Context and challenges

The client is an industry leader in power and propulsion systems. The client needed to assess the feasibility of a SMR concept consisting of a close-coupled arrangement of the main vessels.

This consisted of examining the SMR’s close-coupled vessel nozzle arrangement using Finite Element Analysis (FEA) and Reactor Pressure Vessel (RPV) plant design. It also needed to take into account the effects of restraining free thermal expansion due to plant thermal transients and the effect on the nozzles in comparison with a conventional plant layout.

The client called upon Assystem to provide engineering support to assess the feasibility of their SMR concept. Various plant configurations were assessed to review stresses to the pipework, nozzles, and shells for comparison. Loading on the model comprised of normal operation conditions such as pressure, temperature and gravity with an allowance for seismic forces.

Project scope

Assystem conducted a stress assessment of the SMR’s close-coupled vessel nozzle arrangement by FEA. An optimisation approach was taken using Opti-struct software within Hyperworks.

The optimisation study investigated the following parameters:

  • Variation in the distance that the Steam Generator (SG) is positioned from the RPV
  • Increase in wall thickness of the pipework and nozzle reinforced zones
  • Consideration of allowable stress in line with ASME III primary and secondary stress limits
  • Normal operation and representative design basis event seismic loading
  • SG orientation

A more detailed study was conducted on a representative section of primary loop pipework to ascertain the feasibility of a close-coupled SMR arrangement. This took into account NOC loading and a more appropriate estimate of external force from seismic excitation; an extension to the detailed study to determine the maximum allowable movement of the SG for the pipework and nozzles to remain within ASME III allowable stress limits.


Production of a written report related to the work carried out as well as recommendations for the design. The report’s recommendations helped to determine that the close-coupled design was feasible as long as certain criteria such as allowances for expansion were met. The stress assessment enabled to emphasize the benefits offered by the design changes:

  • The optimisation approach allowed a better understanding of the sensitivity and trends of the key design features
  • Assystem ensured the client passed the technical review gate in line with ASME III limits. The report was well-received and the design is currently in progress.

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