Assessment of Concrete Cracking at Nuclear Waste Disposal Facilities via Fiber Optic Sensors
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Assessment of Concrete Cracking at Nuclear Waste Disposal Facilities via Fiber Optic Sensors Sanaan C. Lair, John C. Walton, Arturo Woocay and Antonio Motta University of Texas at El Paso, Civil Engineering Department, 500 W. University Avenue, El Paso, TX 79968-0506, U.S.A. ABSTRACT Fiber optic sensors offer a novel approach to monitoring of fractures in concrete waste disposal vaults and offer the possibility of determining the quantity, width and location of cracks as they form. Fiber optics can directly detect cracks if they form within the path of a fiber optic as well as monitor secondary indicators of cracking such as temperature changes and strain. When cracks form in concrete waste disposal vaults they can fill with water which has a high heat capacity, this enables cracks to be observed by monitoring temperature variations near the crack. An analytical solution for heat transfer is applied to estimate the propagation of temperature waves around cracks. It is demonstrated that discharge rates through the concrete which are less than 10-5 m3/m-s do not produce a meaningful temperature wave through the concrete. Fractures in the concrete must be larger than 0.07 cm to produce a measurable result and temperature sensors must be located within 0.5 meters of a crack to detect a change in temperature produced by seasonal groundwater flow through a crack. A distributed system of fiber optic sensors may be embedded in the concrete vault and used to monitor crack formation, temperature variations and strain. INTRODUCTION Engineered concrete barriers such as vaults are commonly used in the field of radioactive waste disposal. Concrete vaults offer long-term containment of waste and prevent the release of radionuclides into the environment due to concrete’s high strength and low permeability properties. Radioactive waste can remain hazardous for thousands of years; hence waste repositories must be designed to contain the waste for as long as it remains an environmental and human hazard. Radionuclides are capable of escaping a disposal facility as a gas or in the aqueous phase through preferential pathways such as fractures within the concrete vaults. A requirement of nuclear waste disposal facilities is to complete a performance assessment in order to demonstrate that a disposal facility meets performance objectives. The United States Nuclear Regulatory Commission (USNRC) defines a performance assessment as “...a quantitative evaluation of potential releases of radioactivity from a disposal facility into the environment, and assessment of the resultant radiological doses.” A performance assessment, also known as a risk assessment, involves modeling the anticipated performance of the facility over time to help predict how the engineered and natural barriers will perform well into the future. A crucial step of this process is long term verification and monitoring to ensure the facility meets the performance objectives and that the assumptions made during the performance assessment were valid and neither ov
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