Regulatory Perspective on the Fitness-for-Service Requirements for the Pressure Tube to Calandria Tube Contact in CANDU Reactors

Abstract of the technical presentation presented at:
26th ICONE – International Conference on Nuclear Engineering
July 22–26, 2018

Prepared by:
Sankar Laxman and John Jin
Canadian Nuclear Safety Commission

Abstract:

Canadian licensees implement periodic inspections of the CANDU pressure tubes in accordance with clause 12 of CSA standard N285.4. The Zr-2.5% Nb pressure tubes carry the nuclear fuel and are insulated from the cooler moderator by an outer Zircaloy 2 tube, called the calandria tube. The pressure tube is supported on four spacers, or "garter springs", which assist in avoiding any contact between these two tubes. Contact between the hot pressure tube and colder calandria tube can lead to a cold spot in the pressure tube and subsequent formation of a brittle hydride blister at the cold spot, and eventual cracking of the blister during reactor operation.

Among several volumetric and dimensional inspection requirements, clause 12 of CSA N285.4 requires the determination of the pressure tube to calandria tube gap and stipulates that the pressure tubes are considered acceptable for operation only when "no pressure tube to calandria tube contact" is predicted to exist at the end of the next periodic inspection interval.

It should be noted that the majority of the currently operating Canadian CANDU reactors are equipped with a type of spacer that tends to move from its installed position during reactor operation due to its unique loose-fitting design. This movement may lead to contact between the pressure tube and calandria tube earlier than predicted. To address this spacer movement, Industry has developed a methodology to reposition the spacers in a more optimal position based on spacer loading considerations.

The predicted position of the spacers at the end of the next periodic inspection interval plays an important role in the prediction of pressure tube to calandria tube time to first contact.

In this paper, Canadian Nuclear Safety Commission technical staff provide the criteria for meeting the CSA N285.4 requirements regarding the pressure tube to calandria tube contact and the uncertainties associated with the model used in the prediction of time to first contact.

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