Life at Cernavoda 11 October 2003
The first commercial Candu units have already reached their planned 30-year operational life. Cernavoda 1 was the fifth of the original Candu 6 reactors, and a plant life management programme was started in 1999. By Mihail Cojan
Cernavoda 1 is almost identical with Wolsong 1 with regard to technology and construction. It is almost identical to Point Lepreau in terms of commissioning and operation. From the point of view of safety objectives and licensing conditions, Cernavoda 1 is identical with the first four Candu 6 reactors. The Table below displays the evolution of the Candu 6 reactors.
Over the last four years, the Institute for Nuclear Research in Romania (ICN) has been working on R&D programmes to support a comprehensive and integrated plant life management (PLIM) programme for Cernavoda 1 that will extend the operational life of the plant.
The first step in applying this programme to Cernavoda 1 was taken in 1999 by the inaugural inspection of pressure tubes. On this occasion, 14 pressure tubes selected according to well-defined criteria were inspected, out of the 380 pressure tubes of the core.
The main objectives of the PLIM programme at Cernavoda 1 are:
• To maintain the long-term reliability and safety of the plant during the design life.
• To maintain the long-term availability and capacity factors of the plant with controlled and reasonable generating costs during the nominal design life.
• To avoid surprises through identification of potential ageing issues ahead of their occurrence and provide means for monitoring and mitigation to ensure reliable component performance.
• To preserve the option of extending the life of the plant with good safety and availability at a reasonable cost, beyond the nominal design life of 30 years, up to 50 years.
The strategy adopted in preparing the concept of a PLIM programme applicable to Cernavoda 1 involves the following steps:
• Identify critical components.
• Undertake ageing assessment studies of these critical components.
• Implement life management programmes aiming to maximise component life, ensure good performance and monitor plant conditions.
• Plan, scope and implement required programmes for the original design life.
• Prepare economic case studies for rehabilitation and life extension.
• Implement rehabilitation to operate beyond the nominal design life.
Following this strategy, the multiphase approach of a PLIM programme applicable to Cernavoda 1 includes three phases in its structure:
• Phase 1. Detailed studies to identify critical SSCs, evaluate ageing and define credible mechanisms of critical components degrading.
• Phase 2. Defining, planning and implementation of detailed programmes of ageing management to attain the planned life duration.
• Phase 3. Upgrading, replacement and maintenance of critical components in order to ensure extension of the plant life duration.
In May-June 1998, the first inspection of the turbo-generator was carried out. In November 1999, the first inspection of the pressure tubes was completed.
It is thought that a period of 15 years from plant commissioning until the replacement of the first pressure tubes is optimistic for Cernavoda 1, when the experience at Wolsong 1 is taken into account.
Deformation of the fuel channels may limit the useful lifetime of the pressure tubes. Time-dependent deformation (creep) of Candu fuel channels assemblies occurs as a result of thermal creep, irradiation creep, and irradiation growth processes. The deformation of zirconium alloys, from which the assemblies are comprised, is dependent on time, temperature, stress and fast neutron flux. The expected fuel channels elongations (axial creep) at midlife are shown in Figure 2. The values presented in this Figure are based on measurements at Point Lepreau taken in February 1993. The values for Cernavoda 1 may differ somewhat due to variations in operating conditions as well as due to tube-to-tube variations. Cernavoda 1 fuel channels were designed to accommodate average axial elongation of 3.0 ± 10% inches at each end.
The scheduling of the Cernavoda 1 PLIM programme is shown in Figure 1.
The multiphase programme that has been proposed for application at Cernavoda 1 is supported both by the experience of Candu 6 owners and by the results of research conducted within ICN. Thus the first step of Phase 1 has been covered, referring to the studies on the assessment of critical structures, systems and components (CSSC) operation, encompassing the methodology related to the definition of CSSCs. The work was performed between 2000-2003, within the ICN R&D programme on 'process systems and equipment'. This programme deals with the increase of performance of nuclear power plant systems and components their upgrading based on the evaluation of their operational behaviour. Another objective of this programme is assessing and increasing the reliability and maintenance of process systems and equipment in accordance with plant life management considerations.
In order to attain all the objectives of Phase 1, ICN has initiated four other R&D programmes to evaluate ageing and the capability to carry on safe operation within the limits of nuclear safety of the key critical components in Cernavoda 1, such as: fuel channels; steam generators; chemistry, chemical control; and instrumentation and control.
The ICN R&D programmes in support of the first phase of the PLIM programme are focused on:
• Understanding operating environment and degradation mechanisms, and developing models.
• Developing and applying inspection and monitoring technology.
• Applying models to field data to predict component behaviour and recommend maintenance and management activities, and/or develop and qualify improved components or systems.
Until now, considerable progress has been made in understanding the degradation mechanisms and developing ageing management programmes for the major critical components, such as pressure tubes, reactor assemblies, steam generators and feeder assemblies.
Phase 2 of the Cernavoda 1 PLIM programme started with the inspection of the pressure tubes. This phase is concerned with the maintenance and inspection requirements for the beginning, midlife period, and last years of operation. At Cernavoda 1, a special programme for technological surveillance has been initiated. The objectives are focused on the early identification of potential ageing phenomena and of defect modes that could affect the plant performance, together with the inspection and maintenance required to monitor and evaluate the ageing effects. In parallel with this, the ICN initiated the R&D programme 'analysis of nuclear power plant operation events, ageing, environment qualification and increase of nuclear power plant lifetime'. The programme includes the acquisition of international experience related to the categories of events occurring in nuclear power plant operation and to the actions taken to eliminate their causes, and incorporating these into the Cernavoda 1 PLIM programme.
The interactions between the ICN and the utility in the PLIM programme should provide the utility with an in-depth assessment and promising life prognosis for the key CSSCs in the plant. The proposed ICN/utility interaction also demonstrates the synergy of the ICN R&D programmes with the Cernavoda 1 PLIM programme.