Belleville Nuclear Plant is a PWR operated by Électricité de France (EDF). Since beginning commercial operation in 1989, it has been rated at a nominal capacity of 1,300 MW. The turbine unit consists of one high pressure (HP) and three low pressure (LP) turbines.
Belleville station is cooled by two large cooling towers. Each condenser consists of 6 bundles serving the three low-pressure turbines. Each bundle contains 21,242 tubes, that are 13.75 m (45.1´) long.
The work consisted of retubing three condenser bundles, each having 21,414 tubes 13.750 m long (45.1´). Six tubesheets were replaced, each one measuring approximately 4.4 x 4.4 m (14.4´ x 14.4´). See Figure 2.
The tubesheets were reverse engineered from scanned copies of old drawings. EDF required an analysis be performed to verify the tubesheet’s strength with thin wall tubes and with a postulated increase in hydrostatic pressure. A pullout test was performed to demonstrate that leak tight joints could be produced
Materials chosen consisted of two bundles of 1 mm (0.039?) CuZn30As (similar to arsenical admiralty) tubes, and one bundle of 0.5 mm (0.0197?) X2CrMoNiN 22-5-3 duplex stainless steel tubes. Peripheral tubes and air cooler sections in all three bundles were tubed with 0.7 mm (0.0276?) duplex stainless tubes. EDF plans to use duplex stainless tubes for all the tubes in the mid- to long-term; however, they decided to keep two of the three bundles in brass due to its proven biocidal nature until further experience is gained with the stainless steel tube material.
Major technical challenges included:
- Performing a tubesheet stress analysis on the tubesheets using a higher hydrostatic pressure and thinner wall tubes.
- Assuring new tubesheets were drilled identically to the existing tubesheets.
- Aligning new tubesheets to the support plates accurately and efficiently.
- Rolling brass tubes into the tubesheet with acceptable work hardening of the material.
- Selecting torques that would provide leak tight joints and demonstrating this in shop tests.
- Preventing tube vibration.
Meeting technical challenges
Tubesheet stress analysis – The tubesheet stress analysis demonstrated the thin wall tubes would provide adequate support for the tubesheet and ensured an acceptable safety margin to preclude any potential failure. The analysis was performed with a tubeside pressure of 5 bars (72.5 psi), well above the design pressure of 2.9 bars (42 psi), using 0.5 mm stainless tubes in the main condensing zone and one row of 0.7 mm stainless tubes in the periphery.
The Heat Exchange Institute’s Beam Strip Analysis provided the methodology. The tubesheet is modeled as horizontal and vertical strips extending from the edge of the tubesheet. Each tube is modeled as a spring supporting the tubesheet. See Figure 3.
Reverse engineering of tubesheets – Reverse engineering was used to create new CAD based drawings for manufacturing the new tubesheets. In addition to recreating the old drawings, it was necessary to address other key factors. The first issue was that of material. The original tubesheets were manufactured to AFNOR E.26.3-A36; a specification replaced by new European standards. It was recommended that the plates be fabricated of P295GH steel, which is similar to A-516, Grade 70 steel.
The original tubesheets were welded flush on the outer perimeter with the waterbox and shell; however, given access, time and fit-up constraints, the joint was redesigned to utilize a slightly larger tubesheet connected to the shell with a full penetration weld from the inside of the condenser. The additional width of the tubesheet permitted drilling of holes to mount lifting hardware for rigging the tubesheets into position.