PLiM of MAPS units consequent to failure of moderator inlet manifolds

During 1989, failure of inlet manifolds in both the units MAPS was noticed. The failure was to an extent that the zircaloy inlet manifold got ruptured and its broken pieces were found in the moderator circuit. On internal inspection of calandria, extensive damage to the manifolds was discovered. Under these conditions, it was not possible to operate the reactors. Hence, the reactors were shut down. Short term immediate/intermediate measure were carried out to bring the units to power again. As part of permanent rehabilitation these units are now fitted with three numbers of perforated tubes, called spargers, installed after removal of calandria tubes and pressure tubes in three lattice locations at the bottom of Calandria as an alternate moderator inlet path providing full moderator flow in required flow pattern.

Подпись: The job of permanent rehabilitation involved: • Design, engineering and testing of spargers.
Подпись: • Analysis of flow distribution inside calandria with sparger. • Removal of calandria and pressure tubes at three lattice locations and tube sheet machining. • Development of rolled joint of spargers in calandria tube sheet (this also provides capability to change Calandria tubes in a reactor). • Re-routing of moderator system piping from moderator room to north and south fuelling machine vaults.
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• Take moderator outlet from the original calandria outlet, while maintaining design flows through calandria sprays and other auxiliary cooling circuits.

Fig. A. I.2. Moderator Inlets and Outlets 94 After Permanent Rehabilitation

Drilling of pipe penetrations through fuelling machine vault floors for routing the piping.

All the above activities required extensive theoretical and experimental work before its actual implementation in the plant. As the en masse coolant channel replacement (EMCCR) for MAPS units was due, it was decided to carry out this job during the period when the units will have to be shut down for longer time. The installation is successfully completed in MAPS-2 which is now operating at 100% FP.

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