Patent Number: 042279674
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a liquid metal fast breeder reactor of known construction. This reactor includes a core area 8 containing a plurality of fuel elements 10. The power generated by the reactor is regulated by a series of control rods 12 that are raised and lowered with respect to the core by the rod drive mechanisms 14. The core of the reactor is contained within a vessel 16 and the entire nuclear reactor is housed within a primary shield 18. The reactor uses partially enriched uranium (U-235) or plutonium (Pu-239) for fuel and the primary coolant is typically sodium at atmospheric pressure. The reactor of FIG. 1 is cooled by a flow of liquid sodium that enters the reactor vessel through an inlet nozzle 20. The liquid sodium thereafter flows to a plenum located beneath the fuel elements 10. Thereafter, the primary coolant flows upward through the core where the heat generated by the fission reaction is transferred to the primary coolant. Next, the coolant flows out of the vessel through an outlet nozzle 22. The heat in the primary coolant is transferred to either a secondary heat exchanger (not shown) or to a steam generator. In this secondary heat exchanger steam is generated for the production of electrical power. FIG. 2 illustrates a portion of a control rod 12 that is moved into and out of the reactor core 8 to control the power level. The control rod includes a rod drive shaft 28 that is connected to a rod drive mechanism 14, FIG. 1. The rod drive mechanism is an electro-mechanical motor which precisely moves the control rod into and out of the core. The rod drive mechanism also can release the rod drive shaft so that the control rod can be rapidly inserted into the core in order to scram the reactor. The control rod 12, FIG. 2, has an absorber 30 for absorbing neutrons in the reactor. The absorber includes a can 31 fabricated from stainless steel sheet stock that forms a conduit through which the sodium flows. The can also forms a housing for a plurality of elongate circular poison containing rods 32. The poison in these rods is a material that readily absorbs neutrons such as boron carbide (B.sub.4 C). Attached between the rod drive shaft 28 and the absorber 30 is a thermally elongatable member 34 having means for amplifying its thermal elongation. The elongatable member is constructed from a plurality of coaxial cylinders 36 of differing radii. Each cylinder has a principal axis that is oriented parallel to the direction of motion of the rod drive shaft 28 and the absorber 30. The cylinders 36 are submerged in the primary coolant and have a plurality of ports 38 to which permit the primary coolant to flow between the cylinders and cause a variation in length directly corresponding to the variation in primary coolant temperature. The thermally elongatable member 34 amplifies its thermal elongation by incorporating cylinders having differing coefficients of linear thermal expansion and attaching the cylinders together at their ends in an alternating manner. In FIG. 2 the cylinders identified by reference numeral 36 are fabricated from a material having a large coefficient of expansion (.alpha.1), and the remaining cylinders 36' are fabricated from a material having a small coefficient of linear expansion (.alpha.2). Referring to FIG. 2, the innermost cylinder 36 is fabricated from the material having the larger coefficient of expansion (.alpha.1) and is attached at its top end to the rod drive shaft 28. This innermost cylinder, in turn, is attached at its lower end of the next larger cylinder 36' which is fabricated from the material having the smaller coefficient of expansion (.alpha.2). This second cylinder is attached to the next larger cylinder 36 at its upper end near the attachment point of the innermost cylinder to the rod drive shaft 28. The successively larger cylinders alternate in sequence back and forth between the larger and smaller coefficients of thermal expansion and between the top and bottom points of attachment. This variation is illustrated in FIG. 2. In the preferred embodiment the cylinders 36 having the larger coefficient of thermal expansion (.alpha.1) are fabricated from stainless steel and the cylinders 36' having the smaller coefficient of thermal expansion (.alpha.2) are fabricated from Invar. Stainless steel has the coefficient of linear thermal expansion of 10-12.times.10.sup.-6 increase in length per unit of length per degree C. Invar has a coefficient of linear thermal expansion of about 0.9.times.10.sup.-6 increase in length per unit of length per degree C. The amplifying effect of the thermally elongatable member 34 is mathematically described by assuming that the cylinders 36 all have a length L. The series of cylinders 36 have a large coefficient of expansion (.alpha.1) and the cylinders 36' have a smaller coefficient of expansion (.alpha.2). It can be shown that for N cylinders 36 of material .alpha.1 and N-1 cylinders 36' of material .alpha.2 the overall elongation of the member .DELTA.L is given by: EQU .DELTA.L=[N(.alpha..sub.1 -.alpha..sub.2)+.alpha..sub.2 ]L.DELTA.T where .alpha..sub.1 &gt;&gt;.DELTA..sub.2 PA1 and .DELTA.T is the change in temperature. It should be appreciated that the thermal elongation amplifier 34, FIG. 2, is comparable in some respects to systems for compensating pendulums in order to maintain a uniform period. For example, the pendulums used in some late 19th century grandfather clocks incorporate mechanical devices to compensate for changes in ambient temperature. These devices move a counter weight along the principal axis of the supporting arm of the pendulum and compensate for the variation in length of the supporting arm due to changes in the ambient temperature. These pendulum compensating devices typically include a mechanical yoke and two sets of parallel rods each having a different coefficient of thermal expansion. The yoke alternatively engages rods in each set so that a thermal elongation amplifier is formed. In operation, the control rod 12 is installed in a nuclear reactor as illustrated in FIG. 1. The thermal elongation amplifier 34 is exposed to the temperature of the primary coolant leaving the reactor and is positioned in the reactor at a level below the sodium outlet nozzle 22. Depending on the amount of negative reactivity which