I modified the spontaneous fission rates reported, some of which weren't quite right, based on the data on half-lives and branching ratios provided by Brookhaven National Laboratory in their handy-dandy chart of the nuclides (http://www.nndc.bnl.gov/chart/).  I also modified the statement that spontaneous fission can't provide the flux needed for a chain reaction, since, if a critical mass is present, it is possible for an INITIAL spontaneous fission to provide the neutrons to initiate a chain reaction.