Week 2: Intrinsic Reactivity Effects

Question 1

What are the intrinsic reactivity effects present in a reactor core?

Answer 1

MTC
DTC
Voids
Burnup
Poisons

Question 2

How does fuel depletion affect reactivity?

Answer 2

Less fuel over core life, negative reactivity

Question 3

How do fission product neutron absorbers (poisons) affect reactivity?

Answer 3

Increasing concentration, negative reactivity
Decreasing concentration, positive reactivity

Question 4

How does fuel temperature affect reactivity?

Answer 4

Increasing temperature, negative reactivity
Decreasing temperature, positive reactivity

Question 5

How does moderator temperature affect reactivity?

Answer 5

Increasing temperature, negative reactivity
Decreasing temperature, positive reactivity

Question 6

How do moderator voids affect reactivity?

Answer 6

Greater voids, negative reactivity

Question 7

How does control rod motion affect reactivity?

Answer 7

Inward motion, negative reactivity
Outward motion, positive reactivity

Question 8

Define Conversion in a nuclear reactor.

Answer 8

The change of U-238 into Pu-239 via neutron absorption
Pu-239 builds up over the course of a operational cycle, eventually supplying a significant number of fission reactions to the overall neutron life cycle

Question 9

Define Excess Reactivity.

Answer 9

Nuclear reactors are loaded with more fuel than needed to achieve initial criticality
This is to compensate for intrinsic negative reactivity from burnup, poison generation, and temperature effects
Typical value is K-excess = 1.26

Question 10

How is K_excess controlled?

Answer 10

Control Rods, -8000 pcm
Soluble Boron, -20000 pcm
Burnable Poisons (IFBA), -8000 pcm

Question 11

Define Instantaneous Shutdown Margin.

Answer 11

The instantaneous amount of reactivity by which the reactor is subcritical or would be subcritical from its present condition, assuming all RCCAs are inserted minus the WSR and the MTC/DTC are at HZP conditions.

Question 12

Define the Power Coefficient.

Answer 12

The reactivity change per unit change of reactor power, assuming Tavg is on program
Design and operating limits assure that this value is always negative

Question 13

Define the Power Defect.

Answer 13

The reactivity inserted by raising core thermal power above the point of adding heat.

Question 14

What are the two primary fission product poisons?

Answer 14

Xe-135 and Sm-149

Question 15

Describe the Xe transient resulting from a power increase.

Answer 15

Initially, Xe burnout increases (less Xe, positive reactivity)
After 3 hours, Iodine production turns around and Xe increases (negative reactivity)
By 50 hours, Xe has reached a new, higher equilibrium

Question 16

Describe the Xe transient resulting from a power decrease.

Answer 16

Initially, Xe burnout decreases (more Xe, negative reactivity)
After 6 hours, Iodine production falls and Xe decreases (positive reactivity)
By 50 hours, Xe has reached a new, lower equilibrium

Question 17

Describe the Xe transient resulting from a reactor trip.

Answer 17

Initially, Xe burnout is completely removed (more Xe, negative reactivity)
After 9 hours, Iodine production falls and Xe decreases (positive reactivity)
By 90 hours, Xe production goes to 0

Question 18

Describe Samarium production and removal.

Answer 18

Produced from Pm-149, removed by neutron capture only
Therefore, negative reactivity contribution by Sm-149 is constant when the reactor is critical, and will slowly increase to an equilibrium value when the reactor is shutdown.