Patent Number: 052971821
Section: description

DETAILED DESCRIPTION Referring now to the drawings in detail and initially to FIG. 1, a nuclear reactor is indicated generally at 10. The nuclear reactor 10 comprises a vessel, indicated generally at 12, defining a chamber 13, a fuel core 14 situated within the chamber 13, and reactor internals 18 also situated within the chamber 13. The vessel 12 includes a main body portion 20 and a top closure head 22 coupled thereto. The illustrated vertical orientation of the vessel 12 is maintained by a bracing system which includes legs 24 mounted on the floor of the reactor building. The main body portion 20 of the vessel 12 includes inlet/outlet nozzles 26 which are surrounded by thick flanges 27 for integrity purposes and which are connected to the appropriate plant lines 28. In FIG. 2, the nuclear reactor 10 is shown after preparatory steps of the decommissioning process have been completed. These preparatory steps include uncoupling the top closure head 22 from the main body portion 20 and draining the fluid from the reactor chamber 13 through the appropriate nozzle(s) 26. Additionally, the fuel core 14 is removed in any suitable manner, and probably in the same manner as that used during annual replacements. Still further, the plant lines 28 are severed from their respective nozzles 26 and metal plates 30 are welded over the severed open ends of the nozzles 26. Referring now to FIGS. 3-8, a method of decommissioning the nuclear reactor 10 according to the present invention is shown. This method includes the steps of encapsulating portions of the vessel 12 and reactor internals 18 into a solid reactor capsule 50 and then converting this reactor capsule 50 into a plurality of decommissioned segments 100. The conversion step of this decommissioning process includes a cutting stage in which the reactor capsule 50 is cut into transportable-size segments and an encasing stage in which the transportable-size segments are encased to form the decommissioned segments. In some situations, the cutting stage will comprise a primary "capsule-cutting" procedures in which the reactor capsule 50 is cut, preferably sequentially, into a series of sections 50A-50L and a secondary cutting procedures in which each of these sections is further cut into a plurality of transportable-size segments. It should be noted that "cut" in this context corresponds to any sectioning, segmenting, or dividing process in which a component is converted into a plurality of pieces. Decommissioning the nuclear reactor 10 according to the present invention provides several advantages over prior art decommissioning processes. For example, the cutting of the vessel 12 and the reactor internals 18 occurs substantially simultaneously thereby reducing the decommissioning time and cost. In fact, the preferred method provides for removal of both the reactor vessel 12 and the reactor internals 18 for essentially the same cost as individually removing either of these components. Additionally, during the cutting steps, and during all subsequent steps of the decommissioning process, the reactor internals 18 are embedded in a material which functions as a radioactive shield whereby worker interaction and environmental exposure to contaminated components is minimized when compared to prior art procedures involving the cutting of isolated reactor internals. Still further, "in-house" lifting equipment may be used instead of specially fabricated units which will usually substantially reduce the overall cost of the decommissioning project. The encasing step of the method will eliminate the need for shipping casks in many situations thereby reducing total disposal cost. These advantages will become more apparent in the following detailed discussions of the stages of the preferred decommissioning process. i. Encapsulating Stage In the "encapsulating" stage of the decommissioning process, the relevant portions of the vessel 12 and the reactor internals 18 are encapsulated to create the solid reactor capsule 50 shown in FIG. 3. "Encapsulate" in this context corresponds to converting the relevant portions of the nuclear reactor 10 into a solid, substantially integral mass. In the illustrated and preferred embodiment, the relevant portions of the vessel 12 include substantially all of the main body portion 20 and all of the reactor internals 18. The encapsulating stage of the process preferably includes a matrix-forming step in which a matrix 51 is formed within the relevant portions of the reactor chamber 13. This matrix 51 integrally attaches to the vessel 12 and integrally embeds the reactor internals 18 to create the solid reactor capsule 50. When the capsule 50 is created in this manner, it will have an outer shell 52 which substantially encases a solid center 54. The shell 52 is formed from the main body portion 20 of the vessel 12 and the center 54 is formed from the matrix 51 and the