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Products and solutions, 2005-12-30, 10:00 AM

Watertight at 80 degrees

The new nuclear temporary storage site at the Neckarwestheim power plant is being given a watertight inner lining. The two big caverns, all of the connecting tunnels and shafts are being completely sealed against groundwater and precipitation along the walls and ceilings with the aid of multilayered proofing. A precondition for the project is that even at permanent temperatures of 80 °C, the geotextile/plastic sheet acting as waterproofing retains its watertight properties.

"Proofing underground structures against pressurized and unpressurized water" is the specialized task that just a handful of companies in Europe are capable of performing. ThyssenKrupp Xervon commands a special technique whereby an environmentally neutral plastic waterproofing sheet clings to the tunnel tubes much like a watertight skin. And because this technique outperforms in terms of safety and reliability the solution originally proposed for the Neckarwestheim power plant, the specialists were able to secure the contract for themselves. Various tests and sample bondings underscored both the technical competence of the alternative proposal plus its cost efficiency. Impressed by the proposals were EnBW (the power plant operator), the Swiss Gähler und Partner engineering consultants and the contractor in charge Baresel AG.

15,000 m² of proofing

With its new temporary storage site for spent fuel rods, EnBW address the insistence by politicians on having local sites in proximity to the plants themselves. Neckarwestheim has two tunnel tubes driven into the rock located beneath the office building. These accommodate 150 Castor casks, albeit actual storage is confined to a maximum of five per year. Until the two parallel 90-meter long caverns (height: 18 m, width: 14 m) and all the related shafts and connecting tunnels are complete mid-2006, around 75,000 cubic meters of excavator ground will have to be removed and replaced by 33,000 cubic meters of concrete. Other work includes the sealing of around 15,000 square meters against groundwater and so-called meteorwater (resulting from all kinds of precipitation) at a continuous operating temperature of 80 °C. All parts of the new site, except for the tunnel invert, are being proofed. The invert itself comprises a watertight concrete slab secured to the proofing with joint tapes.

"Every waterproofing task is different and demands the adaptation of our multi-layer process to the specific geological and hydrological conditions," explains proofing professional Per König. In Neckarwestheim, the high required temperature resistance is particularly demanding. In contrast, the careful installation of the sealing is a chore that his specialists are confronted with most every day. What makes the new temporary storage site a genuine showcase assignment are the extremely confined space and tight deadlines for the individual work steps. "We have to stay constantly in step with the bar setters and concrete pourers, as we don't have the usual continuous work process as on most tunnel construction sites," adds König.

Simple principle: geotextile, PVC sheet, laminating layer

The principle of the watertight lining and the work steps required for this, on the other hand, is always the same. First of all a protective layer of geotextile ( 1,000 g/m²) is applied to the tunnel tubes lined with shotcrete in order to protect the subsequently fitted plastic waterproofing membrane, a 2 mm PVC sheet able to withstand temperatures of 80 °C (guaranteed temperature resistance) . So that the waterproofing clings without cavities like a skin to the walls and ceiling of the tunnel tubes, it is fastened to the substrate with the aid of previously anchored discs. In a special process, these discs and the sheeting are welding together with hot air to create a perfect fit. Then the overlapping plastic membranes are welded absolutely tightly together - various tests confirm the quality of the welds. Only specially trained sheet welders conforming to DVS Guideline 2225 (German Welding Association) are permitted to carry out this waterproofing process and the associated tests of tightness. Only when the quality testing has been successfully accomplished is a protective layer of self-adhesive sheeting installed as the final step. This laminated layer comes with a releasable marker film and is very carefully bonded over its entire surface to the PVC waterproofing. This way any damage to the waterproofing system is easily spotted. To complete the tunnel carcass, the only task that´s left is the construction of the reinforced concrete inner shell.

<u>Background information Neckarwestheim power plant:</u>
This power plant generates annually over 17 billion kilowatt hours of electricity for the public grid and German Rail - equivalent to around a third of the annual consumption of the German state of Baden-Württemberg. Two pressurized-water reactors located on the right-hand bank of the river Neckar between Heilbronn and Ludwigsburg generate the energy. Unit I dating back to 1976 has an output of 840 MW, unit II with an output of 1,395 MW came on stream in January 1989 and is Germany's youngest and most modern power plant reactor.

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