Abstract: The Decommissioning Phase SAFSTOR for Nuclear Power Plants, lasting for 50 to 60 years before dismantling begins, is to allow for natural decay of 60Co, a constituent of reactor steel to roughly 1/1000th of its original content. This can be sped up partly by minimizing its build-up in the Reactor Pressure Vessel and in the Biological Shield, partly by reducing its higher contents in the former by milling/cutting from its inner side. The outer rise of the activity (65, in FIG. 10) in the Reactor Pressure Vessel is caused by the back flow of thermalized neutrons (66) from the Biological Shield (67). An absorber (68), added to the inner liner (69), causes by blocking off this flow of neutrons a steady decline of the Co-distribution with its minimum at the outer side of the wall (70). Then, milling/cutting on the inside removes most of the 60Co exclusively from the inside of the Reactor Pressure Vessel.