Program improves reactor recirculation pumps

India`s Tarapur Atomic Power Station has been improving maintenance procedures for these critical components

By A.K. Singh

Tarapur Atomic Power Station

Tarapur Atomic Power Station (TAPS) has twin boiling water reactor units supplied and built by General Electric (USA) and had completed 26 years of successful commercial operation. Each unit generates 160 MWe, derated from an original 210 MWe. These units are light-water-moderated and cooled reactors using slightly enriched uranium oxide fuel.

TAPS reactors operate using a forced circulation-type nuclear steam supply system with steam separation taking place inside the reactor vessel. The forced circulation flow is achieved by means of reactor recirculation systems. This system consists of a reactor vessel and two recirculation loops as shown in Figure 1.

Each recirculation loop is provided with a constant speed vertical centrifugal pump (called a reactor recirculation pump in a BWR), a secondary steam generator (SSG) and motor-operated isolation gate valves. These recirculation pumps take suction from the downcomer annulus at the bottom shell of the reactor vessel, circulate the water through the primary side (tube side) of the SSG and then return it to the lower head of the reactor vessel and on to the core inlet.

Reactor recirculation pump

Reactor recirculation pumps are vertical, single-stage, double-volute, double-suction, centrifugal pumps with mechanical shaft seals driven by 3.3 KV, 1,500-hp constant-speed electric motors. The pump is designed to operate on a wide range of reactor water temperature from 40 C to 304 C. Pump suction pressure is 1,015 psig at 270 C. Each pump delivers 32, 600 gpm flow with 165 feet of water head (both pumps running).

Figure 2 shows the scheme of reactor recirculation pump and motor. The recirculation pump`s impeller is located at the lower end of the pump shaft. The motor shaft is rigidly coupled to the pump shaft from below by a precision spacer coupling that provides proper alignment of pump and motor shafts. It transmits pump axial thrust to the motor shaft and a thrust-type, balanced mechanical shaft seal cartridge. Each seal has one-half of the system pressure across its face. The pressure balancing between seals is accomplished with an integral two-stage pressure breakdown orifice in the seal assembly itself. An integral heat exchanger takes care of seal and bearing cooling. The seal chamber water (primary coolant) is internally circulated with the help of a shaft-mounted coolant recirculation impeller. High-pressure coolant injection has been added to the system as shown in Figure 2 to improve seal performance. This scheme was not envisaged in the original design previously and will be explained later in this article.

The 990 rpm recirculation pump motor has an upper thrust bearing and upper- and lower-babbitted radial bearings. All bearings are oil-lubricated; and each motor is provided with a flywheel that, due to its stored energy, provides an extended coast downtime for the pump to provide adequate circulation and prevent damage to the fuel cladding in the event of an electric power failure. Also during this time, the isolation gate valves close to avoid reverse rotation of the pump because there is no check valve in the recirculation loop.

Because these pumps form a part of primary pressure boundary and are located inside primary containment (drywell), they are not accessible during normal unit operation. Therefore, the pumps and motors are provided with various supervisory instruments as shown in Figure 2 and are continuously monitored in the control room.

Maintenance experience

The four reactor recirculation pumps at TAPS have suffered different types of problems and failures during the last 26 years of operation. Preventive maintenance on recirculation pumps have been systematically followed at specified intervals (during refueling outages of the units). Yet these pumps, at times, have failed due to various reasons.

Failures and problems

Problems observed on TAPS reactor recirculation pumps so far have been mainly the mechanical shaft seal failures, pump bearing failures and motor failures. Pump mechanical shaft seal failures have resulted in high seal leakage to primary containment. Pump bearing damage has caused shaft seizure as well. Motor tripping due to low insulation resistance of its stator winding and motor bearing problems also have contributed to pump unavailability and hence, reduction in plant output.

Shaft seal problems

The mechanical seal failures as indicated by temperature, pressure and flow parameters in the seal-cooling and bleed-off system were too frequent to go unnoticed and gave rise to unscheduled breakdowns. Detailed analysis indicated various causes of mechanical seal failures. Pump bearing failures and other component failures in the seal cavity also affected mechanical seal performance. Mechanical shaft seal cover bolt breakage was noticed in the beginning. The root cause of this bolt breakage was ultimately connected with improper heat treatment, mainly a manufacturing defect. Subsequently, all the cover bolts on the seals were replaced.

