Patent Number: 046408134
Section: description

DETAILED DESCRIPTION OF THE INVENTI0N In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. The fuel assembly 10 is the type used in a pressurized water reactor (PWR) and basically includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 attached to the upper ends of the guide thimbles 14. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 16 spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and the opposite ends of the rod are closed by upper and lower end plugs 26,28. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract some of the heat generated therein for the production of useful work. Improved Burnable Absorber Rod In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. The present invention relates to such means in the form of an improved soluble burnable poison or neutron absorber rod 30, as seen in FIG. 1, inserted into one of the guide thimbles 14. At least one, and preferably several of the rods 30 are stationarily supported by a spider 32 in the guide thimbles 14 of some of the fuel assemblies 10 to assist the movable control rods in the guide thimbles of other fuel assembles (not shown) in maintaining a substantially constant level of neutron flux or reactivity in the core throughout its operating cycle. Referring to FIG. 2, the improved burnable absorber rod 30 basically includes an elongated hollow tubular member 34 having upper and lower opposite ends 36,38 and a hermetically sealed chamber 40 defined within the tubular member between its ends. A neutron absorber material 42 in liquid form is contained in the sealed chamber 40 of the tubular member 34. At the upper end 36 of the tubular member 34 is means 44 providing a hydrogen getter and being disposed in communication with the sealed chamber 40 for a purpose to be described later below. At the other, lower end 38 of the tubular member 34 is means 46 providing a hydride sink and also being disposed in communication with the sealed chamber 40 for a purpose also to be described hereafter. The tubular member 34 is formed by a tubular body 48 and a pair of upper and lower end plugs 50,52, which are composed of any suitable material, but preferably a zirconium-based alloy such as Zircaloy-4. The plugs 50,52 are rigidly attached, such as by girth welds 54,56, to the opposite ends of the tubular body 48 so as to hermetically seal the same. The liquid neutron absorber material 42 contained in the chamber 40 is preferably composed of boron dissolved in water (boric acid) with the boron enriched with B-10 over and above that proportion naturally present in boron. The use of enriched boron material in the improved rod 30 addresses and resolves one of the concerns with the use of boron: its solubility limit in water at room temperature. Its solubility limit is approximately 15,000 ppm at 100 degrees F. (at 325 degrees F. the limit is 125,000 ppm). To achieve its desired function, the rod 30 would require substantially more boron loading than 15,000 ppm. For the prior art fixed type absorber rod, the loading would be about 79,000 ppm; however, the improved absorber rod 30 would require only about 64,000 ppm of natural boron (or 12,800 ppm of B-10). But this is still higher than the low temperature solubility limit. Therefore, the boron would need to be enriched in B-10 as compared to natural boron which contains about 20% B-10 (the isotope of interest). A boron concentration using 85% enriched boron would be about 15,000 ppm natural boron (12,800 ppm B-10) which would stay in solution at the lower temperature. In normal operation of the core, temperature gradients are formed along the length of the rod 30, with temperatures being cooler at the lower portion of the rod and hotter at the upper portion of the rod. The liquid absorber material 42 therefore spontaneously circulates within the sealed chamber 40 due to the presence of these gradients. Such circulation tends to self-equalize the rate of depletion of the neutron absorber material throughout the length of the chamber 40. The tubular body 48 of the rod 30 has one or more reinforcing convolutions formed therein which enhance the structural rigidity and integrity of the rod so as to enable it to better withstand both high internal and external pressures acting on it. The reinforcing convolutions can take on the shape of a recess or groove 58 formed in the body 48 so as to extend along a spiraling path between the ends of the tubular member 34. Or, alternatively, the convolutions can be a series of ring-like circular grooves 60 formed in the body so as to extend circumferentially about and be spaced axially along the tubular member 34 between the ends thereof. Still further, some combination of both can be used. The use of spiral and/or circular convolutions 58,60 formed in the body 48 addresses and resolves another concern with the use of boron in the rod 30: how to handle the release of helium gas within the chamber 40 which results from the boron interaction with a neutron. Parenthetically, it should be mentioned that the liquid boron solution does not fill the entire chamber 40 during non-operating periods, but only up to the level indicated by the numeral 62. This is so because the specific volume of the solution will change with temperature. In going from 100 degrees F. to 600 degrees F, water increases in volume by 45%. Since the rod 30 is designed to have absorber material at full core height during operation, the absorber column in the rod must be reduced during non-operating periods (refueling, etc.). Thus, there is a vapor space 64 present in the chamber 40. Returning now to the concern with the release of helium gas and its solution by the use of convolutions, it should be noted that the helium released increases the end of life (EOL) pressure in the rod 30. The operating vapor pressure within the chamber 40 due to the water being at 600 degrees F. is about 1550 psi. Since the EOL internal pressure must be limited to approximately the reactor coolant pressure (2250 psi) to avoid outward creep of the tubular body (or cladding) 48, the final pressure of the released helium and backfill gas should be about 700 psi to equalize internal with external pressure. Thus, the beginning of life (BOL) backfill pressure must be reduced to around 300 psi. The reduced backfill pressure results in a fairly high pressure acting on the body 48 at BOL which can cause creep collapse of the rod 30. This dilemna is resolved by providing the above-mentioned convolutions, spiral groove 58 and/or circular grooves 60, which increase the collapse strength of the tubular body 48 due to BOL external pressure. If the circular grooves 60 are used, they would preferably be spaced about every one inch increment on the rod 30. Calculations show that a 0.039 inch deep convolution will double the collapse pressure of a 0.45 inch diameter x 0.024 inch wall tube. A 0.059 inch deep convolution will triple the collapse pressure. In summary, therefore, the convolutions 58 and 60 add sufficient strength to the thin wall of the tubular body 48 to withstand the external pressure at BOL. To reduce corrosion of the inside of the tubular member 34, its material is beta quenched. This also reduces the hydride pickup in the member due to free hydrogen from the oxidation process. The solid lower end plug 52 also serves as the means 46 providing the hydride sink. Since this end of the member 34, being the lower one, is the cooler, the hydrogen will tend to migrate toward it. The upper end plug 50 is an attachment fitting for connecting the rod 30 to the spider 32, and adjacent to it is positioned the hydrogen getter means 44. The means 44 takes the form of a Zircaloy sponge 66 adapted to remove hydrogen from the vapor space 64 in the chamber 40 of the member 34. The sponge 66 is retained adjacent the upper end plug 50 by an annular disc 68 which has a central opening 70 for allowing passage of the hydrogen gas to the sponge 66. The disc 68 is held against the sponge 66 by a circumferential bulge 72 formed in the body 48 of the tubular member 34. The lower end plug 52 of the tubular member 34 has a reduced diameter end portion 74 which adapts it to fit within a dashpot (not shown) at the bottomn of the guide thimble 14. The dashpot functions as a shock absorber when the rods are inserted during a scram operation. The lower end plug 52 also has a fill passage 76 extending axially through it which is used to prepressurize the rod 30 with helium and then it is closed by weld 78. It is thought that the improved soluble burnable absorber rod of the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.