Technology Review of Nondestructive Methods for Examination of Water Intrusion Areas on Hanford’s Double-Shell Waste Tanks

Under a contract with CH2M Hill Hanford Group, Inc., PNNL has performed a review of the NDE technology and methods for examination of the concrete dome structure of Hanford’s double-shell tanks. The objective was to provide a matrix of methodologies that could be evaluated based on applicability, ease of deployment, and results that could provide information that could be used in the ongoing structural analysis of the tank dome. PNNL performed a technology evaluation with the objective of providing a critical literature review for all applicable technologies based on constraints provided by CH2M HILL. These constraints were not mandatory, but were desired. These constraints included performing the evaluation without removing any soil from the top of the tank, or if necessary, requesting that the hole diameter needed to gain access to evaluate the top of the tank structure to be no greater than approximately 12-in. in diameter. PNNL did not address the details of statistical sampling requirements as they depend on an unspecified risk tolerance. PNNL considered these during the technology evaluation and have reported the results in the remainder of this document. Many of the basic approaches to concrete inspection that were reviewed in previous efforts are still in use. These include electromagnetic, acoustic, radiographic, etc. The primary improvements in these tools have focused on providing quantitative image reconstruction, thus providing inspectors and analysts with three-dimensional data sets that allow for operator visualization of relevant abnormalities and analytical integration into structural performance models. Available instruments, such as radar used for bridge deck inspections, rely on post-processing algorithms and do not provide real-time visualization. Commercially available equipment only provides qualitative indications of relative concrete damage. It cannot be used as direct input for structural analysis to assess fitness for use and if necessary to de-rate critical components. There are currently no tools that automatically convert the NDE data to formats compatible with structural analysis programs. While radiographic techniques still provide significant advantages in spatial resolution, non-ionizing techniques are still preferred. Radar imagining in the 1–5 GHz has become the most useful. Unfortunately the algorithms and underlying assumptions used in these reconstructions are proprietary, and it is not possible to assess the quality and limitations of the analytical methods used to generate the derived structural data. The hypothesis that water intrusion may contribute to potential rebar corrosion of the tank domes provided the primary guidance in reviewing and evaluating available NDE technologies. Of primary concern is the need to employ technologies that provide the best opportunity for visualizing the rebar and providing quantitative data that can be integrated into structural analysis efforts to better understand and quantify the structural capacity of the domes. The conclusion is that an imaging system capable of locating and quantifying the distribution and conditions of the cement, aggregate, and rebar will provide the most valuable baseline upon which to build a case for the integrity of the structure. If successful, such a system would fulfill the need to incorporate valuable data into current structural load capacity analysis