| Poison | Form | Typical Loading | Behaviour |
|---|---|---|---|
| Gadolinium | GdO mixed into fuel pellets | Up to ~3 g oxide per kg of fuel (~6 wt%) | Requires slightly higher fuel enrichment to compensate. After burn-up of about 17 GWd/t, retains ~4% of its absorptive effect and does not decrease further. Key absorbing isotopes are Gd and Gd. |
| Zirconium diboride (ZrB) | Thin coating on normal pellets (IFBA) | Thin surface coating | Burns away more steadily and completely than gadolinium. Has no impact on fuel pellet properties. Now used in most US reactors and some in Asia. |
Tip: Do not confuse burnable poisons with control rods. Both absorb neutrons, but burnable poisons are built into the fuel and are consumed over time, while control rods are mechanically inserted and withdrawn and are not consumed.