Valery Legasov briefly explains how a nuclear reactor works to Boris Shcherbina. The bullet refers to the fast neutrons release from the fission process of Uranium 235 where free radicals either hits DNA molecule directly or indirectly, resulting in damage. The fission cross section for Uranium 235 is high and the chance of fission is much higher when neutrons energy is low. A moderator is required to slow down (moderate) neutrons from the previous fission process. The graphite refers to the fragments from the explosion of the RBMK reactor which uses carbon as a neutron moderator.

This clip was taken from 2019 HBO miniseries, Chernobyl. I do not own any of the footage and all credit goes to HBO and their talented team.

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