The field of the invention is the one of primary motor-driven pump units of pressurized water nuclear reactors (EPR). More precisely, the present invention relates to the active surfaces, also referred to as active faces, of the main mechanical packing seal of the primary motor-driven pump unit.
In nuclear reactors, the primary pump generates the water circulation in the primary circuit of pressurized water reactors. A shaft dynamic sealing system ensures the sealing between the primary circuit and the atmosphere. This shaft sealing system is a controlled leakage system. It includes three seals disposed in series. Each seal includes two active surfaces which ensure the main sealing. One of the active surfaces, referred to as a rotary active surface, is mounted in a rotary assembly attached to the shaft, the other active surface, known as a floating active surface, is mounted in a non-rotary assembly but which is free to be displaced axially to follow the possible axial displacements of the shaft.
Seal n°1 ensures the greatest part of the pressure drop between the primary circuit and the atmosphere. It is of the hydrostatic type, with a water film of a thickness in the order of 10 μm. The particular geometry of the faces of the active surfaces ensuring the main sealing enables, at shutdown as in rotation, the automatic adjustment of their spacing which only depends on the ΔP of the seal. The active surfaces were initially made of alumina, but they are more and more often made of silicon nitride, which is more abrasion-resistant.
Seal n°1 operates with a controlled leakage rate, in the order of 600 l/h in operation, thanks to the specific profile machined on its active faces. It makes it possible to switch from a pressure of 155 bars to a pressure of about 2 bars.
However, in the seals n°1 of prior art, it is noticed that a significant deposition of iron oxide fouls the active faces and modifies the gradient of these faces, resulting in a modification in the leakage rate.
Document “Deposition of hematite particles on alumina seal faceplates of nuclear reactor coolant pumps-laboratory experiments and industrial feedback” by Gregory Lefèvre, Ljiljana S. Zivkovic and Anne Jaubertie, Hem. Ind., 2012, explains that this fouling phenomenon is due to a two-steps phenomenon:                the particles are transported from the solution towards the active surfaces by hydrodynamic, electrophoretic and thermophoretic phenomena;        they then adhere to the sealing faces by physico-chemical interactions. In prior art, these interactions are considered as been essentially due to the fact that the hematite particles are positively charged, whereas the faces of the active surfaces are negatively charged.        
To overcome this problem, document U.S. Pat. No. 7,287,756 suggests the addition of a catalyst at the face of the active surfaces. This catalyst is preferably a following component or a mixture of the following components: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold. From document U.S. Pat. No. 7,287,756, iron is present in solution as FeOOH (goethite) and as Fe2+ ions. Goethite would deposit at the face of the packing seals. In parallel, the Fe2+ ions would be oxidized by dioxygen into Fe3+ ions which would precipitate and consolidate the deposition. The deposition would then evolve into hematite (Fe2O3). The use of catalysts would make it possible to dissociate the dihydrogen therein, resulting in reducing the chemical potential, to prevent Fe2+ ions from oxidizing and to reduce the present Fe3+ ions into Fe2+, therefore preventing the oxide deposition from occurring.
The applicant has identified another mechanism for forming an oxide deposition on the faces of the active surfaces. We therefore suggest a solution which takes this forming mechanism into account.