A research reactor consists of a reactor core submerged in a deep pool of water. The reactor part is an assembly of metal plates that are uranium oxide in a metal or ceramic matrix, clad with metal (usually a zirconium or aluminum alloy). The power output is sufficiently low that the natural circulation of water in the pool (warm water rises, cool water sinks) keeps the reactor cool. The fuel elements require fairly precise spacing to sustain a chain reaction - too close or too distant and the reaction will stop.As you can see, spending the time and effort to put together an explanation like this is rather considerable, and it will always be easier to play on people's fears of the unknown, or simply what isn't understood, than to explain the science involved.
Throwing a bomb into the pool will damage the core, and may break fuel elements. This, in turn, would release some radioactivity - primarily within the reactor building. The reason for this is that the fuel elements are metal - they are more likely to be bent or twisted than to be broken in half, and they will certainly not be vaporized. This is important because it means that the amount of radioactivity that can escape is limited - a cracked fuel element, even a broken one, will release only as much radioactivity as is exposed at that point. Think of cutting into a pie - some of the filling leaks out into the cut area, but the pie does not spontaneously empty when it's cut open. Similarly, if a fuel element is broken or cracked, some of the radioactivity will leak out, but only a fraction. In other words, there will likely be a release of radioactivity, but most of the radioactivity will remain contained within the fuel elements. Of the activity that escapes from the fuel elements, much would be entrained in the water, and would end up in the reactor building, not on campus. Some contamination would likely be released, and some would likely enter the environment. However, the risk from this would be low because radiation is less dangerous than many tend to believe. I would refer you to the papers I sent you earlier for more on the effects of low-level radiation exposure.
The bottom line is that throwing a small bomb into a reactor pool is likely to damage the core, and likely to cause some radioactivity to be released. However, I would not expect this contamination to pose a health risk to people nearby. Similarly, a truck bomb would likely damage the core and could release larger amounts of radioactivity to the environment, but much of the blast would be diverted by the concrete "swimming pool," reducing the amount of damage.
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