The reactor core includes at least one module, a solid neutron moderator, and a liquid neutron moderators. Each module contains a casing, at least one heat pipe, one fuel element and thermal insulation. The heat pipe comprises a casing, a wick, and a coolant. The fuel element is made of nuclear fuel, arranged along an evaporation area of the heat pipe, around the heat pipe casing, in thermal contact with the heat pipe casing, and enclosed in a can. Liquid metals are used as the coolant of the heat pipe. Thermal insulation is arranged between the can and the module casing. At least one hole is made in the solid neutron moderator. Each module is arranged within a respective hole of the solid neutron moderator. The space between the module casing and the solid neutron moderator is filled with a liquid neutron moderator.