Patent Number: 050892142
Section: summary

BACKGROUND OF THE INVENTION This invention generally relates to pressure monitors, and is specifically concerned with both an apparatus and a method for directly and reliably monitoring the pressure of helium gas within a cask containing radioactive materials. Devices for monitoring the pressure of the helium gas that is typically present within casks used to store or transport radioactive materials are known in the prior art. One of the primary purposes of such devices is to generate a warning signal when a leakage condition occurs which would allow the pressurized helium gas contained within such casks to escape in the ambient atmosphere. There are many reasons why the persons responsible for the maintenance and operation of such casks would want to be immediately informed of such a leakage condition. First, while the helium gas itself is not harmful, the leakage of such gas might carry out very fine radioactive particulates which may in turn pose a radiation hazard. Secondly, the loss of helium gas within such casks interferes with the ability of the cask to dissipate the heat generated by the decay of the radioisotopes disposed within the cask, as helium is a far better thermal conductor than air. Thirdly, the loss of helium in the cask might imply the introduction of ambient air into the cask interior, which is potentially corrosive due to the oxygen content of air. In one type of prior art pressure monitoring device, the space that is normally present between the inner and outer lids that cover the cask is pressurized to a higher pressure than the helium gas within the cask interior. This pressurized space is connected to a differential pressure switch that monitors the pressure difference between the gas in the cask and the gas in the space. A second switch monitors the pressure in the pressurized space between the inner and outer lids, and is used to determine whether or not a change in the differential pressure between the cask interior and the pressurized space is the result of a leak in the seals between the cask interior and the ambient atmosphere, or a leak in the seals between the pressurized space and the ambient atmosphere. While such prior art pressure monitoring devices are capable of fulfilling their intended purpose, the applicants have noted a number of areas in the design of these devices which could bear improvement. For example, in order to obtain access to these particular pressure monitoring devices, the lid of the cask itself must be completely removed. Such lid removal is not only troublesome in view of the size and weight of the lid, and the number of bolts used to fasten it to the cask, but further presents a radiation hazard since it results in the exposure of radioactive materials to the ambient atmosphere. Hence, whenever it becomes necessary to perform a maintenance operation or to replace a component in one of these prior art pressure monitoring devices, the cask must be moved into an area of containment, the heavy lid removed, and either the radioactive materials disposed inside must be removed, or the maintenance or replacement operation must be done remotely through the use of robotic tools so that maintenance personnel are not exposed to potentially harmful radiation. Still another shortcoming in the design of such prior art pressure monitoring devices is the fact that there is no practical way to test the operability of the pressure sensors once the cask is sealed, or to confirm the reliability of the pressure readings generated by the two pressure sensors when these sensors indicate that a leakage condition has occurred. Hence, if one or more of the pressure switches generates a spurious leakage signal as a result of drift in its set point or some other malfunction, the entire cask might be put through some unnecessary and expensive repair operation. Still a third shortcoming in the design of such prior art pressure monitoring devices is the fact that neither of the pressure switches makes a direct measurement of the actual pressure of the pressurized helium gas inside the cask. The lack of any such direct measurement adversely effects the reliability of the pressure readings generated by the switches. Clearly, what is needed is an improved pressure monitoring device which is easily accessible in the event that a repair or a maintenance operation is necessary, but yet does not adversely effect the shielding efficacy of the cask as a whole. Ideally, the readings of such a pressure monitoring device should be readily testable at any time during the operation of the device, and should be further verifiable in the event that a signal indicative of a leakage condition is generated. Finally, such a device should directly measure the pressure of the gas disposed within the cask without compromising the gas seals in the cask so that the output of the device may be as accurate and as reliable as possible. SUMMARY OF THE INVENTION The invention is both an apparatus and method for monitoring the pressure within a cask containing a hazardous gas that eliminates or at least ameliorates the aforementioned shortcomings of the prior art. Specifically, the invention is an apparatus that comprises a differential pressure sensor sealingly connected to an outer end of a bore that penetrates through a wall of the cask for both directly measuring the pressure of the hazardous gas and providing a first barrier between the gas and the ambient atmosphere, and an evacuated sensor chamber that contains the outer end of the through-wall bore as well as the differential pressure sensor for providing a second barrier between the gas and the ambient atmosphere. Preferably, the apparatus further comprises an absolute pressure sensor that is connected in parallel to the through-wall bore so that, in the event that the differential pressure sensor indicates a leak condition has occurred, the system operator may determine whether or not the leakage condition sensed is a result of a leakage condition in the cask, or in the evacuated sensor chamber. Both sensors may be switches. A wall of the evacuated sensor chamber may further include a test port coupling, and the apparatus may further comprise an auxiliary pressure sensor that is detachably and sealingly connectable to the test port coupling for measuring the pressure within the sensor chamber such a measurement may be made through the test port coupling in the event that the absolute pressure sensor indicates that the loss of differential pressure sensed by the differential pressure sensor is a result of leakage in the evacuated sensor chamber. Such a direct pressure measurement would either confirm that the leakage condition arose as a result of a leak in the evacuated pressure sensor, or would indicate that the leakage condition was a false alarm caused by a faulty differential pressure sensor. To facilitate the replacement of either the differential or the absolute pressure sensor, the evacuated sensor chamber is defined in part by a removable outer cover. Additionally, both the differential and the absolute pressure sensors may be connected to the aforementioned through-wall bore in the walls of the cask by means of a gas-conducting conduit which in turn includes at least one isolation valve for isolating the through-wall bore from the pressure sensors during a pressure sensor replacement operation. As an added safety measure in the event that a pressure sensor replacement operation become necessary, the apparatus may further include a venting assembly in the gas conducting conduit between the previously mentioned isolation valve, and the inputs of both the differential pressure sensor and the absolute pressure sensor. This venting assembly may include a vent port, and a vent valve fluidly connected within the segment of the gas-conducting conduit that connects the isolation valve to the inputs of the differential and absolute pressure sensors. Both the differential pressure sensor and the absolute pressure sensor generate electrical signals corresponding to a pressure output that are transmitted through wires which extend through the removable cover of the evacuated sensor chamber through a sealed electrical penetration. These wires are in turn connected to an electrical connector assembly or socket that is mounted in a hole in the housing of the pressure monitoring assembly. An output cable having a plug which is receivable within the socket transmits the electrical signals generated by the differential and absolute pressure sensors to an appropriate monitoring circuit, which converts these signals into pressure values and which is programmed to generate an alarm signal upon the receipt of a pressure signal indicative of a leakage condition. The apparatus provides a highly reliable system whose outputs are directly and reliably confirmable at all times during the operation of the device, and whose structure provides multiple safeguards against the release of any hazardous gases into the ambient atmosphere.