Abstract
The spent fuel channel ball valve is a critical component in heavy water reactors, serving both as a pressure boundary and an unloading channel. To address issues with the spent fuel channel ball valve, an analysis was conducted on failure modes, including high driving torque and leakage test failures. Based on this analysis, corrective actions and improvements for preventive maintenance tasks and spare parts management were proposed. These measures provide valuable guidance for equipment management and maintenance, ensuring the reliable operation of the spent fuel channel in heavy water reactors.
Introduction
The 700,000-kWh unit at Qinsan Plant is modeled after Canada's heavy water reactor design. Its key feature is continuous refueling without reactor shutdown. The spent fuel channel ball valve, comprising an inner and outer ball valve, is installed in series at the end of the spent fuel channel to facilitate spent fuel unloading. The spent fuel channel ball valve remains closed when the underwater or air gate is opened, ensuring the integrity of the containment pressure boundary.
The spent fuel channel ball valve, a 4-inch (10.16 cm) valve manufactured by Velan, has encountered issues in recent years. These include increased driving torque, which prevents valve movement, and frequent failures in leakage rate tests. As a critical component in nuclear power plants, reduced reliability poses significant risks to daily refueling operations and can severely affect the unit's safe and stable operation. According to the Final Safety Analysis Report, leakage rate tests for the spent fuel channel ball valves are performed every six months, with at least one of the two valves required to remain operational. If both valves fail, all spent fuel transmission must cease immediately, and at least one valve must be restored to operation within eight hours.
1. Structure and Working Principle of the Spent Fuel Channel Ball Valve
1.1 Ball Valve Structure
The spent fuel channel ball valve is classified according to nuclear safety guidelines as level 2 and incorporates a metal-seated design. It is designed to operate at a pressure of 3.447 MPa, with a leakage rate of less than 10 ml/min under design conditions. The valve consists of four main components: body, end cap, stem assembly, and ball assembly. The stem assembly includes the valve stem, packing, and gland, while the ball assembly consists of the ball, valve seat, and sealing components. When in the closed position, the ball forms a metal seal against valve seats (09A and 09B) on either side. A graphite gasket (20A) between the valve body end and the valve seat ensures sealing at the ball's end, while another graphite gasket (20B) between the ball and valve seat (09B) seals the valve body side of the ball. The body-side seal assembly includes the valve seat (09B), butterfly spring, positioning ring, and graphite gasket (20B). A spiral-wound graphite gasket is installed between the valve body and end cap, secured with 12 unevenly spaced bolts, with nuts torqued to 230 N•m. The drive stem connects to the ball and features multiple flexible graphite seals, secured by a packing nut.
1.2 Sealing Principle
When the valve body and end cap are fastened together with bolts, the applied pre-load compresses the butterfly spring. This force is transferred from the valve seat to the ball and the opposing valve seat, creating a metal-to-metal seal between the ball and the valve seats. In the outer ball valve, the ball moves due to the combined forces of the butterfly spring and external pressure, compressing the graphite seals (20A and 20B) to create a sealing surface. In the inner ball valve, the ball moves against the restoring force of the spring, compressing the graphite gasket to form a sealing surface.
3. Arrangement of Preventive Maintenance Items and Optimization of Spare Parts for the Spent Fuel Channel Ball Valve
3.1 Arrangement and Optimization of Preventive Maintenance Items
(1) The initial identification and scheduling of preventive maintenance tasks for the spent fuel channel ball valve are based on the guidelines outlined in the maintenance manual (Table 2).
(2) Preventive measures for the spent fuel channel ball valves are developed based on operational data and experiences from nuclear power plants, recommendations from the manufacturer's technical manual, and best practices from international nuclear plants. These are further informed by the "General Piston Pneumatic Valve Equipment Failure Mode and Cause Manual," which details common failure modes and their causes (Table 3).
(3) Based on these principles, the preventive maintenance strategy for the spent fuel channel ball valves has been refined, as detailed in Table 4. The updated strategy addresses typical failure modes and incorporates practical operational insights.
