Challenges to the integrity of calandria, the CANDU PHWR moderator vessel, under serious accident loads and consequences of its failure

Abstract

This paper summarizes the technical and analytical basis for a more advanced examination of structural integrity of a CANDU PHWR Calandria vessel under certain reactor accident conditions. Of foremost interest is the early Boiling Liquid Expanding Vapour Explosions (BLEVEs) induced challenge to vessel structural integrity following a spontaneous in-core rupture of a fuel channel. A sudden, flashing jet discharge of over 100 kg/s of the 10 MPa, 300 °C saturated primary coolant into its 70 °C subcooled near atmospheric pressure moderator water inventory will cause an explosive, shock wave impact. This results from rapid nucleation and vaporization of the discharged superheated liquid. Consequential pressure pulse, pipe-whip and bundle debris impact will induce damage to adjacent channels and cause widespread failure of fuel channel calandria tube rolled joints. These thin tubes inevitably deform to create serious challenges to vessel integrity. On the other extreme of reactor accidents, is the matter of an in-vessel-retention (IVR) of high temperature molten fuel debris in the reactor vessel after a severe core damage accident, such as one caused by an unmitigated station blackout. This scenario has understandably drawn greater attention after Fukushima and potential IVR in the > 200 mm thick PWR vessels when partly submerged in water is viable.It is highly unlikely that we will see reactor debris melt retention in this 19 to 29 mm thick, stepped shell with a tapestry of welds. This is because partial shell melting, diminished strength beyond 650 °C across 2/3 of the thin shell thickness and severe thermal stresses adds to underlying residual stresses near the welded seams. The result is inevitable catastrophic off-site consequences as energetic interactions upon vessel breach of water with hot debris can cause additional steam & hydrogen explosions, containment failures, and large off-site releases.This paper presents technical arguments to illustrate the problem in accepting faulty assumptions conveniently made in licensing safety assessments to simplistically claim survival of the thin-walled welded structure under these two illustrative accidents. I propose pathways to further investigations. Therefore, the issue of Calandria vessel survival under a wide range of accident conditions must be given a second, thoughtful look. Extending boundaries of a safe operating envelope as pressure tubes age needs to be avoided. We need to be more realistic in our severe accident analysis and consequence assessments. Fukushima taught us that the consequences of continuing to rely on convenient but technically faulty rationale, along with hype of eternal safety, can be very unforgiving.