News & Views, Volume 46 | Assessing Prestress Losses in a Nuclear Containment Structure for License Renewal
By: Eric Kjolsing
Nuclear power plants around the world are approaching the end of their original 40-year design life. Efforts are underway to extend the operating license for these plants to 60 years or beyond. As part of the license extension, it must be demonstrated that the reactor containment building remains able to safely perform its intended functions for the extended duration of operation. Many of these containment buildings utilize a post-tensioned concrete design where the tendons are grouted after tensioning. Since these grouted tendons cannot be re-tensioned, an assessment for the loss in prestress beyond the original design life must be performed.
This article describes a methodology to assess the structural performance of a containment structure over time as a function of confidence in the tendon losses and is split into three parts:
- A description of the methodology
- A representative probabilistic assessment
- Representative analysis results
As part of the general design criteria for nuclear power plants, the primary structures and systems of the plant must be designed to handle postulated accident events, including the dynamic effects of postulated pipe ruptures. For a Boiling Water Reactor, analyzed events include various accident conditions in the recirculation piping, including a Loss of Coolant Accident (LOCA). One postulated LOCA event is assumed to be an instantaneous double-ended guillotine break of the recirculation line. This event causes several loads to be imparted on the reactor vessel, attached piping, and reactor internal components. [Some loads such as jet impingement, annulus pressurization, and pipe whip impart loads on the outside of the reactor vessel and the attached piping.][ Other loads, including flow-induced drag and acoustic loads, transmit loads inside the vessel on critical components such as jet pumps, core shroud, and the shroud support structure.] Figure 1 shows the pipe and resulting loads.
In 2018, Structural Integrity Associates (SI) supported a utility in the structural assessment of a submerged concrete intake tower.
Plant engineers must often answer ‘what damage is this piece of equipment susceptible to, and what can we do about it?’ or ‘how much longer can this component continue operating’? 
