Editing Nuclear reactor (section)

===Fueling of nuclear reactors===
The amount of energy in the reservoir of [[nuclear fuel]] is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refueling outage times) is related to the amount of [[fissile]] [[uranium-235]] (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent", having spent four to six years in the reactor producing power. This spent fuel is discharged and replaced with new (fresh) fuel assemblies.{{citation needed|date=March 2019}} Though considered "spent," these fuel assemblies contain a large quantity of fuel.{{citation needed|date=March 2019}} In practice it is economics that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the reactor is unable to maintain 100%, full output power, and therefore, income for the utility lowers as plant output power lowers. Most nuclear plants operate at a very low profit margin due to operating overhead, mainly regulatory costs, so operating below 100% power is not economically viable for very long.{{citation needed|date=March 2019}} The fraction of the reactor's fuel core replaced during refueling is typically one-third, but depends on how long the plant operates between refueling. Plants typically operate on 18 month refueling cycles, or 24 month refueling cycles. This means that one refueling, replacing only one-third of the fuel, can keep a nuclear reactor at full power for nearly two years.{{citation needed|date=March 2019}} 

The disposition and storage of this spent fuel is one of the most challenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years.<ref name="nuclear_energy"/> After being discharged from the reactor, spent nuclear fuel is transferred to the on-site [[spent fuel pool]]. The spent fuel pool is a large pool of water that provides cooling and shielding of the spent nuclear fuel as well as limit radiation exposure to on-site personnel. Once the energy has decayed somewhat (approximately five years), the fuel can be transferred from the fuel pool to dry shielded casks, that can be safely stored for thousands of years. After loading into dry shielded casks, the casks are stored on-site in a specially guarded facility in impervious concrete bunkers. On-site fuel storage facilities are designed to withstand the impact of commercial airliners, with little to no damage to the spent fuel. An average on-site fuel storage facility can hold 30 years of spent fuel in a space smaller than a football field.{{citation needed|date=March 2019}}

Not all reactors need to be shut down for refueling; for example, [[pebble bed reactor]]s, [[RBMK|RBMK reactors]], [[molten-salt reactor]]s, [[Magnox]], [[Advanced gas-cooled reactor|AGR]] and [[CANDU]] reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its [[burnup]], which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burnup is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.