${ }_{92} \mathrm{U}^{235}$ nucleus absorbs a neutron and disintegrates in ${ }_{54} \mathrm{Xe}^{139},{ }_{38} \mathrm{Sr}^{94}$ and $x$. What will be the product $x$ ?

${ }_{92} \mathrm{U}^{235}+{ }_0 n^1 \rightarrow{ }_{54} \mathrm{Xe}^{139}+{ }_{38} \mathrm{Sr}^{94}+2_0 n^1$ Related Theory Moderators are used to slow down the neutrons produced from fission. This is necessary because many nuclear fuels (uranium-235, for instance) require neutrons to be slow-moving in order to absorb them. Nuclei with low mass numbers are most effective at doing so, therefore materials like water or graphite are often used. Caution In a nuclear fission, a heavy nucleus disintegrates into lighter nuclei. Large amount of energy is released during nuclear fission. Nuclear fission occurs by itself and no additional energy is required for a nuclear fission to occur.