Experimental investigation of the permeability of PWR insulation panels for ex-vessel cooling

The CALO experiments demonstrate that there is no significant risk of vapour accumulation with thermal insulation made of screwed panels. Panel-junction clearance of ∼0.5 mm is sufficient to release the vapour produced around the lower head. If the clearance is smaller, the panel connection would be deformed due to the pressure difference between the inner and outer volumes. The connection would deform sufficiently, or even screws would break, before vapour accumulation between the vessel and the insulation reached the level of the lower head.