MOSCOW, JULY 29 -- In the climax to the first trial of officials held responsible for the world's most serious nuclear accident, the Soviet Supreme Court today sentenced the former director of the Chernobyl nuclear power plant and two of his deputies to maximum terms of 10 years in a labor camp for gross violations of safety rules.

The verdict in the trial, held in the town of Chernobyl, 12 miles from the site of the accident, was announced tonight by the official Soviet news agency Tass, ending a virtual blackout in the central Soviet press since proceedings began July 7.

According to reports from western reporters admitted to the courtroom today, three other defendants who were lower ranking plant officials were found guilty of negligence and given sentences ranging from two to five years.

The sentences were milder than those imposed last March on the captains of two ships that collided in the Black Sea last summer, killing more than 300 crew and passengers. They were given 15-year prison sentences and fined 40,000 rubles (about $64,000) each.

The three Chernobyl defendants given 10-year sentences -- director Viktor Bryukhanov, chief engineer Nikolai Fomin and his deputy, Anatoly Dyatlov -- had pleaded only partly guilty. They said they accepted professional responsibilty for the accident, but denied criminal liability. Bryukhanov was also found guilty of abuse of power and given a concurrent five-year sentence.

"There was an atmosphere of lack of control and lack of responsibility at the plant," Judge Raimond Brize said today in a 90-minute summary read as the six defendants stood with heads bowed.

In government reports following the accident, Soviet scientists concluded that it was caused by careless acts by operators at reactor No. 4 who were conducting an unauthorized experiment on the plant's safety systems.

The three chief defendants have been in custody since June 1986, six weeks after an explosion ripped the roof off the fourth reactor. As a result of the accident, 31 persons have died and more than 200 suffered acute radiation sickness.

According to plant officials, Bryukhanov and Fomin were summoned from their beds to the scene of the accident and neither shirked his duty that night. But in court today, Brize criticized Bryukhanov for the way he organized the evacuation of the plant's staff and said he bore the brunt of the responsibility for the accident.

Brize said the power station had been poorly managed before the accident, noting that workers wrote letters and played cards and dominoes while on duty.

The three defendants who faced lesser charges were shift chief Boris Rogozhin; Alexander Kovalenko, chief of reactor No. 4; and Yuri Laushkin, a senior engineer. All pleaded not guilty. Today, the judge singled out Laushkin as not fully understanding the experiment that he allowed to take place.

Access to the 18-mile-radius zone around the plant is still carefully controlled, with soldiers at road blocks checking vehicles leaving the area for radiation. As of June, 16 of the zone's 179 inhabitated areas had been resettled and 55 more were being readied.

Since the accident, 27 persons from the Chernobyl area have been expelled from the Communist Party and 67 more reprimanded for their part in the accident. Kovalenko told reporters three more hearings would be held to examine the responsibility of others involved.

Kovalenko said the hearings would seek to establish responsibility for technical failures in the design and construction of the plant, failings in medical and evacuation procedures and security errors.

When the trial opened three weeks ago, Tass said officials were being tried for "a breach of safety regulations at explosion-prone plants." Several western reporters were also present at opening day, but since then, the trial has gone uncovered in the main Soviet newspapers.

The expectation of more hearings into the evacuations from the Chernobyl area suggests a more thorough, public analysis of the reaction by local officials to the accident. So far, Soviet spokesmen have defended decisions made in the accident's aftermath, although some press reports have criticized the 36-hour delay in informing local residents of the health hazards.

The nuclear accident has been a prime test of Soviet leader Mikhail Gorbachev's openness policy since the explosion spewed a cloud of radiation over northern and western Europe. For the first 2 1/2 days, the government kept silent. On April 28, after radiation was detected in Sweden, Moscow issued a brief statement saying only that an accident had occurred.

A week later, however, the Soviet press began reporting more details of the accident and of the evacuation of 135,000 people from towns and villages in the Ukrainian and Byelorussian republics.

The Soviet power industry has taken steps to improve the response of Chernobyl-type reactors to unstable situations and has decided not to start new construction of Chernobyl-type plants.

Since the accident, two of Chernobyl's four units have been restarted. Tass said tonight that the station has produced 7 billion kilowatt hours of energy since October. The third unit, next to the damaged unit now encased in concrete, is expected to start up again by winter.