Patent Number: 047387998
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to permanent disposal of radioactive waste from nuclear reactors, particularly radioactive wastes from primary coolant fluid systems and steam generator blowdown sludge. In the operation of nuclear power plants, radioactive particular waste develops in the primary coolant fluid and in the sludge produced during steam generator blowdown. In the latter case, primary-to-secondary leakage in the steam generator requires that the blowdown sludge be treated as radioactive waste. All radioactive waste from a nuclear reactor must be processed for disposal in a manner which minimizes the exposure of operating personnel to radiation. The conventional filter cartridges utilized in the auxiliary systems of a nuclear power plant have been found to be difficult to handle and to create a significant radiation exposure hazard for the maintenance personnel. To help alleviate this hazard, replacement of conventional removable cartridge filters with backflushable filters has been proposed. Backflushable filters, however, while convenient, serve only as temporary collection and holding devices. Final removal of the radioactive particulate waste, or "crud", requires backflushing of these filters to convey the waste to a suitable disposal plant for packaging and burial. SUMMARY OF THE INVENTION It is an object of the present invention to prepare such waste for permanent disposal in a manner which minimizes the radiation exposure hazard to operating personnel. Another object of the invention is to transfer such radioactive waste to permanent storage containers by a fully remotely controllable system which enables the operating personnel to be safely isolated from regions where radiation is present. Another object of the invention is to permanently store radioactive particulate waste in relatively inexpensive and structurally simple containers. A further object of the invention is to permanently store such radioactive waste in pre-packaged. ferromagnetic filter matrices disposed within such containers. A further object of the invention is to permanently store such radioactive waste in the form of a uniform, dewatered dispersion of solid radioactive waste, encapsulated in an organic resin solidification system in a liquid-impervious disposal package. Another object of the invention is to permanently store such radioactive waste in a disposable electromagnetic filter cartridge in conjunction with a backflushable filtration system forming part of a nuclear plant fluid system. The above and other objects are achieved, according to the invention, by the provision of a noval cartridge for permanent disposal of radioactive particulate waste, comprising: a liquid impervious casing having an upper end cover, a lower end cover and a side wall extending between the covers, the casing enclosing a waste storage region; ferromagnetic fibrous material defining a waste retaining matrix and filling a major portion of the waste storage region; means defining an inlet conduit extending through the upper end cover and axially of the casing through the waste storage region, and opening into the waste storage region in the vicinity of the lower end cover; and means defining first and second outlet conduits extending through the upper end cover and opening into the waste storage region in the vicinity of the upper end cover. Preferably, the casing and the conduits are all made of a suitable plastic, such as fiberglass, with all components being securely bonded together, for example by means of a suitable adhesive, to form a rigid unit. In accordance with a particular novel feature of the invention, the ferromagnetic fibrous material is constituted by ordinary steel wool, preferably of a fine grade. The grade employed will be determined, at least to a substantial extent, by the size of the particles to be stored, as will be explained in detail below. Preferably, the cartridge further includes two annular, perforated retainer-distribution plates spaced apart in the axial direction of the casing and delimiting the portion of the waste storage region containing the matrix. Between each retainer-distribution plate and an associated cover for the casing, there is thus defined a space for the circulation of fluids between the portion of the waste storage region containing the matrix and the various conduits. Preferably, the upper end cover of the cartridge casing is formed to present a reservoir for holding any liquid which may spill during the introduction of liquid containing the radioactive waste of flushing water to the cartridge. Any spillage can be removed by means of a siphon tube introduced into that reservoir during the filling operations. In addition, the upper end cover is formed, at its top, to present a lateral flange, preferably an inwardly extending annular flange, which partly covers the reservoir and which is used to lock the cartridge to an associated filling system. In the use of this cartridge, a slurry containing the radioactive waste to be stored will be delivered via the inlet conduit, while the liquid filtrate contained in that slurry as well as subsequently delivered flushing water, are expelled via the first output conduit. After the flushing water has been circulated through the waste storage region, dewatering air is introduced via the inlet conduit and is expelled via the first outlet conduit. During this operation, the second outlet conduit will be blocked and the cartridge will be disposed within a magnetic field which acts to trap the particulate waste in the matrix. During the subsequent encapsulation operation, which also takes place while the cartridge is in the magnetic field, encapsulating material is injected via the inlet conduit, while the air previously trapped in the cartridge is expelled via the second outlet conduit, the first outlet conduit then being blocked. Introduction of the encapsulating material continues until the interior of the cartridge is completely filled. In order to monitor the filling of the cartridge with encapsulating material, the second outlet conduit is provided with a check valve which is oriented to be closed by the pressure exerted by the encapsulation material when it enters the second outlet conduit. Closing of the valve actuates a microswitch that is also disposed in the second outlet conduit in order to produce a signal indicating completion of filling with the encapsulating material. The encapsulating material can be of any suitable composition already known in the art, such as known resin-catalyst systems, or even cement. The ends of the conduits which project from the cartridge all project from the upper end thereof and preferably have conically tapered surfaces to define coupling components. While the conically tapered surfaces are preferably exterior surfaces which define male coupling elements, they may also be constituted by interior conduit surfaces to define