The Wigner Effect
In: Physics
The Wigner effect is seen in materials that are exposed to sources of high energy neutrons. Usually in reactors. The neutrons are of course smaller than individual atoms, but in a typical reactor there are literally billions of them shooting out in all directions. These neutrons are moving very fast, so when they hit something head on, things go kablooey. Think of throwing a bare engine block at a car at twice the speed of sound.
Few collisions are as head on as the engine block example, striking only glancing blows to the atoms along the way. In a liquid, that atom gets kicked aside and then is allowed to flow back into position. But in a crystalline substance (like steel) an atom kicked aside can’t easily go back to where it started. So it ends up out of position, causing a defect. The strength of many solids depends on the crystalline structure which spreads out loads. If too many defects occur, the solid can lose a significant amount of strength. This is an obvious problem when dealing with something like a special high strength steel reactor vessel that needs to contain high pressures.
Another issue is that the displaced atoms “want” to go back to their starting point. This means they have a potential energy, just not enough energy individually to manage the relocation. But if enough displaced atoms accumulate, they can collectively have enough energy to move back. When they do this, they release a lot of wasted potential energy as heat. Whether this occurs in steel pressure vessels, carbon moderator rods or whatever, sudden very massive spikes in heat are a Bad Thing. It’s quite possible for the carbon moderators to almost explode with the suddenness of combustion that can occur during these temperature excursions. For carbon rods, the accumulated energy can be safely released by annealing, which is deliberately heating up the carbon fairly high so that it is easier for the displaced atoms to get back into their proper place. Many working reactors use an annealing cycle as part of routine operations. However, not all materials can be annealed. Annealing steel removes a lot of its strength, requiring proper heat treatments to regain it.
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