A CONTROL ARRANGEMENT FOR PROVIDING ENTHALPY-RISE COMPENSATION IN A NUCLEAR FUEL ASSEMBLY

Abstract: The arrangement comprises a cluster of control rodlets containing neutron absorber material and arrayed for insertion thereof into guide thimbles of the fuel assembly. The cluster of control rodlets (32) has a higher axial worth at an upper portion than at a lower portion thereof. The worth of the control rod cluster varies, in one embodiment, in accor­ dance with different lengths of the absorber material contained in the rodlets; in another embodiment, in accordance with different variations in density of the absorber material within the rodlets; in still another embodiment, in accordance with different kinds of absorber material within the rodlets; and in yet another embodiment, in accordance with different varia­ tions in diameter of the absorber material within the rotlets.

Description: The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, to a control arrangement for use with a nuclear fuel assembly. In a typical nuclear reactor, such as one of the pressurized water type, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongate transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongate fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide in the core a neutron flux sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation, and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. One common arrangement utilizing control rods in association with a fuel assembly is disclosed in US-A- 4,326,919 assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongate metallic cladding tube containing a neutron absorbing material and cooled at its opposite ends by means of end plugs. Generally, the neutron absorbing material is in the form of a stack of closely packed ceramic or metallic pellets filling the tube only partially so as to leave between the top of the pellets and the upper end plug a space or plenum chamber for receiving gases generated during the control operation. A coil spring is disposed in the plenum chamber and held in a state of partial compression between the upper end plug and the top pellet so as to maintain the pellets firmly stacked even during stepping of the control rod. Thus, control rods affect the reactivity by changing direct neutron absorption. Control rods are used for fast reactivity control. A chemical shim, such as boric acid dissolved in the coolant, is used to control long-term reactivity changes. More uniformly distributed throughout the core, the boron solution leads to a more uniform power distribution and fuel depletion than do control rods. the concentration of boron is normally decreased with core burnup or time to compensate for fuel depletion and fission product buildup. A pressurized water nuclear reactor (PWR) operating a full power and with no control rods inserted into the fuel assemblies in its core will operate near zero axial offset. If this is not initially true, as burnup progresses, it will be. Also, during full power operation, the coolant density decreases with core height as heat is carried away from the fuel rods. However, if power is then reduced by increasing dissolved boron concentration or some other uniform poison change, the power will swing to the top of the core. This happens because, as power is reduced, the enthalpy rise across the core is reduced. (A rise in enthalpy refers to the rise in heat content of the water as its temperature rises.) Thus, the coolant (or water) density change across the core is reduced which reduces the difference in reactivity between the bottom and top of the core. Relative to the bottom of the core, the top becomes more reactive since the water density is increasing there relative to the bottom. This causes power to shift to the top of the core. One solution to this problem is to insert control rods of the aforementioned conventional design, instead of changing the boron concentration, to change power level. Since the control rods enter the core from the top in the PWR, they will tend to drive the power to the bottom of the core and, thus, will tend to compensate for the tendency of the power to swing to the top of the core at reduced power. While use of conventional control rods makes changes in the power level possible without large swings in axial offset, the increases in axial peaking factor during power changes demonstrates that use of this method is not problem-free. Consequently, a need exists for improvements in enthalpy rise compensation which will effectively solve the problem of skewing reactivity toward the core top but will not at the same time raise a host of new problems. The present invention provides a unique control rod cluster designed to satisfy this need. It is well known that as power is reduced, several phenomena occur. First, the relative axial water (i.e. coolant) density distribution and its associated reactivity are changed because of the change in enthalpy rise occurring as described above. Second, the axial Doppler defect reactivity changes because the absolute axial power is changing as well as perhaps the axial power shape. Third, the reactivity tied up in xenon is changing as power level changes both in absolute terms and spatially. However, underlying the present invention is the observation that if the axial offset is held constant, then neither the Doppler nor the xenon defect reactivity changes in relative magnitude in the axial dimension. Both the reactivity change in the Doppler and xenon defects will remain approximately symmetric from bottom to top. Only the coolant or water density effect will be nonsymmetric. Thus, if the water density effect can be compensated for, then the resultant reactivity changes in Doppler and xenon will be symmetric and can then be compensated for by changing the dissolved boron concentration or some other uniform poison control. It follows that if the water density change can be handled adequately, then any change in axial offset or