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From the article: What van der Worp had suggested, in terms of Galileo, was merely a variant on the now well-known design of the “Fat Boy” plutonium nuclear weapon used against Nagasaki at the end of World War II -- an “implosion” plutonium reaction (below).

The reason this is complete and utter hogwash is that Pu₂₃₈ (Not Pu₂₃₉) is used in RTGs and is not a fissile material.

There are a number of conditions that must be met for a material to be fissile (sustain a nuclear chain reaction):

1) Higher on the periodic table of the elements than Radium.
2) High probability of neutron capture.
3) Release at least two neutrons upon neutron capture.
4) Long enough half life.

A rule of thumb is that the odd mass numbers are fissile while the even ones are not. (This is due to the binding energy in the nucleus) Thusly:

U₂₃₃, U₂₃₅, and Pu₂₃₉ are fissile where U₂₃₄, U₂₃₈, and Pu₂₃₈ are not.

(Note: there is a U₂₃₇, however, the halflife is only about 6 days. Not long enough to be a viable material.)