At full power (200 kilowatts), the MSTR core produces approximately 6.4 trillion fissions per second. Each fission event liberates a tremendous amount of energy, a portion of which is carried away by fission products which then decay and produce high-energy beta particles. Often, these beta particles are emitted with such high kinetic energies that their velocities exceed the speed of light (3.0x108 meters per second) in water. When this occurs, photons, seen to the eye as blue light, are emitted and the reactor core "glows" blue.
While no particle can exceed the speed of light in a vacuum, it is possible for particles to travel faster than light in certain mediums, such as water. The speed of light in a particular medium, v, is related to the speed of light in a vacuum, c, by the index of refraction, n, by v = c/n. Water has an index of refraction of 1.3, thus the speed of light in water is 2.3x108 meters per second. Therefore, beta particles with kinetic energies of 0.26 MeV travel at speeds in excess of 230 million m/s!
When a charged beta particle moves through water it tends to "polarize" (orient) the water molecules in a direction adjacent to its path, thus distorting the local electric charge distribution. After the beta particle has passed, the molecules realign themselves in their original, random charge distribution. A pulse of electromagnetic radiation in the form of blue light is emitted as a result of this reorientation. When the speed of the beta particles is less than the speed of the light in water, the pulses tend to cancel themselves by destructive interference; however when the speed of the beta particle is greater than the speed of light in water, the light pulses are amplified through constructive interference. The phenomenon is analogous to the acoustic "sonic boom" observed when an object exceeds the speed of sound in air.
The intensity of the blue glow is directly proportional to the number of fissions occurring and the reactor power level. This property is utilized in Cerenkov detectors that measure the magnitude of Cerenkov radiation produced in a detector made of lucite. Cerenkov radiation becomes visible in the MSTR core at a power of about 6 kW. At 200 kW the core glows brilliantly with a soft blue glow. The blue glow continues for some time after the reactor has been shut down due to the decay of fission products.
Another explanation of the Cherenkov effect can be found at Wikipedia. The Wikipedia site features the MSTR in its examples to explain the subject.