[en] The first wall of a deuterium--tritium fusion reactor is regenerated in situ. The first wall substantially surrounds an enclosed reaction region confined within the reaction chamber of the reactor. To regenerate a worn first wall without opening the reactor chamber, a gaseous substance is introduced into the chamber, at least a portion of the gaseous substance comprising material, such as low Z refractory material, suitable for forming the first wall. At least a portion of this material is deposited, as by pyrolysis, in solid form on the first wall to regenerate the first wall, and residual gas is removed from the chamber. The chamber is then recharged with a mixture of deuterium and tritium. All the while the inflow of contaminants into the chamber is substantially excluded. Preferably, the pyrolysis of the gaseous substance is effected by energizing the coils used in the operation of the reactor for producing a plasma of deuterium and tritium ions in the reaction chamber