Patent Number: 040615359
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION For a more complete appreciation of the invention, attention is invited to the following description of an illustrative embodiment of the invention, as shown in the attached drawings. As illustrated in FIG. 1, a nuclear reactor pressure vessel 10 having a longitudinally disposed cylindrical envelope or wall 11 closed at each end by a convex base 12 and a domed roof 13 generally includes reactor coolant inlet and outlet nozzles 14 and 15 respectively, protruding from the vessel wall 11 near the domed roof 13. These nozzles, 14 and 15, are generally all disposed in the same lateral plane, that is, transverse to the longitudinal axis of the cylindrical vessel and are separated from each other with an angular displacement. Within the vessel 10, an annular flange 16 formed on the internal surface of the wall 11 serves as a means for supporting a core support cylinder 18 which includes a vertically disposed distribution hoop 19 formed with an opening 20 aligned with the outlet nozzle 15 for reactor coolant discharge. The core support cylinder 18, or more particularly, the distribution hoop 19 is extended by means of a skirt 22 and a thermal shield 24 which in cooperation with the vessel wall 11 serve as an annular hydraulic guide or passageway 26 to accommodate the incoming primary fluid coolant entering the passageway 26 in the direction indicated by means of arrows 28. In addition, the skirt 22 supports the fuel elements in the reactor core (not shown). In operation, the primary coolant enters the pressure vessel 10 through the inlet nozzle 14, flows downwardly through the annulus or passageway 26, rises within the core support cylinder 18 through the hot reactor core (not shown) to the distribution hoop area 19A, whereupon the heated coolant is discharged from the vessel 10 through the hoop opening 20 and the outlet nozzle 15. In accordance with the invention, a plurality of vent assemblies 30, FIG. 2, only one being shown in FIG. 1, are disposed in the distribution hoop 19 in closed sealing engagement with a respective venting passageway 32 through the hoop 19. Preferably, the vent assemblies 30 are disposed in a plane above that of the inlet and outlet lines and, more particularly, disposed near the top of the distribution hoop 19 and vertically distant from emergency core flooding inlets (not shown) disposed in the passageway 26. As illustrated in FIGS. 2,3 and 4, the vent assembly 30 comprises a mounting ring 34 attached to the hoop 19 to establish fluid communication by means of a fluid passageway 35. In addition, the mounting ring 34 includes a radially inward extending lip portion 36, a pair of alignment holes 40 extending through the ring, and a pin 42 inserted into and tack welded to each of the holes 40 for locating purposes. Moreover, one end of the pins 42 extends out of the hole 40 and inwardly towards the hoop area 19A within the passageway 35 having a tapered end 42A for ease of alignment purposes. The vent assembly 30 further comprises a body portion 37 detachably connected to the ring 34 by means of a plurality of mounting bolts 38 circumferentially disposed about the body and including a pair of alignment holes 41 therethrough of sufficient size for ease of alignment of the body portion 37 with the tapered ends of pins 42 of the ring 34 during assembly thereof. Accordingly, the body portion 37 is aligned within the passageway 35 and bolted against the lip portion 36 in seated sealing engagement with the ring 34. The body portion 37, moreover, defines the pressure relief venting passageway 32A, through the hoop 19 for pressure relief purposes during, for example, an accident. A valve plate or disc 43 is suspended across the venting passageway 32A in sealing engagement with a seat portion 37A by means of a hinge strap 44. The strap 44 is connected to a valve hinge pin 45 attached to the body portion 37 by means of valve members 45A and allows outwardly directed rotational movement of the plate. Accordingly, the vent 30, or more particularly, the suspended plate 43, being responsive to forces acting thereon, is ordinarily sealably closed against the body seat portion 37A by the high pressure inlet fluid within the passageway 26, that is, by a radially inwardly directed differential pressure acting upon the plate 43. However, in the event of a failure or accident such that the pressure differential across the vent 30 is directed outwardly, that is the pressure within the hoop 19 is greater than the pressure within the primary fluid passageway 26, the vent plate 43 will automatically rotate about the hinge pin 45 into the passageway 26, opening the area 19A to the passageway 26 and thereby relieving the pressure within the core cylinder 18. Extending outwardly from the plate 43 into the passageway 26 is an energy absorbing column 46 designed to impact the vessel wall 11 as shown in phantom, FIG. 2 at surface 49. The energy absorbing characteristics of the