Abstract
Hold down spring(HDS), clamped between the upper support plate flange and the core barrel flange inside a pressurized reactor vessel, is used to provide a downward force to keep the core barrel stable and from being lifted off from the vessel ledge during reactor normal operation. Spring designer used to think under certain extreme operating conditions, the spring could be lifted off from the ledge because the spring is not stiff enough to prevent the “lift-off”. Therefore, the spring was designed as stiff as it practically can be. However, finite element study indicated that the magnitude of preload is a strong function of friction coefficient. To find the magnitude of the friction coefficient, a series of tests were conducted on 1/10 scale hold down spring samples of three different spring designs. From the test results, it was found that the friction coefficient increases rapidly as the number of loading cycles increases. This implies that the spring preload would increase rapidly during plant operation. Therefore, it is concluded that, because of the strong frictional effect, not only the current design is more than adequate, but also should be made even softer.
Highlights
In a pressurized water reactor (PWR), the core is contained inside a core barrel which is a cylindrical barrel with its top flange resting on the vessel ledge
The amount of spring compression is predetermined by the clearance between the thickness of the flanges plus the thickness of the spring and the distance between the vessel ledges [2]
When friction coefficient is high, for example >0.6, the counterbalancing force increases drastically. This implies that when the friction coefficient is high enough, the core barrel flange will not be lifted off from the vessel ledge
Summary
In a pressurized water reactor (PWR), the core is contained inside a core barrel which is a cylindrical barrel with its top flange resting on the vessel ledge. (1) The spring preload must be large enough to overcome any upward mechanical forces, such as the hydraulic, seismic and flow induced vibration forces, to keep the core barrel flange from lifted off from the vessel ledge. It is known that frictional force, parallel to the spring contacting surfaces, forms a counter-balancing moment against the moment generated by the lift forces, shown in. If the spring were made wider, the spring would be softer
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