Refueling outages in nuclear power plants
In depth - January 23, 2020

Refueling outages in nuclear power plants

The purpose of the refueling outage is to replace the spent fuel, which is approximately one-third of all the fuel in the reactor's core, with new fuel. The removed fuel is then stored in a fuel pool, and the remaining two-thirds of the fuel is repositioned inside the core to optimize its burnup or consumption.

The outage is used as an opportunity to carry out preventive, conservative and modifying maintenance of the equipment, systems and components that are necessary and cannot be carried out during normal operation.

Depending on the plant's characteristics, the interval between each refueling outage is typically 12, 18 or 24 months

The duration of the refueling outage is on average 30 days. Depending on the plant's characteristics, the interval between each refueling outage (known as the operating cycle) is typically 12, 18 or 24 months.

Almaraz I & II stop every 18 months, as do Ascó I & II and Vandellós II. Cofrentes stops every 204 months and Trillo refuels every 12 months.

Activities during the outage

In addition to refueling, the following is also necessary for the correct operation of the plant:

  • Perform corrective, preventive, conservative and modification maintenance on the necessary equipment, systems and components, which cannot be carried out during normal plant operation.
  • Conduct surveillance and required tests according to the Technical Specifications of Operation (TSO).

It is necessary to define the appropriate strategy in order to ensure that all planned activities are carried out while ensuring safety

Refueling planning

When planning the refueling, it is necessary to define the appropriate strategy to ensure that all planned activities that will be taking place are performed while at the same time guaranteeing nuclear safety and personal safety, minimizing the radiological dos received by all personnel and ensuring maximum quality in the execution of the tasks, while optimizing their duration and adhering to the planned schedule.

Trabajos de recarga de combustible en central nuclear
Refueling outage in Cofrentes Nuclear Power Plant


  • Determination of the scope of the operation, considering activities that are significant due to their duration or impact on the program (critical path), the activities to be carried out due to design changes, and the incorporation of lessons learned from previous refueling cycles.
  • Study and preparation of work: spare parts, materials, hiring services, etcetera.
  • Scheduling of refueling work, defining for each task the activity, personnel and expected time; the profile and qualification of personnel; preparation, execution, management and supervision of the work, both internal and contracted; definition of refueling objectives, etcetera.

Refueling scheduling and stages

Refueling outages are composed of three stages:

  1. Shuting down the plant and uncovering the reactor vessel.
  2. Refueling, equipment maintenance, system work and testing.
  3. Covering the vessel and restarting the plant.

Each stage is composed of a sequence of activities. In the case of the first and second stages, these are always the same, in the same order and with a known duration. In the second stage there are also repetitive activities, but, depending on each refueling outload, they have varying duration, and in some cases this implies changing the order of execution, temporarily shutting down complete systems to inspect the equipment and components of each one.

In addition to the tasks, the scheduling must encompass the scope, the sequence of execution of each step, the number of people required and the series of tests to be performed to document that the equipment or system meets the required acceptance criteria.

With all this information, a first draft of the refueling program is created using proprietary design software, obtaining a critical path and a duration that is then sent for review. Once incorporated, all the program details are generated system by system. Once the program is finalized it is necessary to verify that regulatory and normative requirements are met for each situation throughout the refueling event, assisted by the software. One or two more revisions can still be generated before starting the refueling, incorporating last-minute details and scopes that follow the same validation and editing process.

During the refueling, it is necessary to keep track of the execution of all the scheduled activities to verify if they have been carried out according to the program, and their duration. If any changes occur, they may entail modifications for subsequent refuelings.

During the refueling, weekly reports are submitted to the Nuclear Safety Council to inform on the progress of the program

Reports to the Nuclear Safety Council

Four months before starting the refueling outage, it is mandatory to use a Refueling Planning Report to the Nuclear Safety Council (CSN). This report is repeated one month before starting the outage and again one day prior, with any required updates.

During the refueling outage, weekly reports re also submitted to CSN with updates on the progress of the program, incidents, delays, significant changes in staff exposure to radiation, etcetera.

Within three months before the completion of the refueling outage, a final report must be submitted to CSN indicating the results achieved in objectives, radiation doses, program duration, activities not performed and the reason, etcetera.

Refueling outages in nuclear power plants
Refueling outage in Spanish nuclear power plants

Próximas paradas de recarga

Central nuclearLast refueling outageNext programmed outage
Almaraz I16 April - 20 MaySeptember 2024
Almaraz II--April 2024
Ascó I29 April - 16 JuneNovember 2024
Ascó II23 October - 1 DecemberApril 2025
Cofrentes6 October - 12 NovemberJune 2025
Trillo24 May - 23 JuneMay 2024
Vandellós II--April 2024

Table updated on 18/03/2024

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