The energy of the thermal neutrons required for the fission of nuclei is of the order of . However…

The energy of the thermal neutrons required for the fission of $^{235}\text{U}$ nuclei is of the order of $0.025\text{ eV}$. However, the neutrons produced in the fission of $^{235}\text{U}$ possess quite large energy as compared to the energy of thermal neutrons. As these neutrons escape the $^{235}\text{U}$ block without causing the fission of $^{235}\text{U}$ nuclei. Due to this, in a nuclear reactor, it is important that the energy of neutrons is absorbed, and some neutrons are also absorbed. Name the substance used to absorb the energy of the neutrons.