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Cook GFES > Fission Product Poisons > Flashcards

Fission Product Poisons Flashcards

1
Q

What is the most significant fission product poison?

A

Xe-135.
-2700 pcm at 100% power.
-5000 pcm at peak following trip from 100% power.
Produces numerous operational issues.

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2
Q

What is the second most significant fission product poison?

A

Sm-149
-700/-1000 pcm at full power (BOL/EOL)
-1100/-1400 pcm at peak equilibrium after shutdown (BOL/EOL).

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3
Q

How do fission product poisons affect the neutron life cycle?

A

More poison, more absorption of thermal neutrons in poison, thermal utilization factor decreases.

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4
Q

What are the microscopic cross sections for absorption for Xe-135 and Sm-149?

A

Xe-135: 2.6x10^6 barns (huge)
Sm-149: 4.0x10^4 barns (pretty big)

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5
Q

Differences between Xenon and Samarium.

A

Xenon reactivity changes are faster and larger.
Xenon decays to 0 over 3 days after shutdown.
Samarium essentially doesn’t decay and remains in the core forever.

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6
Q

How is Xenon produced?

A

Directly from fission. 0.3% of fission
Indirectly from B- decay of I-135. 5.6% of fission.
Net effect: 5% production from fission, 95% production from decay of I-135.

(I-135 comes from B- of Sb-135 to Te-135 to I-135 which happens in about 20 seconds, therefore Sb and Te are negated when looking at Xe production).

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7
Q

How is Xe-135 removed?

A

Decay. Xe-135 half life is 9.1 hours
Burnout. Neutron absorption by Xe-135.

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8
Q

How is equilibrium Xenon affected by power level?

A

Higher flux removes more Xe thru burnout and also produces more Xe thru decay of I-135.
As power is changed, the production and removal components change which causes changes in Xe concentration until a new equilibrium is reached.

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9
Q

When is decay of Xe at equilibrium vs burnout the most dominant removal process?

A

At 100% power, burnout encompasses about 75% of Xe removal.
Below 10% power decay becomes the dominant removal factor due to much lower flux.

The 2 removal terms are approximately equal at 30% power.

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10
Q

What is equilibrium Iodine and how long does it take to get there?

A

Constant Iodine-135 concentration achieved at a certain stable power level.
Takes 20-25 hours.

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11
Q

What are production and removal mechanisms of I-135?

A

Production: decay of Sb and Te fission products. (~20 seconds)
Removal: Decay if I-135 (6.6 hour half life).

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12
Q

How does the magnitude of equilibrium xenon change over various power levels?

A

Eq [Xe] at 25% is ~50% of the 100% power equilibrium.

Eq [Xe] at 50% is~70% of the 100% power equilibrium.

This concept is due to the increasing burnout removal as flux is increased.

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13
Q

How does the rate at which Eq [Xe] is achieved change at different power levels?

A

Eq [Xe] is reached quicker at higher power levels due to the faster production at higher power.

~40 hours from S/U to full power
~44 hours from S/U to 50% power
Up to 48 hours for lower power levels

NRC questions all assume 40-50 hours for all scenarios.

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14
Q

What is Transient Xenon?

A

How xenon concentration changes when power changes until equilibrium is achieved.

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15
Q

How does [Xe] respond to a trip from 100% power?

A

Initially increase (burnout<production). Peak at 10 hours, then decrease (production is now < decay). At 24 hours concentration is back to prior 100% equilibrium concentration.
After 3 days from trip (70-80 hours), [Xe] is effectively 0.

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16
Q

How does [Xe] respond to S/U and power rise to 100% power?

A

Initially xenon free. Production>removal (no decay yet and burnout is low with low flux).
[Xe] will initially rise quicker and then rise slower as power rises (due to increased burnout) and eventually come to equilibrium (40-50 hours later)

17
Q

How does [Xe] respond to a down power from 100% to 50% power?

A

Initially increase for 7 hours until peak. Then decrease.
Lower equilibrium reached 40-50 hours after down power.

18
Q

Thumb rules for time peak [Xe] is reached following power change

A

Amount of time (in hours) is equal to the square root of the power change.

Ex. For a trip from 100% power, sq rt of 100 is 10. Therefore 10 hours after a trip, [Xe] will be at its peak.

19
Q

What is a Xenon precluded Startup?

A

A startup that cannot be achieved due to the negative reactivity added by Xenon within A24 hours of a Rx trip.

More likely to happen late in core life when trip from 100% due to lack of kexcess

20
Q

What are Xenon Oscillations?

A

Varying flux distribution due to varying production and removal of Xe throughout the core. Oscillates back and forth.

21
Q

How long does it take for a Xe oscillation to shift from a peak at the top of the core to the bottom of the core?

A

12-14 hours (nominally 13 hours)

22
Q

How long does it take for a Xe oscillation to go from peak at the top of the core, down to the bottom and then back to the top again?

A

Double the time when going from top to bottom.
24-28 hours (nominally 26 hours).

23
Q

How does [Xe] change over core life?

A

Flux increases over core life due to fuel burn up. Less fuel means more flux is needed to maintain 100% power.
More flux burns out more Xe, therefore [Xe] lowers over core life.
~10% lower from BOL to EOL.

24
Q

How does Xe reactivity worth change over core life?

A

BOL: more boron, more competition, less worth.
EOL: less boron, less competition, more worth.

25
Q

How is Samarium-149 produced?

A

Negligible from fission (negated).

1.1% of fissions produce Nd-149 or Pm-149 which decay to Sm-149 over ~55 hours.

26
Q

How is Sm-149 removed?

A

Not from decay (very long half life)

Only removed by neutron capture (burnout).

27
Q

When is equilibrium Sm-159 reached?

A

20-35 days after startup (assuming new core, then remains for remainder of reactor life)

28
Q

How does [Sm] change after shutdown?

A

No more burnout. Am rises due to continued decay of Pm-149. Sm peaks ~20 days after S/D.

29
Q

How does [Sm] change after S/U in a core with Sm already in it?

A

Initially lower significantly from burnout then rise slightly from production until the same previous 100% power equilibrium concentration is reached.

30
Q

Is [Sm] power dependent?

A

No

Cook GFES (9 decks)

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