Patent Number: 047675934
Section: summary

BACKGROUND OF THE PRIOR ART This invention relates generally to pressure vessels and in particular to pressure vessels for withstanding cyclic pressure and temperature extremes such as those found in nuclear reactor pressure vessels. Pressure vessels used for containing a nuclear reactor must be designed for operating conditions involving a combination of high temperatures and high pressures along with strong radiation in the form of neutrons and gamma rays. The pressure vessel must also be designed for emergency conditions such as Loss of Coolant Accidents (LOCA) and Pressurized Thermal Shock (PTS). The LOCA conditions may cause the vessel pressure to drop suddenly while the temperature may remain the same or even increase The PTS condition, which may be caused by the injection of cold "coolant" into the pressure vessel from the emergency core cooling system (ECCS), may cause steep temperature gradients, such as, a sudden drop in temperature of the inside surface of the pressure vessel and a portion of the vessel wall, while the pressure may remain constant or even increase. When pressure vessels are subjected to these pressure and temperature extremes, high tensile stresses are produced inside the vessel wall, particularly in the region of wall penetrations, such as, coolant inlet and outlet ports of the pressure vessel. During pressurized thermal shock (PTS) (as might be caused by a loss of coolant accident (LOCA)) where cold coolant is introduced to replace the lost coolant, steep thermal gradients in the vessel wall may cause large tensile stresses to occur in the crotch region of the vessel-outlet and vessel-inlet port intersections which may result in cracking of the vessel wall. In addition, crack initiation may be aided due to embrittlement of the vessel wall by the high level of neutron and gamma radiation being absorbed. To alleviate this condition, some multiple layer pressure vessels of the prior art utilized concentrically disposed multiple shell pressure vessels shrunk fit onto each other. Other pressure vessels utilized multiple layers of sheet metal spirally wrapped around the outside of an inner pressure vessel. In some of the pressure vessels using spaced apart shells, the space between the shell was filled with a neutron absorbing material. One prior art device utilized spaced apart pressure vessel shells filled with coolant or a low melting point material to effect a uniform pressure distribution. The filler material was not maintained under pressure. SUMMARY OF THE INVENTION The multiple shell pressure vessel of the present invention utilizes a set of concentrically disposed, spaced apart pressure vessel shells surrounding an inner pressure vessel, the spaces between the pressure vessel shells being filled with a low melting point, high boiling point material, selected from the group consisting of lead, tin, antimony, bismuth, or sodium and potassium, and mixtures thereof, pressurized to a pressure whereby the wall of the innermost pressure vessel shell is in compression or very low tension during steady state operation. Chemical compositions or compounds containing boron or cadmium may also be added to the molten filler material. The pressure vessel of the present invention may also include devices for maintaining the pressure in the space between the innermost pressure vessel shell and the next inner pressure vessel shell at a constant predetermined multiple of the pressure in the innermost pressure vessel The present invention also includes a configuration of concentric conduit pressure vessel penetration that reduces tensile stresses resulting from transient thermal and pressure variations. The present invention also includes a method of fabricating the multiple shell pressure vessel utilizing concentric, spaced apart pressure vessel shells and the subsequent (or simultaneous) introduction of molten filler materials and the pressurization thereof. It is, therefore, an object of the present invention to provide a pressure vessel offering greater safety with respect to initiation and propagation of cracking due to pressurized thermal shock (PTS) It is a further object of the present invention to provide a multiple shell pressure vessel in which the wall of the innermost pressure vessel shell is maintained in compression during steady state operation of the pressure vessel It is still another object of the present invention to provide a multiple shell pressure vessel in which the wall of the innermost pressure vessel is always maintained in compression or in low tension below the yield point of the wall material of the innermost pressure vessel during a temperature or pressure transient within the innermost pressure vessel shell It is yet a further object of the present invention to provide a multiple shell pressure vessel in which the spaces between the pressure vessel shells are filled with a low melting point, high boiling point material that is pressurized to a point whereby the wall of the innermost pressure vessel is maintained in compression during normal operation. It is also another object of the present invention to provide a device whereby the compression in the wall of the innermost pressure vessel shell is maintained relatively constant for variations in pressure within the innermost pressure vessel. It is yet another object of the present invention to provide a method of fabricating a multiple shell pressure vessel with pre-stressed (or pre-pressurized) fillers. These and other objects of the present invention will become manifest upon study of the following specification when taken together with the drawings .