Pressure tubes: the heart of the CANDU

When I attended and presented at the Canadian Nuclear Safety Commission – Commission canadienne de s?reté nucléaire hearing for the Pickering extension along with Tom Hess and Jules Besseling , the main topic of discussion was pressure tubes. Were they fit for continued operation? Ontario Power Generation of course argued yes, opponents argued no.

Isn't this easy to determine? Let's explore.

Early pressure tubes were made of Zircaloy-2. The material was later upgraded to Zr-2.5Nb, strengthened by Controlled Cold Work.

Instead of a pressure vessel, the CANDU has an array of pressure tubes, fitted inside calandria tubes with end fittings on each end. This constitutes a "fuel channel". For the Pickering B and C6 units, there are 380 of these per unit, for Bruce and Darlington, 480.

The pressure tubes house the fuel bundles and, through the end fittings, which are attached to the feeders, receive the coolant use to create steam in the boilers, driving a turbine to create electricity.

The pressure tubes are a wear item, meaning they eventually need replacement. But how and when are quite different from other reactor components.

The exposure to neutron flux in the core "ages" the pressure tubes. This drives a key parameter in determining the life of the pressure tube:

Hydrogen Equivalent Concentration, or "Heq".

The zirconium alloy picks up hydrogen in service, due to the fission taking place around it, and this drives a change in the metallurgy.

When virgin, zirconium alloy is quite ductile and not prone to fracture. This is a key part of the "leak before break" philosophy of the design, where the annulus space (the space between the pressure tube and the calandria tube) is constantly monitored for moisture.

However, as the hydrogen concentration increases, the ductility drops and the alloy becomes more brittle. This pushes it toward an inclination to fracture rather than deform and leak.

The established limit for Heq is 120ppm. Pressure tubes are measured regularly to ensure they are below this limit.

This is also extensively modelled, as you cannot measure every tube on every outage, so random scrapes are performed of tubes most likely to have higher concentration levels.

The standard model predicts reaching the ~120ppm threshold at around 230,000 equivalent full power hours (EFPH), which is approximately 30 years.

This is where Pickering is different.

The CANDU 6, Darlington and Bruce all use a 37-element fuel bundle, which produces a given amount of neutron flux in the core. Pickering does not.

Pickering uses an older 28-element bundle, which produces lower levels of flux, and despite Pickering B being based on the CANDU 6, because the heat transport system was modelled after the older A units, thus capping thermal and electrical output, the 28-element bundle was retained.

So, when somebody points out that Darlington and Lepreau were both refurb'd at 30-years and state that Pickering pressure tubes are "past end of life", this is not an accurate representation of the situation.

The EFPH model that works for Darlington and Lepreau doesn't work for Pickering.

The attached table shows the higher EFPH for the Pickering B units, but the lower levels of Heq when compared to Darlington. It also shows that even at 295,000 EFPH, Heq will be below 100ppm.

So, increasing the EFPH limit to 305,000 on the B units, and thereby extending the operating life, still keep the units well below the 120ppm limit. This is the argument that justifies their continued operation.

There are also other parameters influenced by operation of the core:

- axial growth (elongation)

- radial growth (increase in diameter)

- sag

Sag is the only one that Ontario Power Generation has had to keep an eye on, as nozzles that are used to inject the gadolinium nitrate into the core (LISS nozzles) are situated under the pressure tubes, and sag of the tubes brings the nozzles and the tubes closer together.

OPG has implemented a "gap adjustment and monitoring program" to ensure that the proper space between the LISS nozzles and the PT's is maintained.

Looking forward, as can be seen in the pressure tube alloy evolution slide shown earlier, the current generation of PT's has low hydrogen and is expected to age more slowly than its predecessor, extending the mean time between refurbs.

Simultaneously, Bruce Power and Ontario Power Generation have undertaken extensive testing to establish a revised Heq limit of 140ppm, which will extend that time even further. This could potentially mean 40 years between refurbs on the 37-element bundle units and 50+ years at Pickering.