Repair and maintenance
Pump help27 August 2009
Delays in draining water out of the reactor vessel during power outages prompted an engineering team to build a suction tube to speed up the process. It is now being used during outages at all 58 French nuclear reactors.
EDF’s Cruas-Meysse plant comprises four 900MWe PWR units. The plant lies on the western bank of the river Rhône, in the Ardèche département, between the villages of Cruas and Meysse, France. The plant was commissioned in February 1985 and generates, on average, 5% of national electricity output. It employs 1170 EDF staff and some 250 contractors. The plant performed its second 10-year inspections from 2004 to 2007 and steam generator cleaning was carried out in 2007 and 2008. An IAEA operational safety or OSART review was conducted from 24 November to 11 December 2008.
During outages when the reactor was fully unloaded, maintenance and operations staff observed two reoccurring problems. Even after 18 hours, drain-down time for the main coolant piping was still not complete. As a consequence water flowed from the reactor vessel towards low points in the main coolant piping. This water disrupted non-destructive testing. In particular, films from radiographic tests were no longer readable. On-call maintenance staff would have to be brought in at night to drain the piping. Radiographic testing would then have to be re-scheduled, which sometimes meant halting reactor building daytime activities to repeat tests.
The original procedure to drain water out of the reactor was a two-step process. First, the residual heat removal pump and reactor coolant pump drain line drained down water to about a height of 9m. This operation took about eight hours. When the residual heat removal pump was stopped, the reactor coolant pump drain line was used for draining the main coolant down to bottom of the cold leg. This draining phase lasted about 10 hours. Below the 9.28m level (at the level of the primary pump flow nozzle), pump flow is limited to 3m3/h. The reactor coolant system is drained via an narrow opening located on the primary pump suction adapter (or the suction skirt).
A potential solution to the problem would need to take into account the needs and wishes of two sets of outage personnel. The maintenance organisation needed to drain the coolant in the reactor vessel to below the bottom of main coolant pipe to prevent reverse water flow disrupting non-destructive tests. The operations staff were focused on reducing draining time.
Patrick Simon from the Cruas mechanical maintenance department and Pierre Romain from operations came up with an idea that would satisfy both teams. With the help of colleague Jean-Luc Weynant from the I&C, electricity and tools department, they devised a suction pump tool to draw water directly from the reactor vessel and expel it to the core internal support structure, enabling the level in the main coolant piping to be lowered slightly (20mm-30mm) below the bottom of the main coolant piping. The draining tool comprises a telescopic plunge tube with strainer fixed on a dummy reactor vessel head, a submersible pump, and a control panel and packaging case.
The tool is used when the reactor is fully unloaded. The plunge tube, internally equipped with the submersible pump and strainer, is installed through the dummy reactor vessel head. Draining starts after the steam generator tubes have been drained and the residual heat removal system is shut down.
The tool takes two hours to lower the water level from 9m to 8.5m, eight hours faster than using the reactor coolant pump drain line alone. During that time, other pumps can drain other elements such as the safety injection system, saving a further two hours.
The tool has cut the draining time, a critical path process, in half. Radiographic testing work is safer and because the tests are no longer repeated, valve and piping work have benefited from a shorter exposure time. Finally, the implementation of the tool has helped human resource management during outages; on-call staff who would have been called up in the night are now available for work in the day.
The reactor vessel draining tool has been used on Cruas outages since 2006, and was integrated into all French outages in 2008. It was also recognised as a good practice by OSART.