The integrity of the reactor building for the additional operational life period is an important PLiM consideration for LTO decisions. Extensive analysis and studies of CANDU6 concrete containment structures have already been completed, including an IAEA TECDOC that covers some CANDU containment considerations [I.4]. Typically, a comprehensive PLiM Life Assessment (LA) specific to the individual plant is completed and then a detailed Ageing Management Plan (AMP) is prepared. These are then factored into the in-service inspection and maintenance to ensure plant life attainment. These plans are updated periodically as part of the overall plant life management programme.

The plausible degradation mechanisms for the containment structure have been identified; the most important being minor concrete cracking and a slight increase in permeability of containment, and changes in construction joints and cold joints. The main ageing mechanisms causing the degradation were freeze/thaw cycles, concrete shrinkage and creep and the repeated containment leak rate test. In addition the Alkali Aggregate Reaction specific to one plant and the chloride penetration at another contributed to the degradation. Corrosion of reinforced steel or corrosion and loss of pre-stressing force in pre-stressed containments where ever used are in general to be assessed for PLiM and LTO.

For LTO a detailed ageing management plan has been developed to better understand the impact of degradation mechanisms on the long term performance of the containment structure. The work includes a thorough review of site documentation, of world experience of the various ageing degradation mechanisms that could affect containment performance with time, and those applicable to the plants under consideration. Current knowledge is supplemented by an enhanced inspection and monitoring programme. At one plant, this includes design, data acquisition and installation of a system of specialized instrumentation to obtain detailed information about the behaviour of the reactor building prior to, during and after the containment building is pressurized for an in-service leak-rate test. Indian PHWRS recently constructed have provided instrumentation to monitor pre-stressing and health of cables. Typically, the Reactor Building leak-rate test is performed once every 3 to 5 years.

In developing ageing management strategies for CANDU 6 concrete containment buildings, concrete ageing experience gained at other facilities is being used. For instance, at the Gentilly site in Quebec, there are two reactor buildings. In addition to the Gentilly-2 plant, owned by Hydro Quebec, there is an earlier prototype CANDU system including a containment building (Gentilly-1) that is owned by AECL. The reactor at Gentilly-1 has been decommissioned and most of the radioactive materials removed to other sites.

The containment structure at Gentilly-1 is now over 30 years old. Importantly, there are many design and construction similarities to CANDU 6 reactor buildings. Observations made on the effects of ageing of the Gentilly-1 structure have been used to guide ageing assessments on other CANDU concrete containment structures. In addition, because there are no concerns for adverse effects arising from an accidental event, Gentilly-1 has been used to safely test and develop technologies that are appropriate for the assessment of ageing effects on structures at operating plants. For instance, conventional techniques including coring and testing the concrete and visual observations on the condition of the structure were supplemented with special methods developed to measure the state of stress in the structure. The techniques for these latter measurements were developed in connection with its fuel-waste management studies.

Subsequently, the technologies developed at Gentilly-1 have been applied at other HWR NPPs. For instance, at one HWR NPP, new instrumentation was applied to the containment structure alongside older mechanisms and a PC-based data acquisition system was installed, so that instruments could be monitored simultaneously and in real time. These instruments were placed to measure the stresses and deformations that occurred in the structure during the pressurized leak-rate test. Equally important, the measurements provided information on the effects of the environment on the structure. Figure 11 shows the mechanisms affecting the long performance of containment concrete building.

Concrete

Metallic Materials

Fig. 11. Mechanisms affecting the long term performance of containment concrete buildings.