RQ-C-4814 (Core Poisons)

Question 1

Unit 1 tripped after operating for 200 days at 99.8% power. Xe-135 concentration will:

A) Rise due to Iodine decay in the core.
B) Lower, because fission production has stopped.
C) Remain stable, because I-135 and Xe-135 decay at about the same rate.
D) Lower initially, then rise because of the difference in I-135 and Xe-135 half-lives.

Answer 1

A) Rise due to Iodine decay in the core.

Question 2

Two characteristics of Xe-135 that result in it being a major reactor poison are its relatively _________ production from fission and relatively __________ absorption cross section.

a. low; large

b. low; small

c. high; small

d. high; large

Answer 2

d. high; large

Question 3

Describe the Xe-135 behavior during a power reduction from 100% down to 50%.

Answer 3

Xe-135 levels will first increase as the burn-out is reduced with the production remaining the same. It will peak about 7-8 hours after the reduction. The Xe-135 level then decreases to a new lower steady state value, settling out about 50 hours after the decrease.

Question 4

How does the equilibrium Xenon concentration at 100% power compare to that at 50% power?

Answer 4

Xe-135 level at 100% will be less than double the value at 50% power. The burn-out becomes a larger portion of the total depletion term making the equilibrium value closer to a constant.

Question 5

A reactor has been operating at 50% power for one week when power is ramped up in four hours to 100% power. Which statement best describes the new equilibrium Xenon concentration?
a. The new equilibrium Xenon will be twice the 50% value.
b. The new equilibrium Xenon value will be less than twice the 50% value.
c. The new equilibrium Xenon value will be more than twice the 50% value.
d. The new equilibrium Xenon value will remain the same since it is independent of power.

Answer 5

b. The new equilibrium Xenon value will be less than twice the 50% value.

Question 6

Two identical reactors have been operating at a constant power level for one week. Reactor A is at 50% power and Reactor B is at 100% power. If both reactors trip at the same time, Xe-135 will peak first in reactor ______, and the highest Xe-135 reactivity peak will occur in reactor _________.
a) A; A

b) A; B

c) B; A

d) B; B

Answer 6

b) A; B

Question 7

A reactor startup (S/U) to full power starts five hours after a trip from full power equilibrium conditions. If a 2%/min ramp were used rather than a 0.5%/min ramp, the Xenon peak would occur ________, and the magnitude of the peak would be _________.

a) sooner, larger

b) later, larger

c) sooner, smaller

d) later, smaller

Answer 7

c) sooner, smaller

Question 8

The reactor is near the end of its operating cycle. To stay critical, power, and temperature have been allowed to “coastdown.” Why is Boron dilution no longer used to compensate for fuel depletion?

a. Boron concentration has become so low that very large amounts of water must be added to produce a small change in Boron concentration.
b. The reactivity worth of the Boron has decreased to such a low value that very large amounts of water must be added to produce a small change in reactivity.
c. Boron concentration has become so high that very large amounts of Boron must be added to produce a small change in Boron concentration.
d. The reactivity worth of the Boron has increased so much that the operator cannot safely control the reactivity changes via Boron dilution.

Answer 8

a. Boron concentration has become so low that very large amounts of water must be added to produce a small change in Boron concentration.

Question 9

During a six-month period of continuous full power reactor operation, the reactor coolant Boron concentration must be decreased steadily to compensate for:
a. Buildup of fission product poisons and decreasing control rod worth
b. Fuel depletion and buildup of fission product poisons
c. Decreasing control rod worth and burnable poison burnout
d. Burnable poison burnout and fuel depletion

Answer 9

b. Fuel depletion and buildup of fission product poisons

Question 10

Select the statement which most accurately describes how the amount of burnable poison in the core changes from beginning to end of cycle.

a. Increases continually
b. Increases briefly, then levels off
c. Decreases continually until fully depleted
d. Decreases briefly, then levels off

Answer 10

c. Decreases continually until fully depleted

Question 11

The amount of boric acid required to increase the coolant Boron concentration by 50 ppm at BOL (1,200 ppm) is approximately _____________ as the amount of boric acid required to increase Boron concentration by 50 ppm at EOL (100 ppm).
a. twelve times as large
b. eight times as large
c. four times as large
d. the same

Answer 11

d. the same

Question 12

At EOL, a certain amount of pure water is required for a 20 ppm decrease in Boron concentration (from 100 ppm to 80 ppm). The amount of pure water required for a 20 ppm decrease at BOL (from 1000 ppm to 980 ppm) is approximately ____________ as the EOL amount.
a. one-tenth as large
b. one-fifth as large
c. one-half as large
d. the same

Answer 12

a. one-tenth as large

Question 13

A reactor has been operating at 100% power for 2 weeks. Power is then decreased over a 1-hour period to 10%.

Assuming manual rod control, which one of the following operator actions is required to maintain a constant reactor coolant temperature at 10% power during the next 24 hours?
a. Insert negative reactivity during the entire period.
b. Insert positive reactivity during the entire period.
c. Insert positive reactivity, then negative reactivity.
d. Insert negative reactivity, then positive reactivity.

Answer 13

c. Insert positive reactivity, then negative reactivity.