Table 2 Preventive maintenance project formulation for spent fuel ball valves
|
Project |
Content |
Cycle |
|
External Ball Valve Inspection |
Check valve stem nut, lubricate worm gearbox, check actuator and accessories. |
2 years |
|
External Ball Valve Overhaul |
Replace valve stem seal, thrust pad, valve seat, and valve body seal. |
10 years |
|
External Ball Valve Cylinder Overhaul |
Disassemble cylinder and worm gearbox and its accessories, replace seals; replace cylinder seal and grease. |
10 years |
|
Ball Valve Limit Switch Inspection |
Check the limit switch and ensure the switch action is normal. |
2 years |
Table 3 Common failures and causes related to the ball valve in the spent fuel Here is the table formatted for better readability:
|
Failure Location |
Failure Mode |
Failure Cause |
|
Speed Control Valve |
Inconsistent response |
Imbalance |
|
Travel Switch |
Corrosion of cable terminal connection |
Environment |
|
Pneumatic Pipeline |
Assembly leakage |
Vibration |
|
Actuator |
Piston sleeve cylinder seal loses elasticity |
Aging |
|
Valve Seat |
Wear and erosion |
Insufficient valve seat pressure load |
|
Valve Core |
Dirt accumulation |
Water quality |
|
Valve Core |
Wear and erosion |
Insufficient valve seat pressure load |
Table 4 Spent fuel channel ball valve optimization project
|
Period |
Inspection/Task |
Description |
|
2 years |
Ball Valve Inspection |
Clean the valve surface and the pneumatic head surface, and check the tightness of the components. |
|
Check the sealing of the air supply pipeline. |
||
|
Check the internal and external leakage of the valve. |
||
|
Check the tightness of the following valve bolts: bolts between the inner ball valve and the channel, bolts between the inner ball valve and the outer ball valve, fastening bolts between the valve body and the drive mechanism of the inner ball valve, fastening bolts between the valve body and the drive mechanism of the inner ball valve, fastening bolts between the valve body and the drive mechanism of the outer ball valve, drive mechanism bolts of the outer ball valve |
||
|
Valve action test to confirm that the valve position indication is correct and there is no obstruction in the operation of the on-site valve. |
||
|
8 years |
Disassemble and Overhaul the Ball Valve |
Check the valve stem seal, replace the valve stem seal, disassemble the cylinder and worm gear box and its accessories, replace the seals, lubricate and assemble, and test and adjust the whole set. |
|
Disassemble and overhaul the ball net; check the spherical surface; check the valve stem surface, and straightness; check the flange bolts; replace the valve stem seal, thrust pad, valve seat, valve body seal, and other internal parts. |
||
|
Disassemble and Overhaul the Gas Cylinder Seal |
Replace the cylinder seal, and the cylinder grease; check the sealing of the gas cylinder. |
3.2 Spare Parts Preparation and Optimization
(1) The procurement of a full set of spare parts for the ball valve and drive mechanism is critical. This ensures that in the event of a major failure of the spent fuel channel ball valve, at least one of the two ball valves can be brought back online within eight hours, minimizing downtime and preventing a plant shutdown.
(2) Localization of the ball and valve seat has been achieved. Localization provides three significant advantages: ① Mastery of domestic manufacturing technology for the spent fuel channel ball valve, reducing reliance on the original manufacturer's support; ② Lower procurement costs, resulting in savings on plant operating expenses; ③ Faster delivery times due to local sourcing.
(3) Customized pure graphite gaskets of varying thicknesses are offered for ball valve applications. These gaskets can be chosen during valve overhauls, ensuring spare parts remain in optimal condition and ready for immediate use when needed.
4. Conclusion
During the overhaul of the spent fuel channel ball valve, procurement specifications for the valve components, roundness inspections, repair requirements, and applicable standards were defined. Additionally, the tightening method for the flange bolts and torque specifications for preventive inspections conducted online were established. Furthermore, the graphite gasket design was revised, and the installation dimensions for the gasket were defined. Over a period of more than two years, no operational failures were reported, confirming the effectiveness of the analysis and corrective actions described in this paper. The optimization of the maintenance procedures ensures ongoing monitoring and control of the equipment's status, maintaining the spent fuel channel ball valve in optimal condition and supporting the safe and stable operation of the power plant.