reactor internals 18 embedded therein. The matrix 51 is preferably formed by providing a fluidized matrix-creating material which may be predictably solidified and which functions as a radioactive shield in its solid state. The matrix-creating material is introduced into the chamber 13 by pouring it through the open top of the main body portion 20 or, alternatively, by pumping it through one or more of the nozzles 26. The introduced matrix-creating material is then solidified in such a manner that it integrally attaches to the vessel 12 and integrally embeds the reactor internals 18. (In some instances the integral attachment to the vessel may be accomplished simply by the "tight fit" of the matrix material within the vessel). Concrete is the illustrated and preferred matrix-creating material because it performs well as a matrix, it is compatible with the subsequent cutting steps and, perhaps most importantly, it functions quite effectively as a radioactive shield. When concrete is used as the matrix-creating material, the "solidifying" step entails simply waiting for the concrete to cure. The relevant portions of the nuclear reactor 10, such as the main body portion 20 of the vessel 12 and the reactor internals 18, preferably remain in their operating orientation and operating location during the encapsulating stage. For these reasons, applicants believe that in most, if not all, nuclear settings, the support system of the nuclear reactor will be sufficient to securely hold the capsule 50 in this position. In this regard, it is interesting to note that reactor capsule 50, although carrying the additional burden of the matrix 51, will be relieved of the weight of the reactor core and the circulating fluid sustained by the reactor 10 during on-line operation. Consequently, the weight differential between the on-line nuclear reactor 10 and the reactor capsule 50 will be substantially less than the weight of the matrix 51. Moreover, nuclear regulatory codes consistently require that reactors be supported in a manner which is capable of withstanding pressures substantially exceeding operating parameters and seismic loading. In any event, the encapsulating step of the decommissioning process creates a solid reactor capsule 50 having a shell 52, which is formed from a portion of the vessel 12, and a solid center 54 which is formed from the radioactive shielding matrix 51 and the reactor internals 18 embedded therein. ii. Capsule-Cutting Stage After completion of the encapsulating stage of the decommissioning process, the reactor capsule 50 is cut into sections 50A-50L as is shown schematically in FIG. 4. It is important to note that during this cutting step, and during all subsequent steps of the decommissioning process, the reactor internals 18 are encased in the matrix 51 which functions as a radioactive shield in its solid state. Thus, worker interaction and environmental exposure to contaminated components is minimized when compared to prior art procedures involving the cutting of isolated reactor internals. As is shown in FIG. 4, the sections are essentially "coin-shaped38 and the size, or more particularly, thickness, of these sections is preferably chosen so that they may be manipulated within the reactor building with existing equipment. Thus, in most instances, the size of the sections will be determined by the lifting capacity of a semi-permanent building crane, which in a typical nuclear power plant would probably be less than 400 tons. The use of "in-house" lifting equipment, rather than specially fabricated units, will usually serve to substantially reduce the overall cost of the decommissioning project. The preferred relative size and shape of the sections 50A-50L is illustrated schematically in FIG. 4, and, as shown, the sections 50A, 50B, 50D, 50E, 50F, 50H, 50I, 50J and 50K are formed by substantially horizontal cutting lines. In such a cutting arrangement, the shell 52 of the reactor capsule 50 (which is formed by the portions of the vessel 12) and the matrix 51 of the capsule 50 (which includes the reactor internals 18) will both be cut by a single cutting line. Thus, in contrast to prior art decommissioning methods, the cutting of the vessel 12 and the reactor internals 18 occurs substantially simultaneously. The remaining illustrated sections 50C, 50G and 50L are also formed by substantially horizontal cutting lines which create coin-shaped sections. However, the sections 50C, 50G, and 50L, which are substantially thicker than the other sections, are then cut into semi-sections by vertical cutting lines. More particularly, section 50C is cut into semi-sections 50C.sub.1 and 50C.sub.2, section 50G is cut into semi-sections 50G.sub.1 and 50G.sub.2, and section 50L is cut into semi-sections 50L.sub.1 and 50L.sub.2. The reason for the different treatment of these sections 50C, 50G and 50L, is that they each contain at least one inlet/outlet nozzle 26 and thus, a