Patent Publication Number: US-2005135547-A1

Title: Control element for a nuclear reactor

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of U.S. application Ser. No. 09/765,135 originally filed Jan. 18, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/265,738, filed Mar. 10, 1999, now abandoned. 
    
    
     TECHNICAL FIELD  
      The present invention relates to a control element for a nuclear reactor. More particularly, the present invention relates to an absorber rod with at least one outer absorber enclosure and an inner absorber enclosure for receiving absorber material.  
     BACKGROUND AND RELATED ART  
      In the present invention, the term “control element” quite generally denotes control assemblies for pressurized water reactors (PWR) and control rods for boiling water reactors (BWR). These control elements are required for controlling the reactor power and also have to be capable of safely shutting down the reactor under any operating condition. The control elements are inserted in or between the fuel elements or nuclear fuel rods in order to absorb neutrons and to control in this way the chain reaction.  
      In boiling water reactors, the control elements are raised into the bottom of the fuel elements, for example in large power nuclear reactors just to such an extent that of the neutrons liberated in a nuclear fission, exactly one neutron on the average will again induce a further nuclear fission.  
      The nuclear lifetime of an original equipment boiling water reactor control element is reached, when the top quarter segment reaches a 10% reduction in reactivity cold worth  
       (         Δ   ⁢           ⁢   k     k     &lt;     10   ⁢   %       )       
 
 The end-of-life reactivity worth reduction will account for any effects of absorber depletion. In the instant case by using B-10 as neutron absorber material, absorber depletion is defined as the ratio of the amount of B-10 atoms that “burn up” in relation to the original amount of B-10 atoms. 
 
      The most direct comprehensive experiments concerning the burn up of B-10 in B 4 C and the tritium release from B 4 C were conducted by Miles, C. C., Wexler, S., Ebersole, E. R. in “TRITIUM RETENTION IN EBR-II-IRRADIATED BORON CARBIDE” in ANL-8107, JUNE 1974. Which showed by careful analytical chemistry measurements that the tritium in B 4 C is formed in a proportional relationship to the B-10 burn up according to the following reactions: 
 
 10 B+ n → 7 Li+ 4 He+2,79 MeV (thermal neutrons);  (1) 
 
 7 Li+ n → 4 He+ 3 H+ n  (fast neutrons)  (1a) 
 
 10 B+ n →2 4 He+ 3 H+0,23 MeV (fast neutrons)  (2) 
 
      The experimental database shows that the irradiation-induced damage to B 4 C is primarily due to the formation of large amounts of helium and lithium and the subsequent accommodation of these atoms in the B 4 C structure. The control element end-of-life depletion values can be converted to fluence (snvt) values consistent with the process computer TCREX array.  
      The process computer accumulates the fuel exposure adjacent to each quarter axial segment of every control element and converts the control element exposure to fluence values. The fluence values are accumulated for each quarter axial control element segment and represent the fuel and control element smeared thermal fluence. This smeared thermal fluence is in a proportional relationship to the axial control element quarter segment averaged B-10 depletion. This includes the B-10 depletion averaged over every absorber rod with respect to the four wings within a control element quarter segment.  
      The control element, which is adapted to be inserted into and extracted from the nuclear core, is not uniformly exposed to neutrons. For instance, the rate of neutron exposure rate is high at the side edges and the upper end of each of the four blades. This means, that these portions of the control blade absorb greater amounts of neutrons than other portions of the control blade. This local depletion effects are taken into account by defining axial and radial peaking factors named f ax  and f rad . So a local B-10 depletion or a local B-10 burn up axially and radially located at a special point of a special concentric (boron) absorber means the number of B-10 atoms in relation to the original amount of B-10 atoms defined as the “local” burn up percentage a m .  
      But even since the “local” burn up percentage a m  differs radially with respect to its absorber diameter, a m  represents an averaged B-10 burn up profile in the radial direction to the center of the cylindrical absorber section. This relationship has been verified by basic irradiation tests conducted in the Hanford KE and KW production reactors Washington, USA by A. L. Pitner and G. E. Russcher, described in “IRRADIATION OF BORON CARBIDE PELLETS AND POWDERS IN HANFORD THERMAL REACTORS”, Wadco Corporation, Richland, Washington December 1970, UC-25, Metals, Ceramics and Materials before the year 1970.  
      According to these irradiation tests for determining burn up levels and reaction profiles, 90 cylindrical B 4 C samples were exposed to thermal neutrons. Burn up had been measured by spectrometric analysis of the  10 B/ 11 B ratio at the beginning and after exposure increments.  
      To establish the relationships between irradiation exposure and average  10 B burn up a m  for these specimen, 500 different cylindrical shells were used to calculate the mentioned averaged relative burn-up percentage a m  for the entire B 4 C sample. This calculation may be performed by using the Shell Model and taking into account self shielding kernels.  
      This relationship between irradiation exposure φ and averaged B-10 burn up percentage a m  with respect to a cylindrical absorber specimen represents the basic expression  
       ϕ   =       a   m     α         
 
 whereas α represents a proportionality factor to correlate or to translate the accumulated neutron fluences φ in the fuel bundles adjacent to the control element to absorptions in the cylindrical absorbers. 
 
      However, reanalysis of those very first investigations showed that the relationship between specimen average burn up percentage a m  and the local burn up distribution a(r), a burn up function with respect to the absorber radius, calculated by the help of the “shell model” needs to be modified by using the “Microscopic Burn-up Theory” developed by the applicant and described in his dissertation in detail. This modification is verified to be in agreement according to the equation of reactions by Miles, C. C., Wexler, S., Ebersole, E. R describing the different B-10 burn up reactions.  
