At the beginning of this year, Framatome ANP carried out extensive replacements and repairs in the external reactor water recirculation loops connected to the reactor pressure vessel (RPV) at the Swedish BWR unit Oskarshamn 1 as part of the plant's "MOD" modernization program. The work was completed, except for a few minor items, in 47 days. Anders Helmersson and Anders Ahrenius, project managers for the plant operator, OKG, commented: "We are very satisfied with your work and how professionally you performed all of your activities, from engineering up to installation on site."

Cracks in Internal Surfaces of Pumps and Valves

Following discovery in the mid-1990s of crack-type manufacturing defects on the inside surfaces of the pumps and valves installed in the reactor water recirc loops, OK decided to have these defects repaired to reduce the scope of in-service inspections (ISI). In December 1997, Framatome ANP was awarded a contract to supply and install new pump casings complete with all internals. The company's supplies and services also included the supply of spare parts and the refurbishment of six pump motors (two of which were spares). In February 2000, Framatome ANP was awarded another contract to replace a number of other components. The workscope consisted of replacing eight inspection valves in the recirc loops and reducing the nominal diameters of the bypass lines and their valves from DN 100 (4") to DN 50 (2"). Also, two valves and a venturi flowmeter had to be replaced in the reactor vessel head spray cooling system as well.

The new pump casings were manufactured by the original pump manufacturer, the English company HaywardTyler. The casings were forged from 20MnMoNi55 steel, a service-proven material often used for PWR reactor coolant pumps, and were provided with austenitic cladding on the interior.

The valves were supplied by the German company KSB. The new components - manufactured to Swedish codes and standards and using state-of-the-art techniques - have low carbon and cobalt contents.

Replacement of the pump casings and the other components as well as reduction of the diameters in the bypass lines will serve to reduce ISI efforts in the future, and thus also the radiation exposure of plant personnel.

Installation of a pump impeller complete with pump motor

Additional Repairs Also Performed

The actual replacement work started in mid-January 2002 after OKG, supervised by Framatome ANP, had decontaminated the old pump casings up to the suction and discharge gate valves. While replacement was in progress, other repairs also became necessary as a result of cladding delamination at the remaining venturi tubes on the pump discharge sides and manufacturing defects in the safe ends of the old valves. Since Framatome ANP already was prepared for these repairs, they were able to quickly carry out the additional work at short notice.

Refurbishment of the six pump motors was performed in parallel with installation of the new pump casings.