In practice you wouldn’t send a giant amount of fissile material in one shipment from the earth’s surface, you would put small amounts up at a time. Multiple trips to the moon would be needed anyway.
That reduces the maximal extent of the problem if something goes amiss, but increases the total risk of at least one launch going amiss.
In practice, if something like that happened, environmental organizations would lobby hard to put a permanent stop on launching radioactive fuel from Earth, regardless of the actual extent of the contamination.
