Patent Number: 050892142
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to FIGS. 1 and 2, the principal purpose of the pressure monitoring apparatus 1 of the invention is to monitor the pressure of the helium gas contained within the interior 3 of a cask 5 used to either store or to transport radioactive materials. Such helium is typically pressurized to approximately 1.5 atmospheres, and serves the two-fold purpose of enhancing the expulsion of heat out of the cask 5 generated by the decay-down of the radioactive materials contained therein, and further retarding the occurrence of corrosion within the cask interior 3. Such casks 5 generally comprise a cylindrical wall 7 whose lower portion 9 includes a floor plate 11 welded therearound, and whose upper portion 13 includes a stepped rim 14 for receiving a lid assembly 15. The stepped rim 14 includes a plurality of uniformly-spaced bolt holes 16 for receiving bolts (not shown) that secure the lid assembly 15 into sealing engagement with the stepped rim 14 of the cask 5. While the pressure monitoring apparatus 1 of the invention may be used in conjunction with any transportation and storage cask, the cask illustrated in this example of the invention is a peripheral-finned transportation and storage cask of the type disclosed and claimed in U.S. Pat. No. 4,997,618, and assigned to the Westinghouse Electric Corporation. With reference now to FIGS. 2, 3A and 3B, the pressure monitoring apparatus includes an annular housing 20 whose inner edge is disposed within an annular recess 22 in the upper portion 13 of the wall 7 of cask 5. The housing 20 includes an lower annular member 24 whose inner edge 25 is welded around the recess 22 to form a gas-tight seal. The housing 20 further includes an upper annular member 26 disposed over the outer edge of the lower annular member 24 as shown. A removable inner cover 28 is disposed between the outer edges of the lower annular member 24, and the inner edges of the upper annular member 26. A gasket 29 is disposed between the inner edge of the inner cover 28 and the outer edge of the lower annular member 24, and a plurality of bolts 30 compresses the inner edge of the inner cover 28 in gas-tight relationship with this gasket 29. These bolts 30 are screwed into threaded bores 32 present around the circumference of the lower annular member 24. Together, the recess 22, the lower annular member 24 and the removable cover 28 define an evacuated sensor chamber 35 within the housing 20 of the pressure monitoring apparatus 1. As will be described in more detail presently, this evacuated sensor chamber 35 contains most of the sensor assembly 50 (illustrated in phantom 3A) of the apparatus 1, and further provides a second barrier between the pressurized helium gas disposed within the interior 3 of the cask 5, and the ambient atmosphere. With reference again to FIGS. 2, 3A and 3B, the housing 20 further includes a removable outer cover 36 that is secured to the outer edges of the upper annular member 26 by means of bolts 37. Together, the upper annular member 26, the removable inner cover 28, and the removable outer cover 36 define an access chamber 38 which contains the electrical terminals leading to the sensor assembly 50 disposed within the evacuated sensor chamber 35, as well as a test conduit that is connected to a test port which allows a direct pressure measurement to be made of the pressure within the evacuated sensor chamber 35. With specific reference now to FIG. 3A, the evacuated sensor chamber 35 surrounds the outer end 40 of a bore 41 that penetrates completely through the cylindrical wall 7 of the cask 5. A shielding insert 43 is disposed within the bore 41 to prevent "streaming" of any radiation which may be emitted by radioactive materials disposed within the interior 3 of the cask 5. The principal purpose of the shielding insert 43 is to define tortuous path 44 which may be easily traversed by the compressed helium within the interior 3 of the cask 5, but which does not provide a straight path for any radiation emanating from the interior 3 of the cask 5. This tortuous path 44 is defined by a combination of intercommunicating radially disposed bores 45 and longitudinally disposed bores 47 as shown. While it is within the scope of the instant invention that the through-wall bore 41 might be placed in positions other than the upper portion 13 of the cylindrical wall 7 of the cask 5, the upper portion 13 is preferred due to the fact that the density of radiation emanating out of the cask interior 3 is considerably less at the upper portion 13 of the cask than it is in the middle portion, since the uppermost height of the radioactive material in the cask is always below the stepped rim 14. While such a low density of radiation is also present near the lower portion 9 of the cask 5, this location is not preferred