Krypton-85, the 3rd most active MLFP, is a noble gas which is allowed to escape during current nuclear reprocessing ; however, its inertness means that it does not concentrate in the environment, but diffuses to a uniform low concentration in the atmosphere. Spent fuel in the U.S. and some other countries is not likely to be reprocessed until decades after use, and by that time most of the Kr will have decayed.