column 46 are provided by the generally annular construction, which by variation of the size length and wall thickness of the column 46 will cause mechanical compression thereof at predetermined loads. Furthermore, hydraulic absorption of the impact energy is also provided by the column 46 by means of the compression of the fluid coolant trapped within the hollow portion of the column and violently expelled outwardly therefrom, as a result of the impact and mechanical deformation of the column, through an orifice 47. In addition, the outwardly extending end face 46A of the column 46 may be formed at an angle to the column's longitudinal center line, as shown in FIG. 2, such that a substantially flush or fluid tight contact between the face 46A and wall surface 49 is made. In this way, the fluid trapped within the column at the instant of contact may only escape through the orifice 47. Furthermore, as is common in the art, portions of the face 46A may be ground to prevent the formation of a sharp cutting edge thereon. Accordingly, during normal operation of the nuclear reactor system, the primary coolant pressure sealably closes the valve plate 43 against the body portion 37 due to an inwardly directed pressure differential. However, as explained above, during an accident or failure, such as a LOCA, the very rapid pressure rise in the cylinder "explosively" opens the valve plate 43 which allows the pressurized fluid to escape through passageway 32 into the primary coolant annulus 26. Moreover, the impact of the valve of this invention and, more particularly, the column 46 with the wall 11 is reduced due to the "light" weight valve system of this invention as compared to the "heavy" prior art systems. That is, at the same impact velocity, the sonic speed of the escaping fluid, a lighter plate has less kinetic energy than a heavy plate. In addition, energy absorption is also accomplished, according to this invention, by the mechanical deformation or crushing of the column 46 as it contacts the wall 11, locally deforms the wall and imbeds into the wall. The insertia of the plate 43, moreover, not only bends or deforms the plate, but also laterally displaces the plate in a direction tangential to the arcuate path of travel at impact. The motion of the plate thereby causes distortion of the strap 44 or further energy absorption. Hence, the bending or deformation load of the strap, which may be varied according to the material employed and the size thereof, is the maximum load transferred to the hinge pin 45. In addition, bending of the "light" weight disc or plate 43 of this invention will also absorb some of the impact energy thereby further reducing the stress on the wall 11. In the illustrative embodiment of this invention, as shown in the accompanying drawings, the plate 43 is hung at a slight angle to the vertical, approximately 5 degrees, so that the gravitational force acting on the plate ensures the initial seating of the plate to the seat portion 37A, and two straps 44 are provided as a redundant safety feature. Furthermore, the plate 43 also includes a stem 50 extending inwardly therefrom. As a practical matter, the stem 50 is provided, as a safety feature, to prevent a detached valve plate from progressing through the primary coolant annulus 26 during normal reactor operation into destructive contact with the core. That is, the stem 50 is of sufficient length such that the width of the valve plate, from the end of the stem 50 to the end of the column 46 is larger than the annulus between the vent 30 and the vessel wall 11. In practice, the responsiveness of the valve plate 43 to an imposed load may be tested, for safety considerations and to ensure the operation or opening of the valve in an emergency or accident situation, in the following manner. A passageway 51, FIG. 2, provided in the hoop allows access to the vent 30 by means of an exercising tool (not shown), e.g. a hooked rod. The hooked rod engages means on the strap 44, such as a pin 52, FIGS. 2 and 4, and at a certain minimum imposed load, as measured by a spring balance, lifts the plate 43 off the seat 37A. Furthermore, the minimum imposed load for operation of the valve in response to a minimum pressure differential may be calculated by one skilled in the art, to establish the imposed load for testing purposes. In accordance with this invention, vent relief of the core support cylinder during an accident or failure is provided by a vent assembly attached to the distribution hoop and, in addition, this vent assembly provides energy absorbing means which during operation of the valve, that is, upon impact of the valve with the reactor wall will absorb some of the impact energy and thereby preclude the possibility of locally overstressing the reactor wall. Furthermore, the energy absorbing means also includes means which limit the load on the valve's hinge thereby preventing deformation of the hinge and ensuring continued responsiveness and operation of the valve if subsequently closed.