      So in the present invention, the term “burn up percentage a m ” quite generally denotes a special point at a control element where the B-10 burn up averaged in the radial direction over its cylindrical absorber section generally shows a maximum in contrast to other locations of the control element.  
      The local burn-up percentage a generally denotes in the present invention a local burn up percentage at one point on the radial burn-up distribution a(r) with respect to the absorber axis of a cylindrical absorber having an averaged burn up percentage a m  at the mentioned special point of the control element, where the B-10 burn up averaged in the radial direction over its cylindrical absorber section generally shows a maximum in contrast to other locations of the control element.  
      If the control elements are to be employed for a reactor service life of about 40 operating years, it must be possible to load the control elements with a certain neutron fluence without causing the efficiency of the control elements to decrease by more than 10% as compared to their original efficiency. Furthermore, the control elements are expected to contribute to satisfying the overriding protective goals for maintaining the integrity of the barriers against radioactivity releases from the reactor cooling system.  
      It has been found that nuclear control elements, in particular the absorber enclosures and absorption material, should be categorized as consumed or consumable materials, because the absorber material strongly expands due to the capturing of neutrons. This often leads to physical and mechanical damage to the absorber enclosure with subsequent leaching, washout, or destruction of the absorber material. If such a control element is used further in the active core zone, this leads to an increase in the local power density distribution in the core. Under certain circumstances, it may even cause damage to the fuel rods in addition to the release of tritium into the biosphere.  
      An adaptation and optimization of the control elements in view of their useful life, however, is limited by the preset geometry of the reactor, in particular by the geometry of the free water gaps between the fuel element boxes in boiling water reactors, and by the geometry of the control rod guide tubes in pressurized water reactors. Attempts have previously been made to prolong the useful life of the absorber enclosures and thus of the control elements as much as possible. This is typically accomplished through selection and variation of the material of the absorber enclosure and of the wall thickness of the absorber enclosure. The success of these attempts, however, was limited.  
      A control element for a nuclear reactor is known from DE 39 03 844 A1. Inner tubes receiving the absorber are inserted in this reactor into a receiving hole. It is proposed according to DE 41 38 030 A1 to make Provision in the control rods for elongated ducts or channels, in which a swelling material can expand.  
      Other control elements are described, for example in EP 0 143 661; U.S. Pat. No. 4,861,544; and U.S. Pat. No. 4,929,412. Control elements especially for pressurized water reactors are described also, for example in “ Design of Siemens Control Assemblies For Pressurized Water Reactors and Operational Experience ” by L. Heins, W. Dambietz, and H. P. Fuchs in Kerntechnik 57 (1992), No. 2, pages 84 to 89 (Carl Hanser Verlag, Munich). A further description of control elements of this type is contained in “ ABB Control Rods ” by G. Vesterlund et al, in Kerntechnik 57 (1992), No. 2, pp 105 and 106 (Carl Hanser Verlag, Munich).  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a control element for a nuclear reactor which can be subjected to particularly high burn up percentage a m  and in that way to particularly high axial control element quarter segment smeared B-10 depletion. A control element for a nuclear reactor includes an absorber rod having at least three absorber enclosures for receiving the absorber or absorber material. The absorber enclosures are constructed in such a way that the received absorber can be subjected to local relative burn up of more than 90% without the burnt-off absorber material getting into the reactor coolant. The control element is constructed in such a way that there are at least three absorber enclosures and a predetermined spacing between the enclosures, so that each absorber enclosure forms a mechanical resistance for the absorber and is removable from the starting position in the event of expansion of the absorber.  
      According to the present invention, the absorber enclosure is acting as a limiting device with respect to the swelling behavior of the neutron absorbing material or absorber. The limiting device, which in the present invention is generally being denoted as the absorber enclosure, is arranged in a starting position adjoining the absorber material. Thus it forms a mechanical resistance for the absorber. In one embodiment, the absorber enclosure is removable from the starting position when the absorber expands. Overall, an absorber enclosure is created and configured such that the absorber enclosure through counter pressure prevents uncontrolled and very rapid swelling of the absorber. However, upon reaching a containment threshold, the absorber enclosure is no longer capable of withstanding the swelling absorber and the absorber enclosure yields once a certain expansion is reached.  
      The absorber enclosure may then be removable from the starting position in that it breaks or is at least partially destroyed. The term “removable”, therefore, is to be understood to mean that the absorber enclosure is either actually spatially removed from its starting or initial position or that the absorber enclosure loses its directly limiting function in which the absorber enclosure exerts pressure inwardly. Thus in one embodiment, the absorber enclosure breaks up or gets destroyed in some other way. Whereas in another embodiment, residual parts of the used absorber enclosures, however, remain physically in the initial position and may become part of the next absorber enclosures as the absorber presses the residual parts into the outer absorber enclosures.  
      Control elements created in this manner with multiple absorber enclosures may be subjected to burn up percentage a m  of almost 100% without burnt-off absorber material getting into the reactor coolant.  
      In another embodiment, an absorber enclosure that has yielded to absorber material that has already expanded beyond a containment threshold may be removed. The absorber enclosure that has yielded may have formed at least one axial crack. In one embodiment, the yielded absorber enclosure is removable by guiding the enclosure in a controlled way outside of an expansion zone of the absorber material. The expansion zone is generally the area where the absorber enclosure continues to form a mechanical resistance for the absorber rod against the absorber material. The remaining inner absorber enclosures may also form, for example a semicircle adjacent to the outer absorber enclosure.  
