History
Post tensioning, as a concept, dates back to 1928 when France's Eugene Freyssinet developed a method for prestressing cast-in-place concrete.  But the process was thrust into prominence in Europe, after World War II, when scarce steel was supplanted by concrete as the principal structural building material.

Post tensioning is a method in which high strength steel reinforcement is tensioned after the concrete has set.  The first serious use of this technique in the United States was for the Walnut Street Bridge in Philadelphia in 1949.  Since then, there has been a steady and dramatic growth in its use for all types of concrete construction projects: office buildings, stadiums, parking structures, high-rise apartments, bridges of all sizes, and, of course, nuclear power plant containments. 

The popularity of post tensioning has been earned by the benefits it affords concrete construction.

   - It permits the reduction of the structural depth of  members.
   - It virtually eliminates cracks in slabs, making them essentially water tight.
   - It controls the deflection of structural members.
   - It makes possible economical,longer spans.    

Circumferential post tensioning
In the design of nuclear reactor containments, post tensioning enables the concrete to withstand high tensile forces, while satisfying the requirements for sheilding. In a non-post tensioning vessel, internal pressures will cause cracks, since concrete has poor resistance to tensile forces.

By reinforcing the vessel with post-tensioned steel tendons, a compressive stress is intentionally applied to the concrete. When internal pressures are applied to the post-tensioned concrete, they are offset by the previously applied prestressing forces, resulting in the desired stress condition.

The horizontal tendons which encircle the containment structure are anchored at the buttresses.  The number of buttresses may vary from six, at 60 degrees spacing, to two, at 180 degrees spacing.  Today, the most popular designs employ three buttresses, spaced at 120 degrees.

Six-buttress containment
Tendons run 120 degrees around the circumference, anchoring into two of the six buttresses. Six tendons form a band of two loops.

Three-buttress containment
Tendons run 240 degrees around the circumference, anchoring into two of the three buttresses.  Three tendons form a band of two loops.

Two-buttress containment
Tendons run 180 or 360 degrees around the circumference.  The particular system selected depends upon an evaluation of tendon efficiency.