SBO-qualified RCP seal

The new 120-hour, SBO-qualified hydrodynamic (HD) seal is one of the solutions developed by AREVA’s Safety Alliance to help utilities worldwide demonstrate and upgrade the safety of their nuclear plant fleets.

Safety Authorities now recommend that utilities increase the safety margins of their nuclear power plants, especially with regard to reactor cooling functions, by improving their ability to cope with beyond-design-basis events, notably extended SBO—that is, complete loss of all sources of power. The US NRC’s post-Fukushima taskforce recommend, for instance, extending the coping time from 8 hours to at least 72 hours.

In pressurized water reactors (PWRs), the reactor coolant pump (RCP), and more particularly the seal system located between the motor and the impeller, are directly impacted by these recommendations, as they play a crucial role in preserving reactor core cooling functions. In heavy-duty operation 24/7, 365 days a year for 40 to 60 years (excluding during outage periods), the RCP ensures the circulation of the primary circuit’s coolant fluid between the reactor vessel and the steam generators in order to cool the reactor core, while the seal system ensures the leak-tightness of the primary circuit even under extreme conditions

The 9m-high reactor coolant pump (RCP) can drive primary coolant flow at rates up to 28,000 m3/hr to cool the reactor core.

Thanks to years of R&D combined with 40 years’ maintenance experience on more than 400 RCPs worldwide, AREVA, through its wholly-owned subsidiary AREVA JSPM based in Jeumont, France, has qualified a new seal technology which they say represents a major industry step forward. AREVA JSPM’s new hydrodynamic shaft seal technology goes even beyond today’s Safety Authorities’ recommendations and has just been qualified to withstand severe SBO conditions for 120 hours.

The shaft seal system

The leak-tightness of the reactor coolant pump, essential for the safety of the power plant, is ensured by a set of three stages of mechanical seals. Located between the rotating shaft and the static pressure-retaining parts, they enable gradual isolation of the primary circuit from the ambient environment. By their nature (they are so-called ‘controlled leakage’ seals in which leaking water is collected through leakoff lines), and because of the particularly high stress levels that they are subjected to, they require special attention to ensure that they are in good working order.

Cutaway view of hydrodynamic seal system, mounted at the bottom of the RCP. Outer diameter of the shaft is about 260mm.

With current hydrostatic technology, the first seal level is subject to the primary circuit pressure. Its role is to ‘break’ the pressure from 155 bars (2249 psi) to 2.1 bars (30 psi). This self-regulated clearance seal enables the control of the leakage flow. The second and third seals level operate at a pressure of less than 2 bars (29 psi) and act as an ultimate bulwark between the primary circuit and the reactor building environment.

The new AREVA JSPM hydrodynamic shaft seal system comprises three identical stages, each composed of an HD seal and a staging coil that ensures the pressure drop from one stage to the other, allowing the pressure to be distributed evenly between the stages of the seal system.

Benefits of new SBO-qualified HD seal

First installed in 1988, the hydrodynamic seal is a proven technology. The first HD seal system has now been in service for more than 140,000 hours and many others have been installed since. AREVA JSPM capitalized on this experience to develop its new HD seal. It provides higher safety, is more reliable and more stable in operation, has an extended service life, and requires minimal maintenance.

In the case of an SBO, all of the reactor coolant pump shaft seal system cooling water sources are lost. With the current hydrostatic shaft seal technology, large amounts of hot reactor coolant water flow through the shaft seal system in such situations and dramatically increase the shaft seal leakage. Representative tests made in the 1980s at the Montereau thermal power plant in France demonstrated that the shaft seal system, tested during less than 20 hours under hot conditions, was leaking cooling water at an average rate of 3.5m3/hr. Under such conditions, the loss of reactor coolant inventory may lead to a higher risk of uncovering the core if none of the shaft seal cooling source is recovered quickly.

With AREVA JSPM new HD seal, each of the three stages is designed and qualified to withstand, on its own, the full pressure of the primary circuit, offering a multiple redundancy. Extensive tests in both static and dynamic conditions (see also below) have shown that in the event of failure of one stage, the system pressure is split between the remaining seal stages, and the RCP can operate until the next planned outage. The plant availability is thus optimized. In the unlikely event of failure of two stages, the RCP can still operate for 24 hours, enabling the utility to better prepare for forced outage operations.

Thanks to the way the seal spreads the load over three identical stages, the mechanical stresses to which each component in the system is subjected are significantly levelled, thus mitigating damage and wear phenomenon so that maintenance intervals can be spaced out. Consequently, and based on AREVA JSPM’s substantial operating experience accumulated over the last 20 years for this type of mechanical seal, the system’s service life is expected to reach a minimum of 10 years.

Finally, being only dependent on the pressure drop in the staging coils, the design of the new HD seal also ensures that leakage is stable and constant under all operating conditions and less influenced by small variations of seal behaviour. Test sequences have even highlighted that the seal itself has stable behaviour under pressure and temperature variation transients.

