RNPP Turbine Deck Foundation: Ultimate Facilitator ofTurbo-Generator System

Introduction: In a modern nuclear power plant territory, just after the Reactor Building, the most important premise is the Turbine Hall. This massive structure facilitates the Turbo-Generator Units which engenders huge electric energy output i.e. 1200MW for RNPP. The turbo-generator unit is supported by a turbine deck foundation, a very complex, critical, and most important structural part of the Turbine Hall. The self-weight of the turbine deck foundation is around 4500 tonnes and it bears 2500 tonnes imposed load of the turbo-generator unit from the design to construction, a turbine deck foundation requires an excellent and special form of knowledge as well as experience. However, the successful completion of the turbine building depends on the success of the construction of the turbine deck foundation.

For the instance of the Turbine Hall of RNPP, the turbine deck foundation is 72m in length and 13m in width in the turbine zone while 8m in the generator zone. This deck, a variable depth from 3.6m to 4.3m, is separated from the main building slabs and oil channel by 50mm to avoid the transfer of vibration to the floor. The deck has been designed in such a way that it will be supported by 94 Vibro-isolator springs to alleviate the massive vibratory and impact load from the turbo-generator set. After installation of the Vibro-isolators, installation of the bottom formworks, reinforcements, and embedded parts are completed sequentially. The concreting is done in a single pouring without allowing any construction joint. Immediately after completion of concreting, the curing of concrete is done following the stipulated methodology. 

Construction Methodology: The detailed, very complex work of the construction of the turbine deck foundation has been stipulated in Work Execution Plans (WEP), followed by Working Drawings (WD). As previously mentioned this 4.3m depth of TG deck starts from elevation +12.800 and it goes up to +17.180. Turbo-generator set, linking with condenser units from the bottom of the TG deck at the low-pressure cylinder unit, has limited access to getting support from the foundation slab. So, to provide load-bearing support, a temporary steel framing has been introduced at the elevation of +12.400. Formwork of bottom parts is started immediately after the installation and commissioning of the Vibro-isolating springs. Later, reinforcements along with embedded parts and all other elements were installed on the bottom firm platform. Concreting was finished through continuous pouring following a proper concrete pouring plan. Finally, just after finishing concrete work, thermal curing started and the concrete temperature was periodically inspected.

Fig 1: 3D model of TG Deck

3. Construction Complexity:  Construction of turbine deck foundation is a very complex and critical work as it allows very small deviation in dimension, and again, limited access for the support structure renders it more complex. Firstly, the TG construction requires a temporary steel structure on which it will rest during the construction phases. The very first challenge for the executor of the work is to create a 10.2m span of a steel frame. Another fundamental work is the installation and commissioning of 94 Vibro-isolating springs that require a high level of engineering excellency. From reinforcement work to installation of pipe penetration for turbo-generator sets all are undoubtedly an assiduous job, less allowable deviation makes it so. Finally, the most tedious and complex job is finishing this massive structure in a single concrete pour; no vertical and horizontal construction joint is allowed for any stage of concrete.

4. Construction Sequences: The turbine sets the lower foundation, starting from elevation -7.250 and ending at the crossbeam at elevation +12.400, which is conventional construction. These crossbeams (10 Nos.) support the whole TG deck foundation and span 24m where the maximum spacing is 10.2m. After the completion of these crossbeams, the top surfaces were prepared for Vibro-isolator springs installation.

4.1. Temporary Steel Support Installation: 200 tonnes of temporary steel supports were installed around the whole TG area. These elements consist of I-beam, C-Channels, and Box sections that were prefabricated. The primary function of this frame is to provide support to the bottom part of the formworks and give simultaneous access to the below part for installation of the condenser upper part.    

                                           Fig 3: Temporary Steel Support Installation

4.2. Vibro-isolator Springs Installation: The Vibro-isolators were supplied, installed and commissioned by renowned German company GERB Gmbh. These dampers sit on the cross beams of elevation +12.400 with rubber gasket pads. There were types of Vibro-isolators installed, one TN series, and another of TVN series, which were installed according to the instruction of WEP and the installation manual from the GERB.

Fig 2: Vibro-isolator Spring

The maximum allowable deviation for these dampers was +5mm and -2mm, so special care was taken during the installation of these Vibro-isolators. After completion of installation, these items were covered with a polyethylene bag for protection from any loose concrete or dust coming to it. 

4.3. Bottom Formwork Installation: Immediately after the commissioning of Vibro-isolators, the erection of bottom formworks on the previously installed steel support was started.

20mm thick ply boards were used to serve this purpose as the load coming from fresh concrete is very high for conventional 10mm ply boards. Following the work execution plan, for a reasonable settlement, these plies were set at +10mm higher elevation which might be compensated by the load coming from freshly laid concrete. An executive scheme was done and inspected just after the completion of the bottom formwork installation.  

