THE nation's oldest nuclear power plant, the Yankee-Rowe reactor in western Massachusetts, has been voluntarily shut down because owners failed to satisfy safety concerns raised by federal regulators.The Nuclear Regulatory Commission (NRC) said the 31-year-old facility, operated by the Yankee Atomic Electric Company, was taken off line on Tuesday, the day before the NRC was to review a staff recommendation that the reactor be shut down. The NRC said that review has been postponed indefinitely. "They have acted very responsibly," said NRC chairman Ivan Selin. "We will continue to work with the utility in assessing this important matter." There was no immediate comment from Yankee-Rowe. Yankee-Rowe's safety review is considered a test case for the nuclear industry, which will face closer scrutiny about safety issues as operating licenses come up for renewal. Yankee Rowe - whose license expires in 2000 - is the first to face that scrutiny. The plant's closure is linked to NRC questions about the reliability of the cooling system to adequately control temperatures in the reactor during an emergency. If that system fails in a crisis, questions then focus on the strength of the vessel housing the reactor to contain radiation. The NRC staff concluded the plant's operators failed to meet a regulatory order to establish a 5 to 10 percent reduction in the probability that the reactor vessel would fail during an emergency. In June, the NRC staff rejected requests to close the plant immediately, saying evidence showed that it was safe to operate until next April when a shutdown for refueling was scheduled. In July, the NRC voted to allow the plant to remain in operation. A nuclear watchdog group, Union of Concerned Scientists, argued for closure earlier this year, saying the reactor vessel was too brittle.