Risk analysis in post-tensioning system surveillance program of nuclear power plants

Abstract

Verification of prestressing forces by lift-off test is one of the aspects ussualy included in the surveillance programs of nuclear power plants when containment building is on prestressed concrete. The test consists of pulling out the end of the tendon until the force applied to the anchorage is released completely. One of the main points in this kind of tests is to establish the maximum difference between the values obtained from the test and the one derived theoreticaly which is admisible for accepting or rejecting the tendon. This problem can be studied as a problem of decision taking in an uncertainty context. In the case analyzed, the cost and probability of each decision (accepting or rejecting the tendon) has been evaluated 'a priori', by assuming some some simplifying hypotesis. From the study, the fact emerges that the optimization of the average costs leads to relatively large tolerances for the lift-off test. The reason for that follows from the fact that the rejection of a surveilled tendon directly generates a high cost of replacement, whereas its acceptance can cause a damage that, through a high economic cost, is of a very low probability of ocurrence. As a consequence, the main conclusion can be drawn that lift-off tests with very wide tolerances of acceptance do not satisfy the pursued objective and, therefore, this type of tests could be replaced by some other kind of surveillance with lower risk of generating high costs and better posoibilities of really improving the safety condition in this a kind of structures.