We are thrilled to announce that our SCHEDULE project (completed in June 2023) has won the 2024 Technological Innovation Award (PRIX INNOVATION TECHNOLOGIQUE) from Sfen.

The prize is awarded for the team that completes the best innovative project in the nuclear sector in the previous 12 months. The award ceremony will take place in Paris on 4 July.

Double skin steel concrete composite construction (DSC) is made of two steel plates connected by a grid of tie bars with concrete between the plates. Composite action between the steel plates and the concrete is provided by shear connectors welded to the steel plates. DSC construction is typically used in a vertical orientation as walls but may be used in a horizontal orientation as floors (e.g. in a spent fuel pool where a watertight membrane is required). Single skin steel concrete composite construction (SSC) comprises a single steel plate reinforced by steel sections (typically T-stiffeners) which provide stiffness during construction and shear resistance to the section against out of plane loading in the permanent condition. Shear stud connectors are welded to the plate and the flange of the T-stiffeners to achieve composite action with the concrete which is placed on the plate. SSC construction is used in a horizontal orientation as floors. DSC and SSC construction are collectively referred to as SC construction.

SC construction has found appeal in the nuclear sector as it offers significant schedule advantages over reinforced concrete (RC) construction. SC modules can be fabricated offsite and assembled onsite, conventional rebar reinforcement is significantly reduced or eliminated, there is no formwork removal and embedded plates are eliminated.

SCHEDULE was a joint European project which studied the design and the manufacture and construction at full scale of a replica of the EDF DUS building using SC construction. The objective was to quantify the benefits of SC construction and acquire design, manufacture and construction expertise using this technology. The project also investigated the potential for SC in the construction of spent fuel pools and APC shells.

Dividing the building into modules that can be fabricated offsite and delivered to site required a balance between the requirements of the building geometry (e.g. building shape, location of penetrations, location of floors), manufacturing considerations (e.g. crane capacity, component sizes, storage, access), transportation (which dictated the maximum module size) and site requirements (storage, methods of joining modules, crane capacity). Building Information Modelling (BIM) was used throughout the different stages of the project – from conceptual design to construction on site to communicate the design and ensure that clashes are avoided.

The building was analysed using a 3-D finite element model. The analysis was performed under the same combinations of actions used in the design of the DUS building (and commonly used for safety critical structures in nuclear power plant). These included the final permanent condition of the building and accidental actions (such as earthquakes, fires, explosions, impact, etc.). In addition, actions which arise during construction and which are specific to SC structures were also considered.

A comprehensive set of tolerances was defined covering both local module geometry (e.g. position of tie bars within modules, separation between module plates, plate flatness between tie bars) and global module geometry (e.g. module size, overall module straightness, ‘squareness’ of corner modules). This attention to tolerances early in the project paid off, as modules were manufactured to a high degree of accuracy. Compliance with tolerance specifications was verified by manual measurement and, for some modules, also by laser scanning.

Manufacturing trials were performed to optimise production sequence. There was a range of module types in the project (standard wall modules, APC shell modules, pool floor and wall modules, standard floor modules) as well as a range of wall module shapes (planar, L-shape, T-shape and TT-shape). Different wall module joining methods on site (by dowel bars, butt welds, fillet welds and Hollo-bolts) were planned as well as joints of wall to floor module, floor to floor module and wall to foundation. These were reflected in the module detailing and manufacturing. In addition, ancillary components identified in the site construction methodology were fabricated and details were incorporated for their attachment to the modules on site. During and on completion of production, module tolerances were checked (and rectification performed where necessary) and welds tested (and repairs performed if defects were found). In addition, a trial assembly of part of the first level of the building was carried out in the fabrication shop to verify module tolerances.  Maximum module height was 3.98 m and maximum length was 10.57 m.

The building foundation comprised a reinforced concrete raft. Construction of the raft required very accurate positioning of the starter bars to avoid any clashes with the module steelwork. The building elevations were divided into 6 levels of wall modules. To investigate the practicalities of different module joining methods, the first four levels were joined by dowel bars embedded in the module concrete while the upper two levels were welded.

The building was constructed one level at a time: the sequence of construction at each level comprised (i) wall module installation and joining, (ii) slab module installation and joining to walls, (iii) concreting of wall modules and (iv) concreting of floor modules. Prior to installation, the modules were fitted with construction ancillary components to facilitate assembly. These included levelling jacks at the top and bottom and installation guides at the top of each wall module. Each module was placed in position by the tower crane, guided by two workers using ropes to accurately place the module in its final position in the building.

A number of the building modules were instrumented with the objective of collecting data on the performance of SC construction in a number of respects. These included the monitoring of concrete pressure on module walls during concreting and hydration of concrete. Other instrumentation will be used to gather data beyond the end of the construction project; these include data from future mechanical tests on wall and floor modules as well as ongoing monitoring of module plate corrosion rates.

A great deal of expertise was gained in the design, BIM modelling, manufacture and construction of buildings using SC. The project highlighted the importance of early involvement of the designer, manufacturer and contractor as decisions made about module joining on site and the construction methodology have an impact on design and manufacture of the modules. Similarly, the outcomes of the modularisation study (module sizes and shapes) have an impact on both module manufacture and onsite construction.

The project demonstrated and quantified the time saving that can be achieved on site using SC construction – the first four levels of the building were completed in 3.75 months compared with 5.4 months for levels 1 – 4 of the RC DUS building (both buildings were identical up to this level). The total time taken for the whole pilot building was 5.6 months (including the complexities of the pool and APC shell) compared with 7.2 months for the RC DUS.

The project also demonstrated and quantified the saving in the number of persons required on site. Only 8 people where needed for the SC building (plus a welding team of 5 people for one month when site welding operations were being performed). In the case of the RC DUS, the number of workers is 30. The RC DUS building required a total of 23500 person-hours of effort on site whereas the SC pilot building required 7300 person-hours.

The project was undertaken by project partners Steel Construction Institute, EDF, CEA, EGIS, Bouygues, Peikko and ArcelorMittal. It was funded by the European Commission (grant no. 800732), ENRESA, Framatome, ORANO, ADF and the project partners.