LOnge RAnge Navigation is the source of the LORAN acronym. It was a terrestial radio-based navigation system used by the Coast Guard prior to using more modern GPS systems.
These systems used vacuum tubes. In a vacuum tube, electrons flow from a cathode to the anode within a sealed, airless tube. The electrons emit X-rays as a result of a phenomenon called bremsstrahlung ("breaking radiation"). This usually isn't a safety problem until the voltage exceeds about 15 keV, because until then, the tube itself provides sufficient shielding of the X-rays.
As the voltage increases, additional shielding is required in order to minimize the X-ray dose (since X-ray energy is proportional to the voltage which is causing electron acceleration).
It looks like the Coast Guard didn't inform the LORAN workers of the X-ray risk, and some workers may have been overexposed. Though it's hard to tell from the article which doesn't give any dose estimates.
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