The U.S. Nuclear Regulatory Commission (NRC) is withdrawing Regulatory Guide 3.65, which previously provided guidance on the decommissioning of nuclear facilities to support license termination. This guide is being withdrawn because the NRC now relies on a more current document called NUREG-1757 to provide guidance for these processes. The withdrawal does not change any existing NRC licenses or their approval and does not prevent current licensees from continuing to use RG 3.65. However, for future applications, applicants and licensees are encouraged to refer primarily to NUREG-1757.
Simple Explanation
The people who make rules about nuclear stuff are switching to using a new guidebook because it's more up-to-date, kind of like how you might stop using an old map when a new one comes out.