Patent Number: 047708450
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a self-actuated reactor shutdown system (SASS) utilizing hydrostatic supported absorber elements. While the invention is particularly applicable for use in a liquid metal fast breeder reactor (LMFBR), it can be utilized in other types of reactors, such as the gas-cooled fast reactor (GCFR). A SASS is defined as a control-rod system that can scram the reactor automatically without either a signal from an external control circuit or an operator action. Initiation of the scram in accordance with the present invention is entirely from direct sensing of inadequate flow and/or an over-power condition. Particular requirements of a SASS are as follows: 1. It must be capable of operating automatically; 2. It must be failsafe, such that no malfunction of the SASS can cause a hazardous condition; 3. It must not impose excessive restrictions on normal operation of the reactor; 4. It must have as little as possible adverse effect upon plant availability; and 5. It must contribute substantially to the overall safety of the reactor. The SASS of this invention satisfies each of the above requirements and employs reactor pressure differentials and a thermionic diode to activate a control rod scram without a signal from the reactor operating control system. The use of hydrostatic supported absorber elements wherein, during normal operation, the control rod is held above the reactor core and is dropped into the core when the hydrostatic pressure is decreased below a specified minimum, such as the weight of the absorber element, are known in the art as pointed out above. While the present invention utilizes this known principle of operation, the invention also incorporates the use of a thermionic diode which is responsive to high neutron flux (over-power) and coolant temperature (undercooling) conditions of the reactor. The diode functions to control an electromagnetically attracted slide valve which, in turn, controls the hydrostatic pressure supporting the absorber elements, whereby the SASS of this invention provides a system responsive to both low-coolant flow, high-neutron flux (over-power) and coolant over-temperature. The SASS incorporating the present invention cannot be overridden by external control, either from operators or plant control systems, with the intent to hold off a scram. Further, the SASS of this invention is able to be restored to operational or cocked condition only by deliberate operator action, and only when the reactor conditions have been corrected and will permit reactivation. In addition, the SASS of this invention is responsive to scram signals generated by the plant protection systems. Referring now to FIG. 1, a SASS incorporating the present invention is illustrated. While not shown, it is known in the art that control rods or elements of the SASS are positioned within a fuel bundle containing a plurality of fuel rods. The fuel bundles are located in the core of the reactor, while the control rod or neutron absorber element of that bundle is maintained above the core during normal reactor operation. As shown in FIG. 1, the SASS comprises a control assembly channel or casing 10 secured at the lower end to an inlet nozzle 11 and provided with an absorber element 12 composed of neutron absorbing material, as known in the art, and mechanism for controlling the location of the element 12 with respect to a reactor core region indicated at 13. A retriever rod 14 is positioned in casing 10 and extends through element 12 and longitudinally through the casing. The lower end of rod 14 is provided with a ring or member 14' which serves to raise element 12 to its ready position, and cooperates with a control assembly snubber, or dash-pot 15, or other kinetic energy absorbing means to slow the descent of the absorber element 12 after it enters the core region 13 and to return the element 12 to its ready position. The upper end of retriever rod 14 is adapted to be connected to drive grapple or mechanism (not shown) supported on the reactor top shield to perform upward movement of the absorber element and to reposition a magnetically retained slide valve 16, as described hereinafter. The absorber element 12 is provided at the lower end with plate 17 having a plurality of orifices to control cooling flow therethrough. As shown, the element 12 is in its ready or cocked position above the core region 13 and is retained hydrostatically against an absorber up-stop or face seal 18 fixedly secured to casing 10. Element 12 is held against up-stop 18 by the pressure differential across element 12 created by coolant flowing upwardly under pressure through inlet 19 in nozzle assembly 11 from a pressure plenum (not shown), as indicated by the flow arrows. The pressure differential which retains the absorber element 12 against up-stop or face seal 18 is produced, as known in the art, by the difference in surface area at the top and bottom of the elements 12 on which the pressurized coolant may act. Since the surface area at the lower end of the element 12 is greater than that at the upper end thereof, due to the element abutting against up-stop 18, the element 12 is hydrostatically retained in its up or cocked position. Any decrease in pressure differential below the minimum required to support the weight of element 12 will cause the element to fall towards core region 13. As soon as the element is separated from the face seal or up-stop 18, essentially all the pressure differential is lost, since the coolant can act against the entire upper surface of the element causing the pressure above and below the element to equalize, and the element will fall freely into the core region 13 under the influence of gravity, the drop stroke of element 12 being illustrated by legend. The fall of the absorber element 12 will be retarded only by flow resistance of the displaced fluid with casing 10, and near the bottom of its stroke or fall by the snubber or dashpot assembly 15 for absorbing the kinetic energy. The pressure differential holding the element 12 in its upper position is a function of the total core pressure drop and the relative flow resistances of any active cooling passages in the absorber element and of the inlet orifice. Since pressure drop across the core region 13 varies with the square of the flow, the available presure will decrease rapidly as flow decreases. A valve (not shown) for by-passing the face seal or up-stop 18 can be utilized to provide a control element scram as a result of excessive core outlet temperature. Such a valve is normally closed, and is designed to open on an over-temperature signal. It can be actuated, for example, by melting a fusible material, a thermionic diode without fissionable material, or by an electromagnetic device, as known in the art. A