Gorbachev's strategy for tackling tough issues. Vast Chernobyl cleanup effort is being slowed by technical problems

The difficult and dangerous job of cleaning up the stricken Chernobyl nuclear plant and its surroundings is now proceeding slower than expected. Next week an international meeting in Vienna will take a hard look at the accident and how it occurred. So far, the Soviet explanations have been less than explicit.

Meanwhile, the Soviets are putting concrete walls along the nearby riverbanks, treating the affected buildings, and removing radioactive material by robot in an effort to contain further spread of radioactive contamination. But they are meeting difficulties in sealing off the shattered power plant itself.

Moscow's Aug. 14 report on the accident to the UN's International Atomic Energy Agency in Vienna identified human error as the primary cause. That report will be the working document for IAEA discussions.

Early translations of the report, which was in Russian and not also in English as promised, have failed to clarify many of the questions about why the reactor behaved as it did.

The report did say the accident occurred after a test was conducted to see how the unit would perform with:

The reactor at low power.

The turbine generator disconnected, but rotating because of momentum and thus still generating some power.

And the emergency core cooling, power regulation, and automatic shutdown systems switched off.

During the experiment, a reactor power surge occurred and the manual insertion of the control rods was unsuccessful. The result was a massive explosion.

But Western experts say it is not clear why the power surge occurred, or why it happened so rapidly. These highly technical questions will be put to the Soviet team during the IAEA session (next Monday through Friday).

At the time of the accident, there were four operating units at the station, with two more under construction. None are now operating.

Though the planned restart in October of the Chernobyl Units 1 and 2 indicates that rapid initial progress was made by the Soviets in cleaning up the less heavily contaminated areas around the plant, the work is slowing. This is because an increasing amount of work is being devoted to the more heavily contaminated areas around the reactor, where men can work for only a few minutes at a time. A wall is being built to isolate Units 1 and 2 from the more seriously contaminated areas.

The recovery work under way at Chernobyl is rather different from that at Three Mile Island over the past seven years. For one thing, there was little or no external ground contamination at Harrisburg, Pa., in 1979, whereas there has been gross contamination of an area of about 600 square miles around Chernobyl. This has meant that the Soviets are having to protect local water supplies from pollution, and to dispose of vast amounts of contaminated soil and vegetation.

Also, for psychological and research reasons, the United States decided to clean up Three Mile Island completely, even removing the fuel from the core (a process that is now under way). The Soviets are proposing, at least for the foreseeable future, merely to entomb the reactor in what they call a concrete ``sarcophagus,'' which will begin 300 feet below ground and cover the top of the shattered reactor.

Earth and water. Protection of local rivers has been a key priority for the Soviets, both to protect water supplies and to prevent contamination from being spread farther afield. Immediately after the accident, the Soviets began to build concrete embankments along the adjoining Pripyat River to prevent contamination from leaching into it from the ground.

To prevent the surface contamination on the ground and on buildings from being washed down into the soil, the Soviets have been spraying special solutions from helicopters. These solutions -- of liquid synthetic rubber, potassium or sodium silicate (water glass), and plastic film -- bond with the surface contamination as they set. The resulting films can then be rolled up and disposed of, either by burial or incineration. Some of these supplies are coming from the West. In addition to this process, the Soviets are also washing down the reactor buildings, structures, and equipment with special decontamination solutions.

Highly contaminated soil is shoveled up by radio-controlled mechanical diggers, put in special containers, and taken to one of several waste dumps that have been set up at various unspecified locations in the Soviet Union. Less-contaminated material can be shoveled up by shielded, manually operated diggers in the same way.

Containing the core. Another key priority for the Soviets after the accident was to ensure that the seriously damaged reactor core was contained.

To put out the fire and to stop the radioactive emissions during the accident, Army helicopters employing World War II bombing techniques dumped 4,000 tons of sand, lead, and boron shielding material in sacks on top of the reactor. Because the reactor building was also carrying the weight of heavy equipment that had fallen on top of it after the initial explosion, there were fears that it might collapse.

Thus, one of the first parts of the recovery program was to use liquid nitrogen to freeze the ground around the reactor. This was done to prevent contamination of water supplies, should the core containment structure break up. Gaseous nitrogen was also pumped in to cool the concrete around the reactor and the reactor core.

To prevent any chance of ground water contamination, a 2-mile-long concrete diaphragm wall is being built deep into the soil around the reactor. Italian machinery is being used; it can bore 300 feet into the ground and then pour in concrete. This wall is part of the sarcophagus.

Completion of the concrete tomb is being delayed, partly because of inadequate supplies of concrete and partly because the Soviets are not sure how to design it. The design problem lies in finding a way to cool the core over a period of decades. One tactic being considered is ventilating the new containment through high-efficiency filters, but a final choice has apparently not yet been made.

Also, a massive concrete slab, fitted with an active radiator cooling system, has been built under the reactor. A tunnel was dug so that the base could be constructed. Miners from various regions of the Soviet Union worked around the clock in eight 3-hour shifts to limit their exposure to radiation. Each team worked for 15 days.

The new slab is just one element of a triple concrete base under the reactor. The second element is the original concrete foundation on which the unit was built. In addition, what is known as a ``pressure suppression pool'' -- a pool of water that lies under the reactor -- has been drained and filled with concrete to provide a third protective layer.

The draining of that pool was one of many examples of heroism exhibited during the accident.

At the height of the crisis, there was concern that molten fuel would burn its way through to the suppression pool, creating a massive steam explosion that would have blasted the complete contents of the core into the environment. (In the end, only 1 to 3 percent actually escaped.) A small number of divers went into the pitch-black pool to open the valves manually. A few feet above them the core, then at a temperature of about 2,000 degrees C. (3,632 degrees F.), threatened to fall in. Fortunately, the fuel never burned through.

Robots and hot suits. To conduct operations in and around the wrecked Unit 4, personnel wearing totally enclosed air-cooled suits can work for only a few minutes before receiving their maximum allowable dose of radiation. ``Hot suits'' sent from Britain, though, can provide protection against temperatures of some 60 degrees C. (140 degrees F.) for up to two hours.

In the most contaminated areas, robots are needed. Three robots sent from West Germany arrived May 10. Two of the robots, which are tracked vehicles carrying cameras and a manipulator arm, are being used to conduct surveillance and collect samples. These robots are controlled and powered via umbilical cables. The third robot, a radio-controlled bucket loader, is being used to remove highly active debris.

Perhaps the biggest ``robots'' in use around Chernobyl are 19-ton remote-controlled bulldozers from the Chelyabinsk Tractor Works. They are being used to remove heavily contaminated topsoil.

Radiation levels at the reactor are being monitored by a 59-foot steel tube packed with instruments, which has recently been inserted by three helicopters. Although little is known about current levels at the reactor site, the levels are still high enough to cause the breakdown of remotely operated equipment because of radiation damage to their electronics. The Soviets are looking for ways to prevent this.

The author is deputy editor of Nuclear Engineering International.