It depends on various assumptions. For some people low-level radiation is troublesome; for others, they don't believe it is anything to worry about. For some people additional radiation of any kind is unacceptable; for others it is. Technically -- at least the materials I work with -- there are different decay times, and in fact some of them peak much later than 20,000 years (peaking here being relative to a low level start).

The management problem is more like: can we actually manage anything to stay put without leakage, etc., for any length of time that matters (not 20,000 years but, let's say 1000)? So far, no one has come up with anything that everyone -- including reasonable engineers -- can agree on is perfectly safe. If perfection is what you are after.....

yours,

Peter T.