Integrated head package for top mounted nuclear instrumentation

An integrated head package for a nuclear reactor having a reactor vessel containing a nuclear fuel is described, comprising: a reactor vessel head including means for sealingly engaging the reactor vessel; a control rod guide mechanism operable to raise and lower control rods relative to the fuel, the control rod guide mechanism including control rod positioning means movably traversing penetrations in the reactor vessel head, and a control rod drive coupled to the control rod positioning means; at least one instrumentation tube structure including a sensor arrangement disposed in proximity with the fuel, and a coupling for the sensor arrangement extending upwardly, the instrumentation tube structure traversing a penetration in the reactor vessel head and the sensor arrangement being retractable relative to the fuel; and, a shroud attached to the reactor vessel head and substantially enclosing the control rod guide mechanism and at least a portion of the instrumentation tube structure when retracted, the shroud comprising a shielding material of a sufficient thickness to limit passage of nuclear radiation and forming a structural element of sufficient strength to support the vessel head, the control rod guide mechanism and the instrumentation tube structure; whereby the integrated head package is removable from the reactor as a unit for servicing