The reactor pressure vessel in nuclear power plants needs to be maintained at intervals of approximately twelve to eighteen months for replacing spent fuel elements by new ones, inspection and to carry out the tests and inspections stipulated by the government authorities. The reactor consists of the bottom part, the so-called reactor vessel, and the top part, the cover. Up to 72 studs and nuts distributed at equal intervals on the circumference of the vessel join and press these two parts together.
To open the reactor, the up to 2 meters long studs are simultaneously stretched in length by nearly 4 millimetres, with hydraulic pressures of approximately 1,000 t applied to each stud and pressures of up to 2,000 bar. So the vessel nuts (size between M130 and M210) can be released without a large torque. This procedure is called stud tensioning. Thereafter the Reactor Studs are turned out by Double Stud Turning Robots automatically and either with the Multi Stud Tensioner or separately transported to a storage location in the reactor building. Afterwards the vessel cover is lifted off the reactor vessel, and the reactor vessel is in the opened state.
In the latest generation of the Multi Stud Tensioners the simultaneous hydraulic elongation and spinning in/out of the studs is only one of many available functions. The transport of the reactor studs and cover nuts, as well as elongation measurement, collection and recording of all relevant operating conditions and parameters, are integrated in the control system. For example, during a tensioning process, the actual stud elongation at all reactor studs is electronically monitored, displayed on a screen in the control console and recorded on IPC. Thus, after completing a tensioning process, a protocol of the proper closing of the reactor vessel can be called up.
