RBM

Question 1

What is the purpose of the RBM?

Answer 1

Prevents local power in fuel bundles surrounding a control rod which is being withdrawn from approaching MCPR limits

Question 2

When is RBM not required? WHY?

Answer 2

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.

Question 3

How does RBM receive inputs?

Answer 3

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.

Question 4

WHAT IS THE TS DESIGN BASIS FOR RBM?

Answer 4

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.

Question 5

When is RBM AUTOMATICALLY Bypassed?

Answer 5

When Rx Power <30%

When an edge rod is selected - adjacent to only 1 LPRM, therefore thermal limits will NEVER be reached.

Question 6

What are the RBM channels and power supplies?

Answer 6

RBM 7 - RPS A

RBM 8 - RPS B

Question 7

What LPRMS go to RBM 7?

Answer 7

LPRM detectors A&C

Question 8

What LPRMs go to RBM 8?

Answer 8

Detector Levels B&D of surrounding LPRMs

Question 9

How many LPRM assemblies feed RBM when central rod is selected? peripheral? edge?

Answer 9

Central is fed by 4 LPRMs
Peripheral - 3 or 2 depending on location
edge - typically only 1

Question 10

What are the INOP signals for RBM associated with the Select Matrix?

Answer 10

No rod selected - this is a SILENT rod block

More than 1 rod selected - INOP alarm and rod block

Question 11

Where does the output of the RBM Select Matrix route it’s signal?

Answer 11

LPRM outputs routed to:
4 rod display
count circuit
Downscale trip unit - then averaging circuit

Question 12

WHat is the purpose of the RBM Count circuit?

Answer 12

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

Question 13

What LPRM signal is required by Count Circuit/Downscale Trip Unit to declare it’s output OPERABLE?

Answer 13

LPRM has low setpoint of 8watts/cm^2. If less than this, it will be bypassed reducing the number of LPRMs used.

Question 14

What is the purpose of the RBM Averaging Circuit?

Answer 14

Produces a signal proportional to the thermal power produced in fuel bundles surrounding the selected control rod

Question 15

How does the RBM Averaging Circuit work?

Answer 15

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

Question 16

WHat is the RBM Gain Change Circuit?

Answer 16

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.

Question 17

What are the Gain Circuit Reference APRMs?

Answer 17

RBM Channel 7 - Normal Ref. APRM 3; Backup Ref. APRM 2

RBM Channel 8 - Normal Ref. APRM 5; Backup Ref. APRM 4

Question 18

When is the RBM Gain Circuit Backup APRM reference used?

Answer 18

When the normal is bypassed

Question 19

Once Gain Balance is achieved, when does it change again?

Answer 19

When a different rod is selected

Question 20

What happens when the gain change circuit CANNOT balance the APRM reference and the averaging circuit?

Answer 20

A no balance condition is sent to Null Sequence control circuit and yields a failure to null INOP rod block

Question 21

What is the purpose of the Nulling Sequence Control Circuit?

Answer 21

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.

Question 22

What is the RBM Slope and Bias Circuit?

Answer 22

Used to generate flow-biased trip reference signals using flow signal from APRM converter.

Question 23

What are the Dual Loop Flow Bias Setpoints?

Answer 23

Hi - 0.625W + 53%
Backup - 0.625W + 55%
Backup Clamp - 120%

Question 24

What are the Single Loop Flow Bias Setpoints?

Answer 24

Hi - 0.625W + 49%
Backup - 0.635W + 51%
Clamp - 116%

Question 25

Which APRM Flow unit is used by which RBM Channel?

Answer 25

RBM 7 uses Flow Unit 1

RBM 8 uses Flow Unit 2

Question 26

What signals are sent by the RBM Nulling Sequence Control Circuit during the nulling sequence?

Answer 26

Nulling INOP Trip - Silent alarm while Nulling occurs

Trip Inhibit - prevents spurious trip by inhibiting all trips during nulling sequence

Question 27

When is RBM Bypassed?

Answer 27

<28% Power per APRM reference output
Edge Rod selected
Manual bypass - recorder goes full downscale and BYPASS light on

Question 28

What is needed to trip an entire RBM channel?

Answer 28

place RBM Mode switch in Standby

Question 29

What happens when RBM Reference APRM fails downscale?

Answer 29

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

Question 30

What happens when RBM fails upscale?

Answer 30

RBM Hi/INOP alarm
Rod Block Alarm
Rod out Permit light goes out

Question 31

What happens when RBM fails Downscale?

Answer 31

At ~7%, recorder indication (5/125 scale per TS)
and APRM >28%:
RBM Dwnscale Alarm
Rod Block Alarm
Rod Out Permit Light goes out

Question 32

What is the RBM Downscale Trip Setpoint?

Answer 32

Averaging Circuit ~5% (5/125)

Question 33

Which RBM INOP Alarms come from the Rod select matrix?

Answer 33

No Rod Selected (silent)

> 1 Rod selected (Alarm)

Question 34

What is the power for the RBM Recorders?

Answer 34

Channel 7 - ESS Bus

Channel 8 - I/B - also powered 902-5 alarm lights

Question 35

How may RBM Power level change with CR going in?

Answer 35

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.