A nuclear reactor module (100, 300, 610, 620, 700) comprising in-core instrumentation (330, 730, 830), the nuclear reactor module (100, 300, 610, 620, 700) comprising: a containment vessel (54, 340, 740, 940) comprising an upper containment head (355, 745) and a lower containment head (345, 545), the lower containment head (345, 545) being a removably attached part of the containment vessel (54, 340, 740, 940); a reactor pressure vessel (52, 320, 720, 920) comprising an upper vessel head and a lower vessel head (325, 525), the lower vessel head (325, 525) being a removably attached part of the reactor pressure vessel (52, 320, 720, 920), wherein, when the containment vessel (54, 340, 740, 940) has its lower containment head (345, 545) attached and the reactor pressure vessel (52, 320, 720, 920) has its lower vessel head (325, 525) attached, the reactor pressure vessel (52, 320, 720, 920) is entirely housed within the containment vessel (54, 340, 740, 940); a reactor core (6, 360, 990) housed within the lower vessel head (325, 525); and a plurality of in-core instruments operably coupled to external wiring via a sealed penetration through the upper containment head (355, 745) of the containment vessel (54, 340, 740, 940), wherein lower ends of the in-core instruments are located within the reactor core (6, 360, 990) when the reactor pressure vessel (52, 320, 720, 920) has its lower vessel head (325, 525) attached, and wherein the in-core instruments are configured such that the lower ends are concurrently removed from the reactor core (6, 360, 990) as a result of separating the lower vessel head (325, 525) with the reactor core (6, 360, 990) housed therein from the reactor pressure vessel (52, 320, 720, 920). a containment vessel (54, 340, 740, 940) comprising an upper containment head (355, 745) and a lower containment head (345, 545), the lower containment head (345, 545) being a removably attached part of the containment vessel (54, 340, 740, 940); a reactor pressure vessel (52, 320, 720, 920) comprising an upper vessel head and a lower vessel head (325, 525), the lower vessel head (325, 525) being a removably attached part of the reactor pressure vessel (52, 320, 720, 920), wherein, when the containment vessel (54, 340, 740, 940) has its lower containment head (345, 545) attached and the reactor pressure vessel (52, 320, 720, 920) has its lower vessel head (325, 525) attached, the reactor pressure vessel (52, 320, 720, 920) is entirely housed within the containment vessel (54, 340, 740, 940); a reactor core (6, 360, 990) housed within the lower vessel head (325, 525); and a plurality of in-core instruments operably coupled to external wiring via a sealed penetration through the upper containment head (355, 745) of the containment vessel (54, 340, 740, 940), wherein lower ends of the in-core instruments are located within the reactor core (6, 360, 990) when the reactor pressure vessel (52, 320, 720, 920) has its lower vessel head (325, 525) attached, and wherein the in-core instruments are configured such that the lower ends are concurrently removed from the reactor core (6, 360, 990) as a result of separating the lower vessel head (325, 525) with the reactor core (6, 360, 990) housed therein from the reactor pressure vessel (52, 320, 720, 920).