      In another embodiment of the present invention, provision is made for at least three absorber enclosures. The at least three absorber enclosures having a predetermined spacing between each of the absorber enclosures to increase the burn-up percentage of the absorber. The predetermined spacing is based in part on the selection of material for the absorber enclosure and the wall thickness of the absorber enclosure. The at least two outer absorber enclosures surround and enclose the inner absorber enclosure. Each of the absorber enclosures embracing one another. The outer absorber enclosures form solid outer jackets, whereas the inner absorber enclosure initially abuts the absorber and offers resistance to the swelling absorber. However, in one embodiment, the inner absorber enclosure breaks apart at a defined pressure, so that the absorber can continue to expand in the direction of the remaining outer absorber enclosures.  
      Upon expansion of the absorber or absorber material in the inner absorber enclosure beyond a containment threshold, the respective absorber enclosure of the absorber rod breaks and is removable from its initial position and mechanical resistance is provided for the absorber by the next absorber enclosure.  
      In a further embodiment of the present invention, provision is made for three or more absorber enclosures surrounding and adjacent to one another because it is possible in this way to offer the swelling absorber several resistances in a staggered and easily preset manner. These resistances will yield one after the other and in this way will permit a particular operating duration of the control element, during which the absorber material can be completely burned up. In one embodiment, the predetermined spacing is provided and maintained by end caps with radial-spaced grooves that receive each of the absorber tubes and position the absorber tubes in fixed positions relative to the other absorber tubes. In another embodiment, each end cap couples and positions the absorber enclosures adjacent to one another. The predetermined spacing is selected such that the absorber enclosure disposed in the innermost position at a given time can break or be destroyed in some other way without damaging in this process the adjacent next outer absorber enclosure.  
      The predetermined spacing is determined depending on the effective creep deformation ε up to breakage of the material employed. Thus the absorber enclosure is first still capable of expanding within the predetermined spacing under the pressure of the absorber material before breakage of the absorber enclosure can occur.  
      It is a particularly preferred embodiment of the invention that the absorber enclosures are designed in such a way that the outer absorber enclosure completely envelops and surrounds the inner absorber enclosure. Therefore, this system has a plurality of absorber enclosures that are fitted or nested into each other, which initially offer resistance pressure to the absorber material expanding from the inside outwardly. These enclosures are preferably concentric and then yield to the expansion pressure and break. Thus the absorber enclosure disposed next to the further swelling absorber establishes mechanical pressure compressing the absorber and effectively prevents uncontrolled swelling expansion of the absorber.  
      In a preferred embodiment of the invention, the dimensions of the inner absorber enclosure are selected in such a way that the inner absorber enclosure is suitable for receiving preset sintered absorber tablets. Also, it is possible to employ a powder as the absorber. B 4 C is preferably used as the absorber. B 4 C shows an excellent neutron-absorbing cross-section, in particular for thermal neutrons. However, like all known absorbers, it exhibits strong neutron-induced expansion, which, in the long term, leads to destruction of the absorber enclosure.  
      It is particularly preferred to employ B 4 C with less than 70% of a theoretic density, particularly of less than 60% because swelling of the absorber can be initially prevented in this way, and a particularly high burn up percentage a m  of the employed absorber material is achieved. However, when optimizing the initial or starting B 4 C-density, it is necessary to make sure that the absorber still has a sufficient amount of B-10 atoms so that the criterion of the effectiveness can be satisfied.  
      Another absorber, which can be employed, is, for example, Ag In Cd, or a material containing boron, which is enriched with the isotope B- 10 . While the expansion characteristics of AG-In—Cd are lower than those described for B 4 C, an absorber using AG-In—Cd may be granulated to achieve the same capabilities or the pellets should have dimensions. Additionally, the predetermined spacing in the absorber enclosures and the volume of the innermost tube may be dimensioned in a way to achieve a lower smeared theoretical density and obtain more free space for expansion.  
      The absorber enclosures are preferably constructed in such a way that they include a plurality of part segments, whereby the part segments of the absorber enclosures are dimensioned in such a way that the part segments of the adjacent absorber enclosures, in particular their abutting surfaces are arranged displaced against each other. The use of several part segments permits easier handling, and by providing the part segments or at least one part segment that is inserted first with different dimensions, the abutting surfaces of adjacent absorber enclosures will not be disposed directly next to each other, which avoids creating a weak point.  
      Such control elements are preferably employable in boiling water reactors and pressurized water reactors. In boiling water reactors, the control elements are normally constructed from four wings arranged in the form of a cross (see  FIGS. 2 and 8 B), such wings having up to 21 absorber enclosures structured in a concentric form. On the other hand, control elements referred to as control assemblies (see  FIGS. 7A-7C ) are normally employed for pressurized water reactors and are driven into the core from the top. The control element as defined by the invention can basically be employed for all types of reactors in which such an absorber is used.  
      With the control element of the invention, it is possible to accommodate the swelling of the absorber without the failure of the outer most absorber enclosure. Thus the control element as a whole can be used for an extended working life and almost all of the absorber material can be exploited in the ideal case at locations, where the neutron fluence shows a maximum. Model computations have shown that it is possible with the control element of the invention to achieve with the use of B 4 C as the absorber a burn up percentage am between 90% and 100%, and under favorable conditions a burn-up percentage of 100% is actually attainable.  
      Other features which are considered as characteristic for the invention are set forth in the appended claims.  
