Application of leak-before-break to primary loop piping to eliminate pipe whip restraints in a Spanish nuclear power plant

The Spanish plant described in this study is a 982 MWe PWR plant with a three-loop primary circuit of piping made from centrifugally-cast stainless steel SA351 CF8A.

The licensee requested an exemption to GDC-4, from CSN, so as to avoid the postulation of guillotine rupture of the primary loop piping. The request was based on the generic work performed for a US PWR plant group to have such exemption to GDC-4. As the piping material in the Spanish plant is different from that in the plants included in the generic work, CSN performed a review of the applicability of the generic results to the Spanish plant. Also, aspects such as fatigue evaluation, net section collapse, crack growth and leak detection, specifically analyzed for the Spanish plant, were reviewed.

CSN found that fracture toughness test results from generic work are applicable to the Spanish plant; sufficient margin exists against unstable crack extension, and adequate leak detection capability exists with the leakage detection systems available in the plant.

Exemption to GDC-4 was approved and CSN authorized the licensee to remove protection devices against dynamic loads from guillotine breaks in the primary coolant loops.