Patent Number: 056335083
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a method of shielding a source of nuclear radiation such as an above-ground, dry nuclear storage cask. The invention provides a granite shielding structure surrounding an above-ground cask containing nuclear waste. The shielding structure includes a construction which minimizes accumulation of water, thereby contributing to an increase in durability. In shielding radiation, the present invention utilizes previously unknown properties of granite. In particular, it has been found that granite has exceptional radiation attenuation properties. As understood, the principal radiation admitted by spent nuclear fuel contained in dry casks is gamma rays from fission byproducts. These fission byproducts typically come from spent fuel consisting of small uranium pellets. These fission byproducts include .sup.137 Cs and .sup.154,155 Eu. It has been found that granite significantly attenuates gamma rays whose energy span the likely range of spent fuel. The range of gamma radiation is 76 keV to 1764 keV. Below, are two tables that summarize the gamma attenuation factors and dose reduction factors for the particular granite of the preferred embodiment. TABLE 1 __________________________________________________________________________ (Rockville Granite) 90% reduction thickness in Gamma energy = cm (in.).sup.1 76 keV 352 keV 609 keV 662 keV 1120 keV 1764 keV __________________________________________________________________________ Photopeak.sup.2 2 (1) 4 (1.5) 4 (1.5) 4 (1.5) 4 (1.5) 3 (1.2) Dose rate.sup.3 3 (1) 12 (5) 6 (2.5) __________________________________________________________________________ TABLE 2 ______________________________________ (Rockville Granite) Thickness: in inches.sup.1 Relative Dose ______________________________________ 0 1 5 0.1 10 0.01 30 0.000001 60 10.sup.-12 ______________________________________ .sup.1 Thickness uncertainties are typically on the order of 25%. These photopeak attenuation and dose reduction factors indicate that walls of granite a few feet thick would provide excellent shielding from normal and most accidental radiation leakage. For example, if on one side of a 4 foot .times. 8 inch thick granite wall, the thickness of the preferred embodiment, there was a 100-year-old unshielded spent nuclear fuel assembly, the dose rate next to the assembly would be approximately 300 rem/hr and would be lethal after a few minutes of exposure time. However, the amount of radiation that would pass through a granite wall of the construction described below and having a thickness of 5 to 10 inches would be only a fraction of that 3 .mu.rem/hr, which is no greater than anywhere else on earth. The significance of these attenuation properties are multiplied by the extraordinary durability properties of granite. Granite structures have withstood the test of time as evidenced by the remaining existence of castles in Europe which are hundreds of years old. These granite walls include a simple block construction. However, the block construction previously used in these granite structures is not adequate to shield dry casks containing spent nuclear fuel. The granite blocks of the prior art may tend to separate slightly, thereby permitting radiation to escape through the cracks. The block construction also does not provide enough structural integrity to withstand attack, sabotage or earthquakes. The present invention does not allow the structure to separate and has exceptional structural integrity by providing a unique construction which provides significant advantages necessary to protect the dry casks containing the spent nuclear fuel. With reference to FIGS. 1 and 2, the construction of the present invention includes a granite wall 10 which includes block sections 11, foundation sections 13, and top blocks 18. Each of these include mating sections formed by flanges 12 and recesses 14. The wall is constructed by interlocking the flanges 12 into recesses 14, as illustrated in FIGS. 1 and 2. The interlocking flange construction of the present invention eliminates gaps in wall 10, thereby preventing the blocks from separating and inhibiting and attenuating the gamma radiation. The interlocking flange construction provides structural advantages not necessary in prior granite walls. This additional structural integrity assists in resistance to attack and sabotage. The interlocking flange construction of the preferred embodiment also secures the blocks in place in the event of a seismic load (earthquake). Because the construction of the preferred embodiment does not use mortar or bonding agents, the blocks are allowed to absorb the energy of an earthquake without being ridged