Statement I : Heavy water is used as moderator in nuclear reactor. Statement I : Thermal neutrons are used for fission reaction in a reactor.

Statement I is true as heavy water (D2O) is a widely used moderator in nuclear reactors, particularly in Pressurized Heavy Water Reactors (PHWRs) like the CANDU design [t3][t8]. Statement II is also true; thermal (slow) neutrons are essential for sustaining fission in reactors because the fission cross-section of uranium-235 is significantly higher at lower energies [t4][t7]. Fast neutrons released during fission are too energetic to reliably trigger further reactions [t2][t3]. Therefore, a moderator is required to slow these fast neutrons down to thermal levels through elastic collisions [t5][t6]. Heavy water is an excellent moderator because it effectively slows neutrons while having a very low neutron absorption cross-section compared to light water [t1][t6]. Thus, Statement II provides the scientific rationale for why a moderator like heavy water is employed in the reactor core, making it the correct explanation for Statement I.