Patent Number: 054935903
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT The increase in CCP can be achieved by raising the CHF of the fuel bundle in such a way that it will not cause a significant increase in hydraulic resistance in the fuel channels. FIG. 2A schematically shows a coolant flow in a fuel channel defined by pressure tube 24 which contains fuel bundles 20. Curve I in FIG. 2B represents schematically the variation in fuel channel flow with channel power, based on the hydraulic characteristics of the fuel channel. For a given channel inlet temperature T.sub.IN and channel outlet pressure P.sub.OUT as shown in FIG. 2A, the variation of CHF with channel flow is shown by Curve II, so that CHF occurs at condition A. As can be seen in FIG. 2B, an increase in channel hydraulic resistance will increase the coolant pressure-drop and thus causes the flow to decrease (Curve IlI). This change in the fuel channel's hydraulic characteristics may shift the occurrence of CHF in the fuel channel from condition A (on Curve I) to condition B (on Curve III). The combined effect of the increase in CHF (a shift from Curve II to Curve IV) and the increase in hydraulic resistance may bring the CHF to occur at condition C. The difference between the fuel channel power corresponding to condition C and that corresponding to condition A is the net gain in CCP. If, however, a different method of producing the same increase in CHF were used that produced a smaller increase in hydraulic resistance (Curve V), then CHF may occur at condition D, resulting in a much increased net gain in CCP. In a critical power enhancement system for a pressurized fuel channel type nuclear reactor in accordance with the present invention, certain fuel elements are each provided with at least one CHF enhancement appendage, or "button", at predetermined strategic locations along the length of the fuel elements. The CHF enhancement appendages project into a subchannel, which is a coolant flow space surrounded by fuel elements, and generates turbulence in the coolant flowing within the subchannel downstream of the locations of the appendages. The provision of the CHF enhancement appendages according to the present invention increases a turbulence level of the coolant at locations within the fuel bundle where CHF is most likely to occur without significantly increasing the hydraulic resistance in the fuel channel. For the flow conditions corresponding to the liquid depletion type of CHF, a large fraction of the coolant is in the form of vapour, with a large number of liquid droplets flows dispersed in it. A thin layer of the liquid exists on the outer surface of the fuel elements. The liquid removes heat from a fuel element surface much more effectively than vapour. The droplets in the vapour flow are continuously being deposited onto the liquid film, hence increasing the thickness of the liquid film. On the other hand, the liquid film is continuously being sheared off by turbulent force (this is called entrainment) and liquid is continuously being evaporated from its surface by the heat from the hot surface. CHF occurs when there is a depletion of the liquid film. The CHF enhancement appendages suppress the occurrence of the liquid depletion type of CHF as the presence of the appendages increases the turbulence level in the coolant flow which, in turn, increases the heat transfer rate. This, in general, lowers the fuel element surface temperature for a given fuel bundle power. Therefore, dryout of the liquid film on the fuel element surface will occur at a higher power level, hence resulting in a higher value of CHF. Furthermore, the increase in turbulence level increases the rate of entrainment as well as increase the rate of droplet deposition. These two mechanisms have opposite effects in sustaining the liquid film thickness, and hence in suppressing CHF. For CHF of the DNB type, the presence of the CHF enhancement appendages increases turbulence, which causes partial or complete destruction of the insulating bubbly layer adjacent to the hot fuel element surface, thus suppressing CHF at these conditions. Each of the appendages is attached only to individual fuel elements, without coming into any mechanical contact with other fuel elements. Unlike the spacers or bearing-pads, these appendages are divorced from the functions of spacing the fuel elements or providing load beating to the fuel elements. Accordingly, the present critical power enhancement system does not alter the mechanical structure of the fuel bundle. When compared with the attachment of spacers or bearing-pads which are subject to mechanical interactions, this allows significant cost savings, and avoids the undesirable "braze heat-affected zones" that are associated with the brazing process required for the attachment of appendages subject to mechanical interactions such as the spacers and bearing pads. The actual location of the CHF enhancement appendages is determined according to predicted locations of the occurrence of potential CHF within the fuel bundle. The actual shape and size of the CHF enhancement appendages are determined according to predicted heat transfer and flow conditions of the coolant along the fuel bundle prior to the occurrence of the potential CHF. In turn, the location of the CHF and the heat transfer and flow conditions of the coolant prior to the occurrence of a CHF are predetermined for a specific bundle design, actual power distribution within the fuel bundle that will be appropriate for the intended use of the fuel bundle, and reactor conditions. The CHF enhancement appendages can be re-arranged to provide an optimum CCP for different fuel bundle designs, different power distribution within the fuel bundle, or specific reactor operational requirements. Thus, the increase in hydraulic resistance in the fuel channel can be systematically minimized, resulting in the maximum gain in the critical power, as described above referring to FIG. 2B. Thus, the present critical power enhancement system is sufficiently flexible that it can be applied to any fuel bundle of the pressurized fuel channel type reactors. Specifically, the heat transfer and flow conditions of the coolant prior to a CHF within a specific design of