to two reasons. First, a lower location of the housing 20 of the pressure monitoring apparatus 1 on the cask wall 7 renders it more exposed to mechanical shock from fork lifts, etc., when the cask 5 is being handled. Secondly, if any significant amount of liquid should accumulate within the cask interior 3, these liquids might flow up through the bore 41, and damage the components of the sensor assembly 50 disposed therein. With reference now to FIGS. 4A and 4B, the sensor assembly 50 of the apparatus generally includes a mounting plate 52 having three bolt holes 54a,b,c for receiving bolts (not shown) when the assembly 50 is mounted within the evacuated sensor chamber 35 of the housing 20. The two key components of the sensor assembly 50 are a differential pressure sensor 55, and an absolute pressure sensor 56, each of which are secured (either directly or indirectly) to the mounting plate 52. In the preferred embodiment, the differential pressure sensor is an Aschcroft.RTM. Model No. B427S XG9 differential pressure sensor manufactured by Dresser Industries located in Milford, Conn., having a stainless steel diaphragm and a setpoint of up to 30 psi (differential). The absolute pressure sensor 6 is preferably a model no. 211-75-700 pressure sensor Corporation located in Seattle, Wash. A conduit 57 connects both the differential pressure sensor 55 and the absolute pressure sensor 56 in parallel to the outer end 40 of the through-wall bore 41. The conduit 57 includes an intake tube 58 whose upstream end 59 is brazed or welded to the outer end 40 of the through-wall bore 41, and whose downstream end is connected to two-serially connected isolation valves 60 and 62. In the preferred embodiment, isolation valves 60 and 62 (as well as all of the other isolation valves 83 and 91 discussed later) are preferably model no. SS-4H-TW "H" Series, bellows-type valves manufactured by the Nupro Company located in Willoughby, Ohio. The purpose of the isolation valves 60 and 62 is to completely isolate both the differential pressure sensor 55 and the absolute pressure sensor 56 from the outer end 40 of the through-wall bore 41 in the event that either of these two components requires replacement or maintenance. While one such isolation valve would be adequate for this purpose, two serially-connected valves 60 and 62 are preferred due to the extra measure of safety that the use of two valves provides against leakage during a replacement operation. The outlet of the second isolation valve 62 is connected to the inlet 63 of a T fitting 64 by way of a tube elbow 66 as shown. The first outlet 67 of the T fitting 65 is connected to another T coupling 69 which fluidly connects the differential pressure sensor 55 to the gas conduit 57. For this purpose, a conduit segment 71 is disposed between the first outlet 67 of the T fitting 64, and the T coupling 69. On its downstream side, the T coupling 69 is connected to an elbow coupling 73 which in turn leads to the absolute pressure sensor 56, thereby connecting absolute pressure sensor 56 to the gas conducting conduit 57. A conduit segment 75 interconnects the downstream side of the T coupling 69 and the elbow coupling 73 in the manner shown. The second outlet 77 of the T fitting 64 ultimately leads to a vent plug 79 by way of a conduit segment 81. Conduit segment 81 further includes a vent valve 83 which, in the preferred embodiment, is the same type of isolation valve as described with respect to valve 60 and 62. The purpose of the vent plug 79, conduit segment 81 and vent valve 83 is to provide a controlled venting of any gas trapped between the isolation valve 62 and the differential and absolute pressure sensors 55 and 56 in the event that either of these two components requires maintenance or replacement. More specifically, the provision of these vent components allows any helium that contains radioactive particulate material to be sucked out of the segment of the gas conducting conduit 57 disposed between the isolation valve 62 and the pressure sensors 55 and 56 prior to the de-coupling of these sensors 55 and 56 from their respective fittings 69 and 73. The balance of the components that form the sensor assembly 50 are mounted on the outer surface of the removal inner cover 28, and are illustrated in FIG. 3B. These components include a chamber pressure test port 85 (indicated partially in phantom) which in turn is welded or brazed in a gas-tight relationship to a testing conduit 87 that terminates in a test cap 89. The testing conduit 87 preferably includes an isolation valve 91 of the same type as previously described with respect to the isolation valve 60 and 62. The purpose of the test port 85, testing conduit 87, test cap 89 and isolation valve 91 is to allow the system operator to make a direct measurement of the pressure of the evacuated sensor chamber 35 for