In certain industries, pumps are used to circulate fluid that may contain potentially harmful or hazardous substances. Nuclear reactor and chemical installations are general examples of facilities that circulate such fluids.
Nuclear reactor installations circulate fluids that serve as moderators or coolant as well as working fluid that drives a turbine. In the discussion to follow, the use of such fluids is described in conjunction with a boiling water reactor form of nuclear power installation. Boiling water reactors (BWRs) are designed such that water, the primary coolant, serves both as a moderator and as a working fluid driving a turbine or the like. Thus, a single coolant loop is involved with these installations and the utilization of separate steam generators and the like is eliminated. Because of the boiling activity in the BWR core, such reactors are operated at advantageously lower system pressures than, for example, pressurized water reactors (PWRs). However, since the working fluid (water) passes through the reactor core in this direct cycle and out of the containment structure to drive a turbine or the like, particular care is paid to the avoidance of radiation hazards. While the primary water coolant is highly purified in such reactors to prevent activation of impurities when subjected to the high neutron fluxes extant in the core, induced radioactivity is witnessed in the circulating coolant. Thus, maintenance activities for BWR installations necessarily become more rigorous in order to account for these radioactive conditions.
In more typical BWR installations, water coolant is heated in the reactor core to rise within the reactor vessel as a two-phase mixture of water and steam. This dual phase mixture then passes upwardly through a steam separator assembly and steam dryer structure to enter a steam line leading to the turbine. Following turbine drive, the steam is condensed to water and returned to the reactor by quite large condensate and feedwater pumps. Generally large vertically oriented recirculation pumps are used to circulate the water through the reactor core. In some installations, cooler water from the circulating pumps is directed to the inputs of vertical jet pumps located between the core shroud and vessel wall (downcomer annulus) which provide a forced coolant circulation and effect a mixing of recirculating coolant with water returning from the steam separation and drying functions of the reactor vessel.
Proper inspection and maintenance of the coolant recirculating system is, of course, of high importance and certain of the procedures necessarily are involved due to radiation hazards. Inspection of the very large recirculation pumps typically requires removal of their large drive motors accompanied by a temporary storage thereof. Because of the size of the components, such storage typically is out of the reactor containment building itself. Following motor removal, to access the elongate drive shaft-impeller blade assembly of the pumps, the pump cover is unbolted and removed, whereupon a large radioactive shielding cask is positioned at the top of the pump and a grapple or hook then pulls the shaft into the cask. The cask and thus shielded drive shaft then are removed to another region of the reactor building whereupon some radiation contaminated depositions on the pump shaft are removed by a high pressure water jet system sometimes referred to as a "hydrolaser". Upon thus decontaminating the shaft, the now contaminated waste water from the cleaning procedure must be dealt with and the pump then is ready for inspection. Following inspection, for example employing any of a variety of non-destructive procedures, for example ultrasonic testing and the like, the drive shaft then can be reassembled within the pump structure and the motor is recoupled to the drive shaft. Typically this coupling takes place at a spacer coupling or connector link extending between the drive output of the motor and the driven input of the drive shaft of the pump. Such a link or coupling may have a length, for example, of about 12 inches as a minimum. The entire inspection procedure typically requires from several days to a matter of weeks to accomplish depending upon the difficulties encountered.
Similar inspection and maintenance problems also can occur in other reactor installation designs in which the moderator and working fluid are circulated independently, as in the pressurized water reactor (PWR). In a PWR, the primary loop that circulates water to the core is subject to radioactive conditions.
Inspection and maintenance of other pumps, as in chemical or petroleum facilities, often poses similar problems and hazards.