The Krsko nuclear power plant, which houses a 2-loop pressurised water reactor, is located in Krsko in Slovenia. The plant went into commercial operation in January 1983. The steam generators are W-model D4-2 with Inconel 600 MA tubing (3/4 inch OD).

Since 1985, after only a short time in operation, the Krsko steam generators experienced tube degradation which required plugging of affected tubes. In 1987, Nuklearna Elektrarna Krsko (NEK) signed a long-term agreement with ABB Reaktor for steam generator repair services, mainly for the installation of ABB removable plugs. This agreement was extended in 1992 for the period 1993 – 1997 and again in 1997 for 1998, including the installation of sleeves.

Until 1992, the plugged tubes did not exceed the plugging margin, but from 1993 NPP Krsko became concerned with crack propagation not only in the tube sheet roll transition, but also in the tube supports.

To operate the plant without decreasing the power output, NEK decided to install sleeves during the 1993 outage. Based on the experience with welded sleeves within the ABB Nuclear Group, ABB Reaktor, together with ABB Combustion Engineering, was chosen as contractor.

In 1993 ABB installed 164 welded sleeves [160 roll transition sleeves (RTZ) and 14 tube support sleeves (TSP) in TSP 3] in Steam Generator No 1. As the tube degradation continued,1 NEK decided on another sleeving campaign in the 1996 outage and ABB installed 461 welded sleeves in both steam generators (188 RTZ, 84 TSP 3, 190 TSP 5).

The ABB welded sleeve is a proven steam generator repair solution, but welded sleeves in general have some constraints:2

• Limitation in installation height.

• Sensitive welding process.

• Critical path process.

• Inspection in the tube weld areas does not give reliable results.

To avoid these constraints, ABB Reaktor started the development of a weldless sleeve (PLUSS sleeve) by applying a simple mechanical expansion process.

Theoretical investigations and feasibility tests showed that with a sleeve of a much lower yield strength and a higher coefficient of thermal expansion than those of the steam generator tube, a leaktight expansion joint can be achieved.

As for the sleeve material, the consideration of steam generator operation conditions has led to the selection of Incoloy 800 as the sleeve material for repairing a tube of Inconel 600.

PLUSS sleeves were successfully installed in 1996 in the Kori 1 nuclear plant (Korea) and in Tihange 3 (Belgium), and in 1997 in Tihange 2 (Belgium) in the roll transition zone.

Based on these good results, NEK decided in the autumn of 1997 to start the licensing process for this operation with the Slovenian Licensing Authority (URSJV) The licence was granted in spring 1998 and accordingly installation of PLUSS sleeves was performed during the 1998 outage in both steam generators.

A total of 135 PLUSS sleeves were successfully installed, 46 of them tube sheet sleeves, 89 were installed in the tube supports, covering defects in TSP 3 (34), TSP 5 (41), TSP 7 (12), TSP 8 (1), TSP 9 (1).

This was the first time that mechanical sleeves were installed up to 6 m in a steam generator tube. The total sleeving campaign was performed with a 100% success rate and no single installed sleeve failed the acceptance criteria.