Fission Product Poisons Flashcards
What is the most significant fission product poison?
Xe-135.
-2700 pcm at 100% power.
-5000 pcm at peak following trip from 100% power.
Produces numerous operational issues.
What is the second most significant fission product poison?
Sm-149
-700/-1000 pcm at full power (BOL/EOL)
-1100/-1400 pcm at peak equilibrium after shutdown (BOL/EOL).
How do fission product poisons affect the neutron life cycle?
More poison, more absorption of thermal neutrons in poison, thermal utilization factor decreases.
What are the microscopic cross sections for absorption for Xe-135 and Sm-149?
Xe-135: 2.6x10^6 barns (huge)
Sm-149: 4.0x10^4 barns (pretty big)
Differences between Xenon and Samarium.
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.
How is Xenon produced?
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).
How is Xe-135 removed?
Decay. Xe-135 half life is 9.1 hours
Burnout. Neutron absorption by Xe-135.
How is equilibrium Xenon affected by power level?
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.
When is decay of Xe at equilibrium vs burnout the most dominant removal process?
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.
What is equilibrium Iodine and how long does it take to get there?
Constant Iodine-135 concentration achieved at a certain stable power level.
Takes 20-25 hours.
What are production and removal mechanisms of I-135?
Production: decay of Sb and Te fission products. (~20 seconds)
Removal: Decay if I-135 (6.6 hour half life).
How does the magnitude of equilibrium xenon change over various power levels?
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.
How does the rate at which Eq [Xe] is achieved change at different power levels?
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.
What is Transient Xenon?
How xenon concentration changes when power changes until equilibrium is achieved.
How does [Xe] respond to a trip from 100% power?
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.
How does [Xe] respond to S/U and power rise to 100% power?
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)
How does [Xe] respond to a down power from 100% to 50% power?
Initially increase for 7 hours until peak. Then decrease.
Lower equilibrium reached 40-50 hours after down power.
Thumb rules for time peak [Xe] is reached following power change
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.
What is a Xenon precluded Startup?
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
What are Xenon Oscillations?
Varying flux distribution due to varying production and removal of Xe throughout the core. Oscillates back and forth.
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?
12-14 hours (nominally 13 hours)
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?
Double the time when going from top to bottom.
24-28 hours (nominally 26 hours).
How does [Xe] change over core life?
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.
How does Xe reactivity worth change over core life?
BOL: more boron, more competition, less worth.
EOL: less boron, less competition, more worth.
How is Samarium-149 produced?
Negligible from fission (negated).
1.1% of fissions produce Nd-149 or Pm-149 which decay to Sm-149 over ~55 hours.
How is Sm-149 removed?
Not from decay (very long half life)
Only removed by neutron capture (burnout).
When is equilibrium Sm-159 reached?
20-35 days after startup (assuming new core, then remains for remainder of reactor life)
How does [Sm] change after shutdown?
No more burnout. Am rises due to continued decay of Pm-149. Sm peaks ~20 days after S/D.
How does [Sm] change after S/U in a core with Sm already in it?
Initially lower significantly from burnout then rise slightly from production until the same previous 100% power equilibrium concentration is reached.
Is [Sm] power dependent?
No
