United States Nuclear Regulatory Commission - Protecting People and the Environment

Study on Post Tensioning Methods (NUREG/CR-7208)

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Publication Information

Manuscript Completed: November 2015
Date Published: November 2015

Prepared by:
Christopher A. Jones and Lili A. A. Heitman
Sandia National Laboratories
1515 Eubank
Albuquerque, NM 87123

Robert Dameron
Moffatt and Nichol
1660 Hotel Circle North
San Diego, CA 92108

Madhumita Sircar, NRC Project Manager and Technical Lead

NRC Job Code V6186-00

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

In order to assess the effects of using grouted prestressing tendon systems in nuclear power containment vessels, a containment vessel is modeled in three dimensions using finite element analysis and different assumptions relating to grouted and ungrouted prestressing systems are applied. The particular containment vessel is the 1:4 Scale Prestressed Concrete Containment Vessel, for which extensive data exists. Such data can be used to benchmark and validate the modeling. The results from the grouted and ungrouted models indicate that greater localized maximum stresses and strains are predicted for the grouted case because the tendon system is not permitted to slip and redistribute loads as the vessel deforms. Accordingly it is noted that the analyses predict failure of the vessel at a slightly lower internal pressure in the case of the grouted system. Additionally, the response of the containment structure to tendon corrosion is investigated for both grouted and ungrouted systems while the prestressing system is subject to corrosion. Contrary to the results from the uncorroded tendon simulations, when corrosion impacts the prestressing system, the grouted system offers better performance owing to isolation of the corrosive effects and better preservation of prestressing force.

A post-tensioned containment is not only subject to normal material degradation, but also to a continuous loss of prestressing force. The losses result from inherent material characteristics. Because loss of prestress is not completely predictable, loss of prestress or residual strength must be measured at regular intervals to determine the containment strength to resist accident pressure and design loads with acceptable margins. The US Nuclear Regulatory Commission and Sandia National Laboratories have investigated and documented current procedures for monitoring and evaluation of post-tensioning to verify the effectiveness of inservice inspection examination methods for grouted and ungrouted tendon systems. Questions addressed include: (1) are current inservice inspection and examination methods adequate and (2) if corrective measures are used, are they adequate. In general, nondestructive evaluation and sensing methods are the only way to monitor the structural health of grouted tendons, and improvements to available methods should be investigated. Similarly, remediation of problems such as corrosion with grouted systems is difficult. The evaluation and repair of ungrouted prestressing systems is more straightforward.

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