Engineering consideration and experience related to refurbishment and safety up-gradation of 40 MWT research reactor CIRUS

CIRUS, a 40 MWt research reactor located at the Bhabha Atomic Research Centre, Trombay, went into operation in 1960. CIRUS is a vertical tank type reactor using natural uranium metal as fuel, heavy water as moderator, helium as cover gas and light water as primary coolant flowing in a closed re-circulating loop. Heat from the primary coolant is transferred to seawater flowing in a once-through mode. After over 3 decades of satisfactory operation, signs of ageing related degradation started appearing in some of the reactor systems. Detailed ageing studies of the systems, structures and components of the reactor were carried out during the years 1992-95 to assess the condition of their health and to identify refurbishing requirements towards safe life extension and safety up-gradation for continued safe operation and better utilization of the reactor. Based on the results of the detailed ageing studies, refurbishing requirements were finalized and a refurbishment outage of the reactor was taken up towards the end of 1997. Additional inspections, which could not be done earlier with irradiated fuel present in the core, were also undertaken after the reactor shut-down. This mainly involved: extensive examination of subsoil (buried) sections of primary coolant carbon steel piping all over its length by employing various non-destructive techniques like in-situ metallography, advanced ultrasonic flaw detector cum imaging technique; examination of emergency core cooling storage tank, a pre-stressed concrete structure using rebound hammer test, ultrasonic pulse velocity test, corrosion potential measurement and assessment of compressive strength of concrete by core sampling; inspection of lattice tubes of the reactor vessel by eddy current technique. During refurbishment, many repair activities required considerable engineering development work. One of the most challenging work was rectification of leaks from helium pipeline flange joints located in the reactor structure region above reactor vessel. An innovative and intricate remote repair technique was developed to rectify leakage of helium cover gas from the 8 Nos. flange joints. These flange joints are situated deep inside the reactor structure at a distance of about 5 m below the operating platform in a 200 mm vertical gap between the steel thermal shield and concrete biological shield above the reactor vessel. In the absence of innovative repair techniques, these flange joints could have been approached only after dismantling top primary coolant distribution headers, biological shields, thermal shields etc. involving handling of massive tonnage of radioactive components which would have resulted in high manrem consumption and possible irreversible damage to the structural components. The repair technique involved design and development of special split sealing clamps, remote tightening tools and remote gauging assemblies. The split sealing clamps were required to be lowered through the central lattice tube of the reactor vessel up to the leaking flange joint 5 m below top of the reactor pile block. There after the split clamps had to be dragged sideways for a distance of about 1.5 m to reach the respective flange location. The split clamps were then made to encircle the leaky flange and tightened remotely to stop the leak. All activities of lowering, shifting and manoeuvring the split clamp around the flange, were done from the top operating platform using nylon ropes by viewing through remote viewing cameras placed near flange location. Detailed finite element based stress analysis was carried out to arrive at the safe compression and torque to be applied to the clamps without causing any deterioration to structural components. A full-scale mock-up station was also set-up for training of personnel and qualification and calibration of tools and procedures to ensure that torque applied during tightening does not exceed the permissible values. Remote repairs to all the eight leaky flange joints were successfully carried out using this technique. Another challenging repair activity was to rectify the leak from 1 1/4'' dia. cooling water inlet piping of top aluminium thermal shield located, inside the reactor structure. This involved design and development of an expandable hollow metallic plug which was installed successfully inside the leaky coolant piping using remotely operated tools operated under extreme space constraints. This repair job also involved setting up of a mock-up station for qualifying the hollow plug, the installation procedure and training of personnel. Together with the refurbishing activities, certain safety upgrades which included fire safety improvements, strengthening of emergency core cooling water storage tank to meet present seismic qualification requirements were also incorporated. The emergency ventilation system for iodine removal was re-designed to incorporate an enhanced filtering efficiency. Provisions were also made for integrating a low temperature vacuum evaporation desalination unit coupled to the reactor, for demonstration of desalination by low temperature heat from the primary coolant system of research reactor. After completion of all refurbishment activities, CIRUS research reactor was made critical on 30th October 2002. With the extensive refurbishment carried out, the reactor has got a new lease of life in a highly cost-effective manner and will be able to serve the country for several more years. This paper details the various engineering considerations, experience gained during refurbishment, various improvements and safety upgrades carried out for research reactor CIRUS