axial power distribution will be slight. The solution provided by the present invention is to tailor the axial distribution of control rod worth (neutron absorption capability) in the PWR control rod cluster to match the axial shape of the reactivity associated with the enthalpy rise. The latter increases approximately linearly with core height because the water density decreases approximately linearly. Actually, to be precise, the reactivity increase follows a modified linear curve tending toward a somewhat S-shape since the water density decreases slowly at the bottom of the core where little power is being generated, then decreases faster in the middle of the core where maximum power is being generated, and finally decreases slowly again at the top of the core where again little power is being generated. However, herein it will be understood that the phrase "approximately linearly" includes the slight S-shape configuration of the reactivity and coolant density change curves. Thus, since the solution is to match the control rod cluster worth to compensate for water density and reactivity change, cluster worth should increase approximately linearly with core height. If the worth of the cluster is chosen properly, there would be no relative change in core reactivity as a function of height when the cluster has been inserted to reduce core power. Therefore, there would be no change in axial offset when the cluster is inserted to reduce power. Such a control rod cluster would cancel the axial reactivity change caused by an enthalpy rise change. Accordingly, the present invention is directed to a control arrangement for use with a nuclear fuel assembly having a plurality of guide thimbles to provide enthalpy rise compensation, comprising: (a) a spider assembly; and (b) a plurality of control rods containing neutron absorber material, said control rods being supported at their upper ends from the spider assembly in a pattern which matches that of the guide thimbles, and extending generally parallel to one another downwardly therefrom for insertion into the guide thimbles of the fuel assembly. Each of the control rodlets has a higher axial worth at an upper portion than at a lower portion of the control rod. In a first embodiment of the invention, the worth of the control rod cluster varies axially in accordance with different lengths of the absorber material contained within the rodlets. The absorber material is in the form of pellets of generally the same height. The absorber material in each control rodlet is different in length from the absorber material in any other control rodlet, each rodlet having a progressively longer absorber stack length. In a second embodiment, the worth of the control rod cluster varies in accordance with different variations in densities of the absorber material contained within the rodlets. In a third embodiment, the worth of the control rod cluster varies in accordance with the different kinds of the absorber material container within the rodlets. Finally, in a fourth embodiment, the worth of the control rod cluster varies in accordance with different variations in diameters of the absorber material contained within the rodlets. These preferred embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which: Figure 1 is a side elevational view, with parts partially sectioned and broken away for the purpose of clarity, of a fuel assembly together with a spider assembly for supporting a control rod cluster embodying the invention; Figure 2 is an elevational detail view of the spider assembly and attached control rod cluster; Figure 3 is a top plan view of the spider assembly and control rod cluster, as taken along line 3-3 of Fig. 2; Figure 4 is an enlarged, vertically foreshortened, sectional view of one of the control rodlets of the cluster of Figs. 2 and 3, containing absorber material in pellet form extending down to the bottom end plug of the rodlet; Figure 5 is a similar view of another one of the rodlet of Figs. 2 and 3, containing absorber material over a length which is shorter than that of the control rodlet of Fig. 4 by an amount giving the whole cluster a worth which varies approximately linearly in the axial direction; Figure 6 is a similar view of still another one of the control rodlets of the cluster of Figs. 2 and 3, containing absorber material which is still shorter in length than that of the control rodlet of Fig. 5; Figure 7 is an enlarged, vertically foreshortened, sectional view of a control rodlet from a cluster constituting a second embodiment in which the rodlets vary in their worth in accordance with different variations of absorber densities; Figure 8 is a similar view of a control rodlet from a cluster constituting a third embodiment in which the worths of the rodlets vary in accordance with different kinds of absorber material; and Figure 9 is a similar view of a control rodlet from a cluster constituting a fourth embodiment in which the worths of the rodlets vary in accordance with different variations of absorber diameters. In the following description, like reference characters designate like or corresponding parts throughout the several views, and terms such as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are employed as words of convenience not to be construed as limiting terms. Referring now to the drawings, and particularly to Fig. 1, the fuel assembly illustrated therein and generally designated with reference numeral 10 is of the type used in pressurized water reactors. Basically, it comprises a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), several guide tubes or thimbles 14 which extend longitudinally upward from the bottom nozzle 12, several transverse grids 16 axially spaced along the guide thimbles 14, an organized array of elongate fuel rods 18 transversely spaced and supported by the grids 16, an instrumentation tube 20 located in the center of the fuel assembly, and an upper end structure or top nozzle 22 removably attached to the upper ends of the guide thimbles 14 to form an integral assembly capable of being conventionally handled without damaging the assembly parts. Each of the fuel rods 18 contains nuclear fuel pellets 24 and is hermetically sealed at its opposite ends by means of end plugs 26, 28. