A nuclear reactor comprising: an elongated reactor vessel enclosed at a lower end and having an open upper end on which an annular flange is formed and a central axis extending along an elongated dimension; a core enclosed within the elongated reactor vessel, the core comprised of a plurality of parallel, vertical co-extending fuel assemblies having an instrument thimble extending down into the fuel assemblies; a reactor vessel head having an annular portion on an underside of the head that is machined to form a sealing surface; a first removable annular seal ring, sized to seat on the reactor vessel flange between the flange and the sealing surface on the underside of the reactor vessel head, the seal ring being interposed between the sealing surface on the underside of the vessel head and the flange on the reactor vessel and sealably accomodating one or more radial passages through which one or more instrument conduits pass from outside of the reactor vessel to an interior thereof to communicate instrumentation signals out of the core of the nuclear reactor; an upper internals package supported above the core within the reactor vessel and having a plurality of hollow support columns respectively having a substantially vertical passage therethrough, that extends through and between an upper core plate and an upper support plate of the upper internals package, the passage through the upper core plate being aligned with a corresponding instrument thimble within one of the fuel assemblies; and a plurality of hollow tubes fixedly connected to the first removable annular seal ring, each of the hollow tubes being slidably mounted within the passage of one of the support columns with at least one of the instrument conduits extending axially through the hollow tube into the corresponding support column, the hollow tube being slidable within the support column between a fully inserted position and a fully extended position wherein in the fully inserted position the instrument conduit enters the instrument thimble and in the fully extended position the instrument conduit is withdrawn from the core. an elongated reactor vessel enclosed at a lower end and having an open upper end on which an annular flange is formed and a central axis extending along an elongated dimension; a core enclosed within the elongated reactor vessel, the core comprised of a plurality of parallel, vertical co-extending fuel assemblies having an instrument thimble extending down into the fuel assemblies; a reactor vessel head having an annular portion on an underside of the head that is machined to form a sealing surface; a first removable annular seal ring, sized to seat on the reactor vessel flange between the flange and the sealing surface on the underside of the reactor vessel head, the seal ring being interposed between the sealing surface on the underside of the vessel head and the flange on the reactor vessel and sealably accomodating one or more radial passages through which one or more instrument conduits pass from outside of the reactor vessel to an interior thereof to communicate instrumentation signals out of the core of the nuclear reactor; an upper internals package supported above the core within the reactor vessel and having a plurality of hollow support columns respectively having a substantially vertical passage therethrough, that extends through and between an upper core plate and an upper support plate of the upper internals package, the passage through the upper core plate being aligned with a corresponding instrument thimble within one of the fuel assemblies; and a plurality of hollow tubes fixedly connected to the first removable annular seal ring, each of the hollow tubes being slidably mounted within the passage of one of the support columns with at least one of the instrument conduits extending axially through the hollow tube into the corresponding support column, the hollow tube being slidable within the support column between a fully inserted position and a fully extended position wherein in the fully inserted position the instrument conduit enters the instrument thimble and in the fully extended position the instrument conduit is withdrawn from the core.