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TSS - Terminal Solid Solubility

Advanced Non-Invasive Hydrogen Concentration Measurements for CANDU Pressure Tubes


CANDU nuclear reactors contain fuel-filled zirconium pressure tubes which slowly absorb hydrogen from the primary heat transport coolant while in service. The hydrogen concentration is an important factor in the life cycle management of the pressure tubes, as it affects the remaining service life. Hence, there is an ongoing requirement for utilities to periodically inspect the pressure tubes to monitor their service condition. 

The hydrogen concentration measurements are performed during a unit outage, typically by defueling and isolating a pressure tube and obtaining a scrape sample of the zirconium material for chemical analysis. This process is time consuming and dose intensive, due to the need for channel isolation (ice plugging) and reactor face work by personnel.

The TSS (Terminal Solid Solubility) Tool uses an entirely different approach, which avoids these disadvantages. The TSS tool performs hydrogen concentration measurements in-situ on a “wet” fuel channel, by determining the TSS temperature of the pressure tube material. There is a well known and accurate relationship between the TSS temperature and hydrogen concentration, which allows the latter to be determined.

Scope of work

To deploy the TSS tool on a CANDU unit and successfully measure hydrogen concentrations at four locations in each of four pressure tubes.


Kinectrics mobilized its team of specialized professionals to provide a complete turnkey inspection services solution. The interface between the TSS tool and its delivery machine was designed and manufactured and the components functionally tested.  Equipment and resources were then shipped overseas to the utility site, where mobilization and setup were completed.

TSS was then deployed on-reactor, where it successfully, reliably and safely completed all required measurements, on schedule and off critical path. This was the first deployment of the TSS technology to a CANDU6 reactor, and the first outside of Canada.

Upon completion of the on-site work, the TSS tooling was shipped back to Kinectrics’ Decontamination and Refurbishment Facility (DRF), where the tooling was decontaminated and the tool heads refurbished for subsequent deployments.

Data quality from the inspections was excellent, and the utility Customers were extremely satisfied with the results obtained.


Accurate measurement of hydrogen concentrations in CANDU pressure tubes provided the assurance of their integrity for the Customer and the local nuclear regulatory authority. The work was completed safely, on time and with high quality results.