Editing Boiling water reactor (section)

=== Refueling systems ===
The reactor fuel rods are occasionally replaced by moving them from the reactor pressure vessel to the spent fuel pool. A typical fuel cycle lasts 12–24 months, with about one third to one fifth of fuel assemblies being replaced during a refueling outage. The remaining fuel assemblies are shuffled to new core locations to maximize the efficiency and power produced in the next fuel cycle.

Because they are hot both radioactively and thermally, this is done via cranes and under water. For this reason the spent fuel storage pools are above the reactor in typical installations. They are shielded by water several times their height, and stored in rigid arrays in which their geometry is controlled to avoid criticality. In the [[Fukushima Daiichi nuclear disaster]] this became problematic because water was lost (as it was heated by the spent fuel) from one or more spent fuel pools and the earthquake could have altered the geometry. The fact that the fuel rods' cladding is a zirconium alloy was also problematic since this element can react with steam at temperatures above {{convert|1500|K|C}} to produce hydrogen,<ref>{{Cite book
| last = Kuan
| first = P.
|author2=Hanson, D. J. |author3=Odar, F.
| title = Managing water addition to a degraded core
| year = 1991
| osti = 5642843
}}</ref><ref name="HaskinStages">{{cite book
|title=Perspectives on Reactor Safety (NUREG/CR-6042) (Reactor Safety Course R-800), 1st Edition
|year=1994
|publisher=U.S. Nuclear Regulatory Commission
|url=https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6042/
|author1=Haskin, F.E. |author2=Camp, A.L. |access-date=23 November 2010
|location=Beltsville, MD
|page=3.1–5
}}</ref>
which can ignite with oxygen in the air. Normally the fuel rods are kept sufficiently cool in the reactor and spent fuel pools that this is not a concern, and the cladding remains intact for the life of the rod.