RBM Flashcards
What is the purpose of the RBM?
Prevents local power in fuel bundles surrounding a control rod which is being withdrawn from approaching MCPR limits
When is RBM not required? WHY?
in LOW power operation (<30%), even the worst withdrawal of one control rod would not result in violating MCPR limits, therefore RBM is not required.
How does RBM receive inputs?
When the RMCS selects a control rod, the inputs from the surrounding LPRMs is routed to the RBM averaging circuit.
This means that no specific LPRMs are assigned for the RBM, but are selected based on CR location.
WHAT IS THE TS DESIGN BASIS FOR RBM?
RBM is designed to prevent violation of MCPR safety limit and the cladding 1% plastic strain fuel design limit that may result from a single control rod withdrawal error event.
When is RBM AUTOMATICALLY Bypassed?
When Rx Power <30%
When an edge rod is selected - adjacent to only 1 LPRM, therefore thermal limits will NEVER be reached.
What are the RBM channels and power supplies?
RBM 7 - RPS A
RBM 8 - RPS B
What LPRMS go to RBM 7?
LPRM detectors A&C
What LPRMs go to RBM 8?
Detector Levels B&D of surrounding LPRMs
How many LPRM assemblies feed RBM when central rod is selected? peripheral? edge?
Central is fed by 4 LPRMs
Peripheral - 3 or 2 depending on location
edge - typically only 1
What are the INOP signals for RBM associated with the Select Matrix?
No rod selected - this is a SILENT rod block
More than 1 rod selected - INOP alarm and rod block
Where does the output of the RBM Select Matrix route it’s signal?
LPRM outputs routed to:
4 rod display
count circuit
Downscale trip unit - then averaging circuit
WHat is the purpose of the RBM Count circuit?
Ensures that at least 50% of assigned LPRM inputs are operable to ensure adequate monitoring of area around rod.
If less than 50% - INOP Trip rod block
What LPRM signal is required by Count Circuit/Downscale Trip Unit to declare it’s output OPERABLE?
LPRM has low setpoint of 8watts/cm^2. If less than this, it will be bypassed reducing the number of LPRMs used.
What is the purpose of the RBM Averaging Circuit?
Produces a signal proportional to the thermal power produced in fuel bundles surrounding the selected control rod
How does the RBM Averaging Circuit work?
It totals and averages the input LPRM signals. The gain circuit provides a gain to make sure that the averaging circuit is greater than or equal to the reference APRM output
Output sent to trip reference level select
WHat is the RBM Gain Change Circuit?
Used to adjust gain of the averaging amplifier so that RBM output is equal to or greater than reference APRM output.
Gain circuit tries to balance the APRM reference and the averaging circuit.
What are the Gain Circuit Reference APRMs?
RBM Channel 7 - Normal Ref. APRM 3; Backup Ref. APRM 2
RBM Channel 8 - Normal Ref. APRM 5; Backup Ref. APRM 4
When is the RBM Gain Circuit Backup APRM reference used?
When the normal is bypassed
Once Gain Balance is achieved, when does it change again?
When a different rod is selected
What happens when the gain change circuit CANNOT balance the APRM reference and the averaging circuit?
A no balance condition is sent to Null Sequence control circuit and yields a failure to null INOP rod block
What is the purpose of the Nulling Sequence Control Circuit?
Controls the sequence of events necessary to reset RBM upon selection of another rod. Sequence takes about 0.5 sec
During the nulling, a silent rod block occurs - seen on rod out permit light delay after rod is selected.
What is the RBM Slope and Bias Circuit?
Used to generate flow-biased trip reference signals using flow signal from APRM converter.
What are the Dual Loop Flow Bias Setpoints?
Hi - 0.625W + 53%
Backup - 0.625W + 55%
Backup Clamp - 120%
What are the Single Loop Flow Bias Setpoints?
Hi - 0.625W + 49%
Backup - 0.635W + 51%
Clamp - 116%
Which APRM Flow unit is used by which RBM Channel?
RBM 7 uses Flow Unit 1
RBM 8 uses Flow Unit 2
What signals are sent by the RBM Nulling Sequence Control Circuit during the nulling sequence?
Nulling INOP Trip - Silent alarm while Nulling occurs
Trip Inhibit - prevents spurious trip by inhibiting all trips during nulling sequence
When is RBM Bypassed?
<28% Power per APRM reference output
Edge Rod selected
Manual bypass - recorder goes full downscale and BYPASS light on
What is needed to trip an entire RBM channel?
place RBM Mode switch in Standby
What happens when RBM Reference APRM fails downscale?
When ref APRM reaches ~28%, associated RBM automatically bypassed and RBM recorder goes downscale. Associated RBM INOP - RBM thinks power <30%
Manual Bypass downscale APRM to use alt. reference APRM
What happens when RBM fails upscale?
RBM Hi/INOP alarm
Rod Block Alarm
Rod out Permit light goes out
What happens when RBM fails Downscale?
At ~7%, recorder indication (5/125 scale per TS) and APRM >28%: RBM Dwnscale Alarm Rod Block Alarm Rod Out Permit Light goes out
What is the RBM Downscale Trip Setpoint?
Averaging Circuit ~5% (5/125)
Which RBM INOP Alarms come from the Rod select matrix?
No Rod Selected (silent)
> 1 Rod selected (Alarm)
What is the power for the RBM Recorders?
Channel 7 - ESS Bus
Channel 8 - I/B - also powered 902-5 alarm lights
How may RBM Power level change with CR going in?
Is possible for power to actually show it going up because CR still selected the gain does not change.
As CR goes in, the “covered” detectors now read low and are kicked out by averaging circuit - so gain is applied to a “higher average” so power on RBM shows going up until deselected and reselected.
