Spent nuclear fuel stays intensely radioactive for a very long time and keeps producing heat, so it must be kept cooled and shielded, normally under water in a pool or inside a thick cask. Without power for the pumps a pool can boil away within days to weeks, and exposed fuel then releases intense gamma radiation and contaminates the surroundings, which is why the response is distance and not investigation.
Part of Radiation Safety
Spent fuel is reactor fuel that has been removed after use; it is no longer useful for power but still carries large quantities of radioactive fission products.
Why Spent Fuel Is Worse Than It Sounds
Fuel removed from a reactor holds the fission products made in the reactor. They decay, giving off gamma and beta radiation and heat, and the longest-lived of them last for decades to centuries; the parent article, Nuclear Safety, lists caesium-137 at 30 years, strontium-90 at 29 years and plutonium-239 at 24,100 years. See Half-Life. A recently removed fuel assembly can give a lethal dose to a person standing near it within minutes if unshielded (recalled general knowledge, not from the corpus; no figure is given here).
Pools: Water Is Both Coolant And Shield
Fuel is first held in a pool, where water carries heat away and also shields the people nearby. The parent says without cooling pumps the pool water boils off within days to weeks (its Why This Matters section says within weeks of losing power). Exposed rods release intense gamma radiation and contaminate the surrounding area. If you suspect a pool has boiled dry, with steam rising from the building or extremely high readings on an electroscope at a distance, the parent advises a 30+ kilometre exclusion zone downwind. See Safe Distances.
Dry Storage: Passive, Not Harmless
After years in a pool, fuel may be moved to dry casks of steel and concrete. They are designed to cool by natural air flow and to shield, with no pumps needed (recalled general knowledge, not from the corpus). An intact cask is a heavy shielded container; it is not a source to open, move, drill or scavenge. Damaged casks or ones opened by scavengers are a source of contamination and gamma radiation.
Worked Example: How Time Helps A Little
A single half-life of 30 years for caesium-137 means after 60 years a quarter remains: (1/2)^2 = 0.25. After ten half-lives, 300 years, 0.098 percent remains. So spent fuel falls in activity over generations, not seasons; no settlement can wait it out. Different isotopes dominate at different times, so this is one isotope’s illustration, not a rule for the fuel as a whole.
Common Mistakes
- Thinking dry casks can be opened for their metal.
- Thinking that a quiet, still pool means safe. Water may be cooling and shielding; or may have gone, and you cannot tell from outside.
- Counting on time. Spent fuel stays dangerous for a very long time without active cooling or shielding.
- Approaching to look. If the shielding is gone, the first sign may be a dose.
Safety
A dose near unshielded fuel can come in minutes
Do not approach pools, cask yards or storage buildings. If steam is rising or the surroundings show signs of damage, move away, preferably upwind of the site, and shelter if the wind carries toward you; see Shelter Design.
What’s Next
- Learn next: Waste Site Identification — other long-lived hazards
- Combine with: Reactor Status Assessment — why status can’t be judged from outside