Get the latest updates as we post them — right on your browser

. Last Updated: 07/27/2016

One Button Ends a Nightmare

CHERNOBYL, Ukraine — At midday Friday, a duty engineer at Chernobyl nuclear power station will press an inconspicuous button and begin the shutdown of the plant that caused the world's worst civil nuclear disaster.

That is if the station is operating — it has been forced to shut down twice in the last three weeks due to collapsed power lines and leaking steam, highlighting the jitters Chernobyl causes around the world.

After years of talks between Ukraine and Western countries, President Leonid Kuchma promised in June to close Chernobyl on Dec, 15, pledging never to turn it back on.

The No. 4 reactor at Chernobyl exploded April 26, 1986, at 1:26 a.m., following a controversial experiment when staff temporarily cut off safety systems during a test of the unit's capacity.

Shortly afterward, a series of powerful blasts caused by overheated steam inside the reactor completely ruined the unit, sending a huge, highly radioactive cloud across Ukraine, Belarus, Russia and much of Europe.

Thirty firemen were killed in the immediate aftermath, but thousands of deaths have since been linked to radiation from the station. Cases of thyroid cancer and other diseases have soared.

Fourteen years on, the last functioning reactor, No. 3, has been churning out 5 percent of the country's energy, but the plant has been dogged by minor accidents.

The process of decommissioning the plant is a long one — it will not be until 2008 that the last fuel rods can be removed and it will take a much longer time for radiation to die down.

Chernobyl operated RBMK-1000 nuclear reactors, the second major type of reactor built in the Soviet Union, designed in the 1960s to 1970s. Fourteen RBMK reactors are currently in service, practically all of them in Russia.

The RBMK is, in the technical jargon, a force-tube type, cooled with boiling water (water and steam). Its nuclear reaction is graphite-moderated.

"In line with our rules, the procedure of shutting down the reactor will take about two hours," said Yury Neretin, Chernobyl's chief engineer.

"We have to transfer the reactor into a so-called sub-critical condition, when a nuclear reaction becomes impossible," he said.

An engineer will give the order and a technician will press a button marked BAZ, short for "rapid emergency defense."

This button forces slow moving 211-cadmium-boron and boron-carbide control rods into the reactor to catch neutrons emitted by the nuclear fuel — the effect is to slowly stop the chain reaction happening inside.

One hour after the BAZ button has been pressed, an operator will press another button marked AZ-5 — "emergency defense No. 5."

This rapidly drops all control rods to the bottom of the reactor's active area — a process that takes between 1.8 and 2.5 seconds and suspends the reaction completely.

At the same time, operators have to cut the capacity of two turbo-generators and, later, to cut off the unit from the national power grid.

"It will be an ordinary stop procedure, and we will do everything possible to maintain nuclear safety during the process," said Vadym Hryshchenko, deputy head of Ukraine's nuclear power regulator.

"But it will be a very morbid moment — like burying a near relation or friend," said Volodymyr Korovkin, director of another nuclear power station, at Rivne in western Ukraine.

"I don't understand why we had to rebuild the station after its explosion in 1986, only to shut it down in 2000," he added.

Chernobyl officials say the No. 3 reactor could be safely operated until 2011 because a lot of work has been done to modernize it since 1986.

The thousands of people who work at the plant are also worried as it means their livelihoods are about to disappear.

"The station is good and reliable. It has been working at 82 percent of its designed capacity. This is higher than the world average and at the level of Japanese reactors," Neretin said.

But Western experts argue that RBMK units are affected by generic defects related to their design, their poor quality of construction and their operating safety.

A report by the Institute for Nuclear Protection and Safety, published several years ago, said: "It should not be forgotten that it is the whole RBMK reactor system that raises very serious safety problems."

Nuclear experts say the emergency safety systems of the Soviet RBMK-type reactor operating at Chernobyl and a handful of other stations in Lithuania and Russia are not as reliable and lack multiple back-up systems.

In 1999, Chernobyl spent more than nine months offline for repairs to pipelines in its cooling system. Thousands of faults were detected in welding, and 260 similar welding faults had already been spotted in 1997.

Experts say that besides welding faults, RBMK reactors have some irresolvable technical problems, especially in their safety systems.

"Even though the capacities of the safeguard systems provided for at the design phase vary from generation to generation, they remain notoriously limited," the report said.

Engineers at Chernobyl shut down the No. 3 reactor a day earlier than planned Thursday in an impromptu attempt to impress Kuchma, who was visiting the plant.

But officials said the plant would be restarted so as not to spoil the televised button-pushing ceremony Friday.