Spent nuclear fuel (SNF) is fuel that has been used in a reactor to the point where it is no longer efficient for sustaining the chain reaction, even though it still contains a substantial amount of fissile material. Spent fuel is highly radioactive and thermally hot when discharged, and its management is one of the longest-lived obligations created by nuclear power.

Inventory and characteristics

A typical light-water-reactor fuel assembly is discharged after three to five years in core, with a burn-up of 40-55 GWd/tU. After discharge it is roughly 95% uranium (mostly U-238, somewhat depleted in U-235), 4% fission products, and 1% transuranic elements (plutonium isotopes, neptunium, americium, curium). Heat output decays sharply: at one day after shutdown decay heat is around 1% of operating power; after a year roughly 0.1%; after ten years roughly 0.01% — but never reaches zero.

Wet storage in fuel pools

Initial cooling occurs in the spent-fuel pool (SFP) at the reactor site. Pools provide:

  • Cooling — convective and conductive heat transfer to water and then to the ultimate heat sink.
  • Shielding — typically several metres of water above and around fuel.
  • Criticality safety — neutron-absorbing racks, soluble boron in PWR fuel pools, and storage geometry.
  • Fission-product containment — water effectively retains fission products in fuel-cladding-failure scenarios.

The post-Fukushima period saw significant investment in spent-fuel pool resilience: cooling redundancy, instrumentation, makeup capability, and structural margin against beyond-design-basis seismic and external events.

Dry cask storage

After 5-10+ years in the pool, spent fuel can be transferred to dry cask storage. Dual-purpose casks (storage + transport) are increasingly common. Dry storage is now standard practice at most reactor sites in the US (Independent Spent Fuel Storage Installations under 10 CFR Part 72), Germany, Spain, Switzerland, and elsewhere.

Centralised interim storage

Some countries operate a single centralised spent-fuel interim storage facility:

  • Sweden — Clab (Centralt mellanlager för använt kärnbränsle, Oskarshamn). Wet storage at depth. ~8,000 tonnes capacity, currently being extended.
  • Switzerland — Zwilag (Würenlingen). Dry storage.
  • Spain — ATC project. Centralised facility under regulatory review.
  • Hungary — KKÁT at Paks site.

Final disposal: deep geological disposal

Under current scientific and regulatory consensus, the long-term solution is deep geological disposal (DGD) — emplacement at several hundred metres depth in a stable geological formation, with engineered and natural barriers to provide isolation over the half-life of the longest-lived radionuclides of concern (Np-237 at 2.1 Myr; I-129 at 15.7 Myr).

KBS-3

KBS-3 is the spent-fuel-disposal concept developed by Swedish SKB and adopted by Finnish Posiva. Three engineered barriers in addition to the host rock:

  1. Copper canister with cast-iron insert (50 mm copper outer wall) — corrosion-resistant in the expected reducing groundwater chemistry of granitic bedrock.
  2. Bentonite buffer around each canister — swells on water contact to provide low-permeability seal and chemical buffering.
  3. Crystalline bedrock (Fennoscandian shield) — low groundwater flow, stable mechanically, predictable evolution.

The Finnish Onkalo facility (Posiva, near Olkiluoto) is the most advanced KBS-3 implementation worldwide and has received construction licence approval; operating licence review is in progress. Swedish SKB's planned facility at Forsmark received government clearance in 2022 and is in regulatory licensing toward construction.

Open vs. closed fuel cycle

In a closed cycle (France, Russia, Japan to a limited degree) spent fuel is reprocessed — uranium and plutonium are separated and recycled into new fuel (MOX). Reprocessing reduces the long-term radiotoxicity inventory but introduces additional fuel-cycle facilities and waste streams (vitrified high-level waste, intermediate-level waste, low-level operational waste). The open cycle (Sweden, Finland, the US in practice) treats discharged fuel directly as waste. See Nuclear Fuel Cycle for more.

Retrievability

Most modern programmes provide for retrievability during the operational and post-closure-but-still-monitored phase — up to several hundred years — so that future generations can change strategy if new disposal methods become available or if better evidence on safety emerges.