CONTROL ROD EXPERIMENT WENT WRONG. SAY RUSSIANS Chernobyl: The fatal error

Press, 31 May 1986, Page 19

CONTROL ROD EXPERIMENT WENT WRONG. SAY RUSSIANS Chernobyl: The fatal error

By

GEOFFREY LEAN and ROBIN McKIE

in an exclusive report for the London “Observer.”

human error was responsible for the Chernobyl disaster, Soviet Union and international nuclear investigators believe.

An operator’s mistake — in tfie positioning of rods used to keep the nuclear reaction'under control — led to a chain of events which caused a 200tonne crane to crash on to the intensely radioactive core of the reactor — and to the tyorld’s worst disaster in a nuclear power plant. 1 The investigators are just beginning their work and refuse to speculate in public on the cause of the catastrophe. But they are privately building up a picture of what they believe happened. J, Just after midnight on the morning of Saturday, April 26, the No. 4 reactor at Chernobyl was idling at 7 per cent of its normal power. It was effec-

tlvely shut down for maintenance; but it had to be kept going at this level to prevent the build-up of radioactive gas that would delay its re-start. At precisely 40 seconds past 1.23 a.m. — without any warning — the power in the reactor suddenly surged from 7 per cent to half its normal level. The investigators believe this can only have happened through an operator altering the position of the control rods.

They believe the operator may have been conducting experiments on the reactor — either officially or on his own Initiative — to see what would happen if the rods were adjusted. This fits in with a cryptic Russian statement last week that “experimental research work” was going on at Chernobyl at the time.

This is a particularly perilous undertaking on a Russian RBMK-1000 reactor like Chernobyl, experts say, because the nuclear reaction in the core (which is 40 feet across) is very delicately balanced, particularly at such low power levels. The investigators think the operator pulled out some control rods and upset this delicate balance. Alarmed, he moved some more rods to try to get the situation back under control — and this caused part of the reactor to “go critical.” His instruments were probably not sensitive enough to warn him in time about what was

happening. The runaway section, near the top of the reactor core, rapidly heated up. This caused both the uranium fuel and its zirconium containers to become dangerously hot. Normally the containers are cooled by water, but as they became extremely hot the zirconium, a metal, reacted with the water to produce hydrogen. The hydrogen leaked out into the giant hall above the reactor core and exploded.

A 200-tonne mobile crane used to refuel the reactor, which was suspended above the core, crashed down on it, caus-

Ing enormous damage. Fire broke out in the reactor hall, and a second fire ignited huge blocks of graphite in the core itself.

Firemen struggled to put out the fire in the reactor hall, which threatened to spread to a second reactor next door, causing an even greater catastrophe. Within 90 minutes they had put it out, at great personal cost Many of the firemen are among the 15 who have died so far.

The fire in the graphite was much harder to control. A video taken a day after the accident shows a quarter of the core as red-hot “grilling like a charcoal fire,” according to one witness. The heat melted part of the nuclear fuel and pushed radioactive materials 2000 to 3000 feet into the air to form the giant radioactive cloud which drifted across Europe. The Rus-

sian investigators privately guess that about a tenth of the radioactive contents of the core literally went up in smoke. At this stage the Russians briefly panicked. No-one knew how to put out a fire in the graphite. But international officials are full of praise for the way they recovered, improvised, and eventually brought the disaster under control by smothering the core under tonnes of boron, lead, and sand.

For days the Russians were concerned that the melting fuel would bore its way through the floor of the reactor down into the earth, in the so-called “China Syndrome,” polluting the water table. This did not happen; but to make doubly sure, they have been pumping concrete under the reactor to entomb it.