Chernobyl: Reality and Myth

What really happened at the Chernobyl Nuclear Power Station on April 26, 1986?

Located on the banks of the Pripyat River, sixty miles north of the Ukraine capital of Kiev, Chernobyl was a major civilian nuclear power station for the Soviet Union. The Soviets designed the Chernobyl reactors according to the RBMK model, which included natural uranium reactor fuel, a water-based cooling system and control rods and a reactor core casing made of graphite.

This reactor model had one significant advantage over other models: It produced on average ten percent more power. Unfortunately, it had one significant disadvantage: On failure the reactor core would go "supercritical." In event of a mishap involving the control system, the reactor would heat up to the point where its fuel would melt down into a heap of slag.

Unrelated to the basic design problem, these reactors had no containment buildings. Although they were shielded by heavy, reinforced concrete, the units were not surrounded by buildings designed to withstand a reactor core explosion.

So why did the Soviets employ this risky design? In the old Soviet Union, like everywhere else it was all about money. To the managers of their centrally-controlled economy, the ten percent additional power production glittered brightly against a backdrop of what turned out to be inferior Soviet technology and engineering. They took a chance, a calculated risk.

Sure it was a stupid thing to do, but they did it. I met with three of the investigating Hanford engineers after they returned from Chernobyl.

They told of a Deputy Chief Engineer who had the previous year convinced Moscow to let him run an experiment that had the potential for allowing power to be drawn from the spinning turbine of a reactor that had just shut down, emergency power that could be used to run emergency coolant pumps during the interim while the emergency diesel generators were coming on line.

Although this engineer was not nuclear trained and really knew nothing about nuclear reactors, the idea seemed to have merit, and a successful prosecution of this concept would elevate this Deputy Chief Engineer to the top of his peer group. He eventually received permission and some time before the accident attempted to run the experiment. Unexpected problems caused the reactor safety systems to shut it down before he could run the experiment. Although red-faced with embarrassment, he petitioned for, and eventually received a second chance, but apparently was also told about dire consequences should he fail again.

When the time came, and in order to prevent the reactor from shutting down during the second run, he ordered all five safety systems bypassed, and he also had all the backup electrical systems shut down, including the emergency diesel generators that could have powered the reactor controls in an emergency.

He probably felt safe doing this because he did not intend on running the reactor for more than a few minutes under load. After all, what could possibly happen in a few short minutes? And, not being nuclear-trained, he had no idea of what unintended consequences could result from disconnecting these systems. Although we will never know for sure, he may have been thinking that the worst-case scenario would be a complete shutdown of the reactor as would happen if the fuel supply were cut off from a conventional boiler.

As luck would have it, unexpected power demand that afternoon delayed the onset of the experiment until late in the evening. In order to get the experiment underway, the engineers needed to reduce reactor power to minimum, and because they were behind schedule, they reduced the power level more rapidly than this reactor design could handle. This caused a buildup of neutron-absorbing fission byproducts which poisoned the reaction process and threatened to shut it down altogether.

Since that would have spoiled the experiment a second time (Hello Siberia), to compensate, they withdrew most of the control rods. Because of the poisoning, this allowed a power increase to barely 30 megawatts, which was just sufficient to bring the reactor into its most unstable range. Something had to be done immediately.

There were only two choices: do absolutely nothing, and wait twenty-four hours for the poisoning to dissipate, or increase the power immediately.

With the threat of exile to Siberia in the wings, we know what choice they made.

The engineers finally marginally stabilized reactor power at 200 megawatts - one fifth of the unit's design power. But because the reaction was still poisoned, they had pulled all but six control rods from the core. The absolute design minimum for this reactor was thirty rods kept in the core at all times, so the immediate situation was dire.

About a half hour later they decided to commence the actual experiment and shut down the turbine generator. Their intent was to see if the turbine could still supply coolant pump power even though it was only coasting - no longer being driven by the reactor. A successful outcome would prove that they did not need to obtain outside power to maintain proper cooling levels when they decoupled a reactor and its turbine. An engineer with nuclear training could have told them the answer without conducting the experiment. But these guys weren't nukes. With reduced electrical power, the pumps slowed, reducing the flow of cooling water.