fuel bundle, and the location of the CHF in the fuel bundle are predicted through experimental and computer code simulations of a particular use of the specific fuel bundle in a specific reactor. Based on these predictions, a critical power enhancement system suitable for these specific requirements is defined which consists of specifications on the shape, size and locations of CHF enhancement appendages within each of the fuel bundles of this specific design. The specification for this particular critical power enhancement system is then further optimized through experimental testing, where the increase in hydraulic resistance is minimized without significantly jeopardizing its CHF enhancement performance. In order to minimize the increase in hydraulic resistance in the fuel channel due to attachment of the appendages, each appendage has a cylindrical shape or a streamlined shape. The cross-sectional area of each of the appendage at the bottom, where the appendage is attached to the fuel element, is preferably in a range of 3 mm.sup.2 to 11 mm.sup.2, and the height of each of the appendage from the bottom is preferably in a range of 0.6 mm to 2.3 mm. The number of CHF enhancement appendages provided in each subchannel, i.e., flow space surrounded by fuels bundles, is preferably one to four, and is a function of the subchannel flow area and the size of the appendages. When more than two CHF enhancement appendages are provided on each of certain fuel elements at axially symmetrical locations in light of the middle of the length of the fuel bundle, the fuel bundles may be advantageously inserted into a pressure tube in either direction. The distance between the appendages and the nearest end of the fuel elements is preferably in a range of 5 to 20 cm when two appendages are attached to each fuel element. Each CHF enhancement appendage must be attached to the surface of the fuel elements such that it does not come off during the operation of the reactor. The manner of attaching the CHF appendages to the fuel elements is within scope of techniques of those to whom the present invention is addressed. The CHF enhancement appendages may be made of the same material as the fuel elements. FIGS. 3A and 3D show an embodiment of attachment of the appendages to a specific CANDU.TM. fuel bundle, the CANFLEX.TM. Mk. 4 Bundle with natural uranium, to be used in Wolsung Reactor No.1. The CANFLEX.TM. fuel bundle 20 is comprised with 43 fuel elements 22 which are separated from each other by spacers 32. Outer elements 36 (101-121 in FIG. 3D) are further provided with bearing pads 34. The fuel elements of a CANFLEX.TM. Bundle are of two sizes, that is, the centre fuel element 143 and all seven fuel elements 136-142 of the most inner ring adjacent to the centre fuel element 143 have an outside diameter of 13.5 mm, and the remaining fuel elements 101-135, which consists of 21 fuel elements 101-121 in the outer most ring of the fuel bundle and 14 fuel elements 122-135 in the middle ring between the inner ring and the outer ring, have an outside diameter of 11.5 mm. The location of the spacers 32 and bearing pads 34 for CANFLEX.TM. Mk. 4 Bundle is as shown in FIG. 3B and 3C. In this embodiment, 182 CHF enhancement appendages 30 of cylindrical shape are divided equally and provided on two bundle axial locations, each of which locations has a distance of 122.3 mm from each end of the fuel bundle 20, as shown in FIG. 3A. Each of the CHF enhancement appendages 30 is attached to a specific circumferential location on the outer surface of fuel elements and projects toward the centre of a subchannel being defined as the coolant flow space surrounded by neighbouring fuel elements without including an inter-fuel element gap wherein an inter-fuel element gap is closest distance between two neighbouring fuel elements. The arrangement of the attachment of the CHF enhancement appendages 30 at each bundle axial location is identical and can be described in the following. As shown in FIG. 3D, at each bundle axial location, there are two CHF enhancement appendage 30 projecting into each of the six subchannels 40 surrounding the centre fuel element 143, one from the centre fuel element 143 and the other from one of the two inner ring fuel elements 136-142. There are two types of subchannels between the inner ring and the middle ring. The large subchannel 42 is each provided with two CHF enhancement appendages 30, one from each of the large fuel elements 136-142 of the inner ring, and the other from each of the small fuel elements 123, 125, 127, 129, 131, 133, 135 of the middle ring directly opposite to the large fuel elements 136-142. The small subchannel 44 between the inner ring and the middle ring is each provided with one CHF enhancement appendage 30 attached to each of the large fuel elements 136-142 of the inner ring. There are three types of subchannels between the middle ring and the outer ring. The largest subchannel 46 opposite from the large subchannel 42 between the inner ring and the middle ring is each provided with two CHF enhancement appendages 30 each attached to each of the two of the middle ring fuel elements 122-135. One of the small subchannels 48 between the middle ring and the outer ring, which is opposite to the small subchannel 44 between the inner ring and the middle ring, is each provided with one CHF enhancement appendage 30 attached to one of the outer ring fuel elements 101-121. In each of the remaining small subchannels 50 between the middle ring and the outer ring, there is also only one CHF enhancement appendage 30 provided, and each is attached to the middle ring fuel elements 122-135. Finally, in each of the subchannels 52 between the outer ring and the pressure tube inner surface (not shown) surrounding the fuel bundle 20, there is one CHF enhancement appendage 30 provided which is attached to each of the outer ring fuel elements 101-121. As shown in FIGS. 4A and 4B, each appendage 30 has a cylindrical shape and is attached to each fuel element by one end of the cylinder. The diameter of the end of the cylinder is 2.5 mm and the height is 1.5 min. It will be understood that various modifications may be made to the invention without departing from the spirit of the present invention.