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the fuel pellets 24 to maintain the pellets firmly stacked within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating energy in the form of heat which is extracted by pumping a liquid moderator/coolant, such as water or water containing boron, upwards through the fuel assemblies of the reactor core when in use. Referring now to Figs. 2 and 3, there is shown the control arrangement which includes a control rod cluster, generally designated 32, embodying the invention and which can be used in some of the fuel assemblies, such as the assembly 10, of the reactor core to reduce core power, when desired, and to compensate for the occurrence of enthalpy rise across the core due to the reduction in reactivity of the core by insertion of the control rodlets of the cluster 32. As will become clear from the following description, the control rod cluster 32 constitutes means adapted to compensate for reactivity increase in the upper region of the core relative to the lower region thereof in a manner such as results in no relative change in core reactivity as a function of height when the rodlets 32 are inserted to reduce core power. The control rodlets 32 in the cluster embodying the invention are supported from a conventional spider assembly generally designated 34 and including radial flukes 36 connected to the upper ends of the control rodlets 32. The control rodlets 32 which extend from the spider assembly 34 downward in generally parallel spaced relationship with respect to one another are arranged in a pattern matched to that of the guide thimbles 14 of the PWR fuel assembly 10 so as to be insertable therein through the top nozzle 22. The spider assembly 34 is connected to a control mechanism (not shown) which is operable in a known manner to move the control rodlets 32 in and out of the guide thimbles 14 so as to regulate the core power and compensate for the enthalpy rise. The control rodlets 32 in the cluster differ from each with respect to the length of control material contained therein, the lengths of control material being chosen such that the control rod cluster as a whole has a higher axial worth at an upper portion than at a lower portion thereof. Figs. 4 to 6, 7, 8 and 9, respectively, illustrate the four embodiments of control rod clusters 32 embodying the invention. The control rods of the clusters in all embodiments have certain basic elements in common. For instance, all control rods 32 have the same overall length, and each includes an elongate, thin-walled metallic tube 40 together with means in the form of end plugs 42, 44 for closing or sealing the opposite ends of the tube 40. Preferably, the rod tube 40 is made of stainless steel and has a constant diameter. The upper end plug 42 has formed integral therewith an upwardly extending stem which includes an externally threaded end portion 46 enabling the control rod to be connected to one of the flukes 36 of the spider assembly 34. The lower end plug 44 is cone-shaped. Slidably disposed within the tube 40 and resting on the lower end plug 44 is a stack of closely packed neutron absorber pellets which fill the tube 40 only partially so as to leave between the top of the pellets and the upper end plug 42 a void or axial gap defining a plenum chamber 48 for receiving gases generated during operation. As usual, a coil spring 50 is disposed within the plenum chamber 48 and held therein partially compressed between the upper end plug 42 and the top pellet so as to maintain the stack of pellets closely packed also during movement of the control rod cluster 32. It is with respect to the pellets stacked in the various control rodlets 32 that the four embodiments of the invention are different from each other. In the first embodiment shown in Figs. 4 to 6, the pellets 52 are composed of the same absorber material, such as B₄C Ag-In-Cd, or Hf, and they are of generally the same height, for example 15 cm. The basic characteristic of the control rodlets 32 of the first embodiment is that the worth of the control rod cluster varies axially in accordance with different lengths of the absorber material contained within the rodlets 32. The absorber material in one control rodlet 32 is shorter, with regard to the total length of the pellet stack, than the absorber material in another control rodlet by the height of one absorber pellet 52 at the lower portions 54 of the control rodlets. For example, it can be seen that all of the pellets in the control rodlet 32 of Fig. 4 are absorber pellets 52, whereas one pellet 56 lowermost in the rodlet 32 of Fig. 5 and two pellets 56 lowermost in the rodlet 32 of Fig. 6 are inert pellets. Thus, the rodlet of Fig. 5 has an absorber length shorter by the height (length) of one pellet than that of the rodlet of Fig. 4, and the rodlet of Fig. 6 has an absorber length shorter by the height of one pellet than that of the rodlet of Fig. 5. In the second embodiment shown in Fig. 7, the pellets 58 are composed of the same absorber material and are of generally the same height as in the first embodiment. The basic characteristic of the control rodlets 32 of this second embodiment is that the worth of the control rod cluster varies in accordance with different variations in densities of the absorber material contained within the rodlets. In any given control rodlet 32, the densities of the pellets 58 increase from bottom to top of the stack thereof, preferably in a generally linear relationship as a function of core height. The average density of control rodlet 32 is greater at the top of the cluster than at the bottom thereof. In the third embodiment shown in Fig. 8, the pellets 58 again are of