After three balky valves were fixed, Seabrook nuclear power plant operators have begun the final phase of preparations for the first atomic chain reaction at the long-delayed plant. The first fission reaction could start by midweek, though plant spokesman Ron Sher declined Monday to speculate exactly when.

Plant officials had hoped to start atomic operations as early as a week ago, but one of the three valves in a steam-driven auxiliary emergency feedwater pump was opening too quickly. The pump would help cool the reactor in an accident.

Seabrook workers removed the valves Thursday and returned them to the manufacturer for modifications. The valves were reinstalled Saturday and worked properly in tests completed early Monday, Mr. Sher said.

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