A method of constructing a nuclear reactor module includes providing formwork defining a chamber in which is mounted a nuclear reactor comprising a nuclear reactor pressure vessel configured to contain nuclear fuel when in use, the formwork being housed within a containment structure configured to contain an internal pressure generated by an escape of coolant from a reactor coolant circuit. The method further includes filling one or more voids within the formwork with concrete through at least one concrete supply pipe that extends from outside of the containment structure, through the containment structure, and to the formwork; and venting the one or more voids within the formwork through one or more vent pipes, thereby forming a concrete support structure for the nuclear reactor.