Rancho Seco Leak Linked to Design Flaw, Staff Training

A radiation leak last December from the Rancho Seco nuclear power plant near Sacramento was caused by design weaknesses in the reactor’s automatic control system and inadequate training of operators to cope with the emergency, the U.S. Nuclear Regulatory Commission said Tuesday.

NRC investigators also reported that a backup system that was to have been installed in 1984 was never added to the plant by the Sacramento Municipal Utility District, which owns and operates Rancho Seco.

“These weaknesses and vulnerabilities were largely known to SMUD and the NRC staff . . . yet adequate plant modifications were not made. . . ,” NRC investigators told commissioners who met Tuesday in Washington.

A reportedly harmless amount of radioactive steam was vented into the atmosphere Dec. 26 in the pre-dawn hours when a seal on a cooling water pump ruptured during an emergency that was caused by the loss of electrical power to the plant’s integrated control system. The system automatically controls plant operations, including the regulation of pumps that channel cooling water to the plant’s reactor vessel, which contains the radioactive material.

Sacramento Municipal Utility District spokesman Brad Thomas said in a telephone interview from the Rancho Seco plant that he could offer no comment as to why the utility has not taken steps to install a backup system that might have prevented the accident.

However, he said that since the accident, various steps have been proposed and that the training of all plant operators is being updated.

“We’ve submitted an action report to the NRC which we think addresses all the major issues. We’re continuing to work with them to resolve all those issues,” Thomas said.

According to the report, it appears that electrical wires in the breakers had been improperly installed, causing an erratic current that was detected by power supply monitors that in turn shut off the power, according to NRC spokesman Gregory N. Cook.

Thomas said the circuit breakers were installed 13 to 14 years ago by Bailey Meter Co. of Wickcliffe, Ohio, a subsidiary of Babcock & Wilcox Co., which built the plant.

Rancho Seco has been shut down since the accident, and Thomas said no start-up date has been set.

Meanwhile, Cook said in a telephone interview from NRC regional offices in Walnut Creek that the investigation ruled out similar problems at other Babcock & Wilcox plants, where various measures, including the installation of backup systems, had been taken. After the Rancho Seco accident, there had been concern that the problem could point to similar or more serious operating mistakes at other Babcock & Wilcox plants.