      The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompany drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements. In the drawings:  
       FIG. 1A  is a graphical view from a side of a 99% theoretical density pellet irradiated to about 3.5% B-10 burn-up at 865° F. (80×) and a chart indicating local burn-up percentage of the pellet according to the prior art;  
       FIG. 1B  is a fragmentary, diagrammatic, perspective view of a nuclear cell in a boiling water reactor according to the invention;  
       FIG. 2  is a plan view from above of a control element according to the invention;  
       FIG. 3  is a partially cross-sectional and partially broken away view of an absorber enclosure of a control element according to the invention;  
       FIG. 4  is a graphical view of the “critical” local burn up distribution of a standard control element at the start of washout according to the invention;  
       FIG. 5  is a graphical and fragmentary, sectional view showing burn up percentage a m  and the local burn-up distribution a(r) of a control element according to the invention;  
       FIG. 6  is a graphical view illustrating the achievable burn up according to the invention;  
       FIG. 7A  is a cross-sectional side view of a pressure water reactor control element according to the invention;  
       FIG. 7B  is a top-plan view of the pressure water reactor control element shown in  FIG. 7A ;  
       FIG. 7C  is a side-elevational view of a control rod of the pressure water reactor shown in  FIGS. 7A and 7B ;  
       FIG. 7D  is an enlarged, cross-sectional view of the control rod shown in  FIG. 7C ;  
       FIG. 7E  is a cross-sectional side view of an absorber enclosure of an absorber rod for use with the control rod shown in  FIGS. 7A-7D ;  
       FIG. 7F  is a cross-sectional side view of the control rod shown in  FIG. 7C  showing the placement of absorber rods  FIG. 8A  is a cross-sectional top view of a boiling water reactor control element;  
       FIG. 8B  is a side-elevational view of a boiling water control element according to one embodiment of the invention;  
       FIG. 8C  is a cross-sectional side view of an absorber enclosure for use with the control rod shown in  FIGS. 8A and 8B ;  
       FIG. 8D  is a cross-sectional side view of an absorber rod containing the absorber enclosure of  FIG. 8C ;  
       FIG. 8E  is a partial cross-sectional view of a partially yielded absorber rod illustrating a radial crack; and  
       FIG. 9  is a flowchart of the expansion containment process according to the invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and techniques have not been shown in detail in order not to obscure the understanding of this description.  
      Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment.  
      Although the invention is illustrated and described herein as embodied in “Control Element for a Nuclear Reactor”, the present invention is nevertheless not intended to be limited to the details shown or described, since various modifications and structural changes may be made therein without departing from the spirit of the invention and still be considered within the scope and range of equivalents of the claims.  
      Turning now in detail to the drawings,  FIG. 1A  shows the burn up percentage a m  (originally titled “AVE BURNUP {overscore (BU)}”) of an absorber pellet together with an illustrative local B-10 burn up distribution a(r) with respect to the radial absorber radius as from A. L. Pitner and G. E. Russcher in “IRRADIATION OF BORON CARBIDE PELLETS AND POWDERS IN HANFORD THERMAL REACTORS”, Wadco Corporation, Richland, Wash. December 1970, UC-25, Metals, Ceramics and Materials in 1970. As can be seen from the chart and pellet cross section, the burn-up percentage is dramatically reduced as the radius increases such that the average burn-up percentage for the tablet is only 3.5%.  FIG. 1A  shows the original measured (averaged with respect to the shown cross-section of the absorber pellet) burn up percentage am (original titled “AVE BURNUP {overscore (BU)}”) of an absorber pellet together with an illustrative local B 10 burn up distribution with respect to the radial burn-up location. The related burn-up distributions had been determined for small B 4 C Pellets showing different B 4 C densities and different neutron fluences. As can be seen, the swelling behavior of the B 4 C powder with respect to the local density reduction is adjusted in proportion to the local burn-up distribution and thermal neutron fluence.  
       FIG. 1B  and the following discussion are intended to provide a brief, general description of a suitable operating environment that includes a boiling water reactor. More specifically,  FIG. 1B  shows a representation of a nuclear cell in a boiling water reactor that includes a control element  1 , fuel element boxes  2  and fuel elements  3 . The fuel element boxes  2  each surround a fuel element  3 . The fuel element is arranged in a square, whereby a gap remains between the individual fuel elements  3 . Thus a total of one cross-shaped control element  1  can be moved between the fuel elements. A free water gap remains between the fuel element boxes. A top nuclear grid (or gate)  4  and a bottom nuclear grid  5  support the fuel element boxes  2 . Control element  1  has a multitude of rods  7  (see  FIGS. 2 and 8 B), which are filled with an absorber  8 . Control element  1  is lowered or raised as required between the fuel element boxes  2 , so that a controlled chain reaction (k eff =1) is maintained. Control element  1 , furthermore, is constructed in such a way that it immediately can be completely raised, if necessary, so that the chain reaction can be immediately stopped in any situation.  
       FIG. 2  shows a top view of one embodiment of a control element  1  as it can be employed, for example in a boiling water reactor. In one embodiment, the control element  1  has four wings  6  arranged in the form of a cross. The wings have up to 21 absorber rods  7 , which are filled with an absorber material, such as boron carbide (B 4 C) powder, AG-In—Cd granules, or metallic hafnium (Hf). B 4 C is preferably used as a neutron absorber  8  because of its favorable physical and technological properties. The absorber rod  7  includes multiple absorber enclosures that each have a sheet metal jacket, which assures mechanical integrity. The absorber rods  7  are an elementary component of the control element  1 . This is because, contrary to original assumptions, such absorber rods  7  cannot be used over the entire useful life of the reactor. Instead, the absorber rods  7  must be viewed as a consumable material. Thus after only a few operating cycles, neutron-induced expansion or swelling of absorber  8  of up to 15% by volume causes mechanical damage to absorber rod  7  with a subsequent washout of the absorber material  8 .  
