Two Billion Years Ago, a Natural Nuclear Reactor Formed in a Uranium

Multiple Choice

Two billion years ago, a natural nuclear reactor formed in a uranium deposit in Africa.Why is it not possible for such a natural reactor to form somewhere on Earth today?

A) It is in fact possible for this to occur.
B) Too much uranium has already been mined for this to occur.
C) Too much ²³⁵U has now decayed for a critical mass to form naturally.
D) The remaining uranium deposits are too far underground for this to occur.
E) Uranium in the remaining deposits is mixed with too many other elements for this to occur.