NUKEFACT #62

NRC Generic Fundamentals Examination Question Bank-PWR

ERRONEOUS EXAM QUESTIONS

REACTOR THEORY -- FUEL DEPLETION and BURNABLE POISONS

last update April 5, 2001
The NRC has recently placed the GFE question banks, both BWR and PWR, on their web site. On reviewing the questions, we find that many of the technical errors identified in the Reactor Theory section of the INPO Test Item Catalog also appear in the NRC question bank. In this Nukefact we address the questions under the section Fuel Depletion and Burnable Poisons.
QID: P864 (TOPIC: 192007 KNOWLEDGE: K1.01 [2.1/2.5])

Instead of using a higher concentration of soluble boric acid, burnable poisons are installed in a new reactor core to:

A. prevent boron precipitation during normal operation.
B. develop a less positive moderator temperature coefficient.
C. allow control rods to be withdrawn farther upon initial criticality.
D. maintain reactor coolant pH above a minimum acceptable value.

ANSWER: B.

Comment: The question is technically incorrect because there are no gradations of negativity. Negative is negative and positive is positive. The question refers to the magnitude of the moderator temperature coefficient being smaller.

QID: P1664 (TOPIC: 192007 KNOWLEDGE: K1.01 [2.1/2.5])

Burnable poisons are installed in a new reactor core instead of using a larger soluble boron concentration to:

A. prevent boron precipitation during normal operation.
B. establish a more negative moderator temperature coefficient.
C. minimize the distortion of the neutron flux distribution caused by soluble boron.
D. allow the loading of excessive reactivity in the form of higher fuel enrichment.

ANSWER: B.

Comment: The question suffers the same defect as P864. Negative is negative.

QID: P1264 (B1163) (TOPIC: 192007 KNOWLEDGE: K1.04 [3.1/3.4])

Refer to the graph of critical boron concentration for a reactor core following a refueling outage (See figure below.). Which one of the following is responsible for the majority of the rapid initial decrease in critical boron concentration?

A. Fuel depletion
B. Fission product buildup
C. Burnable poison burnout
D. Conversion of U-238 to Pu-239

ANSWER: B.

Comment: The question is technically incorrect because the indicated correct choice is too general. Fission product poisons build up throughout the entire life cycle. The primary reason for the decrease in Keff from point 1 to point 2 is the build up of steady state Samarium-149.

QID: P1563 (B1563) (TOPIC: 192007 KNOWLEDGE: K1.04 [3.1/3.4])

Refer to the graph of critical boron concentration versus burnup for a reactor core following a refueling outage (See figure below.). Which one of the following is primarily responsible for the shape of the curve from the middle of core life to the end of core life?

A. Fuel depletion
B. Fission product buildup
C. Burnable poison burnout
D. Conversion of U-238 to Pu-239

ANSWER: A.

Comment: The question is technically incorrect because the indicated correct choice is too general. Depletion of fuel is occurring throughout the life cycle. The negative reactivity change from fuel depletion exceeds the positive reactivity change from burnable poison depletion.

If you disagree with any of our comments on these questions, or would care to add further enlightenment, we would appreciate hearing from you. Our E-MAIL EXPRESS is just a click away. In the next issue of NUKEFACTS we will address questions dealing with Reactor Operational Physics.

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