Advanced Simulation of a Reactor Containment Concrete Structure

Olkiluoto 3 (OL3) in Finland is the 3rd most powerful NPP (nuclear power plant) unit in the world. It started regular electricity production in April 2023. OL3 is a PWR (pressurized water reactor) with a net electrical output of about 1600 MW. The OL3 reactor island was constructed by Areva (today known as Orano).

The OL3 unit incorporates modern systems built on proven technology. Significant focus has been placed on safety through measures for preventing and managing severe accidents. The reactor containment structure is an important safety barrier in the radiation safety system which ensures leak tightness and structural integrity in severe accidental conditions.

A long-term commitment

As part of the design and construction of the OL3 Nuclear Power Plant, the TECHNIA Simulation services team was commissioned by the plant owner, TVO (Teollisuuden Voima Oy), to support the review and evaluation of design specifications and structural calculations for the reactor containment.

The assignment covered multiple activities throughout the entire design and construction lifecycle, with the primary objective of contributing to a robust, safe, and reliable containment structure. Key areas of support are described as follows.

Owner’s engineer

As owner’s engineer, TECHNIA provided expert support to TVO on the design and long-term integrity of the reactor containment and internal building structures. This included specialist expertise in structural integrity, load conditions, monitoring and testing, applicable codes and standards, as well as design and construction practices. Particular emphasis was placed on the containment steel liner and the prestressing system, both of which are critical to ensuring concrete containment tightness and overall structural integrity.

Design review

A core part of the assignment was the independent review of supplier containment design documentation, including calculations, construction drawings, and simulation models. This work was essential in verifying compliance with applicable codes and standards. Support was provided throughout the entire design process, ranging from early-phase containment design assessments to detailed third-party design reviews and comparative analyses submitted by the Owner to the Authority.

3D models

To support the review process, detailed 3D FEA (finite element analysis) models of the containment structures were developed to perform comparative studies of critical load cases and validate the design. These advanced non-linear FEA models were also used to assess ultimate structural capacity under extreme conditions, such as severe accident scenarios involving high internal pressure and temperature. Following construction, the 3D reactor containment FEA model was updated to reflect the as-built condition, incorporating final design drawings and tendon prestressing records. This ensures the model remains a valuable tool throughout the entire operational lifetime of the OL3 NPP.

ISIT and ILRT pressure testing

TECHNIA supported both pre- and post-test simulations for the containment ISIT (initial structure integrity test) and the periodic ILRT (integrated leakage rate tests). Comprehensive structural monitoring, combined with the detailed 3D FEA containment model, enables accurate comparison between measured test data and simulation results.

Validation of structural response at over-pressurization test operations

Using the FEA containment structure model developed during the review process, a global model, updated for as-built conditions and aging related changes of material parameters, was assembled to perform pressure test validation analyses. This model provides up to date representation of the containment concrete structure, prestressing tendons, reinforcement bars and steel liner. A particularly challenging aspect of the modelling was to accurately represent the layout of the comprehensive tendon system and the applied tendon prestress, capturing the initial state of stress and deformation in the containment concrete structure.

The containment response measurements, resulting from the pressurization test, in the form of relative strain and displacement was compared with FEA model calculation results in gauge positions. Acceptance criteria from international codes have successfully been applied to verify the reactor containment performance at the initial test and the periodical test sessions before and after the NPP unit start of operation.

Meeting design safety requirements in reactor containment

The assignment at OL3 carried out by the TECHNIA simulation services team contributed to a robust design and construction of the reactor containment structures, reviewing the compliance with codes and standard requirements. This was made possible by TECHNIA’s extensive knowledge and over 25 years of experience in the nuclear industry.

Advanced 3D FEA models played a key role in the review process, supporting the assessment of structural capacity under extreme conditions, including severe accident scenarios with high internal pressure and temperature.

TECHNIA’s extensive nuclear industry experience, particularly in reactor containment technology, formed a critical foundation for meeting stringent safety requirements. This expertise also underpinned the FEA simulations, supporting the initial pressure test and continues to support periodic pressure testing throughout the operational lifetime of the OL3 reactor containment structure, ensuring ongoing verification of structural performance.