Power is a reading. Inventory is a state.
Imagine two tanks with the same outlet flow but different amounts of water stored inside. Matching flow does not mean matching inventory. Reactor power and fission-product inventory have a similar distinction: the present power does not completely describe the material left by earlier operation.
Some fission products absorb neutrons strongly. “Poison” in this context describes neutron absorption, not a claim about chemical toxicity. Xenon-135 and samarium-149 matter because production and removal take different paths. Their inventories can continue changing after the fission rate changes.
Xenon has competing sources and sinks
Iodine-135 decays into xenon-135. Xenon also has a direct fission source. Xenon-135 can disappear through radioactive decay or neutron capture. These processes respond differently when neutron flux changes; a lower fission rate therefore does not guarantee an immediate reduction in xenon inventory.
Xenon-135 + neutron → xenon-136 (capture route)
Think of a factory that stops taking new orders while work already in the queue continues arriving at the next station. The precursor inventory is that queue. The analogy explains delayed production; it does not calculate a reactor transient.
Read the balance before solving it
A simplified, spatially averaged inventory model makes the bookkeeping visible:
- N: isotope number density, atoms/m³.
- SX: direct xenon production rate, atoms/(m³·s).
- λ: decay constant, s⁻¹.
- σa,X: effective absorption cross-section, m².
- φ: neutron flux, neutrons/(m²·s).
Every term is an inventory-change rate. The first two add xenon; the last two remove it. This equation is an explanatory balance, not a validated CANDU prediction tool: flux distribution, spectrum-dependent cross-sections, initial inventories and coupling to other isotopes need a defined model.
Dimensional checking is especially useful here. A decay constant multiplied by number density gives atoms/(m³·s). Cross-section multiplied by flux gives a capture rate per target atom; multiplying by target number density gives the same inventory-rate units. If a calculation mixes cm² with a flux expressed per m², a physically sensible equation can still produce a very wrong number.
Samarium changes the memory
Promethium-149 decays into samarium-149. Samarium-149 is stable, so its balance lacks the radioactive-decay removal term that xenon has. Neutron capture can remove it as an absorber. Waiting and irradiation are therefore not interchangeable mechanisms.
The useful contrast is qualitative: xenon has radioactive decay and capture removal; samarium-149 has no radioactive self-decay removal. A single waiting-time rule cannot describe both.
Spatial behaviour matters too. Equal total power can hide different local power distributions. An average quantity compresses information; it does not prove that each region behaves identically.
Remember: precursor → absorber → removal. Read all three, not just the isotope name. On-power refuelling does not make the inventory balance disappear.
Sources & further reading
- CNSC — Reactor Physics, sections 5.1–5.4; public educational text ↗
- DOE Fundamentals Handbook, Nuclear Physics and Reactor Theory, volume 2 — independently hosted archive of the DOE publication ↗
Public educational explanation. Plant work requires approved procedures, appropriate training and qualified supervision. This article contains no operating instructions.