Assertion : It is desirable to slow down fast moving neutrons to sustain controlled chain reactions. Reason : Slow moving neutrons efficiently collides with U ²³⁵ .

Step-by-step solution

The average energy of a neutron produced in fission of U ²³⁵ is 2 MeV . These neutrons unless slowed down will escape from the reactor without interacting with uranium nuclei, unless a large amount of fissionable material is used for sustaining the chain reaction. What we needs to do is to slow down the fast neutrons by elastic scattering with light nuclei. Chadwick's experiment showed that in an elastic collision with hydrogen neutron almost come to rest.