What is the balance sheet for nuclear safety three years after Three Mile Island? Constructive results from America's worst nuclear power accident have to be weighed along with warnings of new risks from aging nuclear plants. The plus side will prevail if industry and government build on the increased attention to safety into which they were shocked by TMI.
This momentum, so conspicuous during the first year after TMI, seems to have been flagging. It must not be lost in the midst of the economic preoccupations that cloud the future of the nuclear industry despite all-out support from the present admininistration.
Protesters marked Sunday's third anniversary of TMI with reminders of continued public concern about safety. This week congressional committees are working on the still unresolved matter of how TMI's massive cleanup costs should be paid for.
The latter question is significant, to be sure, because its outcome could set a precedent for handling the costs of any future accident. A central issue is whether TMI should be considered a national problem, with federal and state taxpayers contributing along with industry to cleaning up the contaminated site. One proposal would also require electricity users all over the country to contribute through their electric bills.
The Reagan administration, with its market principles, is understandably reluctant to promote a federal bailout, though it talks about some $120 million for ''research'' on the problem. Surely the utmost legislative scrutiny should be provided before establishing a system by which everybody pays for nuclear accidents anywhere. More promising are proposals for an expanded property insurance system, perhaps with government participation, to take care of nuclear utility needs.
But this cleanup-financing question should not overshadow the reverberations from Three Mile Island in the more fundamental realm of preventing or limiting the extent of accidents in the first place. These gain importance in the light of this week's comments by a Nuclear Regulatory Commission safety engineer that the wrong metal was used in a number of nuclear plants, causing the high likelihood of a core meltdown before long.
To minimize accidents and their effects, the improvements spurred by TMI must be furthered: better training of operators, tighter regulatory enforcement procedures, greater focus on emergency planning. These must not be sacrificed in the accompanying effort to make licensing procedures more efficient as the nuclear industry tries to snap back -- not only from Three Mile Island but from the economic vicissitudes that are causing predictions of cancellation for many plants already under construction.