The Nuclear Regulatory Commission yesterday levied a $450,000 fine, the largest in commission history, against the Consumers Power Co. of Michigan for leaving two crucial radiation control valves open for 18 months.

If there had been a nuclear accident at the Palisades plant in South Haven, Mich., the open valves would have allowed radiation to escape freely into the atmosphere, according to Victor Stello, the NRC's chief of inspection and enforcement.

He called the violation "very serious" and said it would have carried a $2.1 million fine, except that federal law restricts NRC penalties to $25,000 a month for any one plant.

John Selby, president of Consumers Power, denounced the fine as "completely unjustified and unreasonable." He said that no radiation had been released and that the open valves had been discovered and reported by the utility. Although he acknowledged that the incident was "potentially the most serious in the history of Palisades," Selby suggested that the fine was "partially the response to recent criticism of the NRC" for being too lax.

Consumers Power has 20 days in which to challenge the fine formally.

The valves in question involved a pipe about 4 inches in diameter that originally was installed in the eight-year-old plant to allow hydrogen gas to be vented from the containment building in the event of an accident. But the plant's design has been changed so that chemical devices would be used to recombine the hydrogen with oxygen to form water, and the vent pipe is no longer necessary.

Stello told a news briefing that the valves apparently were left open when the plant was restarted after a refueling shutdown in April 1978. Consumers Power workers discovered the valves open last Sept. 11 during another refueling period while they were checking for possible leaks in all the valves and pipes leading out of the reactor area.

The NRC found that procedures on the plant checklist neglected to mention the need to reclose the two valves after refueling period tests. There is no way to know their position except by a visual check, Stello said. t

The fine is nearly triple the size of the $155,000 fine levied against Metropolitan Edison Co. of Pennsylvania for violations involved in the March accident at Three Mile Island.

Stello ordered the company to review all its other safety checklists to make sure they are in compliance with the law and to report monthly on its safety controls.

He also dispatched a letter to all other nuclear power plant utilities describing the incidient and suggesting that they also check their list for reliability.