Tighteners

VVER 440 and VVER 1000 reactor main flange joint stud tensioner

ŠKODA JS a.s. designs and manufactures fully automated nut tensioners for the reactor pressure vessel main flange joint.

The device is used for hydraulic pre-tensioning of the main flange joint stud bolts and for tightening / loosening of nuts of the VVER 1000 (or VVER 440) reactor. It consists of a support ring with a hydraulic unit with distribution and fifty-four (or sixty) stud bolt tightening units. The bolts are pre-tensioned by means of hydraulic tightening units. Subsequently, the tensioning nuts of each bolt are tightened. This fixes their pre-tension, which ensures the tightness of the joint during the reactor operation. The elongation of each bolt is sensed by a sensor and displayed on a control panel located outside the reactor shaft. The loosening procedure is similar.

The tensioner allows to perform all operations with high precision and in short time, the design allows to tighten / loosen all nuts simultaneously on one clamping.

The first assembly of the VVER 440 reactor tensioner was delivered to the Hungarian Paks NPP in 1982. The last assembly of this type was manufactured in 1990 for the Mochovce NPP. In the same period, two tensioners were also produced for VVER 1000 reactors at the Belene and Temelín NPPs. Between 2007 and 2013, ŠKODA JS a.s. in cooperation with the company Siempelkamp Tensioning Systems produced four tensioners of a new concept for the Czech and Slovak nuclear power units.

In 2002, we also produced two ABWR reactor main flange joint nut tensioners for GE Nuclear for the Lungmen NPP, Taiwan.

Tihgtening units for reactor upper block flange joints

For VVER 1000 reactors, a combined device (tightener) was originally used to tighten and loosen the flange joints of the reactor lid nozzles for thermocouples (TK), neutron flux measuring channels (KNI) and linear step drive housings (LKP). However, the operation of this tightener was complicated, the work with it was time consuming and the correct tightening of the joints was not guaranteed.

ŠKODA JS therefore proposed the modernization of the flange joint tightener. As part of the modernization, the universal flange joint tightener was replaced by two special tighteners supplemented by state-of-the-art technology. It is a “tensioning” tightener for flange joint nuts of TK, KNI and LKP housings and a torque tightener for TK joint thrust bolts.

The tighteners are easy to operate, speed up the tightening time and increase the operational safety of VVER 1000 reactors.

For VVER 440 reactors, the flange joints were originally tightened by hand using torque wrenches. Thanks to the universal electronically controlled tightening units developed by ŠKODA JS, the complexity of the tightening has been significantly reduced. The time required for tightening is significantly reduced and so is the length of the power plant maintenance shutdown.

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