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

Abstract of the technical presentation to be presented at:
Regulatory Perspective on the CSA N285.4 & N285.8 Requirements for the Pressure Tube to Calandria Tube Contact in CANDU Reactors
May 15-17, 2019

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

Abstract

Canadian licensees periodically inspect CANDU pressure tubes in accordance with clause 12 of CSA standard N285.4. The Zr-2.5% Nb pressure tubes carry 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 help prevent 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.

When the screening criteria of CSA N285.4 are not satisfied, N285.8 provides additional requirements to disposition. Additionally, N285.8 requires a risk assessment for the entire core performed either deterministically or probabilistically.

In this paper, CNSC staff provides the regulatory perspective for meeting CSA N285.4 and N285.8 requirements regarding the pressure tube to calandria tube contact.

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