11.46. Approval for constructing an aboveground monitored retrievable storage (MRS) facility has been complicated by an unwillingness of state governments to accept it. Also, the 1987 amendment to the NWPA links the MRS to progress toward opening a permanent underground geological site, which has had numerous delays.

11.47. Over the years, various designs for a MRS facility have been proposed, varying from a scaled-up pool storage arrangement to a concrete storage cask array. There are two major considerations in the design. Decay heat must be managed and neutron and gamma-ray shielding provided. Heat generation and activity can be predicted accurately by codes such as ORIGIN (§2.213). However, as a rule of thumb, it is helpful to remember that during about 10 years of temporary storage the assemblies will have lost about two orders of magnitude in heat generation rate and activity from the discharge levels. Subsequent loss is fairly slow, being roughly proportional to the 0.2 power of time (§2.217).

11.48. An indication of retrievable storage cask designs is provided by those submitted by utilities for licensing by the NRC for air-cooled on-site storage to supplement pool capacity. A common design arrangement for a vertical cask accommodates 24 assemblies in a basket rack inside a con­crete shield. A typical cask is about 3.66 m (12 ft) in diameter and 6.1 m (20 ft) high.