Operation of the NIST reactor thermal shield under vacuum to mitigate sustained coolant loss

The 20 MWt test reactor operated by the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) provides neutron radiation that benefits a wide variety of biological and materials science research programs. The Reactor is equipped with a Thermal Shield (TS), which consists of a 2.5 m diameter steel tubular structure with a wall thickness of 0.15 m. On the inside the tube is cladded with 0.05 m of lead. Besides acting as a primary gamma shield, the function of the TS is to protect the concrete in the radially adjacent biological shield from disintegrating due to radiation damage. Copper tubes (188 of them) embedded in the lead carry water that serves to cool the lead, protecting it from melting due to gamma radiation heating; Immediately after the reactor's first criticality in the late 1960's, the TS copper tubing began to develop tiny leaks that had to be tended to by applying various stop leak methods. Since the TS cooling system is not a reactor safety system, this never impeded the safe operation of the reactor, but by the early 2000's the maintenance effort involved in coping with the leaks had become a significant operational burden. Particularly the containment of loose contaminated coolant became very time consuming and detrimental to other parts of the facility; In 2006 the decision was made to attempt to stop the leaking by driving the thermal shield coolant flow by vacuum instead of traditional positive pressure. This would result in the admission of the gas surrounding the coolant lines into the liquid coolant streams rather than liquid escaping from them. This has two advantages: no more radioactive coolant loss and no more need to attempt to mend the leaks (which led to further degradation of cooling tubes due to coagulation of various stop-leak products)