       FIG. 3  shows a cross section of  FIG. 2  taken along III-III through an absorber rod  7  according to the invention. The illustrated configuration of the absorber rod  7  includes absorber  8 , inner absorber enclosure  10 , and multiple outer absorber enclosures  11 ,  12 , and  13 . The absorber  8  is contained within the inner absorber enclosure  10 . In this manner, the absorber rod  7 , through the multiple absorber enclosures  10 - 13 , can be safely supported for a particularly long time even in light of a strongly swelling or expanding absorber  8 . The employed absorber material can be subjected to a burn up percentage a m  of almost 100% without the burnt-up B 4 C and thus without the tritium also getting into the reactor coolant. Absorber  8 , which can be used also in the form of a powder, is provided in  FIG. 3  in the form of sintered B 4 C tablets. These tablets are received in the first and inner absorber enclosure  10 .  
      The B-10 reaction will start directly at location of the outer B 4 C surface to indicate massive circumferential stress as described in detail (See  FIG. 1A ): During exposure the reaction plane starts to move to the central absorber axis step by step by following an exponential burn up expression and by forming a solid ceramic structure growing and showing a lower density during this procedure. The original free volume of 30% (B 4 C 70% of a theoretical density) at the beginning of the exposure event is not completely available for swelling accommodation. This is because the forces acting on the inner wall of the absorber enclosure will increase as the solid ceramic structure zone is being formed by changing the original grain size into fragmented small particles being baked together during exposure to thermal neutrons.  
      However, the rate of creep deformation of the absorber enclosure is very low as compared to the rate of absorber growth (Δε (enveloping tube)/Δt&lt;&lt;Δr (absorber radius growth)/Δt).  
      Hence, the swelling behavior of the absorber respectively and, the local density reduction, will be adjusted in a proportional relationship to the radial burn-up distribution a(r) with respect to the absorber axis of the cylindrical absorber and thus to the applied thermal neutron fluence  
       Φ   =       ∫   o   t     ⁢     flux   ⁢       ⅆ   t     .             
 
      After exceeding a critical neutron fluence Φ crit , i.e., the amount of captured neutrons required to cause the absorber to swell by a certain measure, and upon exceeding of the critical expansion limit or containment threshold of the absorber enclosure, the innermost absorber enclosure  10  fails and breaks. This failure may be indicated by a radial crack that forms between the inner wall and the outer wall of the absorber enclosure. The radial crack (shown in  FIG. 8E ) forms after the absorber has expanded and built up a critical pressure at a radial burn-up distribution a(r)=a(r) crit  with respective to a burn up percentage a m =a mcrit . The pressure within the B 4 C absorber  8  is first reduced and then relieved by such failure to a very significant extent. With further exposure to neutrons, absorber  8  again builds up a critical pressure, relative to the next absorber enclosure, after another critical neutron fluence has been absorbed. This critical pressure exceeds the expansion breaking limit or containment threshold of the first middle absorber enclosure  11  and thus causing failure of the first middle absorber enclosure  11  as well.  
      Provision is made for a minimum spacing between inner absorber enclosure  10  and first middle absorber enclosure  11 . This minimum spacing corresponds with the expansion limit of the inner absorber enclosure  10 . Therefore, any failure of the inner absorber enclosure  10  leaves undamaged the first middle absorber enclosure  11 , which surrounds absorber enclosure  10  and which is positioned at a spaced distance from the outside of inner absorber enclosure  10 . In  FIG. 7D  the spacing between absorber enclosures is shown to be at least about 0.015 mm. In another embodiment illustrated in  FIG. 8D , the spacing averages about 0.03 mm.  FIG. 8E  indicates that the tolerance in one embodiment may be between 1.5 and 3.8% of the tube diameter.  
      The expansion of absorber  8  continues to progress in this way also up to the second middle absorber enclosure  12  and then up to the outermost absorber enclosure  13 . Absorber enclosure  13  forms the outer wall of the absorber rod  7  and in some embodiments (for example,  FIG. 7D ) the outer wall has a thicker wall thickness. Therefore, before expansion of absorber  8  due to exposure to thermal neutrons in absorber rod  7  there are several absorber enclosures  10 - 13  nested one inside the other and spaced one after the other.  
      The B-10 burn up percentage a m  increases as the number of absorber enclosures increases by a certain amount. This increase is a function of the absorber enclosure deformatibility with respect to the deformation strain at the expansion limit and the number of absorber enclosures, in order to finally reach the 100% B-110 burn up limit of a m . As described before, B 4 C absorber  8  normally is fixed to a theoretical B 4 C density of 70%. Thus a free volume of up to 30% will be available for swelling accommodation during the exposure-process step by step in accordance with the number of absorber enclosures and their deformatibilities with respect to the deformation strain at the expansion limit.  
       FIG. 4  shows exemplarily in a graph the B-10 burn up distribution a(r) in % as a function of the absorber radius r in millimeters (mm) of the standard absorber rod design where the burn up percentage a m =a mcrit , that means, in time at which the applied neutron fluence is so high that the critical case occurs, i.e., failure of the absorber enclosure. The burn up distribution a(r) is plotted in percent (%) on the y-axis denoted by reference numeral  14 . Reference numeral  15  denotes the x-axis, on which the absorber radius is plotted in millimeters (mm). The line denoted by 17 reflects the critical burn up distribution a(r) crit  for a standard control element with one single absorber enclosure. Absorber  8  has a radius of approximately 1.75 mm. It becomes clear that a 100% burn up region is produced only in an outer marginal zone, where the B 4 C-absorber has caked into a hard ceramic structure, and where the free volume of 30% originally available there has been completely consumed. Adequate free space is still available in the interior, so that the absorber available there has not been used effectively. The fully drawn line  16  shows the burn up percentage a m =a mcrit , based on the absorber cross section. For such an absorber enclosure, the burn up am comes to about 50%.  