and breaking. After an earthquake, the blocks merely settle back into their original position by the guiding force of the interlocking flange. As illustrated in FIGS. 4 and 4A, blocks 11 forming wall 10 include a top surface 30, an opposing bottom surface 31 and two opposing side surfaces 32 and 33, respectively. As stated above, and as illustrated in FIGS. 4A and 5A, the top and bottom surfaces 30 and 31 include cooperating flanges 12 and recesses 14 as do the opposing side walls 32 and 33 of the blocks. As shown in FIGS. 1, 5 and 5A, wall 10 includes angled corner blocks 40 which also include flanges 12 and recesses 14 similar to block 11. Corner blocks 40 also include cooperating flanges and recesses on side surfaces 42 and 44, as best illustrated in FIG. 5 and 5A. The flange and recess construction of the present invention prevents blocks 11 from separating and letting radiation pass through. In the first preferred embodiment, and as illustrated by the arrows in FIG. 4A, top and bottom surfaces 30 and 31 of blocks 11 are sloped to deflect any rain water that may be between the blocks. Adjacent top to bottom blocks have complimentary sloped top and bottom surfaces to ensure proper mating. As shown by the arrows in FIGS. 4 and 5, the top surfaces of flanges 12 are also sloped to deflect water. Corner blocks 40 include sloped surfaces in two directions as shown in FIG. 5. As shown in FIGS. 1 and 7, in the preferred embodiment, an angled joint capblock 16 is placed above top block 18. The joint cap block 16 acts as a deflector for rain, thereby preventing deterioration of the granite. By deflecting the rain, water does not build up between the blocks. By preventing penetration of water between the blocks, deterioration due to freeze/thaw cycles is substantially eliminated. In the preferred embodiment and as best illustrated in FIG. 2, cap block 16 is only placed at joints 20 formed between adjacent top blocks 18 which form wall 10. As best illustrated in FIG. 6 and 6A, top blocks 18 will have opposed sloping top surfaces 20 and 21 to deflect rain. In the first preferred embodiment, top blocks 18 are located at the top of wall 10. Top blocks 18 also include interlocking flanges and recesses. Top block 18 includes a sloped top surface to deflect rain and a sloped bottom surface complimentary to the sloped top surface of block 11 positioned immediately below it. Foundation blocks 13 also include a flange 12. As illustrated in FIGS. 1 and 2, foundation blocks 13 are positioned below a ground surface 24. Prior art granite walls have not utilized foundation blocks which include flanges or recesses. Foundation blocks including flanges or recesses provide additional structural strength. In the first preferred embodiment, foundation blocks 13 include a greater width than those of block sections 11. Having a greater width exposes top sections 26 and 28 of the foundation blocks. In the first preferred embodiment, exposed sections 26 and 28 are sloped away from wall 10 as to deflect rain water away from wall 10. It is understood that various structures may be used to interlock the granite blocks and the claims are not limited to the particular construction illustrated in the drawings. For example, the flanges may have different cross-sectional shapes or may be oriented differently on the blocks. Additionally, separate granite slats may be used to interconnect the blocks. The present invention significantly reduces the present concerns regarding the above-ground storage of spent nuclear fuel and dry casks. Because the wall is constructed of granite, it needs little or no maintenance as compared to secondary shielding constructed of steel or concrete. Further, granite provides excellent radiation attenuation properties, thereby protecting the environment surrounding the dry casks. It is to be understood that the present invention is not limited to the specific construction illustrated in the figures but rather is broadly directed to using granite as a radiation attenuation shield. In the preferred embodiment, other structure is included in addition to the granite wall surrounding dry casks. For example, a chain fence may surround the granite wall or a berm may be erected which surrounds the granite wall. Cameras may also be included which provide further security for the dry casks containing the spent nuclear fuel. While the foregoing detailed description of the present invention describes the invention of the preferred embodiments, it will be appreciated that it is the intent of the invention to include all modifications and equivalent designs. Accordingly, the scope of the present invention is intended to be limited only by the claims which are appended hereto.