Sunday, June 8, 2008

A Primer on Nuclear Safety: 1.1 Heat and Primitive Reactors

Nuclear safety

1.1 Heat and primitive reactors

A nuclear reactor is a structure designed to create the controlled fission of fissionable isotopes. Nuclear fission produces heat. In addition to the heat generated by nuclear fission, the radioactive decay of fission products also produces heat. Thus one of the fundamental challenges of reactor design is the reliable removal of heat from the core of the reactor. Heat must be removed as long as a reactor is critical, that is as long as nuclear chain reactions are taking place. In addition for most reactor designs, heat must be removed from a reactor during the initial period of reactor shutdown.

The removal of heat from a nuclear reactor does not require great sophistication. The 17 natural reactors at Oklo in Gabon, West Africa, had a self-regulating heat-removal system. The system required that subsurface water enter the uranium ore body. Water slows down energetic neutrons, and that promotes the probability of fission reactions. U-235 in the Oklo ore body began to fission and that led to a self-regulating chain reaction. With the chain reaction came heat, and the water began to boil. As the water boiled away, the ore body dried, and the fission process slowed and stopped.

Boiling water is one way to remove heat. Heat escapes in the form of water vapor. The Oklo ore bodies did not melt because the presence of water triggered the fission reactions, the presence of heat made the water in the Oklo reactors boil, the boiling of the water dried the ore bodies, and the removal of water by drying stopped the fission reaction.

This process occurred over and over again for hundreds of thousands of years.

Unlike the Oklo reactors, the first manmade reactor made so little heat that it required essentially no cooling. Fermi's reactor at Stagg Field in Chicago produced about a quarter of a watt of heat and essentially no decay heat.

The isotopes of uranium undergo a natural decay process, and the more fissionable U-235 decays more rapidly than U-238. So, while water could serve as a moderator for a natural uranium ore body reactor 1.7 billion years ago, this has long stopped being the case. But there are two moderators that can be used to achieve criticality with natural uranium. One is a isotope of hydrogen called deuterium. Deuterium is present in water in one part per 6000 water molecules. Although deuterium was of great interest to reactor pioneers, they chose another substance to to moderate the early reactors. That substance was ultra-pure graphite, a form of carbon. Early reactors were large structures built from huge graphite blocks. Holes were drilled in to the graphite blocks, and slugs of uranium, surrounded by aluminum, were inserted into the holes. More holes were drilled into the graphite for cooling air. Air was blown through the cooling holes and extracted the heat. The hot air was literally blown out of the reactor.

Thus there are two simple ways to cool a reactor. One is by blowing air through it. And this was the preferred method for some of the earliest graphite moderated reactors. The second method was the method introduced by that great reactor designer, mother nature, at Oklo. That is the method of circulating water through the reactor, and extracting reactor heat by boiling off the water.

As reactors grew bigger, the advantages of water-cooling became apparent. It had been suggested that the Hanford reactors, designed to produce plutonium for nuclear bombs, should be cooled with helium gas, but Eugene Wigner persuaded everyone that water would work far better. So the Hanford reactors were water-cooled graphite piles.

There is a big safety disadvantage of water-cooling graphite reactors, as Richard Wilson notes:

"Wigner was certainly well aware that there was a positive void coefficient in a water-cooled reactor - boiling of water in a channel would increase the reactivity - and the report has a whole paragraph showing how the water reduced the reactivity constant from 1.10 to 1.07. Inversely, a sudden removal of the water would increase it by 0.03 which is more than the fraction of delayed neutrons which allow control of the reactor. But as laconically noted by Alvin Weinberg later "no one dared to think of the consequences of a complete failure of the cooling".

What no one dared to think of actually happened at a place called Chernobyl, years later. Richard Wilson describes how the Russians some 30 years later still had not "dared to think of the consequences of a complete failure of the cooling" of a water cooled graphite reactor. Those consequences were a profound indictment of the Soviet communist system. For the Chernobyl disaster was the product of numerous systematic failures,that had their root in an ignorance of and disregard for nuclear safety. The Communist officials who had the power to make decisions about reactor design in the Soviet Union were intoxicated by their own ideology, and ultimately failed to understand the dangerous implications of bad decisions about nuclear safety.

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