Patent Number: 042736706
Section: summary

This invention relates to the concentration of low radioactive aqueous waste and apparatus and methods used therein. More particularly, this invention is concerned with improved apparatus for, and methods of, withdrawing concentrated radioactive aqueous waste or bottoms from an evaporator. BACKGROUND OF THE INVENTION The operation of nuclear reactors, especially those utilized for electric power generation, generally employs substantial amounts of water for cooling and fuel handling. This water acquires a significant amount of radioactive dissolved and dispersed solids from activation of impurities and corrosion products in the reactor. Contaminated borated waste water is also obtained from reactor letdown. Condensate polisher regenerant solutions, high conductivity floor drain streams and waste water from contaminated laundry are also sources of radioactive aqueous waste streams. These streams, and others, are collectively often referred to as radwaste streams. Because of the substantial volume of the radwaste streams they cannot be disposed of without being substantially reduced in volume. The radwaste streams are accordingly collected, sometimes stored temporarily, and then processed to a small volume of greatly increased solids content generally called bottoms. The concentrated waste has a significant radioactive level so that it must be properly disposed of. The concentrated or waste or bottoms is, for example, solidified with a binding agent, put in strong containers, and then buried underground. Radwaste volume reduction, or concentration, systems generally employ a steam heated forced circulation evaporator, a feed tank and a conduit for supplying low radioactive aqueous waste from the feed tank to the evaporator. During start-up of the system, the feed stream from the feed tank is supplied to the evaporator and concentrated without removal of bottoms until the solids level reaches the desired concentration. Since the feed stream rate can be as little as one-hundredth the rate at which the liquor is recirculated it is easy to maintain solids equilibrium in the evaporator. The water from the feed stream is, of course, evaporated except for the small amount which is withdrawn with the bottoms. As the concentration of solids or bottoms increases in the evaporator, a level will be reached at which removal of bottoms becomes prudent, if not essential, to maintain evaporator efficiency. Thus, an evaporator may have a capacity of 1,500 gals. of liquor. When the concentration of solids reaches a predetermined level, such as 25% solids by weight, the liquor level is lowered, as for example 2 feet, by removal of 400 to 600 gallons of bottoms. This volume of liquor is then replaced from the feed stream, resulting in dilution of the liquor in the evaporator. Evaporation proceeds, with continuous addition of dilute radioactive waste, until the liquor again reaches the predetermined solids concentration. Bottoms can then be removed as described. In the past, conventional piping and valves have been used to periodically drain bottoms from the evaporator and feed it to a bottoms tank for further handling and processing. The high solids content of the bottoms resulted in plugging of pipes and valves, causing shut downs and necessitating repairs. Because of the high radioactivity of the bottoms, repair work could be done remotely with suitable protection against radioactive exposure of the repairmen. Furthermore, the prior systems of removing bottoms provided no means of fully emptying the drain pipes so that the radioactive residue remaining in the pipes made it unsafe for workers to come into close proximity of the equipment. Isolation of the highly radioactive bottoms withdrawal areas of the plant was thus required, thereby limiting plant access, inspection and repair and providing an additional unsafe area. SUMMARY OF THE INVENTION The subject invention provides an improvement in apparatus for concentrating low radioactive aqueous waste containing dispersed and dissolved materials including a forced circulation evaporator, a feed tank, and a conduit for supplying low radioactive aqueous waste from the feed tank to the evaporator, with the improvement comprising: a fast acting drain valve in the evaporator for removing concentrated radioactive bottoms from the evaporator; a drain conduit from the drain valve to a pump; a rinse water conduit, containing a first valve, communicating with the drain conduit; a delivery conduit, from the pump to a bottoms storage tank, containing a second valve; a branch conduit, from the delivery conduit to a feed tank, containing a third valve; means operating the pump, with the drain valve closed and the drain conduit, delivery conduit and branch conduit full of rinse water, and opening the fast acting drain valve and the third valve to thereby convey the water in the said conduits to the feed tank before bottoms removed from the evaporator reach the third valve; means closing the third valve and opening the second valve to deliver the bottoms removed from the evaporator to the bottoms tank; means opening the first valve in the rinse water conduit; closing the second valve to the bottoms tank and opening the third valve to the feed tank to back flush the fast acting drain valve and dilute bottoms in the conduits and deliver the diluted bottoms to the feed tank; means closing the fast acting drain valve to discontinue removal of bottoms from the evaporator while flushing the conduits with rinse water by maintaining the first valve open and delivering the rinse water to the feed tank; and means closing the first valve, the third valve and stopping the pump with the conduits and pump full of rinse water. The invention also provides a method of withdrawing bottoms comprising: withdrawing concentrated radioactive bottoms from an evaporator through a drain conduit and pumping it to a bottoms storage tank; continuing withdrawal of bottoms from the evaporator and smultaneously feeding an aqueous rinse stream to the drain conduit to dilute the withdrawn bottoms; pumping the diluted withdrawn bottoms to a feed storage tank; discontinuing withdrawal of bottoms from the evaporator; continuing to feed the aqueous rinse stream to the drain conduit until the stream in the drain conduit is very low in radioactivity; pumping the so further diluted stream to the feed storage tank; and discontinuing feeding the rinse stream to the drain conduit and pumping the diluted stream to the feed storage tank but maintaining the drain conduit and pump full of such diluted stream. A further aliquot of bottoms can be subsequently removed from the evaporator by operating the pump and initiating flow of bottoms to the drain conduit, feeding the dilute stream in the drain conduit and pump to the feed tank until the bottoms nearly occupies the drain conduit, and diverting the bottoms in the drain conduit to the bottoms storage tank, and continuing the method as described.