Replacement of VVER-440 Reactor-coolant-pump Main-flange Packing

摘要

Reactor Coolant Pump (RCP) of the VVER-440 nuclear power plant is a vertical centrifugal pump equippedrnwith a mechanical gland, electric drive and auxiliary systems. Up to now, its thrust flange is sealed by a flatrnmetal (austenitic) packing which is situated in the main force flow and forms a cover of the pump.rnIn the working condition, the RCP flange joint is exposed to a force and thermal load action. The force loadrncovers the media internal overpressure effect and forces in stud-bolts. The thermal load results from arntemperature distribution in the pump wall (stationary and non-stationary thermal field), eventually it may berncaused by parts which have dissimilar thermal expansivity and which can’t dilate freely.rnComputing fatigue damage cumulations performed after 10 years of operation at Unit 1 of NPP Dukovany inrnthe mostly stressed parts of the thrust cover and the RCP-casing flange showed out values ranging from 0.908 torn3.49. The results evidenced definitely that it was necessary to take actions which would reduce the tenseness inrncritical parts of the RCP flange and cover. We’ve chosen an option to modify the seating face so that we couldrnseparate the shape stress concentration from the edge stress concentration through a recess machined in the face,rnwhich would enable to remove the stress raiser. It was necessary to design its shape so that it would resolve thernproblem of the reduced life-time and, at the same time, it would be possible to realize the modification right inrnthe power plant.rnFurthermore, we’ve looked for a method of reduction of the tightening forces in the stud-bolts which cause arnplastic deformation of the sealing-surface material (for the austenitic packing it is 1180 kN per 1 stud-bolt).rnA new flange-joint packing (located in the main force flow) has been designed - a graphite ridge-shapedrnpacking consisting of a steel core with grooves machined in its both surfaces (so called ridges), and foils (with arnthickness of 0.75 mm) made of expanded graphite which are pressed into the ridges. This packing is seated at arnwider surface of the pump casing than it was in case of the original austenitic packing. Calculations have beenrnmade according DIN 2505 and EN 1591 standards in order to check a pressure in the packing at all possible loadrncases. In sum, 31 stress fields for 31 stress conditions have been calculated.rnThe calculations have demonstrated that the newly designed ridge-shaped packing together with the graphiternfoils is capable to assure the tightness of the RCP flange joint while at the same time using significantly reducedrntightening forces in the stud-bolts. The newly recommended tightening force is 588 kN compared to the one ofrn1180 kN used in case of the austenitic packing. This way we happen to reduce significantly the tenseness in thernstud-bolts. The reduced tenseness is present also in the pump casing in points where the new packing is installed,rnthus we achieve an extended RCP service life.rnThe packing was tested, according to DIN 28090-1 and prEN 13555 standards, at a testing bench at anrnaccredited laboratory in NRI Rez (Nuclear Research Institute). The tests have demonstrated definitely therntightness of the joint with the use of the ridge-shaped packing. It will enable, together with NDT tests performedrnat the RCP casing (note : no indications found so far), to extend the RCP inspection interval, thus to save thernmaintenance cost and to reduce radiation doses of the personnel.rnThrough resolving the RCP main-flange lifetime, NPP Dukovany has become the first and so far the lastrnVVER-type nuclear power plant that has happened to successfully cope with the problem.