Complex sentence: He joined the Theoretical Physics Group under his brother's guidance, and, he took participation in Fast neutron calculations--a key calculation to developed the weapon--and contributed in the calculations in Neutron fluxes.
Split into simpler sentences: He joined the Theoretical Physics Group under his brother's guidance. And, he took participation in Fast neutron calculations--a key calculation to developed the weapon--and contributed in the calculations in Neutron fluxes.