"R" in the numeric designation signifies that the 1998 standard was reaffirmed (recertified) as an American National Standard in 2007, 2012 and again in 2017. No changes can be made to a standard during reaffirmation.
This standard is applicable to the storage of fissile materials. Mass and spacing limits are tabulated for uranium containing greater than 30 wt-% 235U, for 233U, and for plutonium, as metals and oxides. Criteria for the range of application of these limits are provided.