|Fig. 1: Cerenkov radiation from the Reed Research Reactor, Portland, OR. (Courtesy of the U.S. Nuclear Regulatory Commission.) Source: Wikimedia Commons|
The notion of glowing radioactive material is well established in popular culture, but as with many aspects of nuclear energy, the topic is rife with misinformation. As it turns out, two distinct physical causes give rise to what could be loosely termed as "nuclear blue glow," the blue light given off in certain environments with sufficiently intense radiation. The blue glow seen in water-moderated reactors and the blue glow observed in criticality accidents in the laboratory setting may seem related, but whereas in criticality accidents a blue glow in air is due to the emission of light in the relaxation of excited gas molecules, the blue glow of a reactor has more exotic origins.
Nuclear reactors are often pictured with a cloud of beautifully disquieting blue light (see figure 1), termed Cerenkov radiation. Cerenkov began his first contributions to the understanding of this radiation in 1934 when he observed light from the gamma-ray irradiation of a pure solvent by a radium source. This observation was unexpected, as he was initially attempting to study the light emitted from uranyl salt solutions under gamma-ray irradiation, with no expected background from the solvent. Moreover, the luminescence could not be quenched, by varying the temperature or otherwise, as is possible in the case of light emitted from the decay of an excited state. Cerenkov’s advisor Vavilov postulated that the radiation might be due to the motion of electrons, and further experiments using beta decays provided supporting evidence. 
Not more than three years later, Tamm and Frank published a theory elucidating Cerenkov’s observations, determining its cause to be the motion of charged particles with phase velocity greater than the speed of light in the traversed medium.  Several salient features of the radiation may be observed from the radiation intensity spectrum they derived, where we assume the charged particle is an electron and v is the speed of the electron, e is the electron charge, c is the speed of light, and n is the index of refraction of the medium:
First, it is apparent that the expression is negative and therefore meaningless unless v > c/n, the phase velocity of the medium. Second, it is interesting to note that Cerenkov radiation is readily differentiated from Bremsstrahlung radiation because the intensity does not depend on the mass of the charged particle, whereas Bremsstrahlung radiation does.  Finally, given that the index of refraction of water does not vary much over the visible spectrum, the intensity goes linearly with frequency, explaining why Cerenkov radiation appears blue.  Later experimental studies corroborated agreement with the Frank and Tamm theory, apart from understood deviations such as ultraviolet absorption in water and second order effects. 
It is interesting to consider what the energy of an electron would need to be in order to observe Cerenkov radiation in air. Assuming a constant index of refraction of 1.0003 over the visible spectrum, the threshold velocity would be roughly 0.9997c. In this case, a simple calculation of the relativistic kinetic energy yields the threshold of 20.35 MeV, higher than all but the most energetic beta decays, many of which only occur with rare isotopes.  It is then clear why Cerenkov radiation is primarily associated with the pools of water surrounding reactor cores—the threshold kinetic energy in water, assuming an index of refraction of 4/3, may be readily calculated to be only about 260 keV, an energy scale readily accessible to common beta decays. Furthermore, it is obvious that a blue glow in the air as seen in criticality accidents cannot be explained by Cerenkov radiation. This is not to say that Cerenkov radiation is only found at the core of nuclear reactors; interestingly, Cerenkov radiation has been implicated in the flashes of light that astronauts have reported seeing with their eyes closed while in transit to the moon. 
© Andrew J. Keller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. M. Bolotovskii, "Vavilov-Cherenkov Radiation: Its Discovery and Application," Physics Uspekhi 52, 1099 (2009) [Uspekhi Fizicheskikh Nauk 19, 1161 (2009)].
 J. D. Jackson, Classical Electrodynamics, 3rd Ed. (Wiley, 1999).
 M. Daimon and A. Masumura, "Measurement of the Refractive Index of Distilled Water from the Near-Infrared Region to the Ultraviolet Region," Appl. Optics 46, 3811 (2007).
 J. A. Rich, R. E. Slovacek and F. J Studer, "Cerenkov Radiation From a Co 60 Source in Water," J. Optical Soc. Am. 43, 750 (1953).
 Chart of Nuclides," U.S. National Nuclear Data Center.
 G. G. Fazio, J. V. Jelley and W. N. Charman, "Generation of Cherenkov Light Flashes by Cosmic Radiation Within the Eyes of the Apollo Astronauts," Nature 228, 260 (1970).