According to some embodiments of the invention, transistors of a semiconductor device have a channel region in a channel-portion hole. The undulator further includes a temperature-compensating material selectively arranged to compensate for a temperature-dependent change in the magnetic field of the undulator. A cooling system operatively associated with the core provides coolant to the coolant inlet end and removes heated coolant from the coolant outlet end, thus maintaining a pressure differential therebetween during a normal operating condition of the nuclear reactor system. The second set of conductors is placed within the channels so that no air gaps form between the two sets of conductors.