The tubes are in Unit 3, which has been shut down since Jan. 31 when authorities found a leak.
Southern California Edison says the leak was not a threat and tests have been under way since then to find the cause of the leak. The tubes are being tested with non-radioactive water at up to three times the normal operating pressure.
Unit 2 was taken out of service for a planned inspection and regular maintenance on Jan. 9.
Inspectors from the Nuclear Regulatory Commission are on site for the tests.
Officials say SoCal Edison customers will not be affected while the work is under way.
The Associated Press contributed to this report.