystem. The lowest of the values is selected in the low auctioneer circuit Z34 (bypassed flow channels will go in with a high signal so that low auctioneer will choose the unbypassed channel). The output from the low auctioneer goes through the APRM mode selector switch on backpanel P607 to the flow control trip reference unit Z33, from which outputs go to the upscale alarm quad trip unit Z35A, to the upscale neutron trip quad trip unit Z35B, and to the power range recorders (if so switched). Reference voltages for the quad trip circuits are calculated using two types of adjustment, slope and level. The trip reference circuit uses the trip setdown; if the reactor mode switch is in any position other than run, the dc reference voltage from the trip reference unit is reduced, representing lower power levels to the upscale neutron trip and upscale alarm trip circuits. See Figure 3.3 for the trip reference circuit.

In the plant there are two types of channel trip circuits. One type is actuated from the flux amplifier output on each LPRM card, and one type is actuated from the APRM averaged output signal. The simulated APRM channel trip circuits consist of the Quad Trip card trip circuits Z35A,B,C,D, all of which are simulated. The outputs from the both the quad trip units go to panel lights, and also to relays which go to RPS and annunciators and the computer. See Figure 3.2 for the quad trip circuits.

Each APRM and LPRM channel has a backpanel P607 meter to display selected LPRM output. Only APRM F and LPRM A have a simulated meter; see Figure 3.5. The meter/switch functions are simulated as follows (switch designations may be found in GEK-75715, "Power Range Neutron Monitoring System Operation and Maintenance Instructions") :

The following switches are part of meter panel Z36

There are two bypass switches on panel 603 which will bypass one and only one APRM channel. One bypass switch will bypass either APRM channels A, C, or E, and the second bypass switch will bypass either APRM channels B, D, or F. The APRM C and D averaged core power is output to RBM channel A and B respectively. When the particular APRM channel is bypassed, the RBM channel will read APRM E (instead of C) and APRM F (instead of D). See Figure 3.4 for bypass signals.

The LPRM card upscale and downscale trip circuit outputs, and quad trip module trip circuit outputs will control relays contained on the plant ICPS page. All of these outputs are modeled in the APRM module, except that the LPRM card outputs and associated relays K13 and K15 actuation are modeled in the LPRM module. The upscale, downscale, and inop circuits in the APRM system are applied in series with the RBM upscale, downscale, and inop circuitry to provide rod withdrawal blocks to the Reactor Manual Control System (RMCS). If any of the rod withdrawal inhibit relays are deenergized, then a rod block is imposed by the RMCS system and cannot be cleared until the cause in the APRM or RBM system is cleared. All other interfaces, including RPS, RRCS, annunciator, computer, and lights, are simulated. See Figure 3.4 for APRM aux relays.

Flow Unit Subsystem

The Flow Unit subsystem is composed of four flow channels, A through D. Each flow channel has flow transmitters, a flow unit (ARxx), associated lights and indicators, and bypass logic (resident in panel 603). The flow unit has a square root converter, a summer, trip circuitry, and amplifiers. Instead of doing a square root conversion of the recirc differential pressure, the simulated system will directly use a flow variable. The summer will pass the summed flow (after various mode switch and interlock processing) on to the low auctioneer circuit in the APRM subsystem. The summed flow will also go to the upscale trip circuit and the comparator circuit in the flow unit. The comparator circuit compares the output of its flow unit with the output of the paired flow unit (the summed flow from A goes to B, B to D, D to C, C to A). The circuit provides trip outputs when the difference between the paired units exceeds a predetermined setpoint. If a flow unit is bypassed, the signal which is input to it will be passed to the receiving flow unit (for example, if channel D is bypassed, then the summed signal from flow unit B will be passed to flow unit C for comparison). The output from the comparator, as well as the upscale trip, will actuate relays which give a rod block. The signals from these two will also go to lights, alarms, and the computer. See Figure 3.6 for the flow channel simulation diagram.

 

Rod Block Monitor

The RBM system provides in-core monitoring during power operation. The system provides the operators with accurate and timely information pertinent to its status. This information is provided on a continuous basis so that the operator can have a high degree of confidence that the RBM function is available and operating properly.

The RBM channels operate when the reference Average Power Range Monitor (APRM) is indicating greater than 30 percent. Following a new rod selection, the RBM selects and averages the Local Power Range Monitors (LPRM) around the newly selected control rod. A trip signal is generated if this average LPRM value is greater than the power biased trip setpoint. Note, either of the two channels may be bypassed at any time.

The RBM system includes signal conditioning equipment, averaging circuits, trip circuits, power supplies, a channel bypass switch, and readout equipment. The trip reference of the RBM trip unit is set to ensure that the Neutron Monitoring System (NMS) can block a control rod withdrawal in response to high local neutron flux during reactor power operation. This could result from a control rod withdrawal due to an operator error or equipment malfunction, to which RBM signals respond and prevent local fuel damage and/or a Reactor Protection System (RPS) scram if allowed to proceed.

The software effecting the APRM, RBM, & Technical Specification/Maximum Extended Load Line Limit Analysis (ARTS/MELLLA) modification will be included in this upgrade and by replacing the existing flow biased trip signals with power dependent trip setpoints. Also, some unnecessary indication will be deleted.