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Thermal Shield Cooling System Upgrade; from Positive Pressure to Vacuum

Paul Brand (NIST Center for Neutron Research)

The National Bureau of Standards Reactor (NSBR) is equipped with a thermal shield. This shield is located immediately around the reactor vessel and consists of a thick-walled steel tube (up to 6. inch wall thickness), which is clad on the inside with a 2 inch thick lead layer. Soldered onto the inside diameter of the steel tube (thus surrounded by the lead) are 188 copper cooling lines, which allow the removal of heat deposited into the thermal shield by gamma rays that are generated in the reactor core. Shortly after the first criticality of the NBSR the thermal shield cooling system developed small leaks. The cause of these leaks has been theorized to be related to the thermal expansion and contraction of the thermal shield during normal reactor on and off cycling. These thermal cycles cause the horizontal sections of the copper cooling lines, which are trapped between the inside of the steel tube and the concrete of the biological shield, to be stretched and compressed until they finally crack due to excessive work hardening (as in a low cycle fatigue process). Whatever the cause of the leaks: over the years we dealt with them by treating the lines with a variety of chemicals and compounds. This approach has allowed us to manage the leaks, but not to actually repair them. The situation is slowly becoming less manageable and we find that we spend more and more time maintaining leaking tubes: time that could be spend in other ways... Over the last two years Reactor Engineering has run a test program whereby it was investigated if . and under what circumstances . the thermal shield could be operated under a vacuum, i.e. .sucking. the water through the cooling pipes, rather than .pushing. it through. The advantage of such an approach would be that if the water flow were to encounter a leak, gas from the surroundings would flow in, rather than water from the inside flowing out. This idea has been proposed before, but so far the idea bumped into the limitation that one of the gasses .flowing in. would be carbon dioxide, which would render the cooling water acidic, thus jeopardizing the cooling system plumbing, which mainly consists of copper. It turns out the cooling water can be .neutralized. using magnesium carbonate, which forms a buffer pair with the carbonic acid that is formed by carbon dioxide dissolved in water. The results of the test program will be shared, in support of our conviction that a long lasting solution to the thermal shield problem has indeed been found. The underlying Chemistry, Metallurgy, Process Technology and Radiation Physics will be presented and explained as well as a way in which this idea can be scaled up towards implementation.

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