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BLD-NUCL84-02 · Lesson 7 · 24 clock hours · theory

Xenon and Samarium Poisoning — Fission Product Effects

Objective. Students will explain how xenon-135 and samarium-149 build up and decay in an operating reactor, predict xenon oscillation behavior after a power reduction, and state the operational significance of xenon peak after shutdown.

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Xenon and Samarium Poisoning — Fission Product Effects

When uranium-235 fissions, it does not produce only energy and neutrons. It also produces dozens of radioactive fission products, two families of which have an outsized effect on reactor control: xenon-135 and samarium-149. Understanding how these isotopes build up, reach equilibrium, and behave after a power change is essential for safe reactor operation.

The Xenon-135 Production and Decay Chain

Xenon-135 (Xe-135) is the most powerful neutron absorber known. Its microscopic absorption cross-section is enormous, meaning it captures neutrons very effectively and removes them from the fission chain reaction. This effect is called reactor poisoning.

Xe-135 originates from fission through a chain reaction:

Te-135 → I-135 → Xe-135 → Cs-135 → Ba-135 (stable)

At full power, a steady state called equilibrium xenon is reached. Production from iodine decay and direct fission exactly balances removal by decay and neutron capture. The equilibrium xenon concentration represents a significant negative reactivity — typically expressed in units of percent delta-k/k — that operators must account for when calculating available reactor control.

Xenon Transient Behavior After a Power Change

When reactor power is reduced or the reactor is shut down, the neutron flux drops sharply. This dramatically slows xenon burnout (neutron capture removal). However, the large inventory of I-135 built up during full-power operation continues to decay into Xe-135 at nearly the same rate for several hours.

The result is a xenon peak: Xe-135 concentration rises well above its equilibrium level. In a typical reactor, this peak occurs approximately 6 to 11 hours after shutdown, and the xenon worth (negative reactivity) during the peak can exceed available positive reactivity from control rods and boron. The reactor physically cannot be restarted during this window, regardless of operator intent. Attempting to overcome the peak by withdrawing all control rods and reducing boron to minimum is unsafe and may still be insufficient.

After the peak, Xe-135 continues to decay without significant production (since I-135 is now depleted), and the xenon worth decreases. Eventually — typically 24 to 36 hours after shutdown — xenon returns to a near-zero level and the reactor can be restarted. The exact timing depends on the reactor design and prior power history.

Xenon Spatial Oscillations

In large reactor cores, xenon does not distribute uniformly. If one region of the core has slightly lower flux, xenon builds up there (less burnout). This increases local negative reactivity, suppressing flux further in that region. Simultaneously, neutrons shift toward the higher-flux region, burning more xenon there and raising local power. This self-reinforcing feedback can produce xenon oscillations — slow, periodic power shifts between axial or radial regions of the core. These oscillations have periods of hours and can cause local power to exceed technical specification limits if undetected.

Operators monitor axial flux difference (AFD) or axial offset — the difference between upper and lower core power — to detect and correct developing xenon oscillations. Incore and excore detectors provide continuous flux mapping. Recognizing spatial oscillations early and making small, deliberate control rod adjustments is the correct response. Large, rapid rod movements can make oscillations worse.

The Three Mile Island Unit 1 facility has been used in operations training programs to illustrate how xenon oscillations in large cores can challenge operators if not recognized promptly. This context is strictly for understanding the physics; it is separate from the 1979 TMI-2 accident.

Samarium-149

Samarium-149 (Sm-149) is the second most important fission product poison. Its production chain is:

Pm-149 → Sm-149

Promethium-149 (Pm-149) is produced directly from fission and decays with a half-life of approximately 53 hours into Sm-149. Unlike Xe-135, Sm-149 is stable — it does not decay. It is removed only by neutron absorption.

At full power, equilibrium samarium is reached when production from Pm-149 decay equals neutron absorption. Because Sm-149 has no decay removal pathway, its equilibrium concentration depends only on the fission yield and the Pm-149 decay rate, not directly on flux level.