       FIG. 5  shows in a graph the critical burn up distribution a(r) crit  in % and the related burn up percentage a mcrit  for an absorber enclosure according to the invention. The critical burn up distribution a(r) crit  is plotted in percentage (%) in this graph on the y-axis denoted by reference numeral  28 , versus the radius plotted on the x-axis denoted by reference numeral  18 . After the first critical fluence has been reached and the break up of the inner absorber enclosure  10  connected therewith has occurred, the critical burn up distribution a(r 1 ) crit , reflected by outer line  19  ensues. This distribution substantially corresponds to the one shown in  FIG. 4 . This results in the related critical burn up percentage a m     1crit   , reflected by the fully drawn line  20 , which comes to about 50%. Complete burn up of 100% is present in the outer region from 0 to r 1′ . When a second critical fluence is reached and the break up of the first middle absorber enclosure  11  has occurred in connection therewith, the critical burn up distribution a(r 2 ) crit  denoted by reference numeral  21  ensues. At this point, the hard ceramic zone has then already expanded up to region r 2′ . In the interior region, the local burn up has strongly increased as well as compared to the time of the first critical fluence. The related critical burn up percentage a m     2crit   , already comes to well over 70% and is reflected by line  22 . Breakage of the third enveloping tube causes a critical burn up distribution a(r 3 ) crit  according to line  23 , and a related critical burn up percentage a m     3crit   , according to line  24  amounts to about 90%. When the fourth critical fluence is reached, only a small residual zone of the absorber remains where it has not yet caked into a hard ceramic structure. This corresponds with line  25  in the region of symmetry axis  27 . At this point in time, a related critical burn up percentage a m     4crit    according to line  26  ensues, amounting to almost 100%.  
      For illustration purposes, absorber enclosures  10  to  13  are shown as well, whereby lines  19  and  20  are associated with the inner absorber enclosure  10 ; lines  21  and  22  are associated with the first middle absorber enclosure  11 ; lines  23  and  24  are associated with the second middle absorber enclosure  12 ; and lines  25  and  26  are associated with the outermost fourth absorber enclosure  13 . Spacing E required between the absorber enclosures  10  to  13  are shown again in  FIG. 5  in this representation as well. This spacing is fixed depending on the expansion limit of the progressively closest or next inner absorber enclosure.  
      In  FIG. 6  illustrates the influences of different theoretical absorber densities (50, 57, 70) in % relative to the achievable burn up a m  in % (a mcrit  critical burn up percentage) until absorber wash out starts. The chart depicts absorber rods with an absorber radius R of about 3 mm and multiple absorber enclosures each having a constant wall thickness of 0.1 mm. The burn up a m , is plotted in percentage (%) on the y-axis denoted by reference numeral  30 . The combined wall thickness of all the absorber enclosures is incrementally plotted in millimeters along the x-axis denoted by reference numeral  29 . In the illustrated embodiment, it was assumed that each absorber enclosure had a wall thickness of about 0.1 mm. The number of absorber enclosures is plotted on a second x-axis  39 . The burn up percentage a m  that took place in this connection was computed according to the microscopic burn up theory. Here it was assumed in connection with curve  31  that the absorber material, namely B 4 C, has 70% of a theoretical density. Then 57% of a theoretical density was assumed in connection with curve  32 , and 50% of a theoretical density was assumed in connection with curve  33 . It was assumed, furthermore, that the inside radius of the B 4 C comes to 3 mm in the present case.  
      However, it is noted that the calculations show that the values determined for burn up percentage am, are independent of the inside radius of the B 4 C material, which, therefore, means that absorber enclosure with an inside radius of, for example 2.7 mm can be successfully employed as well. Curve  31  shows that the first absorber enclosure with 0.1 mm wall thickness would already fail at a burn up a m  of about 45% in the position denoted by 34 with 70% of a theoretical density. The second absorber enclosure, which has a wall thickness of 0.1 mm as well, would permit a burn up am of barely 60%, which is shown in the position denoted by  35 . A burn up a m  of about 65% is achieved with a third absorber enclosure, as can be seen in the position denoted by 36. A burn up of approximately 70% ensues for a fourth absorber enclosure in position  37 , which corresponds with an accumulated neutron fluence of about 5.95×10 21  n/cm 2 .  
      By using additional absorber enclosures, it is possible to increase the burn up percentage a m  further, as follows from the line denoted by  31 . The use of lower theoretical densities results in a higher burn up percentage a m , which follows from the curves denoted by  32  and  33 . For example, with 50% of a theoretical density (curve  33 ), a burn up a m  of more than 80% is achieved already with the third absorber enclosure (position  36 ). A burn up a m  of 90% is obtained already with the fourth absorber enclosure, which corresponds with an accumulated neutron fluence of 8.3×10 21  n/cm 2  as follows from the position denoted by  37 . The individual absorber enclosures have a wall thickness of 0.1 mm. A space of about 0.01 mm remains between the absorber enclosures.  
      If the absorber enclosures are designed with different wall thicknesses, for example the inner three absorber enclosures each with a wall thickness of 0.1 mm and a fourth outer absorber enclosure with a wall thickness of 0.5 mm, this would be more favorable overall than having a wall thickness of 0.5 mm for the innermost absorber enclosure and a wall thickness of 0.1 mm for each of the three outer absorber enclosures.  
