Vessels head access hole

In the lower area of the vessel a manhole with the nominal width of DN800 is provided for cleaning and inspection purposes. On the elliptical vessel top, a head access hole with the nominal width of DN300 is provided. The head access hde will be opened to control if the vessel is really empty before the inspection is carried out Unfortunately there have already been accidents when a manhole was opened because a defective level control signalled an empty vessel although it was still full. Furthermore, the head access hole enables the analogous level control to be cahbrated (see section 4.10, Commissioning ). 

To meet safety requirements, the reactor and the major components of the primary loops are enclosed within containment vessels. The containment vessel which houses the pump and heat exchanger of each primary loop is a cylindrical cap.sule, 20 ft in diameter by 30 ft long, including 2 1 elliptical heads. The capsule is equipped with access holes such that certain maintenance operations may be performed [5]. The reactor containment vessel is a right circular cylinder 30 ft in diameter, with a hemispherical top head and a flat bottom head. Access holes are provided in the vessel for maintenance operations. 

IGSCC has been a concern in the BWR community since first detected in the late 1950 s in annealed stainless steel fuel cladding and in the mid-1960 s Type 304 stainless steel recirculation piping. In the late 1980 s IGSCC was detected in reactor internal components, i.e. shroud head bolts, core shrouds, access hole covers, etc. More recent experience from the world wide BWR fleet indicates that cracking of vessel internals has become more widespread than previously thought. 

The effectiveness of dry well flooding could be improved if the reactor vessel support skirt were vented in order to reduce the trapped gas volume and increase the fraction of bottom head surface area contacted by water. Partial venting could be achieved by loosening the cover on the support skirt manhole access hole. This would increase the wetted portion of the bottom head from 55% to 73% of the total outer surface area, which delays the predicted time of bottom head creep rupture by about one hour. The predicted failure times for the basic case without skirt venting and for Ae case of partial venting at the manhole access are indicated in the first two entries of Table 2. 

The closure head is available for inspection whenever it is removed, and its removal makes available the vessel closure flange, the flange-to-shell weld, closure stud holes and ligaments, and the closure studs and nuts. Each control element drive mechanism is removable as a unit through a closure at the top of its housing. Since many of the reactor vessel closure head examinations are done from the underside of the head, the head laydown area provides access for a man to work under this component using manual techniques. 

---