Heavy water is circulated at low temperature and pressure, upward through flow tubes that contain the fuel, and then spills out to fill the reactor vessel and serve as a moderator.

The vessel is separated from a surrounding light-water reflector by an annulus filled with carbon dioxide (CO2). This annulus was originally designed to irradiate two different assemblies – J-rods and Bismuth rods – creating different geometries for the penetrations into the annulus.

Below the CO2-filled annulus is a water-box that serves a cooling function. The top of this lower water-box forms a wide gutter intended to collect any leaking light water and feed it to a dedicated drain system. Throughout the life of the NRU, there have been light-water leaks into the annulus that, while not an immediate safety concern, have contributed to corrosion of components. A gutter lip was welded to the top of the water-box to separate possible heavy-water leaks from any light-water leaks from the reflector system.

The heavy-water gutter is a narrow slot between the reactor vessel wall and the water-box. Any leakage into the heavy-water gutter was designed to feed into a separate drain system. The heavy-water leak site was found to be near the base of the J-rod portion of the annulus.

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