Nuclear reactor and method of opening a nuclear reactor

A method of opening a pressure vessel of a nuclear reactor;

• wherein the reactor pressure vessel houses fuel rods containing fissile material and has an upper, removable, vessel head;
• wherein the reactor includes control rods, each made of a neutron-absorbing material, the control rods being inserted into the reactor through the vessel head and between the fuel rods to control a rate of a fuel rods' fission reaction, whereby the control rods are movable over a first range of insertion positions relative to the vessel head to control a power output of the reactor when it is critical and generating power, and to put the reactor in a sub-critical shutdown state;
• and wherein the reactor further includes control rod drive mechanisms carried by the vessel head and operable to drive movements of the control rods;
• the method including steps of:
• using the control rod drive mechanisms to move the control rods to an insertion position in which the reactor is in the shutdown state;
• releasing the control rods from the control rod drive mechanisms;
• lifting the vessel head upwards from the reactor pressure vessel such that the control rods slide therethrough to remain stationary relative to the fuel rods to maintain the shutdown state;
• monitoring with a monitoring unit for identifying whether a at least one control rod of the control rods is lifting with the vessel head during the lifting of the head to open the pressure vessel;
• and
• terminating, with a control system linked to the monitoring unit, the lifting of the vessel head when the monitoring unit identifies the at least one control rod is being lifted with the vessel head.