the present invention is directed to a self - actuated reactor shutdown system ( sass ). while the invention is particularly applicable for use in a liquid metal fast breeder ( 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 coolant temperature and / or an over - power condition . particular requirements of a sass are as follows : 2 . it must be fail - safe , 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 . 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 an electromagnetic latch mechanism and a thermionic diode to activate a control rod scram without a signal from the reactor operating control system . the use of electromagnetic latch mechanisms to retain absorber elements such that during normal operation the control rod is held above the reactor core and is dropped into the core upon release of the latch mechanism by gravitational force on the absorber element , are known in the art as pointed out above . while the present invention utilizes this known principal of operation , the invention also incorporates the use of a thermionic device which is responsive to high coolant temperature and / or high neutron flux ( over - power ) conditions of the reactor . the diode functions to control an electromagnet which , in turn , releases the absorber element , whereby the sass of this invention provides a system responsive to both coolant temperature and neutron flux . 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 protective systems . referring now to fig1 a and 1b , a sass incorporating the present invention is illustrated . as known in the art and illustrated in the drawings , the control rods or elements of the sass are positioned within a fuel bundle containing a plurality of fuel rods or assemblies . the fuel bundles are located in the core of the reactor , while the control rod or neutron absorber element of that bundle is maintained in a location exterior of the reactor core region under normal reactor operating conditions . as shown in fig1 a - 1b and 2 , the sass or control assembly generally indicated at 10 is positioned centrally within a fuel bundle composed of a plurality of reactor fuel rods or assemblies 11 . the control assembly 10 is encased in guide tube 12 which extends through the reactor core region indicated at 13 and secured in the core at the lower end of the guide tube as known in the art . guide tube 12 is provided at the lower end 14 with a plurality of coolant inlet openings 15 through which reactor coolant under pressure is directed upwardly as indicated by the flow arrows . movably located within the upper end 16 of guide tube 12 are an absorber assembly ( control rod ) 17 and a main driveline assembly 18 , which are spaced from the inner surface of the guide tube so as to provide for coolant flow therebetween as indicated by flow arrows . absorber assembly 17 , containing neutron absorbing material as known in the art , is provided with a plurality of openings 19 in the lower and upper ends thereof to allow coolant to flow therethrough , as indicated by flow arrows . secured to the upper end of absorber assembly 17 is a magnet armature 20 which cooperates with an electromagnet 21 secured to the main driveline 18 to retain the absorber assembly in its ready or cocked position exterior of core region 13 as shown , when electromagnet 21 is energized . positioned in guide tube 12 below the core region 13 is a control assembly snubber or kinetic energy absorbing means 22 which retards the downward movement of the absorber assembly 17 after it passes into the core region . as pointed out above , the direct holding of a reactor control ( absorber ) rod by an electromagnet secured to the end of a control drive similar to the apparatus of fig1 a and 1b thus far described is known . in operation of the apparatus thus far described , the electromagnet 21 is lowered by the driveline 18 to contact the magnet armature 20 on the top of the control rod or absorber assembly 17 , and the electromagnet 21 is energized by application of electrical current from a power source , whereby the assembly 17 is attracted to the electromagnet and is withdrawn from the core region 13 by driveline 18 and positioned in its ready or cocked location above the core region as shown . release ( scram ) of the absorber assembly 17 is obtained by reducing the holding power of the electromagnet 21 . for example , such release may be obtained by a known method where the reactor undergoes a thermal transient and the coolant is heated above normal thereby heating the electromagnet to a calibrated curie point , causing the magnet to release the control rod . release via the curie point approach is effective but slow . the main driveline 18 is actuated by a mechanically driven system supported on the reactor top shield . a variety of such mechanical drive systems are known , such as electrically driven racks and pinions , roller nut and ball nut screws . the driveline 18 is usually sealed by bellows that allow the linear movement to be translated through the reactor containment boundary . release of the absorber element 17 in accordance with the present invention provides a substantially higher speed of response and involves a thermionic device such as one or more thermionic diodes illustrated in fig3 and 4 . the thermionic device is