Patent Number: 046541926
Section: description

DETAILED DESCRIPTION OF THE INVENTION The instant invention is an apparatus for releasably supporting a safety rod, said apparatus being actuated automatically by a pre-determined increase in temperature. The device includes a bimetallic component which comprises two intimately bonded metals having different thermal coefficients of expansion. An increase in the temperature of the component will cause a deflection toward the side of the component having the lower expansion coefficient. If one end of the component is held fast, the free end will deflect from its original configuration. When the temperature of the component returns to normal, the component will resume its original configuration. The instant invention is readily understood by reference to FIGS. 1-3 which show safety rod upper adapter 10 housed within safety rod drive shaft 12. Coolant flows upwardly from the safety rod (not shown) through the hollow interior 14 of upper adapter 10 and out into the reactor vessel through ports 16 in upper adapter 10 and ports 18 in drive shaft 12. Upper adapter 10 is provided with protruding retention shoulder 19. In the embodiment shown in FIGS. 1-3 bimetallic member 20 is substantially cylindrical. Inner layer 22 has a higher thermal coefficient of expansion than outer layer 24. As shown in FIGS. 2 and 3, member 20 is provided with split 25 in substantially the axial direction. Inner layer 22 is provided with ledge 27 which engages retention shoulder 19 of upper adapter 10, thereby supporting the safety rod. The inner edge 28 of ledge 27 defines a circle that is eccentric with the circle defined by the circumference of member 20. Thus ledge 27 is widest at split 25 and narrowest at the point diammetrically opposed thereto. This is most easily seen in FIG. 3. Outer layer 24 is provided with collar 29 which rests on shelf 13 of drive shaft 12 to provide support for member 20. Coolant flowing through ports 16 in upper adapter 10 comes in intimate contact with bimetallic member 20, so that the temperature of member 20 tends to be at or near the temperature of the flowing coolant. In the event of a thermal excursion in the nuclear reactor, the temperature of the coolant may rise about 200.degree. F.; for example, a normal coolant outlet temperature of about 950.degree.-1000.degree. F. may rise to about 1150.degree.-1200.degree. F. The gain in temperature of the flowing coolant will be experienced by bimetallic member 20. Because the thermal coefficient of expansion of inner layer 22 is greater than that of outer layer 24, inner layer 22 will expand more than outer layer 24. This will cause bimetallic member 20 to deform by "opening" at split 25. The term "opening" as used here means that the two halves of bimetallic member 20 defined by split 25 are deflected from one another in a jaw-like manner such that split 25 widens. The extent of the opening will be much greater than that which would occur with simple mono-metallic thermal expansion. It may be seen that the greatest amount of deformation occurs at split 25 where ledge 27 is widest and almost no deformation occurs at the point diametrically opposed thereto where ledge 27 is narrowest. When the temperature is increased by at least a pre-determined amount, the deformation is sufficient such that the opening movement of bimetallic member 20 disengages ledge 27 from retention shoulder 19, thereby releasing the safety rod into the reactor core in automatic response to the thermal excursion. Bimetallic member 20 is provided with spine 30 opposite split 25. Spine 30 is secured to drive shaft 12 by known fastening means such as screws 32. Spine 30 provides a pivot point for the jaw-like opening action of member 20. After the reactor has cooled from its thermal excursion and bimetallic member 20 has resumed its normal configuration it may be easily reengaged with upper adapter 10. This is accomplished while the safety rod with upper adapter 10 is still in its lowered position. Drive shaft 12 supporting member 20 therein is forced downwardly over upper adapter 10. Upper conical portion 15 of upper adapter 10 forces bimetallic member 20 into a slightly open position. As drive shaft 12 moves downward, it brings bimetallic member 20 down along portion 11 of upper adapter 10, with ledge 27 following the contour of said portion until it is driven around and under retention shoulder 19. When member 20 reaches its normal position along upper adapter 10, the opening force is relieved and ledge 27 of member 20 simply snaps into place beneath retention shoulder 19. Thus the instant invention is uniquely simple to reengage. An alternative embodiment of the invention is illustrated in FIGS. 4 and 5. In this embodiment, bimetallic means 120 comprises a plurality of bimetallic strips 121, each strip 121 having an inner layer 122 and an outer layer 124, each inner layer 122 having a larger thermal coefficient of expansion than the corresponding outer layer 124. In the particular embodiment shown, four such strips are provided. The strips 121 are parallel to upper adapter 110, and are suspended from drive shaft 112 by fastening means such as screws 132. The inner layer 122 of each strip 121 is provided with a supporting ledge 127 which engages upper adapter retention shoulder 119, so that the plurality of strips 121 together support upper adapter 110. Coolant flowing upwardly past the safety rod (not shown), through interior 114 of upper adapter 110 and out of the ports 116 comes in intimate contact with bimetallic strips 121. In the event of a thermal excursion in the reactor, the temperature gain of the coolant will be experienced by bimetallic strips 121. The greater expansion of inner layers 122 relative to outer layers 124 causes bimetallic strips 121 to deform by deflecting away from upper adapter 110 near the region of engagement. At a predetermined increase in temperature, the deflection is sufficient such that support ledges 127 become disengaged from retention shoulder 119, thereby releasing the safety rod into the reactor core in automatic response to the thermal excursion. It may be seen that this embodiment of the invention can reengage the upper adapter in a manner analogous to that of the substantially cylindrical embodiment, that is, by pushing the drive shaft down over the upper adapter until ledges 127 snap into position under retention shoulder 119. The instant invention is particularly well suited for use in liquid metal fast breeder reactors (LMFBR). In these reactors the normal coolant outlet temperature is about 950.degree.-1000.degree. F., so the pre-determined automatic release temperature for a thermal excursion should be about 1150.degree.-1200.degree. F. A typical rod assembly is about 18' long and about 6" across the flats of a hexagonal cross-section; the diameter of the upper adapter is typically somewhat narrower. For these dimensions, the overall size of the substantially cylindrical embodiment may be about 12" long and about 5.5" in diameter. The inner layer may be an austenitic stainless steel which has a relatively high thermal coefficient of expansion, and the outer layer may be a high chrome steel having a lower thermal expansion coefficient. For these materials, satisfactory results will be obtained if each layer is about 0.050"-0.100" thick and if the device has a free length of about 4.0"-8.0". The bimetallic components can be manufactured by known methods such as the explosive bonding process. It would be desirable for the non-stressed state of the bimetallic components to be about 1000.degree. F. The invention may be modified within the scope of the above teachings. For example, instead of using a retention shoulder and a supporting ledge, the upper adapter and the bimetallic components may be provided with corresponding ratchet grooves. The invention may also be adapted to different temperatures, may be enlarged or reduced in size, and may be made of different materials, all within the intended scope of the invention. The foregoing description is not intended to limit the invention to the precise forms disclosed. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application.