This invention relates generally to the cleaning of the interior of vessels, and is specifically concerned with the cleaning of sludge, debris and deposits from the shell side of a heat exchanger which has crevice regions. This invention has particular application to the cleaning of nuclear steam generators where the sludge and debris may be radioactive and where deposits in crevice regions may be particularly detrimental to the longevity of the steam generator.
There are many methods known in the prior art for cleaning heat exchangers. Among the known methods are the chemical cleaning processes taught in U.S. Pat. No. 4,686,067 entitled "Process for Eliminating Deposits Formed in a Steam Generator of a Pressurized Water Nuclear Reactor" and U.S. Pat. No. 4,720,306 entitled "Cleaning Method". Chemical cleaning techniques are limited by concerns regarding corrosion of the heat exchanger components. Cleaning agents which are effective in dissolving deposits are generally detrimental to the structures of the heat exchanger. Therefore, the concentration of the cleaning agent and the duration of the cleaning process are typically limited. Furthermore, because the concentration of the cleaning solution is limited, depletion of the active cleaning solution prior to complete cleaning of the crevice regions is common. In order to increase the concentration of cleaning solution in the crevice regions, U.S. Pat. No. 4,632,705 entitled "Process for the Accelerated Cleaning of the Restricted Areas of the Secondary Side of a Steam Generator" teaches a process which induces flash boiling of the cleaning solution by a reduction of pressure in the heated cleaning solution. However, the effectiveness of this technique is also limited when the concentration of active cleaning solution is depleted in the bulk solution.
Mechanical cleaning methods avoid the corrosion concern of the chemical methods. Among the mechanical methods known in the prior art are U.S. Pat. No. 4,756,770 entitled "Water Slap Steam Generator Cleaning Method" and U.S. Pat. No. 4,972,805 entitled "Method and Apparatus for Removing Foreign Matter from Heat Exchanger Tubesheets" and U.S. Pat. No. 5,006,304 entitled "Pressure Pulse Cleaning Method". Such mechanical methods are limited by the amount of force which can be exerted on the accumulated sludge and deposits without creating damage to the structures of the heat exchanger.