Improvements in which area would reduce the possibility of damage to the environment when using uranium as a fuel?
more-efficient extraction techniques
better control of fusion reactions
stronger-pumping wells
thicker shielding around the reactor

The correct option is this: THICKER SHIELDING AROUND THE REACTOR.
Uranium fuel is an alternative to fossils fuels. The fuel is produce via nuclear reactions. In order to protect the environment from the harmful effects of this fuel, it is necessary that the reactors be surrounded with materials that will effectively shield the radioactive rays that are continually been emitted by Uranium dioxide, which is found in the reactor core.