is required to overcome a temperature excursion, the thermal elongation amplifier 34 and the absorber 30 are installed on one or a plurality of the control rods. It should also be noted that the elongation amplifier and the absorber need not be installed on a control rod at all but can be attached to a structural component of the reactor vessel 16. All that is required is positioning the components so that the absorber can move relative to the core as the temperature of the primary coolant varies. In the operation of a nuclear reactor the control rods 12 including the neutron absorbers attached thereto are positioned with respect to the reactor 8, FIG. 2, in a manner to precisely control the growth of the neutron population within the reactor. Typically during start-up the absorbers 30 are partially withdrawn individually and in groups and the fully inserted position in the reactor core. As the temperature of the primary coolant increases, the reactivity in the reactor decreases as a result of the negative temperature coefficient. To compensate for this increase in negative reactivity, the control rods are withdrawn slightly from the core by the rod drive mechanism 14. As a reactor is brought on the line and commences to provide power, the control rods are withdrawn further from the core. When the reactor is operating at full power, some of the control rods 12 are fully withdrawn from the core and others remain partially inserted. Those control rods having a thermal elongation amplifier 34 connected between the rod drive shaft 28 and the absorber 30 are operated in the same manner as those control rods that do not. A loss of coolant flow can occur, for example, from either a rupture in the primary system piping, the stoppage of a main circulating pump (not shown), or the closure of a valve (not shown) in the primary coolant main circulating loop. The primary coolant then ceases to carry away the heat generated by the reactor. Since the reactor continues to generate heat at substantially the same level of power as before the loss of flow, the temperature of the primary coolant commences a thermal excursion. The increase in temperature of the primary coolant causes the thermal elongation amplifier 34 to increase in length and to move the absorber 30 into closer proximity with the core 8 of the reactor. The absorber moves downward relative to the rod drive shaft 28 which remains stationary unless moved by the rod drive mechanism 14. This motion of the absorber causes it to absorb more of the neutrons in the reactor and to correspondingly reduce the level of power. The motion of the absorber into the core is directly proportional to the increase in temperature. The thermal elongation amplifier 34, FIG. 2, is placed in intimate thermal communication with the primary coolant and is sensitive to the temperature of the primary coolant at the point where the coolant leaves the reactor. The elongation amplifier has a plurality of ports 38 that permit a substantially unrestricted flow of primary coolant around the cylinders 36, 36'. As the primary coolant coasts down after the loss of flow, the primary coolant transfers its heat to the elongation amplifier by convection. When the flow of primary coolant essentially stops, the cylinders 36, 36' in the amplifier are heated either by conduction or natural convection. Typically, in a loss of flow accident the sensing devices and scram mechanisms immediately terminate the temperature transient and the reactor is shut down without a significant increase in overall power or temperature. However, the present invention provides additional security because in the event of the failure of all reactor sensors, actuation mechanisms and power supplies, the reactor will be shut down by the insertion of the absorber 30 into the reactor by the thermal elongation amplifier 34. It should be noted that in some reactors it may not be necessary for the absorber 30 to be inserted completely into the core. In these reactors it is sufficient that the level of power in the reactor be reduced to a low steady-state level of less than 15 percent of full power. At this power level the heat load can be removed by natural convection of the coolant and dissipated by a small, emergency cooling heat sink. The present invention has special application in a liquid metal fast breeder reactor because the reactor normally has either a very small negative temperature coefficient of reactivity or a slightly positive coefficient. The thermal elongation amplifier 34 in combination with the absorber 30, in effect, provides a negative temperature coefficient of reactivity and stablizes the reactor during operation. The operation of the breeder reactor becomes self-correcting because the absorber is moved closer to the core and inserts negative reactivity as the temperature of the primary coolant increases. It should also be noted that this device provides a means for minimizing temperature excursions and for terminating over-power transients without having to shut down the reactor. The transient can be overcome without a scram and the reactor can remain on the line producing power. Referring to FIGS. 4 and 5 the elongate members are a plurality of parallel rods 40, 40' that are connected by a mechanical yoke 42 so that thermal expansion of the members is amplified in the direction of relative motion. Each rod has a principal axis oriented parallel to the direction of relative motion. Although the preferred embodiment has been described in use in a liquid metal fast breeder reactor, this invention contemplates application in other reactors such as pressurized water reactors, gas cooled reactors, and boiling water reactors. In addition, the absorber 30 need not be boron carbide but can be any neutron-absorbing material. The present invention also contemplates locating the thermal elongation amplifier 34, FIG. 2, in other locations in the reactor besides in a position to monitor the exiting primary coolant. For example, the amplifier can be positioned in the reactor to sense the incoming primary coolant and in the bottom of the reactor to sense the temperature of the primary coolant below the core. Thus, although the best modes contemplated for carrying out the present invention have been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded as the subject matter of the invention.