Neutron Imaging of Lithium Inside a High-Temperature Heat Pipe

Neutron imaging of a high-temperature Mo-Li [molybdenum-lithium] heat pipe has been performed to assess Li flow visualization. The high neutron attenuation cross section of Li allows for imaging inside the heat pipe shell structure, which has a lower value of neutron attenuation, making neutron radiography an ideal choice for characterizing Li movement.

The gravity assisted, cylindrical heat pipe was heated to 903^oC [degree] using a wire resistance Mo heater operating under vacuum. This Mo-Li heat pipe is 35 cm long with a square internal geometry 2.2 cm per side, utilizing small corner radii as the wicking structure for Li return. The heat pipe is evacuated and back filled with 10 g of Li and has an estimated operational temperature of 900^oC. The neutron imaging prototype facility on the CG-1D beamline at Oak Ridge National Laboratory was used to capture 30 s exposures of heat pipe operations from ambient conditions through startup to steady-state and subsequently through two large power fluctuations. Tomographic imaging of the heat pipe was performed using 180 s exposures at 1.5^o increments with the heat pipe at ambient conditions.

Time lapse reconstruction of the neutron radiography shows a change in the bulk Li level and the meniscus concavity. A change of 9 mm is measured from ambient conditions to steady-state operation. This is a result of Li density changes as well as Li surface tension changes. As the temperature increases, Li held at the top of the pipe through capillary forces flows towards the evaporator increasing the bulk Li level. A decrease in the meniscus concavity confirms this. Dynamic movement is seen on one side of the pipe during both power fluctuations. Tomographic reconstruction shows Li held in the corner channels above the bulk Li level.

Subsequent work has been performed to develop a new test platform to house future neutron radiography testing. This new platform will increase efficiency on the neutron beamline by decreasing heat pipe startup time. It utilizes an induction heating system to achieve surface heating of the pipe and can be adapted to different heat pipes and orientations.