10 C.F.R. § 15.55

The NRC may terminate collection activity when:

(a) The NRC is unable to collect any substantial amount through its own efforts or through the efforts of others;

(b) The NRC is unable to locate the debtor;

(c) Costs of collection are anticipated to exceed the amount recoverable,

(d) The debt is legally without merit or enforcement of the debt is barred by any applicable statute of limitations;

(e) The debt cannot be substantiated; or

(f) The debt against the debtor has been discharged in bankruptcy.