High stress monitoring of prestressing tendons in nuclear concrete vessels using fibre-optic sensors

• We weld radiation-resistant optical fibre strain sensors to steel prestressing tendons.
• We prove the sensors can survive 1300 MPa stress (80% of steel's tensile strength).
• Mechanical relaxation of sensors is characterised under 1300 MPa stress over 10 h.
• Strain transfer between tendon and sensor remains at 69% after relaxation.
• Sensors can withstand and measure deflection of tendon around a 4.5 m bend radius.

Maintaining the structural health of prestressed concrete nuclear containments is a key element in ensuring nuclear reactors are capable of meeting their safety requirements. This paper discusses the attachment, fabrication and characterisation of optical fibre strain sensors suitable for the prestress monitoring of irradiated steel prestressing tendons. The all-metal fabrication and welding process allowed the instrumented strand to simultaneously monitor and apply stresses up to 1300 MPa (80% of steel's ultimate tensile strength). There were no adverse effects to the strand's mechanical properties or integrity. After sensor relaxation through cyclic stress treatment, strain transfer between the optical fibre sensors and the strand remained at 69%. The fibre strain sensors could also withstand the non-axial forces induced as the strand was deflected around a 4.5 m bend radius. Further development of this technology has the potential to augment current prestress monitoring practices, allowing distributed measurements of short- and long-term prestress losses in nuclear prestressed-concrete vessels.