Nuclear reactor reflector

Abstract: A nuclear reactor reflector is disclosed that comprises a stack (11) of reflector blocks (10.1-10.8) with vertical water flow passages to cool the reflector. The interface between blocks is opposite support points for reactor fuel rods (16). Water flows between the reflector and the reactor barrel (20) from passages (30) in a bottom block (10.1). The top block (10.8) contains a flange (10.81) to limit this flow and the flange has a slot (10.83) to receive an alignment pin (10.60) that is welded to the barrel. The pin is held in the slot by two removable shims (62). Alignment bars (80) extend the length of the stack in slots machined in each block when the stack is assembled.

Description: TECHNICAL FIELD This invention relates to nuclear reactors, in particular, to a nuclear reactor having a reflector as set out in the preamble of claim 1. Such a nuclear reactor is disclosed in EP-A-488.890 and EP-A-323.306. BACKGROUND OF THE INVENTION In a typical nuclear reactor, the reactor fuel core comprises rods mounted in modular elongated fuel assemblies located in a cylindrical core barrel that is inside a pressure vessel. In a steam generation system using one of these reactors, water is supplied to the pressure vessel, flows down an annular space between the core barrel and the pressure vessel, reverses direction, and flows upward through openings in a lower core support leading to the reactor core. Filling the space between the reactor core and the barrel, a neutron reflector is supplied with some of this water to cool it. U.S. patent 4,849,162 shows a reflector design, where a plurality of vertically standing rods extend the length of the core tightly packed together, the space between the rods providing a uniform water passage through the reflector. The diameter of the rods are selected to provide a specific area passage in order to achieve a specific heat transfer. U.S. patent 4,701,299 discusses a modular lining around the core, and there the lining consists of assemblies of prismatic shape arranged side by side inside the barrel. The resultant liner consists of modular metal elements "juxtaposed" so that the modular lining occupies virtually all the volume of the annulus around the core. Each of these techniques relies on a heavy, costly and complex reflector having a considerable number of parts, especially fasteners holding the reflector in the barrel. DISCLOSURE OF THE INVENTION Among the objects of the present invention is to provide an uncomplicated, lightweight, reliable reactor reflector. The invention in its broad form is a nuclear reactor comprising a cylindrical barrel, a reactor reflector within the barrel, a reactor core within the reflector and a core support supporting the reflector and the core, the reflector comprising a stack comprising a plurality of generally annular blocks with the core in the center, each block having water passages extending through the block in a direction normal to the core support and in alignment with passages in adjacent blocks to provide a plurality of parallel water flow paths through the stack from the core support, characterized by adjacent blocks meeting on a plane co-planar with support points for fuel rods in the core. According to the invention, a reflector is constructed of generally circular reflector blocks with an interior annulus having orthogonal walls that conform to the plan profile of the reactor core. The blocks, in effect doughnuts, are stacked one on top of the other, forming a reflector stack. The reflector stack rests on the core support. Each block contains vertical water passages, the number and size of these holes selected to provide a specific percentage of void area (e.g. 10%) in a specified region of the block. The water passages between adjacent blocks are aligned creating a plurality of parallel water passages that extends from the bottom to the top of the stack. According to one aspect of the invention, when the blocks are stacked together, there are seams between each layer of the stack formed by each block. These seams are horizontally aligned with support points between the ends of fuel rods in the core. Any water that "jets" from the seams between the blocks strikes the core at the support points not an intermediate point. According to an embodiment of the invention, the outer perimeter of all but the uppermost of these blocks has four straight sides 90 degrees apart each side, creating a space between the outer surface of the block and the inner surface of the core barrel. It is through the area between the barrel and the stack that some of the water from the core support flows upward around the outside of the reflector from passages that are located only in the bottom block, between the inside and outside of the stack. The top reflector block, however, contains a flange or rim that controls the space between the stack and the barrel and thus limits the amount of flow around the reflector. An alignment key extends through the core barrel into a seat on the flange. There are four of these alignment pins which are 90 degrees apart from each other. The alignment pin is welded into the barrel and the seat is a vertical slot in the flange, allowing the stack to be lowered into the barrel. When the stack is resting on the core support, the alignment pins are located in the slot. The alignment pins have opposed vertical flat surfaces at a distance from opposed walls in the slots. When the stack is in place, that distance is measured and shims, machined to that distance less a calculated thermal expansion, are bolted to the flange, filling the space between the pin and the flange with the pins that are on the barrel passing into the slot. The bottom block which rests on the core support, contains a seat for alignment pins which extends from the seat into a seat in the core support. The alignment pin at the core support contains water flow passages which direct water that is flowing horizontally in the core support through the alignment pin and up through and into holes or passages in the core block. The present invention provides a reflector with a minimal number of threaded fasteners, particularly in the high flow region of the reactor core. The reflector is a very reliable, easy to assemble and comparatively inexpensive design compared to state of the art reflectors. A compelling feature is that there are a minimum number of horizontal gaps, which are potential sites for flow jetting which can damage the rods and, because of the invention, any gaps are located at the point where the rods are mechanically supported. Other objects, benefits and features of the invention will be apparent to one skilled in the art from the following discussion. