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SMEPI, the tank stud specialist

Tank studs are essential components in nuclear power plants in order to have a secure and operational site. They must be developed according to strict standards. SMEPI specializes in making this type of stud.

Process for removing studs from reactor vessels

The components of nuclear power plants must be regularly checked and maintained, and mandatory tests must be carried out to ensure the proper functioning of the installations and to reduce the risk of accidents to a minimum. For this, the pressure vessels of the reactors are opened at intervals of about 12 to 18 months. The manufacture of these parts is the business of a professional who masters the standards relating to the high-tech industry.


A nuclear reactor has two parts:

  • the upper part, called the cover;

  • and the lower part: an enclosure formed by a vessel integrating the core of the reactor.

These two parts are held firmly against each other by means of a set of vessel studs (54 for a 900 MW reactor) and nuts screwed and spaced at regular intervals on the flange of the cover.

To open and close a reactor, operators use the so-called "tension" tightening technique. This involves lengthening the studs (with a maximum length of 2 meters) by a few millimeters and exerting hydraulic pressures of around 1,000 tonnes and atmospheric pressures of up to 3,000 bars. In this way, the nuts, some of which have dimensions between M150 and M210, can be released without requiring significant forces. Note that devices such as the tensioner for multi-stud flanges (MST) exist to facilitate the operation.

Then the vessel studs are unscrewed and transported separately or with the multi-stud flange tensioner to a storage location in the reactor building. The container lid is then lifted, leaving the pressure vessel accessible. The latest generations of MST offer many features such as simultaneous hydraulic extension or rotation of the studs.

The transport of reactor studs and cover nuts, as well as the measurement, acquisition and recording of all operating conditions and parameters are integrated into the system.

For example, during a closing process, the actual stud elongation is electronically detected on all studs in the reactor, displayed on a screen in the tensioner control panel, and recorded on a PC. Thus, after each removal of the cover, the device is able to detect whether the tank is properly closed.

Bowl studs with specific technical characteristics

To guarantee the watertightness of the tanks, the bolts (studs and nuts) are made of high-strength alloy steel type 40 NCD 7-03.

Also, the studs must withstand tensions of up to 54 kN, corresponding to an elongation of 1.3 mm.

Finally, as they are intended for use in nuclear reactors, their manufacture must meet the safety standards in force, namely the RCC-MRx code relating to the design and construction of mechanical equipment for nuclear installations. The components made according to strict recommendations - in particular with materials of choice - withstand certain constraints.

It should be noted that the studs are subjected to significant loads, in particular significant variations in temperature and pressure. And therefore, they must be completely reliable to guarantee the safety of an installation.

At SMEPI, we design mechanical parts (vessel studs , bolts, nuts, etc.) that meet the standards applicable to experimental reactors in nuclear power plants. We can machine studs specifically adapted to the fastener or thread. We provide components that are fully adapted to your installations.

Our services

In terms of precision mechanics, particularly in the machining of mechanical components, whatever their size or shape, SMEPI is the best to meet all types of specific needs. In everything that has to do with precision machining, we are capitalizing on more than ten years of presence, we have been able to build an enviable reputation throughout France. Our components are recommended by professionals with a wide variety of technological issues, among others: petrochemicals, special machines, energy, transport, nuclear, chemicals, etc.

SMEPI has all the key resources to assist you throughout your project including: the selection of raw materials, heat and surface treatment, assembly, machining, prototyping, study ... as well as a strict verification of finished parts. We provide you with the know-how and experience of a team of experts and the quality of our materials in order to guarantee the quality of our achievements, speed and respect for deadlines.

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