TWO MILESTONES

Nuclear power passed two milestones last month. In Colorado, America's most efficient nuclear power plant went into operation; and from New Jersey, the AEC received its first application to licence a nuclear plant floating offshore.

The heat produced by nuclear fission in the core of a reactor is always transferred to a coolant, and the heat produces steam to run the turbines driving electric generators. In almost all commercial reactors, the coolant has hitherto been water. The first exception was the High Temperature Gas Reactor (HTGR) at Peach Bottom, Pa., completed in 1967. It uses a gas, helium, as a coolant and produces 40 MW of electric power, making it a small ancestor of the just completed plant at Fort St. Vrain, Colo., with a whopping 330 MW output (itself a stepping stone to the next generation with 1100 MW).

The HTGR, manufactured by Gulf General Atomic, has two advantages over pressurized or boiling water reactors: higher efficiency, and higher safety. A gas allows substantially higher temperatures than is possible when water is used as a coolant, and since thermal efficiency depends on the difference of temperatures of the working medium entering and leaving a heat engine, the efficiency of an HTGR is higher than that of a water-cooled reactor (about 30%). With 39% efficiency, the Fort St. Vrain plant is now America's most efficient nuclear power plant.

The increased safety is somewhat more philosophical, since the probability of a disaster is so small for all types of presently used reactors that comparison becomes difficult. However, unlike water-cooled reactors, whose core is contained in a metal vessel, the Fort St. Vrain plant has both core and steam generators in a giant concrete box, 61 feet across and 106 feet high. It not only contains the helium at 700 psi, but also acts as a radiation shield. A 15 foot thickness of reinforced concrete stands between the core and the anti-nuclear crusaders' most fervent dream of radioactive disaster.

The British have been experimenting with HTGR's using carbon dioxide as a coolant for many years. But the British government has recently decided to abandon further development of these trouble-plagued types, and to order Gulfs helium-cooled HTGR's instead .

On the other hand, a small (25 MW) but more advanced design is now being readied in West Germany. The nuclear part runs on the same principle as in Fort St. Vrain, but it does not produce steam; instead, the hot helium goes directly into the turbine blades, and though this produces some unusual problems, the net result is a capital cost reduction.

The Fort St. Vrain plant was built at a cost of $75 million, some 25% less than it would have cost to build a fossil fuel plant of equal capacity. After extensive tests, the Public Service Co. of Colorado plans to put it on line in June of this year.

The big bogeyman, as usual, is radioactive wastes. But they can be turned into solids now, and all of the wastes produced by all nuclear power plants through the year 2,000, says AEC chairman Dr D.L. Ray, can be stored on a single acre of land.

If she is 100% wrong, they can be stored on two acres.