Modern nuclear reactors used in the United States and the rest of the world control neutron level by absorbing them with Boron or Cadmium control rods. The primary coolant acts as a moderator by slowing the neutrons. The RMBK model, however, works in reverse, using graphite rods to moderate the neutrons, and the primary coolant to absorb them.

At this critical juncture on April 26, 1986, we had a reactor operating at a significant power level with almost all the moderating control rods pulled out. The reactor was still stable - although barely - because the primary coolant was absorbing neutrons as fast as they were being produced. At this point, disaster struck: The coolant pumps slowed as a result of reduced electrical power from the shut down turbine, so the cooling water moved more slowly through the system. It stayed in the reactor core longer, getting hotter, and finally began to boil. But steam cannot absorb neutrons: Suddenly the neutron flux - the total emission of neutrons from the reactor fuel skyrocketed.

The reactor operators immediately hit the emergency button designed to drive all the control rods back into the fuel core, but since all backup power had been shut down, even the emergency diesel generators, the only available electrical power came from the slowing turbine. This meant that the already slow primary coolant pumps had even less power, and so the skyrocketing neutron flux increased even more.

This is when another design problem of the RMBK became evident. The control rods had graphite tips followed by a one meter hollow segment (I don't know why, they just did), followed by a five-meter graphite section. As soon as the rods penetrated the core, they displaced more coolant without themselves absorbing any neutrons, because of the hollow section.

The already skyrocketing neutron flux went ballistic and all hell broke loose. The reactor container exploded - not a nuclear explosion, just a plain, old-fashioned steam boiler explosion. But it was a doozy: Red-hot chunks of highly radioactive reactor fuel and graphite fell everywhere. Fifty tons of nuclear fuel evaporated in the blast and were ejected high into the atmosphere. Another seventy tons of fuel were ejected sideways into the surrounding area. An additional fifty tons of fuel and eight hundred tons of graphite remained in the reactor vault smoldering for days. Experts have placed the release of radioactivity at about ten times the amount generated by the atomic bomb that destroyed Hiroshima.

A plume of radioactive fallout swept across Europe, leaving measurable contamination as far away as Finland. There was a veritable continent-wide panic reminiscent of the response to the Three Mile Island incident seven years earlier.

In the final analysis, however, the health consequences were relatively small. According to the Nuclear Energy Agency (a specialized agency within the Organization for Economic Co-operation and Development, an intergovernmental organization of industrialized countries based in Paris) only 31 persons as of April 2001 had died as a direct consequence of the accident. They were all either plant personnel or directly involved in fighting the fire following the explosion. Another 140 individuals from these same groups suffered varying degrees of radiation sickness and health impairment, but all had recovered fully with no permanent consequences. During the period between 1990 and 1998, in the regions affected by the explosion and subsequent fallout, officials diagnosed 1,791 cases of thyroid cancer that were assumed to have been caused by the radiation release.

The deaths and the injuries are tragic, of course. But this is a far cry from the misinformation contained in a Greenpeace website commemorating the tenth anniversary of the Chernobyl disaster, where they state flatly that 2,500 people were killed, millions were affected, and hundreds of thousands displaced.

A careful examination of the Chernobyl incident reveals that it was a stupid, completely unnecessary accident resulting from gross criminal negligence and total managerial incompetence. This problem could only have happened within a political system that was completely out of contact with the real world. Moreover, the entire tragedy stemmed from what experts call a unique "accident chain" - a series of missteps that as a whole led to a particular breakdown. Chernobyl hinged upon the reactor becoming unstable when the coolant flow slowed, but this can only happen in the RMBK reactor design. All other reactors in use would have shut themselves down.

Those who have waved the banner of Chernobyl in a campaign to ban nuclear power worldwide have ignored the facts of this incident, in particular the reality that such an explosion is impossible in other reactor designs.

Next week: An examination of the politics of nuclear waste.