Feasibility study of a controllable mechanical seal for reactor coolant pumps
Payne, John Wilson
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In a nuclear power plant, one of the most important systems for both safety and performance is the reactor cooling system. The cooling system is generally driven by one or more very large centrifugal pumps. Most reactor coolant pumps utilize a multi-stage mechanical face seal system for fluid containment. As a result, these seal systems are critical to safe, continued operation of a nuclear reactor. Without adequate sealing, loss of coolant volume can occur, and a reactor may be forced to shut down, costing the operating utility significantly until it can be brought online again. The main advantage of mechanical face seals is their self-adjusting properties. These seals are tuned so that they automatically adjust to varying fluid conditions to provide adequate leakage control. Because of the enormous pressures, complicated water chemistry, and possible large temperature transients, the mechanical seals inside a reactor coolant pump must be some of the most robust seals available. In addition, their long service life and continuous operation demand durability and the capability to adjust to a wide range of conditions. However, over time, wear, chemical deposition, or changing operating conditions can alter the face gap, which is the critical geometry between the sealing faces of a seal. An altered face gap can lead to undesirable conditions of too much or not enough leakage, which must be maintained within a certain range to provide lubrication and cooling to the seal faces without resulting in uncontrolled coolant volume loss. Nuclear power plants operate within strict leakage ranges, and long-term effects causing undesirable leakage can eventually necessitate a reactor shutdown if the seal cannot self-adjust to control the leakage. This document will examine possible causes of undesirable leakage rates in a commonly-used reactor coolant pump assembly. These causes will be examined to determine the conditions which promote them, the physical explanation for their effect on the operation of a mechanical seal, and possible methods of mitigation of both the cause and its effect. These findings are based on previous publications by utilities and technical and incident reports from reactor stations which detail actual incidents of abnormal seal performance and their root causes as determined by the utilities. Next, a method of increasing the ability of a mechanical seal to adapt to a wider range of conditions will be proposed. This method involves modifying an existing seal face to include a method of active control. This active control focuses on deliberately deforming one face of the mechanical sealing face pair. This deformation alters the face gap in order to make the fluid conditions inside the face gap more preferable, generating more or less leakage as desired. Two methods of actuation, hydraulic pressure and piezoelectric deformation, will be proposed. Finally, a model of the actively controlled seal faces will be introduced. This model includes a method of numerically solving the Reynolds equation to determine the fluid mechanics that drive the lubrication problem in the seal face and coupling the solution with a deformation analysis in a finite element model of a seal face. The model solves iteratively until a converged solution of a sealed pressure distribution, a resulting face deformation, and a calculated leakage rate is reached. The model includes a study of the effects of induced deformation in the seal via both hydraulic and piezoelectric actuation and the ability of this deformation to control the leakage rate.