Open Top Method: Advanced Construction Technology for Nuclear Power Plants

by Mark Gino Aliperio, BH Song, SH Jeong, UH Jang, E Akpoguma

Industrial technologies are evolving, and in construction, there is no exception. But the most recent developments extend beyond building equipment alone. In this 21st century, technological advancement is the order of the day. New approaches for construction are seen in the field. Such advancement in construction technologies are driving efficiencies which results to better-quality outcomes. With several nuclear power plants (NPPs) presently under construction worldwide, expectations are rising regarding the quality, cost and schedule associated with the construction of new NPPs. Worldwide experience in large construction projects, including NPP projects, has resulted in significant advancements in construction techniques and methods and one of these is the Open Top Method.

Concept of Open Top Method

The containment building is wrapped around the auxiliary building and has bigger and heavier components in it than those of former nuclear power plants. The constraints on the installation of components within the reactor building and containment were a major challenge to the construction schedule. There are two ways to bring big components like steam generators into the containment building and to seat them at their own places. One it a conventional concept of ‘Side Method’. In the past, the reactor building and containment wall were constructed with temporary openings in the side of the buildings to allow entry of bulk commodities and large equipment and seated by using the polar crane. With the Side Method, components can enter the containment building only after providing the polar crane and the area around the equipment hatch on the auxiliary building roof. These may cause serious schedule delay. The other is the ‘Open Top Method’ in which massive equipment come into the containment building in vertical direction from top of the building by large capacity crane.

Recently, mainly due to advances in construction crane technology, significant improvements in schedule have been achieved through installation of heavy equipment through the open top method. Open top installation can provide cost savings and flexibility in delivery of heavy equipment that is not enclosed by ceiling slabs. In the open top installation, the reactor vessel and steam generators are placed into position by using a very heavy lift (VHL) crane after containment wall is built. Once the equipment has been placed inside, work on connecting the equipment to the already built piping and electrical systems can proceed in parallel with the ongoing construction works.

There is a lot of heavy equipment in NPP buildings like reactor pressure vessel (RPV), containment vessel and steam generators. This heavy equipment needs to be installed in the correct position. The open top method allows easier installation of the heavy equipment, as equipment can be installed directly into the final position. In addition, installation of these equipment will affect the construction schedule. The installing containment vessel will be the critical path to the Nuclear Island, and the installing condenser will be the critical path to the Turbine Island. When larger and heavier equipment are installed using this method rather than side method, the construction schedule could be shortened.

Open top method is a construction technique that involves integrating the construction of the walls and slabs of a building with the installation of commodities. The commodities are designed, procured and constructed and installed in their final positions before those areas are fully enclosed by higher elevation slabs and the containment dome. When properly used, this open top construction technique speed the construction process and reduce construction labor cost. Open top method reduces temporary openings, which are used in the side method to carry in equipment after the construction of the buildings. Bulk commodities such as piping and cable trays can be moved by using the open top method.

Field Application

1. Qinshan III 1-2, China

During the construction of Qinshan III Units 1 and 2, approximately 70 items of equipment were set in place using the VHL crane. This equipment included steam generators (220 ton each), pressurizer (103 ton), reactivity mechanisms deck (43 ton), feeder frames (40 ton each), fueling machine bridges (16 ton each) and major heat exchangers. The steam generator took one day to install, compared to the two weeks typically required using the traditional horizontal access method.

2.Tarapur Units 3 and 4, India

During the construction of Tarapur 3 and 4 in India, open top method was used to position about 50 pieces of equipment, including the steam generators, moderator heat exchangers and pressurizer. The positioning of each steam generator was completed in less than a day, much less than the installation time of more than one month required by other methods.

Considerations when the Open Top Method is to be applied

The open top method requires the early procurement of equipment compared to the conventional method which uses temporary openings, because the equipment needs to be installed before the ceiling is installed.

• The equipment to be installed using the open top method should be identified in the earliest stage of the project, and the engineering and procurement schedules should be adjusted to match the open top method schedule.
• The open top method requires strict manufacturing and shipping management of the equipment. Any delay in the equipment causes a delay in the construction of the building, which will affect the entire construction schedule.
• Equipment installed using the open top method should be properly protected against physical impact, the weather, humidity, and construction dust and fumes, because the upper slab is installed after the installation of the equipment.
• Open top method requires very heavy lift (VHL) cranes.
• A large crane is used to lift the modules or equipment into the buildings. Large items of equipment such as the RPV, pre assembled containment vessel, pre assembled condensers and pre assembled modules are lifted ‘over the top’ of the building to avoid interfering with the building construction. The VHL crane needs to have the capacity to lift the large equipment and modules into the building. Current industry practice typically uses VHL cranes with the capacity to lift and place modules with a mass of more than 900 tons.
• Movement of large assemblies, modules and equipment from port to port, port to site, and site to crane location is an additional area where efficiency has increased significantly.
• Just in time delivery should be evaluated on a case by case basis, based on the economics. The concept of ‘single lift,’ which seeks to move material and equipment directly onto the site and into place without any interim lift or storage, should be utilized where possible.
• Very heavy lifting has been used for large scale capital projects. An important consideration in the application of a VHL crane is the area needed for strategic placement of the crane to conduct the numerous lifting activities. Planning for crane placement and movement is a crucial step in open top installation.
• Full exploitation of open top construction is expected to reduce total construction time, but attention must also be focused on the cost of heavy lift equipment and the facilities necessary for the prefabrication of modular components.
• A VHL could be one of the long lead items which may take two to three years, including design and manufacturing.

This work is part of the project report KINGS/PR-PEP02-2018-08 "Advanced Construction Technologies" by the Project Engineer Program II Laboratory of KEPCO International Nuclear Graduate School Class of 2019.