ENERGY: Chernobyl-type Accident Could not Happen Again

By IPS Correspondents Reprint | | Print |

Judith Perera and Andrei Ivanov

MOSCOW, Apr 26 1996 (IPS) - Anti-nuclear protestors chained themselves to a railway line near the Chernobyl plant in Ukraine Friday and demanded its immediate closure, as the 10th anniversary of the world’s worst nuclear diasaster was marked by protests and reflection.

Russian President Boris Yeltsin, in a special message, called for higher safety standards at nuclear power plants. Earlier in the day, there were marches and special functions in Ukraine to mark the occasion.

Two Chernobyl reactors are still in operation, although Ukraine has promised to close the entire operation by 2000.

However, although scientists say many improvements still need to be made to Chernobyl-type RMBK reactors, especially the older models, there is no longer any possibility of an accident similar to the one which destroyed that nuclear station’s fourth Unit in 1986.

This was the main conclusion of a conference held recently at Desnogorsk in Russia where experts from Russia, Europe and the United States discussed the upgrades which have been made to these reactors over the past nine years.

Since 1986 Soviet-designed reactors have been the subject of intense study. Investigations have been conducted by international bodies such as the International Atomic Energy Agency (IAEA), the European Community (EC) and the World Association of Nuclear Operators (WANO).

Studies on the RBMKs, including the International RBMK Safety Review Project funded by the EC’s TACIS Programme and the IAEA’s International Programme on RBMK Safety, generally agree on the problems which need to be addressed.

Luis Lederman, head of the IAEA programme, says safety concerns have been identified and prioritised and recommendations made for addressing each one. “To a large extent they are now being implemented,” he says.

The accident at Chernobyl was due to a combination of bad practices and bad reactor design, experts say.

The RBMK design is based on a military plutonium-producing reactor. It has graphite as a moderator and water as a coolant — a potentially dangerous combination.

However, one of the main design faults of the RBMK stems from the fact that there is a far greater risk of a runaway chain reaction if the coolant is lost — what is known as a positive void coefficient.

Nuclear energy depends on the fission (splitting) of atoms (usually uranium) to release energy. It requires careful control of the number and speed of neutrons produced in the fission process and the amount of heat generated. Uranium fuel is enriched to increase the number of fissile (easily splittable) atoms.

“Moderators” slow down the neutrons to a useful speed and “coolants” carry away excess heat. Control of the process depends on how these factors interact. Reactors are also fitted with control rods which absorb neutrons. Some are fixed in the core but others can be inserted or removed to fine-tune the reaction.

The RBMK in Unit 4 at Chernobyl had 16,000 fuel channels in its graphite core containing 140 tonnes of uranium fuel enriched to about two per cent. The channels are inserted into tubes containing water for cooling which are fixed in the core.

Water in these tubes boils from the heat being produced by the fuel. The reaction is moderated by the graphite. The RBMK is generally acknowledged to be one of the most complex reactors ever designed.

In April 1986 the staff at Chernobyl’s reactor 4 were told to undertake tests to assess its performance at low power, while the unit was being shut down for routine maintenance. At very low power, however, the RBMK becomes unstable, with high risk of a run- away reaction if too much of the coolant water in the core turns to steam too quickly. This is essentially what happened.

To enable power to be run down for the test, operators had shut down the emergency core cooling system and several other safety systems as well as removing most of the control rods — all in violation of regulations.

Once they realised the reaction was out of hand they lowered (scrammed) all the control rods. However because of bad design this simply added to the problem. They could not be lowered quickly and their initial effect was to displace even more of the cooling water, adding to the reactivity.

This sent power to 100 times its design level, shattering the fuel which then reacted with the water. The result was a steam explosion followed by another explosion due to expansion of the fuel vapour. The reactor lid was thrown off letting in air which reacted with the graphite moderator and ejecting an estimated eight tonnes of highly radioactive reactor fuel from the core.

In the first three years after the accident fundamental modifications were made to all operating RBMKs to eliminate the risk of a positive void coefficient. Enrichment of the fuel has been increased to 23.4 per cent and an additional 80 fixed control rods have been installed in the core to inhibit operation at low power.

The rods under manual control have been increased, redesigned and their operation speeded up. Precautions have been taken to prevent unauthorised access to safety systems.

Some problems still remain, however. RBMKs are all built without proper containment buildings which means that there is little to protect the environment in the event of an accident.

They also have inadequate emergency core cooling systems. Little can be done about this, but there is continuing Western involvement in improving safety through the introduction of more sophisticated computerised instrumentation and control systems.

There are currently two RBMKs operating at Chernobyl, two at Ignalina in Lithuania and 11 in Russia (Leningrad, Smolensk and Kursk). A 16Th unit — reactor No. 2 at Chernobyl — was closed down in 1991 after a fire.

One more RBMK — a modified third generation unit with added safety systems — is under construction at Kursk and there are plans for two new design RBMKs designated MKER-800s to be built at Sosnovy Bor near St. Petersburg.

Although some safety concerns remain, earlier this month the European Commission’s RBMK safety review which had been working since 1992 concluded that there was no need to close down the nine newer RMBK units (eight second generation and one third generation) provided they are carefully monitored.

The six older first generation models (two each at Leningrad, Kursk and Smolensk) could also continue to operate provided they were fitted with improved emergency core cooling and containment systems.