the purpose of confirming whether or not a chamber leakage signal generated by the pressure sensors 55 and 56 is the result of a true leakage condition within the chamber 35, or is merely the result of a defective sensor 55, 56. The removable inner cover further includes a sealed electrical penetration 93 which conducts the output signal-carrying cables from the pressure sensors 55 and 56 to a terminal block 95 which in turn is connected to an electrical socket assembly 97 through connecting wires 98. The electric socket assembly 97 receives the plug 98.5 of a monitor cable 99 as shown. This monitor cable 99 is connected to commercially-available read-out circuitry (not shown) which converts the electrical signals generated by the differential and absolute pressure sensors 55, 56 into pressure readings. The final component of the sensor assembly 50 is an auxiliary pressure sensor 100 (indicated in phantom) which is detachably connectable to the testing conduit 87 after the test cap 89 has been removed. The provision of the auxiliary absolute pressure sensor 100 allows the operator of the apparatus to make a direct measurement of the pressure within the evacuated sensor chamber 35 when such a measurement becomes desirable. In operation, the differential pressure sensor 55 continuously generates an electrical signal indicative of the absolute pressure of the helium disposed within the cask interior 3, since the chamber 35 that surrounds the differential pressure sensor 55 is evacuated. As the helium disposed within the interior 3 of such cask 5 is typically pressurized to about 1.5 atmospheres, the output of this sensor 55 will generally read 1.5 atmospheres. Because the absolute pressure sensor 56 is in fact only a differential pressure sensor that uses it own, self-contained evacuated chamber as a reference point for making pressure measurements, the absolute pressure sensor 56 should likewise continuously generate a signal indicative of a pressure reading of 1.5 atmospheres. However, in the event that a leakage condition should occur in either the cask 5, or in the evacuated sensor chamber 35 that surrounds the differential pressure sensor 55, the sensor 55 will begin to generate a signal indicative of the presence of a lower pressure. In the preferred method of the invention, the circuitry (not shown) connected to the output of the differential pressure sensor 55 is programmed to generate an alarm signal when the differential pressure measured falls to about 1.2 atmospheres or lower. When this occurs, the operator of the apparatus immediately checks the pressure read-out generated by the absolute pressure sensor 56. If this pressure sensor 56 likewise indicates a pressure read-out of 1.2 atmospheres or lower, then the system operator concludes that a leakage condition has occurred with respect to the cask 3. If, however, the pressure read-out of the absolute pressure sensor 56 has not fallen and is still substantially at a level of approximately 1.5 atmospheres, then the operator of the apparatus tentatively concludes that a leakage condition has occurred with respect to the evacuated sensor chamber 35. The next step of the method of the invention, the operator of the apparatus confirms whether or not a leakage condition has occurred with respect to the evacuated sensor chamber 35 by removing the outer cover 36, and the test cap 89, and connecting the auxiliary pressure sensor 100 to the testing conduit 87. Once this has been accomplished, the isolation valve 91 is opened. If the resulting pressure reading is 0.30 atmospheres or higher, then the system operator concludes that a leakage condition with respect to the evacuated sensor chamber 35 has, indeed, occurred. If on the other hand the auxiliary pressure sensor 100 indicates that the evacuated sensor chamber 35 is still substantially evacuated, then the operator of the apparatus 1 concludes that the output of the differential pressure sensor 55 is in error, either as the result of drift in its set point, or some other type of mechanical failure. In either case, the operator of the apparatus 1 proceeds to either repair or replace the differential pressure sensor 55 by first removing removable inner cover 28, and then closing the isolation valves 60 and 62, and then effecting a controlled venting of any helium gas disposed within the section of the gas-conducting conduit 57 by removing the vent plug 79, connecting a suction hose to the vent port, and then opening the vent valve 83. After the venting operation has been accomplished, not only is the differential pressure sensor 55 removed and repaired, but the absolute pressure sensor 56 is tested to make sure that the read-out generated thereby is accurate and correct. After the foregoing maintenance operations have been accomplished, the apparatus 1 is reassembled, and placed back into operation.