CRD Mechanism Flashcards
What is the purpose of the CR and CRD Mechanism?
Control Reactor Power
Provide flux profile shaping
Provide adequate negative reactivity to shut down reactor from any NOC or Accident condition
How long does it take to SCRAM?
3-5 seconds
How long is a Control Rod?
144 inches
How many positions are there? How many notches are there?
- 24.
What is different about the notch at position 48 and why is it different?
It is a double taper to allow the rod to settle and let the collet fingers go to a non stressed position.
What are the 5 tubes of the CRD Mechanism?
Outer Tube Inner Tube Index Tube Piston Tube Indicator Tube
Which of the 5 tubes moves?
The index tube
How is the Index Tube Connected to the mechanism?
Upper End threaded to drive piston
Lower End threaded to coupling spud
How do we tell the position of a Control Rod?
Reed switches in the PIP pass a magnet that is in the drive piston.
How does the CR Blade connect to the CRD Mechanism?
The spud couples to the bottom of the blade
How are scram loads absorbed and reduced during a scram?
Bellville washers in and buffer holes (act as hydraulic brake) in the piston tube
How does the CRD Withdraw and what happens to the collet fingers as a result?
The rod must move in partially, then be released. As a result the collet fingers are pushed out by the guide cap as the index tube moves down.
Where do the withdrawal and insert water headers attach?
Attached to the Pover and Punder ports(Above and belwo piston areas)
What are the purpose of vessel drive ports and how many are there per mechanism?
8 per mechanism. rout water to underside of ball check valve for scramming drive using reactor water.
How does CRD water aid in cooling when at position 48?
Rod bears on inner filter and lifts the lock plug to lift valve disc for flow path.
Why is the uncoupling rod length critical?
If rod is too long, might force spud finders out and uncouple drive; or if too short, wont allow valve disc to lift and provide cooling.
What will happen with an uncoupled drive?
Will drive out past 48 and indicate red double dashes.
What part of the CRD Mechanism is connected to the RPV and how?
Guide tube via stub welds
What is the purpose of the thermal sleeve?
Protects housing to stube weld from thermal overstress
protects from thermal stress following scram
What causes thermal overstress during a scram?
CRD Pump continues to pump cool water into drive until scram reset. If seals are worn, excessive amounts of cooling will occur in upper drive area. Cooling water flows between thermal sleeve and outer tube.
What is the Velocity Limiter?
Engineered Safeguard. Limits design basis rod fall rade to 5ft/sec if becomes uncoupled. Limits added reactivity rate.
Why do we need to limit reactivity addition rate?
To prevent fuel damage durign a rod drop accident.
Where does the velocity limit position at 48?
Rests on velocity limiter backseat forming a seal between blade and guide tube.
What will prevent an uncoupled rod from coupling?
Velocity Limiter lower female socket damaged
What do the screens of the CRD Mechanism do?
Prevent FM intrusion to:
Prevent damage to interals.
Protects scram ports and ball-check valve
What is the purpose of scram buffering and where does it occur?
Holes drilled in the piston tube that are progressively covered by index tube travel. This slows the last 10% of travel during a scram.
Minimizes force of drive piston slamming into stop position.
Also aided by bellville washers to absorb shock.
How does CRD Insert?
Drive water enters at P-under port under drive piston, at same time drive water is discharged to exhaust water header via p-over port
How does CRD Withdraw?
Weight on collet fingers must be removed by slight insertion. THen drive header is revers and aplied to p-over port and travels between piston tube and indicator tube out buffer holes to over piston area. At same time water exhausts through P-under port
How is cooling performed with no rod movement?
Flange orifice allows p-under port flow to area between outer tube and thermal sleep then through outer filter.
High temps cause seal wear acceleration
What is the drive water temperature limit?
250F to prevent damage to scram discharge valve
What is the scram flowpath?
Accumulator water and RPV water used - Accumulator starts movement, then when pressure decreases, Rx aids via ball check valve opening to allow RPV water to underpiston area.
How are scram times affected by higher drive pressure?
Decreased
How does crud build up on ball check valve impact scramming?
May prevent proper seating and slower movement due to flow past valve
What is a STUCK ROD?
One that cannot be inserted. If it can be inserted but not withdrawn, then it is not “STUCK”.
SCRAM, DRIVE, and WITHDRAWAL isolated. cooling maintained.
What is the normal drive movement speed?
3in/sec, normal stroke of 48 seconds; 53sec if timed cold
What does scramming too fast cause? and what is it caused by?
Can cause Index tube damage and bellville washer damage.
Can result from excessive accumulator pressure or worn lower stop piston seals.
Which is larger: Underpiston area or Overpiston area?
Underpiston
What is the TS pressure requirement for scram timing?
> =800psig.
What is the limit for the reactor pressure being able to scram alone without accumulator?
400psig
What can cause continuous rod drift out?
FME stuck in collet piston or wedged finger
Cracked collet housing when plant depressurized
What happens with worn drive piston seals?
Drie may not move in or out due to leakage not producing DP needed to move rod, indicated by stall flows.
What fail safe mechanism is designed upon insert line rupture?
RPV pressure will reposition ball check vale, blocking the insert line and limiting coolant loss.
