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The Advanced Test Reactor (ATR) is a research reactor at the Idaho National Laboratory in the United States. Constructed in 1967, the ATR is the second oldest of three reactors still in operation at the site. Its primary function is the intense bombardment of materials and fuels with neutrons to replicate long-term exposure to high levels of radiation, as would be present after years in a commercial nuclear reactor. The ATR is one of only four test reactors in the world with this capability. The reactor also produces rare isotopes for use in medicine and industry.
In this picture the ATR is glowing bright blue. This is due to Cherenkov radiation, which emits photons in the blue and ultraviolet range.Photograph: Argonne National Laboratory