Other problems encountered were mainly maintenance related. For example, immediately after plant commissioning in 1969, mechanical seal failures on all the four reactor recirculation pumps were noticed simultaneously. Root cause was found to be improper seal-loading. The original seal manufacturer was consulted, and a revised seal loading method was incorporated in the existing assembly and installation procedure.

Later problems

For almost 12 to 13 years, there were no major problems with recirculation pump seals. But, from February 1982 when repeated mechanical seal failures were experienced on one of the recirculation pumps (2NGPO1B of Unit 2), frequent shutdowns began to occur. These troubles were attributed to pressure breakdown device failures and seal sleeves rubbing stationary parts.

The mechanical seal`s pressure breakdown device is a closed assembly with two tortuous path stages through which controlled amounts of coolant (0.5 gpm to 0.75 gpm) flow. An increased gap within the pressure breakdown device boosted bleed-off flow causing excessive load on the heat exchangers. Ultimately, the secondary chamber seal`s temperature remained higher than normal, and heat cracks on secondary seal faces (tungsten carbide) confirmed lack of cooling in secondary seal chambers.

Excessive seal sleeve rubbing was observed on three occasions. Evidence of rubbing was seen on the inner diameters of stationary components like the seal cover, the pressure breakdown assembly and the carbon seal rings. It was suspected the pump shaft was bent. The shaft`s inspection required a complete dismantling of the pump, a very high radiation dose-consuming activity. It also required an extended outage period and hence, lost generation. Therefore, as an interim arrangement for this pump, the gap between the seal`s sleeve and other stationary components in the assembly were increased.

Pump bearing problems

Recirculation pump carbon bearing failure was noticed all too frequently, requiring the isolation of recirculation loops and replacement of the bearings. Major bearing failure occurred on the same pump (2NGO1N) during September 1982 when the bearing shaft sleeve seized with carbon bearing.

The bearing housing retainer`s anti-rotation pins and swiveling dowel pins (shown in Figure 3) were also found to be failing frequently. Detailed study and analysis indicated stress and lateral impact loading on the pins due to a (suspected) bent shaft or some misalignment of the housing causing bearing damage. Thorough internal inspection of this pump is planned during next Unit 2 refueling outage.

Motor problems

The reactor recirculation pump motor operates in a saturated humid environment in the primary containment (drywell). These motors started failing in 1978. The root cause was found to be the failure of aging class B stator winding insulation. A program was established to change the stator winding of all the motors by indigenous procurement of coils and stator reconditioning at the site. The new coils have class F insulation, having better resistance to environmental conditions.

There have been two motor bearing failures related to abnormally high operating temperatures. In both the cases, the bottom bearing temperature crossed the upper limiting value (100 C) and the pump had to be tripped. Bearing clearance was found to be less than specified. Human error in dimensional measurement was established to be the root cause.

While conducting the pump motor coupling procedure after replacing damaged bearings on Unit 2`s reactor recirculation pump in May 1992, a crack was observed in the motor shaft coupling keyway region. Dye penetrant testing indicated the crack extended two inches from the root of the keyway toward the shaft surface and had a maximum depth of 0.5 inches (Figure 4). This was a new problem, experienced for the first time on any of the recirculation pump motors. In order to solve the problem the following options were reviewed: Replace the complete rotor, repair/replace the damaged shaft, or accept shaft condition “as is” after a detailed study and evaluation of its physical strength.

The first two options were not immediately possible because a spare rotor was not available, and the handling and repair of such a large rotor (7 tons/9 feet long) would be difficult to contract locally. The last option was accepted. The pump could be run trouble-free for nine months when unit refueling outage was taken up in March 1993.

TAPS then took steps to replace the motor`s rotor shaft. Due to limited facilities at the station, the shaft replacement project was contracted out to an experienced agency from Bombay. Detailed dimensional drawings of the rotor shaft had to be prepared, and the quality-assurance plan worked out. Upon chemical analysis, the motor shaft material was found to be very close to the alloy En-8. En-24, being a superior material and easily available through the Indian market, was recommended for a new shaft. Because the motor is located in a radiation area, dismantling the motor and its subsequent reassembly was carried out departmentally. The shaft was manufactured out of a stepped forging supplied by a nearby sub-contractor.