After shutdown, Pm-149 continues to decay into Sm-149 while neutron burnout stops. Samarium builds up to a peak, then stabilizes at an elevated level until the reactor restarts and burnout resumes. Samarium poisoning after shutdown is a negative reactivity concern operators must account for during restart planning.

Operational Safety Significance

Key terms10

Xenon-135 (Xe-135)A fission product with an extremely large neutron absorption cross-section; the most significant reactor poison, produced mainly from decay of iodine-135.
Reactor PoisoningThe reduction in reactor reactivity caused by neutron-absorbing fission products such as xenon-135 and samarium-149, which compete with fuel for available neutrons.
Equilibrium XenonThe steady-state concentration of Xe-135 reached during sustained full-power operation, where production and removal rates are equal.
Xenon PeakThe transient rise in Xe-135 concentration and associated negative reactivity that occurs several hours after a significant power reduction or reactor shutdown, caused by continued I-135 decay with reduced neutron burnout.
Iodine-135 (I-135)A fission product with a half-life of approximately 6.57 hours that is the primary precursor to Xe-135; its inventory at shutdown determines the magnitude of the subsequent xenon peak.
Samarium-149 (Sm-149)A stable fission product poison produced from decay of promethium-149; removed only by neutron absorption, it accumulates to a peak after shutdown and must be accounted for during restart planning.
Axial Flux Difference (AFD)A monitored parameter representing the difference between upper and lower core neutron flux; used to detect and control xenon spatial oscillations in large reactor cores.
Xenon Spatial OscillationA slow, self-reinforcing oscillation of power distribution within a large reactor core, caused by the interaction between local neutron flux levels and local xenon concentration.
Neutron Burnout (Xenon Burnout)The removal of Xe-135 from the reactor through neutron capture, which converts Xe-135 to Cs-135; the dominant removal mechanism at high power levels.
Reactivity WorthA measure of how much a material or condition changes the multiplication factor of the reactor, typically expressed in units such as percent delta-k/k or pcm (percent mille).