      The design of the control elements according to the invention ( FIGS. 7 and 8 ) can be computed with the help of the microscopic burn up theory, and which has been verified also by different measurements. This shows that the pressure load acting on the control elements is basically determined by the B 4 C expansion due to swelling.  
      The designs to date only permit a B-10 burn up percentage am of approximately 50%. In order to cover local neutron fluence increases, however, it is necessary to make sure that a local B-10 burn up a m  of up to 100% will not lead to failure of the outermost absorber enclosure. This is accomplished with the present invention. If need be, the control element according to the invention also can be fitted with a plurality of absorber enclosures exclusively in the regions where the neutron fluence is excessive. These regions are particularly in the upper regions and in the marginal zones of the wings of the control elements. Thus absorber rods may have more absorber enclosures in the high neutron fluence region than in other regions.  
       FIG. 7  provides various illustrations of a pressure water reactor control element that provides a suitable operating environment for a control element using an absorber rod with multiple absorber enclosures according to one embodiment of the invention.  FIG. 7A  provides a cross-sectional side view of a pressure water reactor control assembly or control element. The control element includes multiple control rods, and a spider assembly, compression springs, a spring cup, a bolt and nut. The spider assembly having a compression spring, bolt and spring cap. The spider assembly is typically machined from a forging using milling and electrical discharge machining to avoid welded or brazed connections for optimum mechanical strength. Each of the individual control rods may be fastened to the spider assembly using a lock-welded nut.  FIG. 7B  is a top view of the control element illustrating how the spider assembly positions the control rods.  
       FIG. 7C  is a plan view of an individual control rod. Generally, each control rod includes absorber material, which is enclosed in a gas-tight stabilized austenitic steel cladding tube with welded end plugs within the control rod.  FIG. 7D  shows a cross-sectional view of the control rod including an absorber rod. Each absorber rod includes multiple absorber enclosures ( 10 - 13 ) surrounding the absorber material  8 . The inner absorber enclosures each have a wall thickness of approximately 0.1 mm and spacing between each absorber enclosure of about 0.015 mm. The outer absorber enclosure has a wall thickness of 0.65 mm. In another embodiment, each outer absorber enclosure exhibits a progressively greater wall thickness than the previous inner absorber enclosure.  
      The materials used to construct the absorber enclosures may be selected from stainless steel (SS) 304, SS-304L, SS-316, SS-316L, SS-347, and other similar materials.  
      As previously discussed, the absorber material may include boron carbide (B 4 C), AG-In—Cd, or metallic hafnium (Hf). The absorber material may take the form of powder, sintered tablets, granules, or other similar configuration. B 4 C is preferably used as a neutron absorber material  8 , because of its favorable physical and technological properties. However, in one embodiment, the alloy Ag 80 In 15 Cd 5  is used as a neutron absorber material  8  and the remaining volume of the control rod is filled with helium at atmospheric pressure.  
      In one embodiment, the combination of different absorber materials in various configurations may help to optimize the control rod performance and life expectancy. However, any combination of absorber materials must consider variations in expansion characteristics and theoretical densities of the components before making the combination. For example, Ag—In—Cd has different expansion characteristics when compared to B 4 C, thus a combination of the two materials should adjust the dimensions of the tablets in relation to the innermost diameter of the absorber tube. Alternatively, the substances could be provided in different forms, such as one in granulated form and one in tablet form.  
       FIG. 7E  is a cross sectional side view of an inner absorber enclosure  10  of an absorber rod. The inner absorber enclosure  10  of the absorber rod includes two end caps or end plugs coupled to the absorber tube to retain the absorber material  8 . The end plugs may be welded, crimped, threadably fastened, or other similar method of fixably fastening the end plug to the absorber enclosure tube.  FIG. 7F  is a cross-sectional view that illustrates the placement of multiple absorber rods within the control rod according to one embodiment of the invention.  
       FIG. 8  provides various illustrations of a boiling water reactor (BWR) control element and the associated absorber rods. The nuclear cell shown in  FIG. 1B  illustrates a suitable operating environment for use with the control element of  FIG. 8  according to one embodiment of the invention.  FIG. 8A  is a cross-sectional top view of a boiling water reactor control element; using an absorber rod with multiple absorber enclosures  10 - 13  to contain absorber  8 .  FIG. 8B  is a schematic side view of the boiling water control element  1  according to one embodiment of the invention.  FIG. 8C  is a cross sectional side view of inner absorber enclosure  10  configured for use with the control rod shown in  FIGS. 8A and 8B .  
       FIG. 8D  is a cross sectional side view of an absorber rod including multiple absorber enclosures of different types. The illustrated absorber rod includes a inner absorber enclosure configured to contain the absorber  8 . Exemplary dimensions for the absorber rod are a length of about 90 mm, an inner absorber enclosure diameter of about 5.5 mm. a middle absorber enclosure diameter of about 5.74 mm and an outer absorber enclosure diameter of 5.98 mm. As previously discussed the preferred material for construction of the absorber enclosures is SS-304, but other types of material may be used. Moreover, the end caps previously discussed in  FIG. 7E  may be similarly connected to the inner absorber enclosure.  