corresponding annular flange 27. The cutting of the sections 50C, 50G and 50L into semi-sections eliminates the need to cut directly through the sometimes massive flanges 27. The capsule-cutting steps are preferably accomplished with a diamond wire cutting system such as that marketed under the name "Trentec." The Trentec diamond wire system comprises a diamond matrix wire made to length for each individual cut and a hydraulic drive apparatus. The diamond wire is routed to envelope the cut area and then the wire is guided back to a drive wheel located on the hydraulic drive apparatus. The drive wheel rotates and pulls the wire through the cut area. The Trentec cutting system is particularly suited for the present method because it lends to substantially remote operation whereby worker interaction with contaminated components may be minimized. Additionally, the dust and/or particles created by the Trentec cutting system are practically nonexistent when compared to other conventionally used cutting techniques. Consequently, HEPA ventilation and the need to wear respirators during cutting operations is eliminated because airborne contamination will not be generated. Furthermore, because the equipment may be lubricated solely with a small volume of cooling water, any liquid radioactive waste may be minimized by recirculating the cooling water through settling drums designed to collect any radioactive debris. Still further, the illustrated cutting arrangement coordinates very efficiently with the Trentec cutting technique. More particularly, the cutting wire may be positioned at a predetermined location or level on the capsule 50 and appropriately pulled to cut the capsule 50 and form the first section 50A. Thereafter, the cutting wire may be moved downwardly by a pulley-system to the next "cutting" level to perform a subsequent cut and form the second section 50B. The same pulleys may be rearranged and used for the vertical cuts on sections 50C, 50G and 50L. When concrete is used as the matrix-creating material, the material make-up of the reactor capsule 50 will be extremely compatible with the Trentec cutting system which is specifically designed for the removal of concrete. It may be additionally noted that in the "water platform" decommissioning method, the Trentec diamond cutting wire was not used to cut the submerged reactor internals because their mounting allowed them to vibrate, or "wobble", during the cutting process. Consequently, the benefits of the Trentec cutting system, such as low airborne contamination and uncomplicated lubrication, could not be enjoyed in the past. However, in the present invention, the embedding of the reactor internals 18 in the matrix material eliminates any vibration whereby the advantages of the Trentec cutting system may be realized. Moreover, a concrete matrix appears to additionally facilitate the sawing process because it provides a substantially uniform density across the cutting line and extremely compatible surface qualities. In the preferred method of decommissioning the nuclear reactor 10, the primary-cutting stage of the process will begin with the capsule 50 being cut to create the top, or first, section 50A. This first section 50A is then transferred to a location away from the direct locality of the remaining portion of the capsule, such as an appropriate secondary cutting station within the reactor building. Upon transfer of the first section 50A, the remaining portion of the reactor capsule 50 is cut to form the second section 50B, and this section is then transferred away from the direct locality of the now remaining portion of the capsule 50. This sequence is repeated until all of the sections 50A-50L have been created and transferred. The transfer of the sections/semi-sections 50A-50H will preferably be performed with existing equipment, such as the building crane discussed above. As such, the sections 50A-50L will have to be rigged to accommodate the crane, or supplied with lifting lugs which may be coupled to the crane. Although the lugs could possibly be attached after a particular section has been cut from the reactor capsule 50, they are preferably attached prior to this cutting step after the location of the cutting lines has been determined. More particularly, the lugs may be welded to the capsule shell 52 prior to the capsule-cutting and/or they may be welded to the vessel 12 prior to the encapsulating step. A representative section, namely the second section 50B, is illustrated in detail in FIG. 5 and, as shown, the section 50B includes an outer annular ring 52B, or more generally a parametrial frame, having an integral filling 54B. The annular ring 52B is formed from a slice of the capsule shell 52, and thus was originally part of the reactor vessel 12. The integral filling 54B is formed from a slab of the capsule center 54, and thus includes a layer of the radioactive shielding matrix 51 and pieces 18B