Patent Number: 
Section: description

FIG. 2 shows a preferred embodiment of a canister for the containment of a radioactive item according to the invention. Canister 101 comprises substantially tubular body 124, interior 128, a first, preferably closed, end 125, and second end 140. Interior 128 is adapted by size and geometric configuration to receive a radioactive item such as an nuclear reactor pressure vessel for storage, transportation, and disposal. Preferably interior 128 is large enough for such radioactive item to be disposed within the canister without difficulty, and to allow the addition of a stabilizer, as described herein, yet simultaneously as small as practicable, in order to reduce the size and weight of the completed package and to ease transportation and handling. Thus length width 134 and length 135 of the canister and in particular the canister interior are large enough to accommodate the item to be contained, but not larger than necessary for the purposes described herein. The selection of suitable dimensions and geometry will not trouble the designer of ordinary skill once he or she has been made familiar with disclosure. For most RPVs substantially circular cross sections will serve satisfactorily, and facilitate easy and inexpensive fabrication and employment of the canister. Body 124 of canister 101 further comprises integral sacrificial fenders 126 located adjacent each end of the canister. Sacrificial fenders 126 are comprised of extensions of body 124 of the canister beyond the end caps or closures of the canister, that is, beyond those portions of the canister actually used for containing the radioactive item, and are adapted to absorb or dissipate shocks administered to the completed containment package by deforming under contact loads. The mechanics of such fenders and their role in attenuating or absorbing shocks are well understood and will be plain to those of skill in the art, given the disclosure herein. The canister 101 shown in FIG. 2 further comprises lid or other closure means 108, which is adapted for attachment of a portion of the item to be contained. Lid or closure 108 can comprise a simple end plate, as shown, or might take the form of a cap-type enclosure sized to encompass the entire end of the canister, or any other suitable means for closing the canister. In the embodiment depicted in FIG. 2 closure 108 is adapted for the attachment of a portion of the contained device by means of holes 141, which are sized and positioned to accept attachment fittings present on the item to be contained, for example, the head attachment posts in a PWR pressure vessel. Preferably any portions of items to be attached to the exterior of the canister are not highly radioactive, or are sealable in their own right. Lid or top plate 108 further comprises optional fill and vent ports 160 and 161. Central ports 160 are provided for optional filling of the interior of the RPV body with grout or other sealant; peripheral ports 161 for filling the gap between the canister interior and the RPV exterior. Canister 101 in FIG. 2 further comprises optional secondary circumferential shield 130. Secondary shields are advantageously employed to provide additional containment of relatively highly radioactive portions of any contained items, such as some of the internal structures in a PWR pressure vessel. Preferably circumferential shields are employed in conjunction with cap or lid-type shields such as shields 122 and 142 shown in FIG. 4. A particular advantage of using substantially cylindrical canisters of the type shown in FIG. 2 is that secondary shields are relatively simple to fabricate and install, and provide substantial structural reinforcement as well. In the case of circumferential shields, open-ended cylinders of nearly the same size as the canister body may be employed, and may be disposed around the inner or outer surfaces of the canister, at any axial position along the canister that may be desired. Cap or lid type shields may be fabricated from flat plate material merely by trimming them to size, and may be placed at any axial location within the canister or covering one or both ends of the canister. In either case it is often suitable, as will be understood by those having ordinary familiarity with the art of radiation shielding, that the same materials as those employed in fabricating the canister body may be used in fabricating secondary shield structures, with substantial savings in cost. Cap or lid shields are particularly useful for providing ALARA (As Low As Reasonably Achievable) shields during stacking of internal parts, external fittings, and/or insulation inside an RPV as described herein, as an added shield against radiation for workers. Canisters or containment vessels according to the invention and as shown in FIG. 2 are substantially easier and less expensive to fabricate than prior art containment vessels. This is in large part due to their simplified construction, as described. They are also economical to use, especially during the containment and removal of decommissioned RPV""s, because they may easily be separated into sections, moved into place for containment of the RPV or other item, and reassembled easily. For example, the containment canister shown in FIG. 2 may be cut anywhere along the length of its body into two or more sections merely by cutting the container, as for example by means of any conventional metal cutting methods, along a circumference such as that shown by reference numeral 138 in FIG. 2, the location of which may be varied anywhere along the body of the canister, such that two sections 136 and 137 result. In such cases reassembly is accomplished merely by replacing second section 137 back in place adjacent to first section 136 and reattaching, as for example by welding. The use of secondary radiation shield 130 also facilitates the use of the canister in this fashion, as it can be used as a doubler or structural reinforcement as well as an additional radiation shield. Canister 101 may be fabricated economically and easily by rolling or