In a nuclear reactor.

Options

A. the chain reaction is kept under control by rods of cadmium, which reduces the rate
B. the thick concrete shield is used to slow down the speed of fast neutrons
C. heavy water (or graphite )moderate the activity of the reactor
D. out of `U^(238)` and `U^(235)` natural uranium has less than `1%` of `U^(235)`

Answer

a,d

Explanation

(a) True, Cd absorb neutrons.
(b) No, concrete reflects, does not slow down,
(c ) 'Moderate the activity' is not correct. 'Moderator' in the sense of slowing the neutrons is different.