attached electrically in parallel with the electromagnet and when the device conducts it shorts the electromagnet current causing it to lose its holding power . the thermionic switched electromagnetic latch of the present invention as illustrated in fig1 a and 1b consists of a flux sensing thermionic switch 23 located above and electrically connected in parallel , as described hereinafter , with the electromagnet 21 and a temperature sensing thermionic switch 24 mounted on main driveline 18 above the top guide tube 18 . note that fig2 illustrates three switches 24 positioned around driveline 18 . thermionic switch 24 is also connected electrically in parallel , as hereinafter described , with electromagnet 21 and is located above the coolant outlet 25 of the fuel assemblies 11 so that heated coolant indicated by the flow arrows passing through coolant outlet 25 is directed onto temperature sensing switch 24 . a neutron shield 26 for flux sensing thermionic switch 23 is positioned about the switch by a neutron shield drive rod 27 operatively connected to the drive mechanism , not shown , for operating the main driveline 18 described above . neutron shield 26 , for example , may be constructed of material such as depleted uranium . main driveline 18 is provided with a plurality of coolant outlets 28 such that coolant from inlet 15 passes under pressure up through guide tube 12 , through openings 19 and around absorber assembly 17 , around electromagnet 21 , around thermionic switch 23 , upwardly through main driveline 18 , and exits via coolant outlets 28 . the flux sensing thermionic switch 23 , which can be electrically identical to temperature sensing switch 24 , is located within the control assembly 10 so that it will not be in direct contact with high temperature coolant from the fuel assemblies 11 . as shown in fig2 a plurality of temperature sensing thermionic switches 24 can be placed around or along the driveline 18 , or the switches 24 can be supported on extensions or arms over the fuel assembly coolant outlets 25 . also , ducts may be provided to direct the coolant flow from outlets 25 onto the temperature sensing thermionic switch 24 . it is within the scope of this invention to utilize a plurality of flux sensing thermionic switches 23 within the control assembly 11 to provide for redundancy , set point , and position adjustment . also , the flux sensing thermionic switch 23 can be placed in a different location than that illustrated , if needed , to more accurately adjust the detection ability . the thermionic switches 23 and 24 of fig1 a and 1b control assembly are embodied in fig3 and 4 as a thermionic diode indicated generally at 30 . the diode 30 consists of a sealed container 31 having therein an emitter 32 and a collector plate 33 separated by a gap 34 , with a uranium blanket 35 positioned around emitter 32 which causes heating of the diode due to neutron flux , and a quantity of thermionic material 36 located within sealed container 31 . emitter 32 and collector plate 33 are connected to an electrical potential , as illustrated in fig5 via electrical leads 37 and 38 , respectively , which extend through insulators 39 in container 31 . the uranium blanket 35 may be replaced by a quantity of uranium attached to the emitter 32 . by way of example , the diode 30 may be constructed of the following material : container 31 is of stainless steel ; emitter 32 is of molybdenum , with a diameter of 0 . 750 in . and wall thickness of 0 . 050 in . ; collector plate 33 is of molybdenum , with a diameter of 0 . 450 in . and wall thickness of 0 . 10 in . ; gap 34 is in the range of 0 . 10 in . ; uranium blanket 35 has a wall thickness of 0 . 10 in . ; thermionic material 36 may be cesium or other metalic vapors at operational temperatures . the electric leads 37 and 38 are of copper ; and the insulators 39 are of alumina . the thermionic material 36 is tailored to ionize at a selected temperature , for example , in the range of 1000 ° f . to 1100 ° f . an electrical potential , from power supply 40 , such as 10 to 15 volts , is applied to the emitter 32 and collector plate 33 and when the ionization temperature of the thermionic material 36 is reached , due to reactor over - power conditions ( high neutron flux ) or coolant temperature , the material changes from high resistance to low resistance thereby conducting more of the available current and , in effect , short - circuits the electromagnet 21 in fig1 a which is connected in parallel with the diode 30 , via the control circuit illustrated in fig5 . fig5 schematically illustrates an embodiment of an electric circuit interconnecting the electromagnet and the thermionic switch means with an external power supply . as shown , the thermionic switches or diodes 23 and 24 are connected in parallel with electromagnet 21 and to current limited , regulated d . c . power supply 40 . it has thus been shown that the present invention provides a self - actuating shutdown system ( sass ) for nuclear reactors , particularly and lmfr , which is responsive to low coolant flow and / or high neutron flux ( over - power ) conditions of the reactor . the sass of this invention satisfies each of the requirements outlined above for such a system . 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 .