One of the coolest things about nuclear reactors is Cerenkov radiation. The Theory of Special Relativity is pretty unambiguous about the speed of light in vacuum. It is unreachable. Any object attempting to approach the speed of light will have to be fed an enormous amount of energy. Since energy and mass are the same, the object will get heavier as it speeds up. This means it takes yet more energy to get it to speed up further, which means it will get heavier still and so on. In order to reach the speed of light, the object will need infinite energy, something which is of course impossible.
So even high energy beta particles spewed from decaying radionuclides, travelling as fast at 290 million metres per second, will still be below the speed of light in vacuum.
However, 200 million metres per second is faster than the speed of light in water. And when an electron travels faster than the speed of light the same thing happens as when an aircraft or a bullet travels faster than the speed of sound. There is a sonic boom, or in this case a luminal boom. This takes the form of bright blue glow known as Cerenkov radiation.
The important question that must now be asked about this phenomenon is, how can this be used for decorative purposes? The obvious answer is in lighting for a swimming pool. A swimming pool illuminated from the bottom by an eerie blue glow would be awesome.
Strontium-90 would be a rather handy isotope to use. It is a pure beta emitter, which means that the radiation will not penetrate beyond a few centimetres. Strontium can also be manufactured into the durable ceramic, strontium titanate. Small sheets of high enriched ceramic could be placed just above the bottom of the pool. A mesh barrier would be placed a few centimetres above the ceramic blocks to ensure no swimmers got within range of the radiation.
So you have a cool lighting setup and free heating to go along with it.
Unfortunately, there is a problem with this. Strontium-90 emits purely beta radiation but it decays to yttrium-90, which emits beta radiation along with some powerful gamma radiation. A few centimetres of water is not going to shield against that.
You could also raise questions about high energy ultraviolet from Cerenkov radiation, but given how many people regularly like to lie in ultraviolet grills, this particular fact wasn't considered a showstopper.
Oh well, back to the drawing board for the next million dollar idea.