Development of a long-term post-closure radiation monitor

The long-term monitoring of a hazardous waste site for migration of radionuclides requires installation of radiation sensors at a large number of subsurface locations. Sensors which can be lowered into boreholes to make radiation measurements are available, but are relatively complex and expensive and generally not suitable for long-term installation underground. This problem is amenable to a solution based on fiber optic technology. The concept under development in the present contract employs a passive in-ground measurement probe which contains a scintillator coupled to optical fibers. Gamma radiation absorbed by the in-ground scintillator generates optical photons which are coupled into the fiber optic cable, conveyed to the surface and detected as an indication of the instantaneous subsurface radiation level. The in-ground probes will be configured to geometrically resemble cone penetrometers or penetrometer-based sampling tools and thus can be installed to depths of up to 50 meters using conventional CPT trucks and methods. The use of the penetrometer eliminates the cuttings normally associated with drilled wells, and thus minimizes worker exposure and eliminates the cost of cutting disposal. A large number of the in-ground passive probes can be multiplexed to a single, aboveground opto-electronics unit to provide for detection and readout of any long-term changes in the distribution of the radionuclides in the vadose zone. The overall goal of the Long-Term Post-Closure Radiation Monitor development program is to confirm a long-term radiation monitor using commercially available, demonstrated components to the largest extent possible. The development program is planned as a three phase program spanning a total time of 53 months