Worked examples3

1. At full power, a reactor reaches equilibrium xenon. The equilibrium xenon worth is expressed using the simplified instructor formula: X_eq = (γ_I × Σ_f × φ) / (λ_Xe + σ_a_Xe × φ), where the instructor has provided evaluated constants that yield X_eq = 3.0 × 10^15 atoms/cm³ at full power flux. After a complete shutdown (φ → 0), xenon removal by neutron capture stops. Only radioactive decay of Xe-135 continues (half-life ≈ 9.17 hours). What fraction of the shutdown xenon inventory remains after one Xe-135 half-life (approximately 9.17 hours) — assuming no further production from I-135 (a simplification to illustrate decay math only)?
  1. Step 1: Identify the decay law. After n half-lives, remaining fraction = (1/2)^n.
  2. Step 2: One half-life has passed, so n = 1.
  3. Step 3: Remaining fraction = (1/2)^1 = 0.5.
  4. Step 4: Remaining xenon = 0.5 × 3.0 × 10^15 = 1.5 × 10^15 atoms/cm³.
  5. Step 5: Note the simplification — in a real transient, I-135 decay adds Xe-135 during this same period, so actual xenon at 9.17 hours after shutdown is higher than this calculation shows, not lower. The xenon peak typically exceeds the equilibrium value.
Answer: After one Xe-135 half-life (≈9.17 hours), approximately 50% of the shutdown xenon would remain by decay alone (1.5 × 10^15 atoms/cm³), but the real xenon inventory is higher during this period because I-135 continues to feed new Xe-135 into the core.
2. I-135 has a half-life of approximately 6.57 hours. At shutdown, the I-135 inventory is 8.0 × 10^16 atoms/cm³. Approximately how much I-135 remains after 13.14 hours (two I-135 half-lives)?
  1. Step 1: Determine the number of half-lives elapsed: 13.14 hours ÷ 6.57 hours per half-life = 2 half-lives.
  2. Step 2: Apply the decay law: remaining fraction = (1/2)^2 = 0.25.
  3. Step 3: Remaining I-135 = 0.25 × 8.0 × 10^16 = 2.0 × 10^16 atoms/cm³.
  4. Step 4: This means 75% of the I-135 inventory has decayed into Xe-135 over this period, contributing heavily to the xenon peak observed in the first 10–15 hours after shutdown.
Answer: Approximately 2.0 × 10^16 atoms/cm³ of I-135 remain after 13.14 hours; the other 6.0 × 10^16 atoms/cm³ has decayed into Xe-135, driving the post-shutdown xenon peak.
3. A reactor xenon transient data table (instructor-provided) shows that xenon worth peaks at −2,500 pcm at hour 8 after shutdown. Available positive reactivity from fully withdrawn control rods and minimum allowed boron concentration is +1,800 pcm. Can the reactor be restarted at hour 8?
  1. Step 1: Compare available positive reactivity to xenon worth magnitude. Available reactivity = +1,800 pcm; xenon negative reactivity = −2,500 pcm.
  2. Step 2: Net reactivity if all positive reactivity is inserted = +1,800 − 2,500 = −700 pcm.
  3. Step 3: A net reactivity of −700 pcm means the reactor remains subcritical. The chain reaction cannot be sustained.
  4. Step 4: Conclusion — the reactor cannot be restarted at hour 8. Operators must wait for xenon to decay to a level where xenon worth is less than +1,800 pcm in magnitude.
Answer: No. The xenon worth (−2,500 pcm) exceeds available positive reactivity (+1,800 pcm) by 700 pcm, leaving the reactor subcritical. Restart is physically impossible at this time without violating design limits.
In class: Lecture with production/decay chain diagrams: students trace the Te-135 → I-135 → Xe-135 → Cs-135 chain and the Pm-149 → Sm-149 chain, annotating half-lives and production rates from fission, then calculate equilibrium Xe-135 concentration at full power using simplified instructor-provided equations. · Xenon transient plot exercise: using pre-calculated data from a standard xenon transient table (instructor-provided), students plot xenon worth vs. time following a 50% power reduction and a full shutdown, identify the xenon peak time and magnitude, and explain why the reactor may not be restartable during the peak. · Operational scenario discussion: instructor presents the Three Mile Island Unit 1 xenon oscillation historical context (operations training context only, not accident); students discuss how xenon spatial oscillations can occur in large cores and what axial flux monitoring is designed to detect.
Materials: Printed xenon transient data tables, semi-log graph paper, scientific calculators, generic nuclear plant operations training text (Westinghouse AP1000 or generic PWR training manual excerpt), whiteboard
Check: Written response: student must correctly draw a xenon peak curve after shutdown, label time to peak, and explain in two sentences why the reactor cannot be restarted during the peak. Plus 5-question quiz at 80% pass mark.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Reactor Physics — NRC Non-Licensed Operator / RO License preparation
1. What is the primary precursor isotope that produces Xe-135 in an operating reactor?
Reactor Physics — NRC Non-Licensed Operator / RO License preparation
2. After a full reactor shutdown, the xenon concentration in the core initially rises rather than falls. What is the best explanation for this behavior?
Reactor Operations — NRC Non-Licensed Operator / RO License preparation
3. Why might an operator be unable to restart a reactor approximately 8 hours after a full-power shutdown?
Reactor Operations / NUCP Uniform Curriculum
4. What operational parameter do reactor operators monitor to detect xenon spatial oscillations in a large core?
Reactor Physics — NRC Non-Licensed Operator / RO License preparation
5. How is samarium-149 removed from the reactor core during power operation?
Reactor Physics — NRC Non-Licensed Operator / RO License preparation
6. A reactor operating at full power for several days is shut down. Approximately how many hours after shutdown does the Xe-135 peak typically occur in a conventional PWR?
Reactor Physics / NUCP Uniform Curriculum
7. Which of the following best describes a xenon spatial oscillation in a large reactor core?
Radiation Protection — NRRPT / OSHA 10 safety awareness
8. From a radiation protection standpoint, why is the post-shutdown period immediately following a reactor trip a high-risk time for plant workers near the primary system?
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