Why is graphite used for melting substances with high boiling point and for moderating the speed of fast moving neutrons in nuclear reactors?

Dear student,

Graphite is used for melting substances with a high boiling point and for moderating the speed of fast-moving neutrons in the nuclear reactor because graphite is thermally stable and it is a good conductor of heat. 

Graphite is made up of boron electrodes and boron is a very good neutron absorber. Graphite as a moderator has the ability to oxidize in the presence of air.

Regards