A moisture probe based upon neutron moderation for in situ use in a cone penetrometer system

A moisture probe based upon neutron moderation has been designed, tested and built for measuring the moisture concentration of hazardous waste stored in the underground waste tanks at the DOE Hanford site. The probe is 1 inch in diameter, 24 inches long, and is deployed using a computer controlled winch into a cone penetrometer for in situ moisture measurements. Two thermal neutron detectors in a near-far geometry and a low-intensity ²⁵²Cf source are used in the probe to count thermalized neutrons for determining the moisture content. Several testbeds were constructed and a variety of experiments were performed to determine the optimal detector-source geometry and to rest that the sensor is capable of operating in the expected high temperature and very high gamma-ray fields within the tanks. Several calibration curves of the near-far count rate ratio were determined for tanks contaminated with various levels of neutron absorbing materials. A field-ready probe and a winch deployment system have been delivered for application of this technology to the Hanford Site tank farms