THE DECISION by five utility companies to do voluntarily what the Nuclear Regulatory Commission wants them to do was wise. It should relieve some of the public fears about a repetition of the Three Mile Island accident. And it should also make more credible the claim by those who operate nuclear reactors that they, too, worry about safety.

The repercussions of that decision, however, will be substantial. Shutting down a power plant is not like shutting down a factory; the utility must either find electricity elsewhere or curtail the amount it delivers to its customers. Either way, somebody suffers.

That was the reason the utilities, particularly Duke Power, fought the original recommendation of the NRC's staff that its three plants be shut down immediately. Those plants generate about 30 percent of the power it provides for 4 million people, and replacing that much electricity is neither easy nor cheap.

The immediate reaction of many people to the idea of higher prices is that they should not be made to pay for the mistakes of the utility companies or the manufacturers of the suspect reactors. The trouble is that as the figures accumulate in the aftermath of Three Mile Island, it is clear that either the customers or the general taxpaylers are going to end up paying most of the bills in that and other related incidents.

General Public Utilities Corporation, the owner of the Three Mile Island plant, claims it is now spending about $800,000 a day buying high-priced electricity from other utilities to replace that plant's production. While it can take some of its additional costs out of its profits -- it has cut its dividends and reduced executive salaries -- it does not have a pot of money sitting around to meet those new bills. Since utilities have few assets they can sell to raise money -- someone has to generate electricity -- it will eventurally pass through some of its higher costs to its customers.

How the burden of paying for these shutdowns, first in Pennsylvania and now in the Carolinas, should be didvided between stockholders and customers will present hard questions for the public utility commissions in those states. The companies ought to bear as much of the burden as they can without undermining their economic ability to function. After that, you know who will pay the rest.

These considerations should not have been a factor in the thinking of the NRC's members as they struggled with their staff's recommendation. But their indecision suggests they were. Safety in a nuclear plant, as in everything else, is a relative matter, and it is hard to keep from putting other costs in the balance, especially when those who will pay them are as vocal as the opponents of a shutdown were last week. By agreeing to do what the NRC thinks should be done, the utilities have shifted that pressure back where it belongs -- on themselves. They may also have freed the NRC to focus on its only job: safety.

That is where the NRC's responsibility in this affair lies, and it is a very heavy one. In this connection, it should not be forgetten that the NRC's investigation of these plants is still incomplete. So everyone should understand that the arrangement that has just been made by the utilities does not foreclose the NRC from insisting on further corrections to those plants at a later date.