The Challenge
The PFR pool reactor used sodium as a coolant, with the reactor and primary coolant being contained within a stainless steel vessel which is set into the concrete reactor vault in the secondary containment building (SCB).
To facilitate the ongoing removal of the bulk sodium coolant and sodium residues from the reactor vessel, it is necessary to ensure the optimum drainage of trapped sodium within the reactor structure is achieved. The objective was to design, fabricate, trial and eventually operate a facility which would be housed on the reactor rotating shield and used to machine penetrations in the reactor plenum floor. These penetrations would allow approximately 1.5 tonnes of residual liquid sodium to drain into the bottom of the reactor containment and subsequently be removed and safely disposed of.

The Solution
Development of the initial concept into a fully detailed design commenced in December 2005 and the complete drilling rig was fully designed, manufactured and assembled by the end of March 2006. Within this timescale, the drilling rig was subject to many development improvements and amendments as the build progressed. After this time the rig underwent a series of accurate drilling trials at T3|UK where a mock up of the actual drilling arrangement that would be seen at the reactor plenum floor was constructed and the drilling machine and all necessary tooling was rigorously tested. All drilling trials, operator training and final rig amendments were complete by the end of May 2006. The process of installing the rig onto the rotating shield at PFR is now underway with the main shield plug (which is part of the plenum drilling rig) installed into the existing CHIP Port by use of the existing 36” flask adaptor. These works are being carried out by UKAEA and RWE Nukem.

Project Benefits
Developing, trialing and testing the rig off site in a clean environment led to significant cost savings and also very significant safety, environmental and time benefits. The complete rig was assembled in an environment where visibility and access to all areas of the rig was available and therefore any assembly problems were very quickly identified and addressed eliminating these unforeseen circumstances from occurring during critical assembly operations at site. Once assembled all seals could be leak tested to eliminate the chance of any containment breach at site improving, environmental safety and reducing cost.
Without offsite trials it is unlikely the timescales would have been met and working in a clean, radiation free area minimises the time operators require to spend on site, helping to satisfy the requirements of the ALARP Principle.