Having been designed to be installed in existing RCPs as a replacement for the current hydrostatic seals, as well as in new third-generation reactor coolant pumps for new nuclear power plants, the new seal system is suitable for both installed-base and new-build markets.

How hydrodynamic seals work

The sealing system developed by AREVA JSPM is based on a hydrodynamically-balanced mechanical seal technology. The main principle of this mechanical seal remains identical to the current hydrostatic seal: a low-thickness fluid film (the primary water)?flows between a rotating seal subassembly and a stationary seal subassembly, which is free to slide axially on an insert.

The technical difference between the two technologies is the balancing forces that drive the stationary subassembly. In addition to the existing hydrostatic forces, a hydrodynamic lift is generated by the combination of shaft rotation and the circumferential wavy profile of the seal ring. When the shaft is stopped, the hydrodynamic seal leakage is nil or very low, in contrast to hydrostatic seal systems, which remain open as long as the system is pressurised, as does the leakoff line. This feature gives hydrodynamic seals a significant advantage in extreme conditions such as station blackout.

The two main hydrodynamic seal technologies in operation on RCPs worldwide are hydrodynamic mechanical seals and thermo-hydrodynamic mechanical seals.

In the thermo-hydrodynamic seal, the lift is dependent on cyclic cooling areas of one seal face, leading to cyclic circumferential thermal deformations of at least one of the rings in operation. A wavy profile of the seal face results from these deformations. In practice, this is a complicated system of mutual dependencies between the seal geometry and its environment, which is difficult to model with computers and hard to predict, AREVA says. In such a system, a mixed friction phenomenon appears during operation when the steady state is not reached.

The hydrodynamic seal technology on which AREVA JSPM’s new SBO-qualified HD seal is based has a specific wavy profile directly machined on to one of the seal faces. Its geometry offers a large cooling capacity that cuts out the dependency on thermal deformation and eases the prediction of fluid mechanics. In such a system, the fluid film between the seal faces remains stable under any modifications of reactor coolant pump operating parameters, such as temperature or pressure. Such stability results in a very low seal wear rate and an extended seal lifetime.

SBO qualification tests

The shaft seal system has been submitted to a long-term qualification test programme. First, both a single hydrodynamic mechanical seal stage and a three-stage shaft seal system were tested to identify the influence of the RCP operating parameters on their behaviour.

Second, all elastomers incorporated into the shaft seal system tested under SBO were aged and irradiated to simulate at least 10 years of operation in the power plant.

After the long-term qualification test, the shaft seal system was installed in AREVA’s Karlstein (Germany) SBO test facility. The test facility mainly consists of an electrically-heated pressurizer, a high-pressure cooling water injection pump and a test frame representative of the shaft seal surrounding an RCP. The test facility was designed for a maximum pressure of 185 bars at 360°C. Furthermore, a device was installed on the bench to verify the ability of the shaft seal system to properly track the upward or downward motion of the RCP shaft. This aims to assess the risk of opening of the seal faces that could result in a failure of the seal which controls the reactor coolant leakage.

Wider-angle view of the test bench

The hydrodynamic shaft seal system was within SBO conditions at the severest conditions of temperature and pressure for 120 hours; during the first 72 hours, the temperature and pressure conditions were maintained respectively higher than 280°C and 165 bars, and, after 72 hours, a mitigation of the test conditions was performed. It has to be pointed out that during the first 24 hours of the test, the temperature of the shaft seal system was maintained at 310°C. Furthermore, prior to the 120-hour SBO test, a preliminary two-hour temperature peak transient was also performed on the shaft seal system; a fast temperature increase to 308°C and a fast temperature decrease (more than 20°C/min).

During the first 30 minutes of the SBO test, the shaft seal leakoff line remained open, allowing hot water to flow up in the full shaft seal system. After closure of the staging coil leakoff line, only some drops per hour were found at the third seal stage. The total leakage was collected and found to be 21.3 litres after 120 hours. This leakage amount is more than 20,000 times lower than with the hydrostatic shaft seal system.

Shaft seal leakage vs SBO test conditions

During a final static test at cold conditions and 158 bars, the right pressure distribution at each seal stage verified the proper function of each component of the hydrodynamic shaft seal system.

After the SBO test, the very good state of the hydrodynamic shaft seal was revealed through inspection. After dismantling, no indications of wear or failure were evident. Even the elastomers showed no signs of damage or extrusion.

The test results, backed by AREVA JSPM maintenance experience, demonstrate that the new shaft seal system will operate satisfactorily in the RCP environment and significantly increase nuclear power plants’ safety margins in case of beyond-design basis events such as extended SBO scenarios thanks to its unequalled ability to withstand loss of cooling capabilities during 120 hours, thus reinforcing primary circuit safety by better preserving its integrity even under severe conditions.

This article first appeared in the May 2012 issue of Nuclear Engineering International