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The Case of the Stealthy Silicon
Kenneth Russell, Professor Emeritus, MIT -- Design News, November 3, 2008
Silicon is primarily known as the basis for most computer chips, but it is also valuable as an alloying element, notably in transformer steels and a wide range of aluminum-based alloys. This column tells of an unusual role of silicon as an alloying element.
Neutrons are very valuable tools in a range of fields, including biology, physics and metallurgy. You can do things with neutrons that cannot be done with light waves, X-rays or ions. Most neutrons are produced in nuclear reactors that fall under the auspices of the Dept. of Energy (DOE).
The Scene of the Crime
Some years ago, the DOE became concerned with a possible loss of coolant accident at the High Flux Beam Reactor (HFBR) on Long Island. A loss of coolant would allow the reactor core to overheat, as occurred at the Three Mile Island reactor. HFBR was ordered shutdown, only to reopen when safety could be ensured.
The Investigation
A very broad-based committee was appointed to study important topics including fluid flow, human factors and metallurgy. I have spent most of my career studying the effects of radiation on alloys, so was asked to do the metallurgy work for the committee. Metallurgy was a particular concern in that some of the aluminum alloys in the reactor core were showing loss of ductility.
Neutrons have no electrical charge, hence they may easily enter the atomic nucleus to give transmutation reactions. In the case of aluminum, the nucleus may simply capture the neutron to move up a notch in the periodic table and become silicon. The absorbed neutron may also induce the atom to split into two nuclei. In the case of aluminum, one of the particles is often a helium nucleus.
Helium is a bad actor in irradiated alloys. The atoms agglomerate to form bubbles that embrittle the material. It was feared the observed loss in ductility was due to such bubble formation. If so, the reactor was doomed and could not be restarted.
I insisted an electron microscope study be done to determine just what the alloy microstructure was. Helium bubbles would be readily visible in this instrument.
The Smoking Gun
The Oak Ridge National Lab. in Tennessee did the study on the highly radioactive samples and found a surprising result. There were no helium bubbles of any size. Instead, there were huge numbers of very fine silicon particles throughout the sample. These particles had strengthened the alloy and given some loss in ductility, which in the present case was tolerable. The helium atoms had been tied up at the interfaces between the silicon particles and the aluminum matrix and rendered harmless.
The alloy was perfectly safe to use. In fact, it was the strongest aluminum alloy that had ever existed. There would be a big commercial market for the stuff were it not so radioactive.
My committee gave the reactor a clean bill of health and all seemed in order for a restart. But, the antinuclear crowd was lurking. They struck at a benefit for international refugees given by financier George Soros. Clinton Secretary of Energy Bill Richardson was there, as were supermodel Christie Brinkley and actor Alec Baldwin. The latter two lobbied the secretary against restart, claiming various dire consequences.
Shortly after the Soros benefit, Richardson announced the reactor would not be restarted. He gave monetary reasons rather than citing safety concerns, but neither side believed him. The research community wailed like banshees, but to no effect. (Read the full Soros story).
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