A. I.2.1. LIFE MANAGEMENT PROGRAMME FOR THE PRESSURE TUBES

The objective of formulating the life management programme for the PT has been the understanding of the various degradation mechanisms and their mitigation through research and development in the fields of design, manufacture, operation, in-service inspection and life extension. Some of the aspects of life management are listed below:

• Design modification and improvement in manufacturing procedures.

• Inspection and monitoring of the degradation related parameters.

• Development of methodologies for assessment of fitness for service with due importance to safety and not being unduly conservative.

• Development of analytical codes for degradation modelling and residual life estimation of PT.

• Development of life extension tools and technologies.

• Post irradiation examination studies.

• Replacement of degraded PTs.

A. I.2.1.1. Design modifications and improvements in manufacturing procedures

Problem due to deuterium pick-up in pressure tube has been mitigated to a large extent by reducing the maximum initial hydrogen in the as-manufactured condition to 5 ppm from the earlier value of 25 ppm and changing the material to cold worked Zr-2.5Nb from cold worked Zircaloy-2. The specification has been modified to limit the content of chlorine, phosphorus and carbon achieved through quadruple melting. Such tubes are being used in all reactor currently under retubing as well as new ones under construction.

The effect of axial creep-growth elongation of the PT on the service life of the coolant channel has been resolved by providing sufficient length for the bearing sleeve. Through suitable design of the coolant channel it has now been ensured that diametral creep-growth and bending creep-sag of the coolant channel do not limit the life of Zr-2.5Nb PT.