The original reactor built in 1942 was just "barely" critical because the natural uranium that was used contained less than 1% of the fissionable isotope U-235 (half life 713 million years).What if,in 1942,the Earth had been 9 billion years old instead of 4.5 billion years old? Would this reactor have reached critical stage with natural uranium? Why?
A)No.The age of the Earth has nothing to do with the reactor reaching critical stage with U-235.
B)No.1% of the fissionable isotope U-235 is insufficient for the reactor to reach critical stage no matter what the age of the Earth.
C)Yes.If the Earth had been twice as old there would have then been twice as much fissionable U-235 present and the reactor would have reached critical stage.
D)No.The increased age of the Earth would mean that there would be a much smaller percentage of U-235,thus not enough for the reactor to reach critical stage.

Question

Quizplus · Accepted Answer

The age of the Earth has no direct relation to the reactor reaching critical stage with natural uranium. However, since U-235 has a half-life of 713 million years, if the Earth had been 9 billion years old instead of 4.5 billion years old, there would have been more time for radioactive decay to occur, resulting in a smaller percentage of U-235. Therefore, there would not be enough U-235 for the reactor to reach critical stage even if the Earth were older. Choice D is the best answer.

The Original Reactor Built in 1942 Was Just "Barely" Critical