otherwise forming tubular body 124 by conventional means from sheet or plate metal, and welding or otherwise attaching a bottom plate at closed end 125 and lid or closure plate 108. Integral fenders 126 are easily formed in such processes by placing the end closures at a suitable distance from the ends of the body structure, leaving the fenders protruding or extending from the body. Provision of optional fillet 143, which is also readily formed by rolling or other conventional means, is particularly beneficial, as it permits provision of integral fenders 126 as described, in such fashion that fenders 126 are able to perform their function with full efficiency, while optionally permitting the canister weight and the weight of any of its contents to be transferred directly to the floor or other surface on which canister 101 and any contents are placed, without passing through and possibly harming the fenders or reducing their capacity to absorb or dissipate shocks. Optionally fenders 126 are of sufficient depth to allow them to provide protection not only to the containment package as a whole, and in particular to the packaged radioactive item, but also to any additional items, such as removed reactor vessel pressure head, which may be attached to the exterior of the canister. An early part of the process for containing a radioactive item according to the invention is preparing the radioactive item for packing. Generally this comprises removing at least one external fitting from the item, and optionally portions of the internals of the item as well. A process of preparing the item for packing is shown in FIGS. 3 and 4. FIG. 3 is a cutaway schematic elevation of an intact radioactive item, specifically a PWR pressure vessel, prior to being processed for containment according to the invention. Reactor pressure vessel (RPV) 102 comprises body 114 and head 115, internals 117, and a number of external fittings 103, including water nozzles 144 and control structures 145. Head 115 is joined to body 114 at flange 146 by means of attachments 132, and insulation 116 is in place around exterior 105 of the RPV. Internals 117 comprise upper internals 147 and lower internals 148. In FIG. 4 the RPV of FIG. 3 has been at least partially processed for containment according to the invention. External fittings 103 have been trimmed so that non-removed portions 104 of the fittings are substantially flush with external surface 105 of body 114. In particular, non-removed portions 104 of the external fittings do not protrude past the outer circumference of flange 146. Moreover, internals portions 148 (FIG. 3), including the central portion of the core barrel and the core baffle assembly, have been removed from interior 109 of the RPV, and upper internals 147 have been stacked on top of support plate 142, which preferably serves also as a secondary radiation shield. Portions 149 of core barrel 151 (FIG. 3) are also disposed within the RPV. Additional portions 119 of the upper internals and insulation 116 removed from the exterior of the RPV are also placed in otherwise vacant space within the RPV body. A secondary support structure and ALARA shield 122 is placed atop internals 117, 147, and removed portions of external fittings 103 are placed thereupon. Additional insulation 116 taken from the exterior of the RPV body is placed atop fittings 103. Penetrations 120 of RPV body 114 have been stopped by means of plugs 121. As described herein, it is beneficial during at least the first portions of this process to leave the reactor coolant fluid or other liquid in the RPV, to serve as a radiation shield for those working on the containment process. Preferably this is accomplished by leaving water or other liquid in the RPV body to at least the level of 123 in FIG. 4 until the more highly-radioactive components of the core have been removed and all penetrations 120, for example, have been plugged; at this point it is advantageous to drain the fluid, install secondary supports and shield structures such as 142 and 122, and proceed with packaging. FIG. 5 is a schematic view of a radioactive item being disposed within a canister in accordance with the invention. Presented is one scenario of placing RPV 102 within canister 101. This scenario can be altered to accommodate plant specific and unique conditions. Internal shields 130 are welded to lower section 136 and upper section 137 of the canister during manufacturing. Lower section 136 of canister 101 has been placed atop transfer cart 160 with a removal frame and trunnions 139. Transfer cart 160 with lower section 136 has initially been placed in position 161 near RPV installation point 152. Head 115 of RPV 102 has been removed and lifting device 163 attached to RPV body 114. RPV 102 has been disconnected from the remainder of the plant of which it formed part by disconnecting external fittings 103, including piping 144, and penetrations 120 in RPV body 114 have been sealed. RPV external surfaces 105 are sealed with paint or other suitable substance to immobilize surface contaminants. RPV body 114 has been removed from its prior position 114xe2x80x2 in RPV installation 152 and raised, whereupon transfer cart 160 with lower section 136 has been moved into loading position 169, and RPV body 114 has been disposed within section 136 of the canister. Canister 101 and RPV body 114 are ready for a stabilizer to be introduced in gap 106 between the RPV body 114 and the interior wall of canister 101. After gap 106 has been filled to a level sufficient to allow the stabilizer to support RPV body 114 and the stabilizer allowed time to set sufficiently, or RPV body 114 otherwise sufficiently supported, upper section 137 of canister 101 is placed over the RPV body and reattached to lower section 136 by welding or other suitable means. RPV studs 185 are installed through top plate 108 of upper section 137 into the flange of RPV body 114. Spray metalizing is used to seal openings between RPV studs 185 and attachment penetrations 141 (FIG. 2) through upper section 137. Head 115 is placed atop external