What Really Happened at Three Mile Island and Chernobyl

by Marc de Piolenc
Cleantech Asia Online
1 July 2009

Everybody thinks they know what's wrong with nuclear power. Ever heard of Chernobyl, and of Three Mile Island? When nuclear power goes wrong, it goes horribly wrong, and even when it works as it's supposed to, it produces dangerous wastes that have to be stored and guarded for thousands of years.

So much for what almost everybody thinks they know. Now for some facts.

The Soviet reactor at Chernobyl literally blew up. It wasn't really a nuclear explosion; it happened when hydrogen, generated by the dissociation of water in the runaway reactor core, recombined explosively with oxygen.

The Soviets didn't believe in containment structures (those great prestressed-concrete golf balls that surround all Western reactors) so there was nothing to prevent the spread of highly radioactive fission products and irradiated reactor components over thousands of square miles. How did that effusion of hydrogen come about? Glad you asked. The reactor operator violated his own rules. The reactor had been undergoing a turbine coastdown test, in which power was quickly reduced, allowing  "fission poisons" - fission products that inhibit the nuclear chain reaction - to accumulate. Safety regulations for this type of reactor require that it be gradually ramped down in power, to prevent the accumulation of fission poisons. The experimental rapid slowdown should have been followed by a long rest period, to allow the accumulated poisons to decay.

But the regional power dispatcher called for full power, and the operator obeyed. To build up reactor power in the face of the inhibiting effect of the poisons, the reactor operator had to take heroic measures. When the reactor finally did react, it did so abruptly, causing a runaway rise in power that was too fast for the operator to control. Steam began forming in the core.

In a typical Western nuclear plant, the presence of steam bubbles would have instantly reduced the reactivity of the core, because the presence of liquid water is needed to "moderate" the neutrons that sustain the nuclear fission chain reaction - that is, to slow them down so they can trigger more fissions. The Chernobyl reactor used graphite as its moderator, so the steam didn't calm the reactor; it only made the reactor cooling system completely ineffective - think of the vapor lock phenomenon that sometimes happens in your car's cooling system.

From that moment on, the doom of the Pripyat Marshes was sealed. The resulting horror cannot be exaggerated. But here's the point: despite the absence of containment, despite the paucity of engineered controls, the explosion of this RBMK (designed originally for plutonium production for nuclear weapons) was completely preventable - only human folly made it possible.Well, how about Three Mile Island (TMI)? TMI Unit 2 was a modern, Western-style pressurized-water reactor with all the bells and whistles, including a containment structure. It all started when a cooling pump failed. The auxiliary pumps kicked in automatically, as they were designed to, but for some reason the operator overrode the controls and turned them off. The water in the core began to boil, which raised the pressure inside and caused the reactor to shut down automatically. This caused the core temperature to rise for a time, as neutrons were absorbed by the safety rods. No problem, except that the pressure relief valve in the reactor vessel failed to close, allowing more of the coolant to escape. At this point the auxiliary cooling pumps came back on, but were throttled back by the RO. Water continued to boil in the reactor core for three hours, causing irreparable damage to the reactor and a loss of nearly a billion dollars to the owner. But it is important to stress that no human deaths or illnesses occurred, either within or outside the TMI grounds.

Let's compare TMI and Chernobyl.

Chernobyl's reactor "ran away," producing excess power. The damage was caused by a hydrogen-oxygen recombination generated by the reactor running well above its rated power, with a completely inoperative cooling system

Three Mile Island (TMI) Unit 2 was successfully shut down by automatic controls; damage was caused by residual heat from neutron absorption, compounded by human interference with a working emergency core cooling system

Chernobyl was over in an instant; once the conditions for an accident were present, the situation was beyond remedy.TMI could have been prevented from causing the total loss of the reactor at any time during a period of hours preceding the collapse of the core.

Human error was a factor in both events, but especially at TMI where multiple consecutive errors of judgment were committed. Damage from TMI was limited to the reactor core primarily due to its safer design.

Now let’s compare casualty counts. As of April 1990, 31 people had died as a result of Chernobyl, and over 200 cases of acute radiation poisoning was reported, some of which would certainly have resulted in death. More people have certainly died since then, and more will die before their time in the years ahead from the delayed effects of exposure to the fallout of that disaster.

Human casualties from TMI are a nice round number: zero.

Lesson: even with old technology, safe nuclear power - that even in the worst case does not harm the general public - is possible. Far from causing a loss of confidence by the public in nuclear power, TMI should have bolstered it!

The designers of nuclear powerplants learned valuable lessons from TMI: newly constructed and future plants include passive safeguards that are inherent in the plant configuration, making a deadly disaster physically impossible, even in the worst case of compounded human errors and a total failure of engineered controls. Future nuclear plants will consist of clusters of smaller reactors, rather than a few giant ones. There will be "nuke farms," just as there are now "wind farms" and solar power arrays. All support services will be on-site, to reduce the need to transport fissionables and radioactive material.

A life-threatening nuclear accident to Western reactors is already unlikely; the safety record so far is one for zero - one accident, zero deaths. The new technologies of the nuclear industry should allay fears of nuclear proliferation and transport accidents as well.

Marc de Piolenc is a mechanical engineer, a former field editor for Gas Turbine World and Cogeneration magazines, who has tracked nuclear power developments since the mid-1980s.

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