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, a sectional view of a reactor vessel, shows a cross-section of a reflector embodying the present invention and the reactor core within the reflector. FIG. 2 is a section along line 2-2 in FIG. 1. FIG. 3 is a plan view showing a portion of the core barrel and adjacent reflective blocks embodying the present invention. FIG. 4 is an elevation of an alignment pin embodying the present invention that is used between the reactor core support and the lowest reflector block. FIG. 5 is an elevation of an alignment pin used between adjacent intermediate reflector blocks according to the present invention. FIG. 6 is a more detailed view of the alignment blocks shown in FIG. 1 and the adjacent core barrel. FIG. 7 is a plan view showing a portion of the core barrel and alignment pin and a portion of the top reflector block. FIG. 8 is a section along line 8-8 in FIG. 7. FIG. 9 is a plan view of a portion of the interface between the lowest reflector block and the core support. FIG. 10 is a section along line 10-10 in FIG. 9. FIG. 11 is a section along line 11-11 in FIG. 9. FIG. 12 is a prospective view of the top or uppermost blocks in a reflector embodying the present invention. BEST MODE FOR CARRYING OUT THE INVENTION In FIG. 1, which shows a typical reactor vessel, there are eight generally annular reflector blocks 10.1-10.8 (see also FIG. 2 and FIG. 12) resting on a core support 12 that supports a reactor core 14 located within a central portion of the blocks. The blocks, when so stacked, define a stack 11. The reactor core, typical of the type in the state of the art, is made up of a plurality of vertically extending fuel rods 16 suspended between rod supports 18. The rods and core support and reflector blocks 10.1 - 10.8 are housed in a cylindrical core barrel 20, located in a reactor vessel 22, through which water is circulated between inlet 24 and outlet 26. The flow of cooling water follows the path shown by arrow 28, passing between the barrel 20 and the inner surface 22.2 of the reactor vessel to a point 35 below the core support where a plurality of holes or passages 12.2 are found that permit water flow up through the reactor core of rods 16 into an upper chamber 37 of the reactor vessel, where the water leaves through the outlet 26. As will be explained in more detail below, the water flow also flows vertically through aligned water passages 30 in the reflector blocks 10.1 - 10.8 but at this juncture it should be observed that water enters the stack at the interface 10.12 at the lowest block 10.1 and the core support 12. In particular, FIG. 9 shows that the lowest block 10.1 contains a plurality of holes 30 that align with holes in the adjacent block 10.2 (see FIG. 1). FIG. 9, which reveals the appearance of the bottom block as seen from the point at which it interfaces with (contacts) core support 12 shows that there is a passage 32, with an outlet 32.1 and inlet 32.2 running through the block from the area occupied by the core 14 to a space 13 between the core barrel 20 and the block 10.1. This space is more easily appreciated in FIG. 2, where it can also be observed that the water that passes from the core area 14 through the passage 32 and into the space 13 flows up along a generally flat portion of the stack. It should be observed that these passages 32 appear at four locations at block 10.1. Water flows up the side of the stack in direction X at the four locations 11.1. FIG. 9 also shows that the holes have different diameters and are dispersed with different densities within the block. The determination of the number of holes and their size is calculated to achieve a specific "void density" in particular areas of the block, e.g. 10%. As FIG. 9 also shows, there is a second passage 33; this slot also allows water to flow between the core base and the lowest block 10.1 where it reaches some of the holes 30. However, these passages 33 do not extend completely through the block but provide a water path to a cavity 32.3 machined in the bottom of the block and also connected to passage 32. The area 10.12 and 10.13, which define the passages 32 and 33 and the cavity abut directly up against the core support. As a result, the wall 10.15 fits closely to the inner wall 20.1 of the core barrel. The hole pattern may be more easily observed in FIG. 2, where the block abuts up virtually against the inner wall 20.1. To control the flow through the passage 32, the exit 32.1 of the passage has a smaller area than the inlet 32.2 (see FIG.10). FIG. 11, a section of the passage 33 shows that it simply leads to the cavity 32.3, providing access to more of the holes 30. For clarity, it is repeated the holes 30 extend linearly through each of the blocks 10.1 - 10.8 allowing the water to flow through the holes and through the stack, exiting at the top of 10.8. The passages 32 and 33 as noted before are only located on the bottom block 10.1. Also to be observed, the pattern of the passages on the straight sections and the curve sections is symmetrically repeated around each block and that the patterns being mirror images between adjacent blocks. With reference to FIGS. 3, 7 and 12 several important features of this embodiment of the invention should be considered. First, the uppermost block 10.8, although having the same hole orientation (flow path X), this block is different from the other blocks in that it contains a flange 10.82 with a vertical slot 10.83 for an alignment pin. The entire stack of blocks is held together by tie rods that are located at four points around the stack. During the construction of the reactor, the stack of blocks 10.1 - 10.8 are tied together with tie rods and lowered into the barrel. The proper location of the stack on the core support is accomplished with an alignment pin 52 which sits within the core support and which as shown in FIG. 4 contains passages 52.1. The inlet of each of these passages is aligned with a corresponding passage 32 or 33 thus allowing water to enter the passage 32 or 33 and flow through the passage 32 or 33 to an approved point which is aligned with one of the holes which is aligned with two of the holes 30 in the lowest block. As the reflector is lowered into the barrel, it is rotated so that the four slots 10.83 in the flange 10.82 each are aligned to receive an alignment pin 60 which is welded into the wall of the core barrel. As FIG. 8 shows, this pin extends into the slot 10.83 and has two flat sides 60.1 which are distance X1 from the edges of the block 10.8. When the stack of blocks is inserted in this manner, shims 62 are inserted between the pin 60 and the block 10.8 and held in place by alignment pins or dowels 64 and bolts 66. When the stack is placed on the core support, the distance X1 for each pin 60 is measured and the actual dimension X1 for each shim 62 is machined to provide a desired tolerance between the shim and pin 60 the purpose being to accommodate a certain level of thermal expansion in the core stack and in the reflector stack as it heats up during reactor operation. The alignment pins 52 (located at four equiangular positions) are located between the core support 12 and the lowest block 10.1, as mentioned previously. Following a similar design philosophy, alignment pins 70 are located between adjacent blocks, e.g., between blocks 10.8 and 10.7 also at four equiangular points, and these blocks contain passages 70.1 which allow the passage 30 in each of the blocks to be in fluid connection with each other when the two blocks are stacked together. Referring to FIG. 6, a method for aligning the blocks is shown where, once again, the stack 11 is present but at four quadrants there is an alignment bar 80 extending from the top block 10.8 to the bottom block 10.1, these being bolted to the blocks. FIG. 6 also shows the alignment pin between the core support 20 and the lowest block. During the assembly of the reflector stack 11, a rectangular slot (for the rectangular alignment bar 80) is machined into the stack when all the blocks are assembled together. The alignment bars fit tightly into the slots of each reflector block and maintains an acceptable size for the fuel core while limiting the displacement of the reflector blocks during when the stack is installed. This allows for the disassembly of the stack and its reassembly with the same alignment at the reactor construction site.

Claims: A nuclear reactor comprising a cylindrical barrel (20), a reactor reflector within the barrel, a reactor core (14) within the reflector and a core support (12) supporting the reflector and the core, the reflector comprising a stack (11) comprising a plurality of generally annular blocks (10.1 - 10.8) with the core in the center, each block having water passages (30) extending through the block in a direction (X) normal to the core support and in alignment with passages in adjacent blocks to provide a plurality of parallel water flow paths through the stack from the core support, characterized by adjacent blocks meeting on a plane co-planar with support points (18) for fuel rods (16) in the core (14). A nuclear reactor as described in claim 1, further characterized by: slots (33) in the interface of a first block (10.1) with the core support (12), said slots extending through the block to a space (13) between the stack (11) and the barrel (20) that extends to a block (10.8) at the top of the stack. A nuclear reactor as described in claim 2, further characterized by: the size and pattern of holes in the blocks establishing a selected void percentage. A nuclear reactor as described in claim 3, further characterized by: a block (10.8) at the top of the stack (11), said block having a flange (10.82) that extends from the block to a wall of the barrel (20) for substantially closing said space (13) except for a selected clearance between the flange and said wall, to allow flow to an area above the flange at a selected rate. A nuclear reactor as described in claim 4, further characterized by: an alignment pin (60) fastened to the barrel (20) and extending into a slot (10.83) in said flange (10.82); and shims (62) attached to the flange and located in diametrically opposed spaces between the pin and the flange, said shims providing a selected clearance in said spaces. A nuclear reactor as described in claim 5, further characterized in that: said slot (10.83) extends through the flange parallel to said wall of the barrel and defines two spaced apart parallel surfaces in the flange; and said alignment pin (60) has two diametrically opposed parallel surfaces (60.1), each spaced apart and parallel to one of said surfaces in the flange. A nuclear reactor according to claim 6, further characterized by: a first plurality of alignment slots that extend in a direction normal to the core support (12), said slots being spaced at equal angles from each other around a circular center of the blocks; and a first plurality of rods (80), each extending through the aligned alignment slots of adjacent blocks and anchored to blocks at the top and bottom of the stack.

Inventors: HOPKINS RONALD JACOBS, LAND JOHN THOMAS, MISVEL MICHAEL C, HOPKINS, RONALD JACOBS, LAND, JOHN THOMAS, MISVEL, MICHAEL C.
Applicants: WESTINGHOUSE ELECTRIC CORP, WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1993
Main classification: G21C11
IPC codes: G21C11, G21C5, G21C1, G21C15