surface 127 of the canister, and fixed thereto, preferably by means of head attachments 132 (FIG. 3) threaded onto RPV studs 185. If necessary, the completed containment package will be turned about its upright longitudinal axis by pivoting the package with trunnions 139 on the removable frame on lower section 136. The package may then be removed from the plant housing and made ready for shipment by removing the frame with trunnions 139. It may be seen that division of canister 101 into two or more sections provides a number of benefits, such as a reduction in clearance height requirements to place RPV body 102 within the canister. This is especially beneficial in the limited workspaces of most nuclear plant installations. FIG. 6 is a cutaway schematic view of a radioactive item packaged in accordance with the invention. In addition to elements shown in other Figures, stabilizer 107 is shown substantially filling gap 106 between canister 101 and RPV 102. RPV head 115 is in place atop lid 108 of the canister, and attached by means of RPV head-body attachments 132, which, together with stabilizer 107, further comprise the sole attachment between the RPV body 114 and the containment canister. Optionally the entire interior of the RPV body is filled with stabilizer 107, to further immobilize contaminants and stored components. A containment package for a PWR pressure vessel is described. This example corresponds to plans for disposal of the Connecticut Yankee PWR. A containment canister according to the invention and as shown generally in FIG. 2, including top and bottom plates and fillet, is fabricated from three-inch thick structural carbon steel. Secondary shielding of two-inch thick carbon steel is placed within the canister body so as to shield the most highly radioactive portions of the completed package. The canister is fabricated in two sections, with the weld seam located behind the secondary shielding on rejoining. Rejoining is accomplished by full penetration weld. The canister, including integral fenders, 35xe2x80x2 3xe2x80x3 feet in length, 17xe2x80x2 10xe2x80x3 diameter, and weighs 190 tons empty. The completed package, with stabilizing grout and externally-attached RPV head, weighs 800 tons. The height of the package, with head attached, is 39xe2x80x2 7xe2x80x3. The head and RPV body are attached to each other, and to the canister, by means of the approximately twelve (12) head closure studs present on the reactor in service, which pass through canister lid and into upper flange of the RPV body. The canister provides containment shielding equivalent to DOT Industrial Package type 2, analyzed to withstand a 1 foot horizontal drop and a 1 foot drop with 2 feet of slap-down at either end. Ninety-nine point eight (99.8) percent of the radioactive material present is intrinsically contained within RPV activated metals themselves; remaining 0.2% is affixed to metal surfaces and is immobilized by grout or epoxy. A method for placing and sealing a PWR pressure within a containment vessel is described. The RPV is disconnected from external piping, controls, and the like, and the head is removed, as described and as shown generally in FIGS. 2-6. Highly radioactive portions of the internals are removed for separate containment and storage. Segmented internals, including particularly upper internal components, are placed inside the RPV body as described. Nominal 30 pcf low-density cellular concrete (LDCC) is placed inside the RPV body to seal and immobilize remaining and relocated internals. The RPV body is lifted over and lowered into position within the canister lower section, with a gap between the RPV and the canister interior. Nominal 70 pcf LDCC is poured into the gap to a sufficient depth to support the RPV after curing, and is allowed to cure. The lifting rig used to position the RPV is removed. Removed portions of the RPV nozzles are placed inside the RPV, atop an ALARA plate. RPV head closure guide studs (ref. 132 in FIG. 6) are installed in some of the RPV head attachment stud holes. The canister upper section is lifted and lowered into place over the guide studs, so that it rests upon the RPV head flange. RPV hold down studs are installed using remaining RPV head attachment stud holes. The canister upper and lower sections are welded together. Openings between the canister top plate and the RPV hold down and guide studs are sealed with metalizing spray. Nominal 70 pcf LDCC is pumped into remaining voids between the canister and the RPV body through peripheral fill ports opened in the top of the canister. Nominal 30 pcf LDCC is pumped into remaining voids inside the RPV body through center fill ports opened in the canister top plate. Fill and vent ports in the canister top plate are plugged and sealed. The RPV head is placed on top of the canister and the guide studs already in place. The guide studs are cut flush with the top of the RPV head flange. All LDCC is allowed to complete curing. The package is rigged for removal from the assembly location by the attachment of lugs and/or other structures to the canister exterior. The package is lifted and turned to a substantially horizontal position, secured to transport conveyance, and transported to a disposal site. Preferred embodiments of the various structures disclosed herein are fabricated from any materials having sufficient strength, durability, corrosion resistance, and radioactive shielding qualities to serve the purposes described for such structures. Suitable materials are known, and have been identified herein where appropriate; but any materials having suitable qualities will serve. While the invention has been described and illustrated in connection with preferred embodiments, many variations and modifications as will be evident to those skilled in this art may be made without departing from the spirit and scope of the invention, and the invention is thus not to be limited to the precise details of methodology or construction set forth above as such variations and modification are intended to be included within the scope of the invention.