4.4. Reinforcement and Embedded Part Installation: Another tedious job of the scope of TG deck construction is to install reinforcement and embedded parts. A total of 260 tonnes of rebar and 50 tonnes of embedded parts were designed to be installed for the TG deck. The maximum diameter of the rebar was 40mm whereas the minimum was 16mm. The rebar size 20mm or higher was connected with specific couplers. There were 130 pieces of pipe sleeves that engulfed the bolts coming with a turbo-generator set. As the maximum deviation of these pipes was +- 2mm, it was also very critical work to be executed. There were a number of running embedded parts throughout the whole TG deck to make the installation easier, the designer considered fabricating the embedded part in pieces and connecting through designed welding.

4.5. Close Circuit Grounding Grid: All the rebar and embedded parts of the TG deck was designed to connect each other to make a close-circuit grounding grid (CCGG).

4.6. Vertical Formwork Installation and Concreting: After completion of rebar, embedded parts and CCGG work, the whole skeleton was covered up with side formworks, there were a total of 1810 square meters of formworks used in the TG deck. The formworks were properly aligned and allowed proper clear cover. The ultimate challenge for the execution of turbine deck construction is concreting this huge structure in a single pouring segment. A specific concrete pouring plan was developed, and the manpower and other resources were properly allocated.

Vibratory, normal B30 w6 concrete was replaced with self-consolidating concrete (SCC) to avoid complexity during concreting. Having restriction of free fall height of SCC concrete within 0.5, tremie pipes were used. For the continuous and seamless flow of concrete, a total of 6 Nos. (5 running, 1 stand-by) of the volume of concrete for 1^st stage was 1412 cubic meters, and it took 24 hours to complete the pouring. There were 36 pouring points throughout the whole TG deck. Before pouring to the site, fresh concrete was periodically monitored to inspect its quality i.e. flow ability L box, V funnel test. The whole turbine deck area was covered up with temporary sheds so that it could serve the uninterrupted pouring in case of rain.

4.7. Post-Concreting Management: The concrete of the turbine deck foundation went through a thermal curing process. Just after finishing the concrete pouring works, the deck was covered up with mineral rock wool to control the skipping of heat from the surface.

Fig 5: Curing by insulation

There were 88 temperature inspection points and a group of specialists was engaged to monitor the temperature periodically. The temperature difference between core and surface was allowed within 15 degrees Celsius, so special care was taken to control the temperature gradient. The removal of formwork was allowed only when the temperature difference between the surface of concrete and the ambient went below 5 degrees Celsius. 

Fig 6: TG Deck after 1^st stage of concreting

After the removal of formworks, several joint inspections were conducted for the surface quality of concrete.

5. Quality Control of TG Deck Construction:The detailed Quality control plan and the program have been stipulated in the quality assurance program for the construction of the turbine building and in the Work Execution Plan for the construction of the turbine deck foundation. Both visual and measuring controls were performed during quality and acceptance control. Before coming to the construction site for execution, all construction material had to go through intensive incoming control. From the very beginning of reinforcement installation to the post-concrete acceptance, every stage of construction had some hold point to check the quality of the work for acceptance. During the inspection of reinforcement, the coupling joint, diameter, spacing and surface of the rebar were inspected properly. As-built schemes for all installed embedded parts were arranged and inspected. After formwork installation, all geometrical parameters including concrete cover and cleanliness were checked jointly. With a view to checking the parameters of fresh concrete (i.e. Flow ability L box, V funnel test)several concrete monitoring teams were formed. Besides checking the quality of fresh concrete, the teams monitored the concrete pouring rate. After design age (i.e. 28 days) NDT tests were performed with the participation of the CRNPP quality control team to monitor the concrete strength in the field.

6. Conclusion: The construction of the turbine deck foundation was planned for 90 days of time exposure and the concreting was finished within this time period. All the supervisors from the sub-contractor, general contractor and customer did their job fantastically to make the work fruitful. This 24 hours of concreting was monitored by the CRNPP personnel through a roaster duty. Right now, the concreting of the 2^nd stage has been completed and prepared for the installation of the turbine set is in the progress.

References:

1.     RPR.0110.10UMA.MPA.KZ.LC001.’Turbine set foundation, Geometry’, version C02

2.     RPR.0110.10UMA.MPA.KZ.LC002 ’Turbine set foundation, Reinforcement’, version C02

3.     RPR.0503.10UMA.0.CS.TZ0004. ‘Arrangement of Turbine set pedestal upper structure, version D02.

4.     RPR-PSAR0101-BAA0001.Preliminary Safety Analysis Report: Introduction and General Description of Nuclear Power plant. Revision B04.

5.     RPR-PSAR0103-BAA0003. Preliminary Safety Analysis Report: Design of Structures, Components, Equipment and systems, Book 3, Revision B05.