mechanical drive or grapple, not shown, is connected to the upper end of retriver rod 14, as set forth above, for raising the absorber element 12 and holding it in its upper position until adequate coolant flow is established to produce the pressure differential discussed above. The grapple must be released before reactor operation. Release of the grapple can be assured after disconnecting by raising the grapple to a higher position. To enable the plant operator to know the location of the absorber element 12 with respect to the core region 13, a plurality of position detection coils 20 (three in this embodiment) are positioned on the casing 10 along the length of the element 12. It is readily seen that the location of element 12 can be determined by the readout from the coils 20. Should the element 12 be in a partially inserted (lower position), for example the readout from the upper coil 20 would differ from that of the two lower coils. Coil readout apparatus is well known in the art and further description of such is deemed unnecessary. Positioned above the up-stop 18 is an instrument and control column 21, including a housing 22 which, at the lower end thereof, is secured in casing 10 and provided with seal means for preventing coolant flow therebetween. Housing 22 includes a chamber 23 within which slide valve 16 is movably positioned. A fluid or coolant passage 24 extends from chamber 23 to an outlet chamber 25 in control column 21, which is provided with coolant flow outlet openings 26. An electromagnetic coil 27 is positioned above slide valve 16 and is connected via an electromagnetic control circuit indicated at 28 to a power supply, not shown. A uranium-blanketed thermionic device 29 mounted in chamber 25 and secured to control column 21 is electrically connected in control circuit 28 so as to be in parallel with coil 27 and is responsive to neutron flux. The electromagnetic coil 27 is normally energized from above the reactor head via control circuit 28 such that slide valve 16 is magnetically retained in its upper position, as shown, whereby coolant flows through passage 24 into chamber 25 and out openings 26, as indicated by the flow arrows. In the event of reactor over-power (high-neutron flux), the thermionic device 29 is heated to a change of state. This change of state causes the electromagnetic coil 27 to be short-circuited and lose its holding power, whereupon the slide valve 16 drops by gravitational force and closes off the flow through passage 24. This change (decrease) in coolant flow above absorber element 12 causes a decrease in the differential pressure across element 12 such that the holding pressure is less than the weight of the element, whereby element 12 moves downwardly with respect to face seal or up-stop 18. As described above, this initial downward movement or drop of absorber element 12 results in a loss of pressure differential or equalization of the coolant pressure above and below the element such that the element drops into reactor core region 13 under full gravitational force. When normal reactor flow conditions have been reestablished, or there has been a sufficient reduction of the neutron flux, the absorber element is returned to its ready or cocked position by means of the retrieval rod 14, as described above. In addition, the retrieval process returns the slide valve 16 to its position against the electromagnet and is retained in the upper section of chamber 23 by magnetic attraction when the electromagnetic coil 27 is re-energized. The retrieval rod 14 is then lowered to permit the full drop stroke of the absorber element 12. The retrieval rod 14 is provided with a member, not shown, such as a ring, which is located on rod 14 so as to simultaneously position slide valve 16 at the top of chamber 23 adjacent electromagnetic coil 27 and element 12 against up-stop 18. The thermionic device 29 of FIG. 1 is embodied in FIGS. 2 and 3 as a thermionic diode 29. The diode 29 consists of a sealed container 30 having therein an emitter 31 and a collector plate 32 separated by a gap 33, with a uranium blanket 34 positioned around emitter 31 which causes heating of diode 29 due to neutron flux, and a quantity of thermionic material 35 located within sealed container 30. Emitter 31 and collector plate 32 are connected to an electrical potential (control circuit 28), as known in the art, via electrical leads 36 and 37, respectively, which extend through insulators 38 in container 30. By way of example, the diode 29 may be constructed of the following material: container 30 is of stainless steel; emitter 31 is of molybdenum with a diameter of 0.750 in. and wall thickness of 0.50 in.; collector plate 32 is of molybdenum with a diameter of 0.450 in. and wall thickness of 0.10 in.; gap 33 is in the range of 0.10 in.; uranium blanket 34 has a wall thickness of 0.10 in.; thermionic material 35 may be cesium or other metalic vapor at operational temperatures. The electric leads 36 and 37 are of copper; and the insulators 38 are of alumina. The thermionic material 35 is tailored to ionize at a selected temperature, for example, in the range of 1000.degree. F. to 1100.degree. F. An electrical potential, such as 10 to 15 volts, is applied to the emitter 31 and collector plate 32 and when the ionization temperature of the thermionic material 35 is reached, due to reactor over-power condition (high neutron flux) or to undesirable coolant temperature conditions, the material changes from high resistance to low resistance, thereby conducting most of the available current and, in effect, short-circuiting the electromagnetic coil 27 in FIG. 1 which is connected in parallel with the diode 29, via control circuit 28, as set forth above. It has thus been shown that the present invention provides a self-actuating shutdown system (SASS) for nuclear reactors, particularly for an LMFR, which is responsive to low coolant flow and/or high-neutron flux (over-power) and/or reactor coolant temperature (under-cooling) conditions of the reactor. The SASS of this invention satisfies each of the requirements outlined above for such a system. The thermionic diode also may be utilized to cause self-actuation of the control element due to reactor coolant over-temperature. While not shown, this may be accomplished via a coolant flow control valve controlled by an electromagnet and a thermionic diode. In a reactor over-temperature condition, the diode will be heated by the coolant to a change of state causing the electromagnet to be shorted thereby actuating the valve which provides a changed flow and pressure condition required for scramming the absorber element. While a particular embodiment of the invention has been illustrated and described, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications as come with the scope of the invention.