Columbia Generating Station, the Northwest's only nuclear power plant, has been shut down for about five weeks for its scheduled biennial refueling outage, Energy Northwest officials said Saturday.

The outage was timed to coincide with spring runoff to minimize the effect of taking the 1,150-megawatt plant offline. It is tentatively scheduled to restart and reconnect to the Northwest power grid in mid-June after refueling and plant maintenance are completed, Energy Northwest officials said.

Originally planned for midnight Friday, the shutdown began several hours early when sensors indicated low pressure in a system that cools the power plant's main generator.

Officials decided to leave the plant offline and continue with preparation for the refueling outage that began several hours later.