The horrors of Chernobyl should not be used as an argument against nuclear power - as was the case last week when the fifth anniversary of the disaster at the Soviet nuclear reactor was widely noted.

A Chernobyl accident cannot happen here.The design of a Chernobyl-type reactor is completely different from any reactor in the West; it would never have been licensed in any Western country.

The Soviet reactor consists of a large block of graphite, in which are embedded about 1,700 tubes containing the uranium that produces fast neutrons.

The tubes are cooled by water flowing through them.

The graphite slows down the neutrons so they can interact with uranium-235, continuing the chain reaction, and the water absorbs some of these slow neutrons, depressing the chain reaction.

The design of the Chernobyl reactor results in an unfortunate instability.

If, for some reason, the reactor produces excess power, more of the cooling water will turn to steam, fewer neutrons will be absorbed by water and more of them will be absorbed by uranium-235.

This increases the already too-high power still further.

That is exactly what happened: The power increased and the reactor became uncontrollable.

In Western countries, most nuclear reactors use water for slowing the neutrons and cooling.

If some water is lost, the chain reaction stops automatically and the reactor shuts down.

The situation at Chernobyl was aggravated by bad management.

On April 26, 1986, an experiment on the reactor's electric generator was entrusted to an electrical engineer who had no training in nuclear engineering and was unaware of the instability problem.

Records show that the operating crew was overtired and was given only a short time to conduct the experiment.

During the experiment the crew blithely disregarded safety rules.

Western reactors are stable; if water boils away they automatically shut down.

Nevertheless, they can have accidents, as we saw at Three Mile Island in Pennsylvania in 1978. Yet the accidents at Three Mile Island and Chernobyl were totally different.

The disaster at Chernobyl happened in a fraction of a second; the Three Mile Island meltdown took hours.

The amount of radioactivity emitted in the Soviet Union was a million times greater than in the United States.

More than 30 persons died of the Chernobyl accident's immediate consequences, and delayed illnesses, including cancers, are reported in areas as far as 80 miles away.

Since the Three Mile Island accident, efforts have been made to assure even greater safety at U.S. nuclear plants.

The Nuclear Regulatory Commission inspects power plants more in depth than before. The Institute for Nuclear Power Operation, an industry organization, makes regular, extended visits to each nuclear plant and instructs operators on the best practices.

Controls have been improved and operators get frequent training in simulated accidents.

Future nuclear plants are designed to be even safer than the present ones.