Bulk Vitrification Sampling System Innovation
CH2M HILL Hanford Group, Inc. are conducting pilot treatment tests of a Bulk Vitrification (BV) technology and system to assist in immobilizing the low-activity tank waste from the underground storage tanks on the Hanford Site. An in-container vitrification process has been designed in which low-activity waste, soil, and glass forming chemicals are mixed, dried, and then melted at approximately 1500°C by electrical resistance. The current BV design uses steel containers 8 ft (tall) x 8 ft (high) x 24 ft (long). Refractory material is used to insulate the steel container walls from the high-temperature glass melt. The layers include a 15.2 cm (6 in) cast refractory, and a 5.1 cm (2 in) layer of duraboard insulation at each end of the box.
The Cast Refractory Block (CRB) is porous and the potential for leachable 99Tc to deposit in the pores of the CRB is high. Thus to better control and understand the complex dynamics and process interactions, representative physical samples will be acquired to produce cross sectional profiles of radiochemical distribution through the CRB, and CRB glass migration interfaces and will guide further developmental modifications to the BV process. Profiling will also help in assessing scale up relationships, verify waste feed parameters in producing acceptable glass and also gather sufficient data to demonstrate compliance with land disposal requirements and establish waste package variance.
Given the depths, the metal and sand outer layers, the paramount dry sample acquisition conditions, expected radiological conditions, extreme physical hardness and the multiple interfaces of the materials used to vitrify and encapsulate the waste, it is not surprising that successful dry sampling (in a safe and total containment of active debris) has only achieved limited success. NMNT has sampled “dry”, a simulated ICV Box configuration of a representative mass of drilled and cored glass and refractory materials and collected hermetically representative incremental powder or whole and intact core samples. This sampling process is operated at present in the horizontal direction perpendicular to the ICV Box metal wall, penetrating through the varying material layers into the waste package, rapidly accessing predetermined matrices and locations in a contained simulated manner.
Increased process sampling credibility will require a program of effective and focused action; one that is paramount to a safe radiological ICV Box sampling regime given the high visibility of the Bulk Vitrification program. Representative and rapid sampling of the vitrified waste containers are of the utmost importance to allow effective verification of the process.