Vessel Steam Pipe Rupture Force on Internals

A postulated steam pipe rupture in a water-moderated nuclear power plant would cause a sonic discharge from the rupture and the formation of a large-amplitude step decompression, which would travel through the pipe to the reactor vessel. When the decompression arrived at the reactor, it would propagate into the vessel where it would exert decompression forces on the internal steam dryer and other surfaces. The magnitude of the decompression can be 40 bars or more when first beginning in the steam line. If the initial magnitude entered the vessel as a step, even though attenuated by expanding, damage could occur to the dryer components. However, solution of the propagation equations show that a step decompression in a compressible fluid spread as it travels so that its arrival at the vessel is a ramp disturbance in time rather than a step. This feature causes a decompression force of smaller impulse to be exerted on vessel internal structures. This study presents an analytical model for quickly estimating conservative decompression forces on a flat wall structure (like a dryer surface) inside a reactor caused by a steam line rupture. It is shown that longer steam line lengths and greater distances between the vessel wall and the dryer surface result in smaller transient load magnitudes. Also, smaller dryer surface dimensions cause the dryer force duration to be decreased.