In March 2002, Kraftwerk Gösgen-Däniken AG, the operator of the Swiss nuclear power plant Gösgen, awarded Framatome ANP a contract to upgrade the plant's overpressure protection system. The modifications will bring the system in line with current state-of-the-art nuclear safety technology and will allow it to control beyond-design-basis accident conditions. The upgrade also will benefit normal plant operation. Similar upgrades were carried out in all of Germany's PWR plants between 1988 and 1999.

The Gösgen plant, built by Framatome ANP, is a 3-loop PWR with a rated capacity of 1020 MWe that began commercial operation in 1979. In connection with the Periodic Safety Review (PSR) conducted for the Gösgen plant, the Swiss Federal Nuclear Safety Inspectorate (HSK) requested that a study be undertaken to investigate different options for upgrading the overpressure protection system of the reactor coolant system. The plant operator decided to go a step further by modifying - with HSK's approval - the entire pressurizer valve station.

The upgraded overpressure protection system will provide Gšsgen with capabilities for reactor coolant system depressurization that were not state-of-the-art when the plant was built. The main benefit, from a safety viewpoint, is that controlled depressurization will be possible in the event of beyond-design-basis accidents to bring the reactor down to pressure levels that permit primary-system feed by the accumulators and later long-term coolant makeup by the low-pressure safety injection systems. This also will serve to enhance overall plant safety.

Innovative Safety Valves in Compact Configuration

To assure maximum reliability along with minimum maintenance requirements, three new innovative safety valves (from the Swiss valve manufacturer Sulzer) will be flanged directly to a new pressurizer dome on top of the pressurizer in a compact configuration and certain functional modifications will be made at the pressurizer valve station. The pressurizer spray valves and the routing of their pipes will be optimized at the same time.

Overpressure protection system after the upgrade

These modifications will reduce the loads imposed on piping and surrounding structures, will cut down the number of high-pressure lines and will ensure highly reliable valve reseating. The changes to be made on the overpressure protection system affect components belonging to the highest safety class.

Safety Upgrade to be Performed in Just 32 Days

Framatome ANP has overall responsibility for all activities performed as part of this project. The scope of work covers not only design and installation of the new equipment but also planning and execution of all related electrical and I&C work as well as startup and functional testing. The entire safety upgrade is scheduled to be performed in just 32 days during the plant's annual refueling outage in 2005.