n117.
The 60 years estimate comes from van Leeuwen, “Nuclear Power: The Energy Balance”, part H, especially H1 and H2.
It is also accessibly summarised in Storm van Leeuwen, “Energy Security and Uranium Reserves”, Factsheet 4, Oxford Research Group, 2006, available at www.oxfordresearchgroup.org.uk/sites/default/files/energyfactsheet4.pdf .