A MORATORIUM on nuclear power plant construction--to provide time for a thorough re-evaluation of both atomic reactor safety and waste disposal--is long overdue.

The recent accident at the Three Mile Island nuclear plant in Pennsylvania clearly demonstrates the combination of design flaws, technical mishaps, and human error that can cause a catastrcphic reactor accident. According to the Nuclear Regulatory Commission (NRC), the reactor's design was ineffective in containing radioactive water; safety and monitoring equipment failed to perform properly when called upon; plant operators apparently forgot to turn on important safety valves deactivated two weeks before the accident and twice turned off the reactor's emergency cooling system prematurely.

The Three Mile Island accident is only the most recent--and best-publicized--in a long series of serious reactor mishaps. In 1975, for example, a fire at the massive Brown's Ferry nuclear plant destroyed safety systems, filled the plant's control room with dense smoke, and threatened a meltdown. Nuclear engineers from the Tennessee Valley Authority, which operates the plant, have stated that a potentially catastrophic radiation release from Brown's Ferry was averted "by sheer luck."

Perhaps the most outrageous example of human error occured when operators inexplicably started up the Vermont Yankee plant with the lid off the reactor's containment vessel.

A catastrophic reactor accident could be more devastating than any previous peacetime event. A U.S. government report, kept secret until a Freedom of Information Act request was filed, predicts that a credible nuclear power plant accident could conceivably kill 45,000 people and create a disaster area the size of Pennsylvania.

The NRC has resorted to arguing that the probability of such an accident is vanishingly small. But a few months ago, the NRC revoked its official support for the study previously used to document that assertion.

The record on nuclear waste is no better. Despite periodic official reassurances, no proven method of safe, permanent radioactive waste disposal exists. A recent report by a federal interagency task force conceded this point, claiming that no available technology can guarantee containment of waste beyond a few thousand years.

UNTIL AND UNLESS safe reactor operation and waste disposal can be guaranteed, no new atomic plants should be built, and nuclear construction now under way--at Seabrook and elsewhere--should be halted.