       FIG. 8E  is a fragmentary cross-sectional view of an exposed absorber tube just after passing the critical burn up margin or containment threshold for the inner absorber enclosure  10 . The absorber  8  in the illustrated embodiment is B 4 C. As previously indicated, the depletion of the absorber  8  is approximately 50% and has expanded from the original position. This expansion has created a radial crack extending from the inner wall to the outer wall of the inner absorber enclosure  10 . To prevent damaging adjacent absorber enclosures when the inner absorber enclosure breaks a fabrication tolerance spacing of about 1.5% and about 3.8% of the diameter of the next absorber enclosure  11  is built into the absorber rod. This variance is based in part on observed dimensional measurements from defective absorber enclosures. In most cases, only one axial crack forms in the inner absorber enclosure  10 , because the single crack appears to relieve the stress in other parts of the circumference of the absorber enclosure. As the absorber enclosure  10  exhibits only a slight diameter increase of about 0.2% prior to cracking, the tolerance spacing helps the absorber rod avoid damaging the adjacent absorber enclosure  11 .  
      Turning now to  FIG. 9 , particular methods of various embodiments are described in terms of operational mechanisms with reference to a flowchart. The methods to be performed by a nuclear device constitute operational programs performed by mechanical devices or computer-controlled machines. Describing the operational methods of the nuclear reactor by reference to a flowchart enables one skilled in the art to develop such operational programs including such instructions to carry out the methods on suitably configured control devices (the absorber rods or control rods of the pressurized water reactors or boiling water reactors).  
      The steps may be monitored and performed in a computer controlled device or may be embodied in a mechanical device. If written in a programming language conforming to a recognized standard, such instructions can be executed on a variety of hardware platforms and for interfaces to a variety of operating systems.  
      It will be appreciated that a variety of devices and methods may be used to implement the control element system for a nuclear reactor as described herein. Furthermore, it is common in the art to speak of flowchart steps, in one form or another (e.g., program, procedure, process, application . . . ), as taking an action or causing a result. Such expressions are merely a shorthand way of saying that execution of the step by a device causes the control element of the nuclear reactor to perform an action or a produce a result.  
       FIG. 9  is a flowchart that illustrates one embodiment of a control element system  900 . The system  900  initially provides absorber material within an innermost absorber enclosure of the absorber rod. Exemplary absorber material may include boron carbide (B 4 C), AG-In—Cd, metallic hafnium (Hf), or combination thereof. The absorber material may be embodied in various forms including powder, granular, solid sintered tablets, or other similar configurations.  
      Upon activating an absorber rod in event block  910 , the system  900  begins neutron absorption in action block  920 . In one embodiment, the absorber rod is activated by exposing the absorber material to neutrons within the nuclear reactor. For example, in boiling water reactors (BWR), control elements with absorber rods are raised into the bottom of the fuel elements to reduce the rate at which the fuel elements are generating neutrons through nuclear fission.  
      As the absorber material absorbs the neutrons in a controlled fashion, the system  900  is able to help control and regulate the nuclear reaction. One effect of neutron absorption is the gradual swelling or expansion of the absorption material. As the absorber material expands, due to neutron absorption, to conform to and/or to match the internal physical dimensions of the innermost absorber enclosure, the system  900  mechanically resists further expansion of the absorber material within the innermost absorber enclosure.  
      Query block  930  indicates whether sufficient contact exists between the absorber enclosure and the absorption material for mechanical resistance to affect the continued expansion of the absorption material. If insufficient contact is made, then the system  900  continues neutron absorption in action block  920  without substantial mechanical resistance from the system  900 .  
      Upon sufficient swelling or expansion of the absorption material due to the neutron absorption in action block  920 , the absorption material creates sufficient contact with the absorber enclosure such that the absorber enclosure begins to provide mechanical resistance in action block  940  against further expansion of the absorption material. This resistance allows the absorber material to be more thoroughly saturated with neutrons and thereby increases the achievable burn up.  
      The expansion of the absorption material from neutron absorption and the mechanical resistance continues in action block  940  until a containment threshold is reached in query block  950 . One exemplary indication that a containment threshold has been reached is the formation of at least one axial crack in the innermost absorber enclosure. In various embodiments, the containment threshold of the absorber enclosures are configured to be an expression of absorber depletion, relative/local burn up percentage, the theoretical density, or other similar factors that describe the usage of the absorber material. For example, a set of absorber enclosures may be configured to break according to a local burn up profile of the absorber material at about regular intervals (e.g., 5-10% intervals).  
      Upon breaking or surpassing a containment threshold for the innermost absorber enclosure in query block  950 , the system  900  may remove the innermost absorber enclosure in event block  960 . If query block  970  detects the final absorber enclosure, the system  900  may remove the absorber rod in event block  980 . If query block  970  determines that there are additional absorber enclosures, a next innermost absorber enclosure of the multiple nested absorber enclosures becomes the innermost absorber enclosure and the system  900  returns to block  920  to begin neutron absorption for the next absorber enclosure.  
      In one embodiment, subsequent absorber enclosures are configured to provide predetermined tolerance spacing between the inner absorber enclosure and each subsequent absorber enclosure. As such, there is a brief period of free expansion, without substantial mechanical resistance from the system  900 , for the absorber material within the tolerance spacing in block  920 . Query  930  again determines whether contact exists between the absorber enclosure and the absorption material due to sufficient swelling or expansion of the absorption material. If no contact is found, the system  900  continues neutron absorption in block  920 .  
      In this manner, via multiple stages of containment within the absorber enclosures, the system  900  allows the absorber material to be more efficiently used, such that the employed absorber material can be subjected to a burn up percentage a m  of almost 100% without the burnt-up B 4 C and thus without the tritium also getting into the reactor coolant.  
      The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. Accordingly, while a few embodiments of the present invention have been shown and described, it is also to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, the scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.