Unit 2 at Diablo Canyon nuclear power plant was shut down Sunday for refueling and installation of several key plant upgrades.

In addition to replacing about a third of the reactor’s fuel assemblies, workers will upgrade a crane system that allows large components to be moved, and they will install a new digital process control system.

Typical of refueling outages, more than 1,000 supplemental workers will be brought in to perform more than 11,000 maintenance and testing activities. This kind of work is done during refueling because many components are more accessible than during regular plant operations.

Each of the plant’s two reactors is refueled every 18 months. Unit 1 was refueled starting in April, an outage that lasted two months and included installation of a new digital process control system.