The most efficient post tensioning system for prestressing nuclear containment vessels is the wire-tendon system utilizing parallel lengths of high-strength wire.

Tendons for nuclear containment work consist of 90, 170, and 212 wires and were fabricated to the designed length. Both ends of each wire terminate in a cold-formed buttonhead after the wire is passed through a machined anchorage fixture. This results in a no-slip anchorage and eliminates potential seating losses common in friction-grip anchors.

The ungrouted post-tensioning tendon system is an unbonded one, which means that there can be no reliance on a grout bond to assist in anchoring the tendon. The buttonhead wire principal provides positive and totally reliable tendon anchorage and delivers the actual ultimate strength of the prestressing steel.

Tendons were installed by pulling them through the semi-rigid metal conduit which had been encased in concrete. After such an installation each tendon was stressed. Finally, grease was pumped into this sheathing conduit, under pressure, so that each tendon was fully and permanently protected against corrosion.

Surveillability
One of the important features of the unbonded system is its total surveillability. Because every tendon is readily accessible and capable of being de-tensioned and retensioned, it can be easily inspected, at any time after installation, for strength and integrity in accordance with the surveillance requirements established by the Nuclear Regulatory Commission.