Control Rods

Question 1

Typical materials used for control rods

Answer 1

Silver
Indium
Cadmium
Boron
Hafnium

Question 2

How are 4 loop Westinghouse rods typically designed?

Answer 2

Cylindrical. Cylinder fits inside guide tubes w/in fuel assembly matrix.
53 full length CRs called Rod Control Cluster Assemblies (RCCAs)
Each RCCA has 24 absorber rods.

Question 3

How many RCCAs are typically in 2 and 3 loop Westinghouse plants?

Answer 3

33 and 45 for 2/3 loop respectively.

Question 4

What are Westinghouse RCCAs typically made of?

Answer 4

Silver-Indium-Cadmium alloy. Ag-In-Cd

Question 5

Typical Combustion Engineering Control Rod design

Answer 5

Who cares

Question 6

What factors of the 6 factor formula is primarily affected by Control Rods?

Answer 6

Thermal Utilization Factor
Thermal Non Leakage Probability

Note:
Thermal Non Leakage Probability is also affected but since commercial reactors are considered to be “infinite” this is negligible and not tested by NRC.

Question 7

How is Thermal Utilization Factor affected by Control Rods?

Answer 7

Withdrawal removes strong neutron absorber which allows more thermal neutrons to reach the fuel and be “utilized”. Therefore positive reactivity is added as thermal utilization rises with Rod withdrawal.

Question 8

How are control rods arranged?

Answer 8

In banks/groups. (S/D and Control banks).
Typical Westinghouse design has 4 Control and 2-5 S/D banks.

Question 9

How do rods affect power in power range?

Answer 9

Withdrawal adds positive reactivity. W/ no change in steam demand, MTC and FTC add negative reactivity until stable.
Rx power increases temporarily and then stabilizes at original level.
Opposite is true for Rod insertion.

Question 10

Typical amount of negative reactivity added upon Rx trip by all control rods.

Answer 10

6000-8000 pcm
Rx power immediately drops to 6-7% rated thermal power. (decay heat)

Question 11

Define Control Rod Worth.

Answer 11

Effectiveness of a specific control Rod in absorbing neutrons.

Question 12

How does neutron flux affect control rod worth?

Answer 12

If flux is higher near the tip, worth is more (middle of core)
If flux is lower near the tip, worth is less (top/bottom of core)

Question 13

How does rod location affect rod worth?

Answer 13

Neutrons at the edge of the core tend to leak out more and neutrons near installed poisons tend to get absorbed more by the poisons, therefore flux at these regions is naturally lower and rods in the same locations have lower worth.

Question 14

What is differential rod worth?

Answer 14

Change in reactivity per unit change in rod height.
DRW=C•[flux(tip)/flux(avg)]•imprtance factor.
DRW is proportional to [flux(tip)/flux(avg)]^2 or
[flux(final)/flux(initial)]^2

Question 15

What is Integral Rod Worth?

Answer 15

Total reactivity added over an entire rod movement.
IRW=DRW(ave)•steps moved

Note: DRW is the slope of IRW

Question 16

Describe the shape of DRW curve.

Answer 16

From bottom to top of core, DRW is low at start and increases until rod is at the middle of the core where DRW peaks. From there DRW decreases until the rod is at the top of the core where DRW is again at a minimum.

Question 17

Describe IRW curve

Answer 17

Maximum negative with rods fully inserted down to minimum (0) with rods fully withdrawn (for rod insertion. Withdrawal is opposite). Slope is smallest at bottom and top of core and largest while rods are traveling thru middle of core.
Shows total amount of reactivity added (positive or negative) as rods are withdrawn or inserted respectively.

Question 18

How is rod worth affected by T(mod)?

Answer 18

As T(mod) rises, density lowers and more neutrons are able to travel to adjacent control rods. Therefore CRW is more negative at higher temps.

Question 19

How do Fission Product Poisons affect CRW

Answer 19

Xe135 building in at the bottom of the core would tend to shift neutron flux upward toward control rods thereby making DRW higher.
Assume homogenous Xe135 in test questions unless stated otherwise.
When homogenous, Xe135 is more competition w/rods and more tends to make DRW less negative.

Question 20

How does Boron Concentration affect DRW?

Answer 20

Boron is competition w/rods for neutrons and with higher boron concentration, DRW is less negative.
Therefore DRW becomes more negative over core life as boron concentration lowers.

Question 21

How does flux change over core life?

Answer 21

Shifts from higher at the middle at BOL to higher towards the edge at EOL.
With this, DRW is greater in rods near edge at EOL.

Question 22

How does Power Level affect DRW?

Answer 22

At Higher power, fuel temp and mod temp rise. Xe135 stabilizes higher and boron concentration is reduced. Flux shifts down as delta T rises. Higher FTC and Xe135 and lower boron lowers DRW. Higher T(mod) and flux raise DRW. Not tested by NRC.

Question 23

What is rod shadowing?

Answer 23

How the reactivity worth of a control rod is affected by the presence of other control rods.

Question 24

What is a Bare (Unreflected) reactor with respect to power distribution?

Answer 24

Flux is minimal at edges, top and bottom. Flux is maximum in the middle/center of the core.
Flux curve looks like a hump for both radial and axial flux distribution.

Question 25

What is a Reflected Reactor?

Answer 25

A portion of neutrons realistically are “reflected” back into the core by moderator at the edge and raises flux slightly near the edge.

Question 26

What is a Heterogeneous Reactor?

Answer 26

Most realistic type of reactor.
Fuel, Control Rods, Moderator, etc. are all contained in the core and are separate entities that affect flux distribution.

Question 27

Describe Flux Shaping.

Answer 27

Method of rod operation used to control radial and axial neutron flux distribution.
Rod sequencing

Question 28

Describe Bank Overlap.

Answer 28

Also part of sequencing. Different banks move at the same time to provide even reactivity addition throughout the core.

Question 29

What are Rod Insertion Limits?

Answer 29

Based on power. Rods are only allowed to be inserted so far as to ensure adequate shutdown margin, reduce effects of rod ejection and keep Axial Flux Difference (AFD) acceptable.

Question 30

Operator responsibilities with control rods.

Answer 30

1: Operate rods with proper bank overlap.
2: Maintain rods above rod insertion limits.
3: properly position rods to maintain AFD within allowed operating range.
4: Maintain all rods w/in specified tolerance (QPTR)
5: Move rods at proper speed and sequence.