EIGHTEEN MONTHS ago, the voters of California rejected a proposal that would have had the effect of shutting down the development of nuclear power plants in the state. Now the state government seems to be moving toward doing by administrative action what the voters refused to do. The immediate decision before the state relates only to a proposal by a group of utilities to build a nuclear plant in the desert some 200 miles east of San Diego - but its repercussions could be far wider than that. For this particular plant is so sound - technically and with respect to its remote location - that, if it is turned down by the state's energy-development commission, or if its approval is delayed for a long period of time, the utilities are unlikely to propose another nuclear plant in California any time in the foreseeable future.

So far, the reception to the proposed plant has not even been what you might call lukewarm. The commission has under consideration proposals that it approve the plant only if the utilities can demonstrate that alternative sources of electricity are impractical. Among the sources some commissioners want considered are a geothermal plant and an oil-fired plant that could be converted to coal gasification by 1990.

The trouble with these alternatives - and with the policy of Gov. Edmund G. Brown Jr., which favors solar and geothermal power to the exclusion of nuclear power - is that no one really knows whether they are feasible. Sooner or later, Californians, like people in the rest of the country, are going to have to face nuclear power squarely instead of playing games with its proponents. If the decision is not to have any more nuclear plants, then the reality of where electricity is going to come from in the future must be faced. It should be obvious, even in the California, that oil is not an alternative. Neither are geothermal hot spots in the ground and energy from the sun, unless there are major technological breakthroughs or a huge reduction in the use of electricity. Even coal, which provides the most likely alternative, has shortcomings in its destruction of land and its risks to human life that must be weighed carefully against the shortcomings of atomic energy.

We can understand the disenchantment with nuclear power. It does present substantial problems, particularly in the disposal of wastes and in the need for tight security. But it is hard to understand why so many opponents of nuclear power regard these problems as insolvable while they are prepared to stake so much on the ability of scientists and engineers to solve equally difficult problems involving the safe and cheap production of electricity from coal or solar or geothermal sources. It would be of enormous help, of course, if the federal government would produce a more plausible plan for the long-term disposal of nuclear wastes than it now has. At present, however, it seems to us that the burden on those who want to close out nuclear power is to tell us how they expect future needs for electricity to be met. Otherwise, the next generation is likely to be sitting around in the dark blaming the utilities for not doing something this generation's officials wouldn't let them do.