Advisor:

Dr. Kimberly E. Kurtis (CEE) and Dr. Lawrence F. Kahn (CEE)

Committee Members:

Dr. Laurence J. Jacobs (CEE/ME), Dr. T. Russell Gentry (ARCH/CEE), and Dr. James J. Wall (EPRI)

Abstract

The U.S. currently has 99 commercial nuclear power reactors licensed for operation and generating approximately 20% of the nation's net electricity. At 33 of these reactors, a cylindrical post-tensioned concrete containment building (PCC) serves as the final barrier to the release of radiation from the enclosed nuclear reactor. As a critical public safety-related structure, the long-term integrity of the PCC is necessary for continued operation of the reactor. In 2009, during preparations for a steam generator replacement, extensive subsurface laminar cracking was identified in a portion of the Crystal River 3 (CR3) PCC in Florida, and the plant was permanently shut down in 2013. This study investigates potential contributing factors to the identified cracking with particular focus on the effects of high early-age temperatures on the cracking risk of the concrete, on the development of the concrete properties, and on the late-age structural behavior of the concrete.

Two planar, full-scale mock-ups of a portion of the CR3 PCC were constructed and instrumented with temperature and strain gauges to monitor the thermal and mechanical behavior during representative concrete curing and post-tensioning loading. Standard- and match-cured concrete specimens were tested for determination of the time- and temperature-dependent development of thermal and mechanical concrete properties, and hydration parameters were determined for the mock-up cement paste for modeling the heat generation in the concrete. These properties and parameters were utilized in 3D finite element analysis of the mock-ups in COMSOL Multiphysics and compared with experimental results. Non-destructive evaluation via ground-penetrating radar and shear wave tomography was conducted on the mock-ups to identify flaws and determine the effectiveness of the methods for identifying delaminations between post-tensioning ducts approximately 10 inches beneath the concrete surface.

Though early-age thermal stresses were determined not to have caused cracking in the mock-ups, the high early-age concrete temperatures resulted in decreased late-age compressive strength, splitting tensile strength, and modulus of elasticity values, which were shown to contribute to greater concrete cracking risk when the mock-up was post-tensioned. Through parametric modeling, the influence of various PCC design factors on the cracking risk was also identified. Additionally, relationships between the mechanical properties for the standard- and match-cured specimens were identified that could enable prediction of in-place or match-cured concrete properties based only on the results of tests on fog-cured specimens.