of reactor internals embedded therein. It may be noted for future reference that the exposed surfaces of the filling 54B, namely the axial end faces 56B, are substantially planar. iii. Secondary-Cutting Stage As was indicated above, the size and shape of the sections/semi-sections 50A-50L will usually be chosen to maximize the efficiency of the primary lifting equipment, i.e., the building crane. However, this particular geometry may substantially exceed the parameters necessary to safely transport the decommissioned reactor components to a disposal site. For example, if the lifting capacity of the utilized crane is 120 tons, this will be reflected in the cutting of the reactor capsule 50 whereby each section/semi-section will preferably weigh approximately 120 tons. At the same time, shipping requirements could very well dictate that the transported pieces weigh a maximum of approximately 30 tons. Additionally, a sometimes related, but often independent, consideration is the sizing of the section to be transported relative to the available access openings in the reactor building, the loading dock area and/or the transporting vehicle. For this reason, the decommissioning method may include secondary-cutting steps in which the sections/semi-sections 50A-50L are further cut into transportable segments. For the purposes of this discussion, it is assumed that the representative section 50B is too heavy/wide for transportation purposes and that segments of approximately a quarter of this weight/width would be transportable. Accordingly, as is shown in FIG. 6, the section 50B is cut, preferably equally, into four segments 50B.sub.1, 50B.sub.2, 50B.sub.3, and 50B.sub.4 at an appropriate cutting station. The appropriate cutting station for the section 50B, as well as sections 50A, 50C-50G and 50L would probably be a dry cutting station. The appropriate cutting station for sections 50H-50K, which correspond to the fuel-containing area of the nuclear reactor 10, would probably be a wet cutting station. In either event, the secondary cutting is preferably accomplished by the Trentec cutting technique due to its low air/water contamination and undemanding lubrication needs. The geometry of the segment 50B.sub.1 is probably best described as being shaped like a "pie-piece." The segment 50B.sub.1 includes a 90.degree. ringlet 52B.sub.1 and a quadrant 54B.sub.1 projecting therefrom, or, in more general terms, a frame fragment 52B.sub.1 and an integral extension 54B.sub.1. The ringlet 52B.sub.1 is formed from a piece of the annular ring 52B of the segment 50B, and thus is formed from the reactor vessel 12. The quadrant 54B.sub.1 is formed by cutting a quarter piece of the integral filling 54B and thus is formed from the radioactive matrix 51 and the reactor internal pieces 18B.sub.1 embedded therein. The ringlet 52B.sub.1 surrounds and covers the curved circumferential surface of the quadrant 54B.sub.1 while its axial end faces 56B.sub.1, as well as its radial side faces 58B.sub.1, are exposed. These exposed surfaces 56B.sub.1 and 58B.sub.1 are all substantially flat or planar, and each is bordered by edges of the ringlet 52B.sub.1. The remaining segments 50B.sub.2, 50B.sub.3, and 50B.sub.4 will have essentially identical characteristics. In the illustrated example, the size and shape of the segments 50B.sub.1, 50B.sub.2, 50B.sub.3, and 50B.sub.4 is assumed to meet transporting requirements whereby no further cutting is necessary. However, in the event that a further reduction in size/weight was necessary, the segments would be further cut until an appropriate geometry was achieved. The secondary cutting steps are preferably performed in a similar manner on the other sections of the reactor capsule 50 so that the exposed surfaces of the respective matrix slabs are all substantially flat or planar, and each is bordered by edges of a ringlet formed from the reactor vessel 12. In other words, the further cutting would result in transportable-size segments which are shaped like a "pie-piece." iv. Encasing Stage The final stage of the conversion of the reactor capsule 50 into a plurality of decommissioned segments entails encasing the transportable-size segments. This encasing stage particularly involves covering the exposed surfaces of the integral extension 54B.sub.1 so that the transportable-size segment is fully encased, or decommissioned. The preferred encasing procedure is shown in FIGS. 7A-7C in which the transportable-size segment 52B.sub.1 is used for the purposes of explanation. The encasing procedure particularly includes providing rectangular steel sheets 60B.sub.1 dimensioned to cover the radial surfaces 58B.sub.1 of the segment 52B.sub.1. (See FIG. 7A). Each of the sheets 60B.sub.1 includes a first pair of opposite edges 62B.sub.1 and a second pair of opposite edges 64B.sub.1. Because the exposed surfaces 58B.sub.1 are flat, the sheets 60B.sub.1 may likewise be flat whereby