Our invention relates to an elevator for moving heavy loads between respective elevations in an industrial facility and a method of operating the elevator. The elevator according to the invention is a buoyancy device which moves in a shaft of liquid between upper and lower elevations.
The idea of elevating and lowering a body in liquid by changing buoyance has been applied in the chemical and metallurgical industries as disclosed for example in U.S. Pat. No. 2,305,823. This reference discloses an apparatus for raising and lowering a vessel for carrying chemical or metallurgical elements.
U.S. Pat. No. 2,470,313 relates to a fluid mechanism for producing mechanical movements wherein a buoyancy elevator is used to pump liquid from one container into another container.
In the medical arts, U.S. Pat. No. 3,801,995 teaches a bath unit for bathing patients wherein a platform is raised and lowered by buoyancy forces. Still other references which disclose the utilization of buoyancy forces to move a body in a liquid are: U.S. Pat. No. 2,968,929 which is directed at an arrangement for raising and lowering massive bodies such as ships; U.S. Pat. No. 3,276,211 relating to a floating drydock; U.S. Pat. No. 3,171,376 which is directed to a bathyscaphe apparatus; and U.S. Pat. No. 2,887,977 which is also directed to a bathyscaphe type apparatus.
The elevator of our invention is especially adaptable for moving casks utilized in the nuclear power industry for shipping irradiated nuclear fuel elements.
The fuel elements commonly employed in BWRs (Boiling Waters Reacters) or PWRs (Pressurized Water Reactors) comprise a fuel assembly unit containing a plurality of long thin fuel rods. The fuel rods comprise a plurality of radioactive uranium compound pellets packed within a steel tube clad with zirconium or a zirconium alloy. The fuel elements become spent requiring replacement and reprocessing when their tubing springs a leak or their reactivity falls below a desired level.
In the current practice, the fuel assembly to be replaced and reprocessed is lifted out of the reactor core and moved through a connecting channel into an adjacent spent fuel storage pool of a fuel handling building where it is cooled down for some time. Next, a suitable shipping cask, usually made of lead or other good shielding material, is removed from its transport vehicle stationed inside the fuel handling building, lifted to the top floor of the fuel handling building, then carried across the top floor and lowered into the spent fuel storage pool, the fuel assembly loaded into the cask, the cask sealed, lifted out of the storage pool to the top floor, lowered to the decontamination area, carried to the transport vehicle, and there secured for conveyance to the reprocessing plant.
At the reprocessing plant, a similar cask transport process is carried out before the fuel rods can be separated from the assembly and taken apart, the radiated pellets removed, the fissionable material separated from the fission products and prepared for forming into new pellets.
As is known, the fuel storage pool of a nuclear power plant and similar pools at fuel processing plants contain the highest quality demineralized water (same water quality as in the reactor). However, the fuel pool water will be radioactively ccontaminated due to leaking fuel rods despite constant purification. Normally the fuel pool water is reprocessed with the twofold purpose of 1, keeping the radioactivity level within bounds and 2, maintaining a high water quality.
Submerging the cask into the spent fuel pool for fuel transfer operation may introduce to the pool water a large number of foreign, substances, such as oil, dust particles, and paint scale. Thus, it becomes extremely difficult for the normal fuel pool purification system to reprocess this water to restore the extremely high water quality normally used for this purpose.
Before the loaded cask can be brought out from the nuclear power station it must be decontaminated if it has been exposed to radioactively contaminated fuel pool water. Demineralized water, together with detergents, are used for cask wash-down. The waste water now containing detergents and radioactive contaminants, in addition to all the other foreign substances above, is also reprocessed (by a special radwaste system) although the water quality may not be restored to the same high level as that of the fuel pool water.
The burden of decontaminating the cask can cause extreme toil and the risks for radiation exposure for the personnel involved is great.
Moreover, contaminated water pools cause contamination of overhead cranes, sheaves and cables used for hoisting the cask in the out of the pool. Decontamination of such equipment is extremely burdensome.
Thus, there is a need in the art for a cask handling system that would avoid exposure of the cask exterior to possibly contaminated pool water as well as the sheaves and cables of the cask crane equipment. Also, relative to cask handling, there is a need to provide a cask handling system which will preclude the danger of accidental cask droppage into the reservoir wherein the spent fuel operation is performed.
It is anticipated that future shipping casks will have a loading capability of 12 to 15 PWR elements or 26 to 32 BWR elements. These casks will have an outer diameter of about 10 feet and a length of more than 20 feet and weigh nearly 125 tons when loaded. In the past, handling methods for shipping casks provided only one line of safety by employing a single load path through an overhead crane to lift and move the cask in the fuel handling area. The frequency with which casks have been thereby handled, coupled with crane performance characteristics, could result in a significant probability that a cask will drop in or into the spent fuel pool. Should the hoist cables fail, the falling cask could have catastrophic effects such as; crushing fuel assemblies stored in the pool, causing release of radioactive gases or cracking of the pool floor, causing a drain of the shielding pool water and exposure of the radioactive fuel assemblies.
In an effort to remedy the situation, cask loading pits were introduced. The cask loading pit is the full depth of the spent fuel pool and is connected thereto by means of a transfer canal. Since the potential drop height, in the case of hoist failure, in some instances can be a height corresponding to the depth of the cask loading pit which could exceed 40 feet, the integrity of the shipping cask would not be guaranteed.
Thus, as seen from the foregoing, cask handling arrangements of the prior art could present first the problem or requiring that the cask itself be immersed in the spend fuel pool making it necessary to wash the cask down to decontaminate the cask exterior to remove radioactive residue ahhering thereto because of its immersion in the spent fuel reservoir and second, the danger of cask droppage with its possible contaminant catastrophic effects.