generally the same height as in the first embodiment. However, here the basic characteristic of the control rodlets 32 is that the worth of the control rod cluster varies in accordance with different kinds of absorber material contained within the rodlets. Different kinds of absorber can be weak absorber material like stainless steel near the bottom, and strong absorber material Hf like at the top of the cluster. In any given control rodlet 32, the strength of the absorber material in the pellets 60 increases from bottom to top of the stack thereof, preferably in a generally linear relationship as a function of core height. The average strength of absorber material in the control rodlets 32 is greater at the top than at the bottom. Strengths can be increased by, for instance, increasing the B-10 enrichment of B₄C axially. Finally, in the fourth embodiment shown in Fig. 9, the pellets 62 have the same axial height and are made of the same absorber material, the basic characteristic of the control rodlets 32 of this embodiment residing in that the worth of the control rod cluster varies in accordance with different variations in diameter of the absorber pellets 62 contained within the rodlets. In any given control rodlet 32, the diameters of the pellets 62 increase from bottom to top of the stack thereof, preferably in a generally linear relationship as a function of core height. It should be understood that increasing control rod cluster worth as a function of core height may be achieved by using a combination of the above-mentioned characteristics of the various embodiments. The total rod cluster worth required for these enthalpy rise compensating control rodlets 32 is about 0.5% ΔK. ΔK being the change in reactivity. This requirement is established as follows. At EOL (end of core life), the moderator temperature coefficient (MTC) is about -54.10⁻³ pc ΔK/°C (-30 pcm/°F) in a PWR. The temperature increase across the core is typically 21.1°C (70°F) at full power. The difference in reactivity due to the enthalpy rise between the top and bottom of the core at EOL (end of core life) is, therefore, about 2% ΔK at full power. To go to 50% power at EOL, a difference of 1% ΔK would have to be compensated for. If the worth of this rod cluster increases linearly starting from zero worth, then the average worth of the rod cluster would be about 0.5% ΔK. Since the required worth would be less earlier in life and reductions below 50% power near EOL are unlikely because xenon buildup would prevent return to full power, it is expected that the enthalpy-rise compensating rods 32 would not need to be worth more than 0.5% 4K. It is also noted that these rods will have to have significantly less worth near BOL (beginning of core life). Here the MTC is less than 18.10⁻³ pcΔK/°C (-10 pcm/°F) so that a worth as low as 0.1% ΔK may be necessary. Lighter banks at BOL are necessary because the enthalpy rise rods need to be approximately fully inserted to work properly. For instance, at BOL a 0. 5% ΔK rod bank would only be inserted about one-third of the way into the core. This may be better than conventional control rods but not nearly as effective as a fully inserted control rod cluster 32 which at each elevation of the core compensates for the core enthalpy rise change exactly. This observation is not a problem since several light banks can be used. For instance, banks of 0.1% ΔK, 0.2% ΔK and 0.2% ΔK would produce worths up to 0.5% ΔK in steps of 0.1% ΔK depending on which banks are inserted. Thus, at any core burnup, a set of rods can be selected which, when approximately fully inserted, would compensate for enthalpy rise changes. The reason this is true is that the rods are not overlapped and, when fully inserted, the rod cluster worths are additive.

Claims: 1. A control arrangement for use in providing enthalpy-rise compensation in a nuclear fuel assembly having a plurality of guide thimbles, said control arrangement comprising a spider assembly (34) and a plurality of control rodlets (32) containing neutron absorber material, said control rodlets being supported from the spider assembly so as to extend therefrom downward in substantially parallel spaced relationship with respect to each other and in a pattern enabling insertion of the control rodlets into said guide thimbles, characterized in that said control rodlets (32) are arranged in a cluster having a higher axial worth at an upper portion than at a lower portion thereof, each of said control rodlets varying in axial worth from every other rodlet in the cluster. 2. A control arrangement according to claim 1, characterized in that the worth of said cluster of control rodlets (32) varies in accordance with different lengths of said absorber material (52) contained within the rodlets (32). 3. A control arrangement according to claim 2, characterized in that said absorber material (52) is in the form of pellets (52) of substantially the same height. 4. A control arrangement according to claim 3, characterized in that the absorber material (52) in each control rodlet (32) is different in length from the absorber material (52) in any other control rodlet (32). 5. A control arrangement according to claim 1, characterized in that the worth of said cluster of control rodlets (32) varies in accordance with different variations in densities of said absorber material (58) contained within said rodlets (32). 6. A control arrangement according to claim 1, characterized in that the worth of said cluster of control rodlets (32) varies in accordance with different kinds of said absorber material (60) contained within said rodlets (32). 7. A control arrangement according to claim 1, characterized in that the worth of said cluster of control rodlets (32) varies in accordance with different variations in diameter of said absorber material (62) contained within said rodlets (32).

Inventors: FREEMAN, THOMAS RAPHAEL
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: G21C7
IPC codes: G21C7