their formation is uncomplicated. The steel sheets 60B.sub.1 are then welded to the segment 52B.sub.1 to cover the exposed surfaces 58B.sub.1. More particularly, one of the sheets 60B.sub.1 is placed over one of the surfaces 58B.sub.1 and the edge 62B.sub.1 abutting the ringlet 52B.sub.1 is welded thereto, and the other sheet 60B.sub.1 is placed over the other surface 58B.sub.1 and its edge 62B.sub.1 abutting the ringlet 52B.sub.1 is welded thereto. In this arrangement, the opposite edges 62B.sub.1 of each of the rectangular sheets will meet at a corner and are welded to each other at this corner. (See FIG. 7B). The encasing procedure further includes providing steel sheets 66B.sub.1 which are shaped and sized to cover the axial end faces 56B.sub.1 of the segment 52B.sub.1. (Again, because the exposed surfaces 56B.sub.1 are flat, the sheets 66B.sub.1 may likewise be flat whereby their formation is uncomplicated.) In the illustrated embodiment, this shape will be in the form of a triangle having two equal sides 68B.sub.1 and a rounded base 69B.sub.1. One sheet 66B.sub.1 is placed over each of the faces 56B.sub.1, and its sides 68B.sub.1 are welded to the abutting edges 64B.sub.1 of the rectangular sheets 60B.sub.1. Additionally, the rounded bases 69B.sub.1 of the sheets 66B.sub.1 will be welded to the abutting edges of the ringlet 52B.sub.1. In this manner, the integral extension 54B.sub.1, and thus the pieces 18B.sub.1 of the reactor internals embedded therein, are totally encased to form the decommissioned segment 100 shown in FIG. 8. If the preferred cutting steps are used, the encasing procedure will be conceptually the same regardless of the sectioning needed to create transportable segments. For example, if further cutting of the segment 52B.sub.1 is necessary to meet transporting requirements, the encasing procedure for the resulting "pie-piece" segments would be essentially identical to that described above, except that the dimensions of the base side 69B.sub.1 of the top/bottom sheets 66B.sub.1 would be appropriately reduced. (The dimensions of the sides 68B.sub.1 of the sheets 66B.sub.1 and the overall shape and size of the rectangular sheets 60B.sub.1 would remain the same). Likewise, if "semi-sections" of the section 50B were of an appropriate transportable size, a single rectangular sheet (which is approximately twice as long as each of the rectangular sheets 60B.sub.1) and two semi-circle sheets would be used in the encasing procedure. Additionally, in the event that section 50B was of a transportable size (and thus itself constituted a transportable-size segment), the encasing procedure would entail placing circular sheets over the exposed faces 56B and the circumferential edges of these circular sheets would be welded to the abutting edges of the ring 52B. Thus, regardless of the size and shape of the transportable segment, the encasing procedure will produce a decommissioned segment 100 having a casing 102 which totally encases a solid interior chamber 104. The casing 102 will include at least one wall 52B.sub.1 which is formed from a portion of the vessel 12 and the interior chamber 104 will include a chunk of the matrix 51 which is integrally attached to the wall 52B.sub.1 and which integrally embeds pieces of the reactor internals 18 therein. This encasing step maintains the intactness of the segment and/or provides radiation shielding of the reactor internals contained therein. In many situations, this encasing in combination with the matrix 51 will eliminate the need for shipping casks thereby reducing total disposal cost. v. Closing One may now appreciate that the present invention provides a method of decommissioning a nuclear reactor 10 which may significantly reduce man-rem exposure and may substantially decrease the time and capital expenditure of a decommissioning project. The cutting of the vessel 12 and the reactor internals 18 occurs substantially simultaneously thereby reducing the decommissioning time and cost. Additionally, during the cutting steps, and during all subsequent steps of the decommissioning process, the reactor internals 18 are embedded in the radioactive shield matrix 51 whereby exposure to contaminated components is minimized. Still further, the method allows the use of "in-house" lifting equipment and eliminates the need for separate shipping casks in many situations. Although the invention has been shown and described with respect to a certain preferred embodiment, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. For example, the method could be used for disassembling any nuclear device, (such as a steam generator used in conjunction with a pressurized water reactor) which includes a receptacle defining a cavity and radiation-exposed internal components positioned within the cavity. The present invention includes all such equivalent alterations and modifications and is limited only by the scope of the following claims.