Though the vendor part of the work was completed on schedule, the whole project got delayed by almost a month. This was mainly due to considerable time consumed in decontamination of the complete rotor assembly at site before sending it to the vendor`s facility. Loose contaminations present in intricate recesses in rotor bars and laminations caused cross-contamination. Minor radiation field due to trapped contaminations beneath the subsequent layer of varnish posed another problem. As local method of decontamination didn`t help much, a 13-pH caustic solution treatment was given to the rotor after removing its flywheel.

Radiation exposure problem

Limiting working personnel`s radiation exposure in the drywell`s radioactive environment created some of the biggest problems in maintaining quality during the assembly and disassembly of the reactor recirculation pump internals and motors. Working personnel had to be reshuffled too frequently, and that affected work quality. In order to share the radiation exposure burden, workmen from an outside agency were hired. Communicating with these workmen from different regions of the country was difficult. To some extent, this created communication gaps among various working personnel. These problems also contributed to a higher manrem consumption on the job and further affected work quality.

Improvements and modifications

Reconditioning of pressure breakdown devices and seal sleeves was taken up as the development program since new components were not available due to import problems. The assembly was broken down and precision-machined components were assembled by shrunk fitting. Stainless-steel casting of radiographic quality had to be procured from local steel manufacturers. The reconditioning trials were satisfactory, and a large number of pressure breakdown devices slated for disposal were recovered. Centrifugally cast alloy 316 stainless-steel shaft sleeves were also developed indigenously from local suppliers.

Improved lapping techniques were developed for mechanical seal-face polishing. Surface flatness of up to 2 helium light bands was achieved on seal faces.

A mechanical shaft seal assembly checklist was developed. A detailed, step-by-step procedure, along with component sketches, was prepared for shaft seal and pump bearing replacements. This type of list was also done for motor overhaul procedure. This helped a great deal to eliminate errors during the installation of mechanical seal, pump bearing and motor assemblies in radiation area.

Additional seal coolant injection

The seal cooling system was originally designed to use reactor water as its coolant, but this was modified to receive filtered and cold demineralized water from the high-pressure control rod drive hydraulic system. This cold filtered water was directly injected into the primary coolant chamber. Injection water flow of 10 liter per minute at 38 C was maintained to each pump seal and extended its life further. Earlier shaft seals were failing frequently. Shaft seal life could be extended beyond four years after carrying out improvements. Actual reactor recirculation pump mechanical seal running life at TAPS has been comparable to any BWR in the world. Maximum seal running life achieved on these recirculation pumps has been more than 50,000 hours (Figure 5).

Simplified new design seals were also developed with the cooperation of a seal manufacturer abroad. The new seal design helped eliminate assembly errors because both seal stages are the same size and are interchangeable. Furthermore, the new design has only four lapped faces compared with 16 lapped faces on the old design. This reduced the reconditioning time considerably.

The bearing housing retainer`s anti-rotation pins and dowel pins were strengthened. The pin system was redesigned and welded in the housing retainer to secure the pins. The diametrically opposite pins were used instead of one. Bearing clearance was also increased to accommodate pump shaft run-out, and these measures extended the bearing life considerably.

This was a major effort to recondition old seal and bearing components extending their life. The reactor recirculation pump failures due to mechanical seals have been reduced to a great extent during the last eight to 10 years.

Reducing exposure

A number of methods to control worker radiation exposure were implemented. Checklists and work procedures were streamlined, and administrative control tightened. Some of the practical approaches for exposure control have included shielding radiation hot spots, mock-up practice and retraining, utilization of highly skilled work forces, avoiding personnel training in the field, and building staff expertise and experience.

The last factor allows for making quick decisions in the field and has reduced equipment trials and repeated procedures to a great extent. For complete motor, shaft seal and pump-bearing overhauls, we have been successful in reducing by one-third total personnel exposure on reactor recirculation pumps.

These improvements resulted in high availability of reactor recirculation system. This is an example of efforts to update component design to improve service life and reliability.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

References

1. “IGE Technology Training Course, Vol. II,” San Jose, 1964, Pages 2-181, 2-182.

2. “GE Design and Analysis Report, Vol. 1,” for Tarapur Atomic Power Station, Page V-2-4, 1966.

3. Byron Jackson, “Type DVDS, Single Stage Vertical Recirculation Pump for Nuclear Application: Installation and Operation Instructions,” Los Angeles, 1964.