female coupling components. Preferably, the bottom cover is provided, at its lower surface, with an alignment groove which will cooperate with a lug provided on an associated conveyor to assure that the cartridge is correctly alinged with conduits of a cartridge filling system, the latter conduits being formed, at their lower ends, to present coupling elements constructed to mate with those of the cartridge conduits. The objects of the invention are further achieved by the provision of a novel system for storage and encapsulation of radioactive particulate waste, comprising: a cartridge having a liquid impervious casing enclosing a waste storage region, a ferromagnetic waste storage matrix housed in the cartridge and occupying at least a major portion of the waste storage region, and an inlet conduit and at least one outlet conduit projecting from the cartridge and communicating with the waste storage region; means for establishing a magnetic field in the matrix; fluid handling means including a source of liquid containing the radioactive waste to be stored in the cartridge, a source of flushing water, a source of air, a source of encapsulating material, and a receptacle of receiving flushing water; cartridge filling means including a plurality of conduits releasably couplable to the conduits associated with the cartridge; and fluid flow control means including a plurality of remotely controllable valves connected between the fluid handling means and the cartridge filling means, the fluid flow control means having a first operating state for selectively supplying liquid containing the radioactive waste, flushing water, or air from their respective sources to the inlet conduit for loading the matrix with radioactive waste, while placing one outlet conduit in communication with the receptacle, and the fluid flow control means having a second operating state, for supplying encapsulating material from its source to the inlet conduit for filling the cartridge with encapsulating material, while permitting air in the cartridge to be expelled via one outlet conduit. Preferably, of course, the cartridge of the above system is of the type described earlier herein. The means for establishing a magnetic field is preferably constituted by an annular solenoid presenting an axial passage dimensioned to permit introduction of the cartridge. Systems employing a solenoid to apply an electromagnetic field to a ferromagnetic storage medium are already known in the art. According to one preferred embodiment of the system according to the invention, the cartridge has first and second outlet conduits, the cartridge filling means is movable between a first operating position associated with the first operating state of the fluid flow control means and second operating position associated with the second operating state of the fluid flow control means, the conduits of the cartridge filling means are grouped into a first set of conduits releasably couplable to the conduits associated with the cartridge when the cartridge filling means are in the first operating position, and a second set of conduits releasably couplable to the conduits associated with the cartridge when the cartridge filling means are in the second operating position, the one outlet conduit which is placed in communication with the receptacle is the first outlet conduit, and the one outlet conduit via which air is permitted to be expelled when the fluid flow control means is in the second operating state is the second outlet conduit. The cartridge filling means includes one conduit which is common to both sets, and the movement of the cartridge filling means between its first and second operating positions is effected by pivoting the filling means about the axis of the common conduit. Preferably, this common conduit is coupled to the second outlet conduit of the cartridge and constitutes the conduit via which air is vented from the waste storage region of the cartridge during filling with encapsulating material. The cartridge filling means is preferably constituted by a turret carrying the various conduits. The turret is supported by a column which is, in turn, supported in a loading head constituted by a cylindrical housing having a closed upper end and an open lower end. The housing is constructed to permit its lower end to form a sealed connection with the top of the cartridge. In addition, the column is movable vertically relative to the loading head to displace the turret between a raised, or retracted, position when the conduits associated with the cartridge filling means are separated from those of the cartridge, and a lowered, or coupled, position in which one set of conduits of the cartridge filling means will be coupled in a sealed manner to the cartridge conduits. The turret further carries a siphon tube which is positioned to be introduced into a reservoir formed at the top of the cartridge, around the projecting ends of the associated conduits, to permit aspiration of any liquid which may spill onto the top of the cartridge during the course of the filling operation. The loading head further carries a group of locking cams arranged to cooperate with a flange formed at the upper end of the cartridge in order to lock the cartridge against the lower end of the loading head during the various filling operations. The valves of the fluid flow control means can all be of a conventional type. These valves are preferably electrically controllable to permit remote-controlled operation of the system. The objects of the invention are further achieved by operating the system defined above as follows: placing the cartridge filling means in the first operating position and coupling the first set of conduits in a sealed manner to the conduits associated with the cartridge; operating the establishing means for establishing a magnetic field in the matrix; while the first set of conduits is coupled to the conduits associated with the cartridge, effecting loading of waste material by operating the fluid flow control means for blocking the second outlet conduit and sequentially supplying liquid containing the radioactive waste via the inlet conduit to the waste storage region until the matrix is loaded with waste while conducting liquid from the waste storage region via the first outlet conduit to the receptacle, supplying flushing water via the inlet conduit to the waste storage region and from the waste storage region via the first outlet conduit to the receptacle, and supplying air via the inlet conduit to the waste storage region and from the waste storage region via the first outlet conduit until substantially all liquid has been removed from the waste storage region; after the step of effecting loading of waste material, decoupling the first set of conduits from the conduits associated with the cartridge, placing the cartridge filling means in the second operating position, and coupling the second set of conduits in a sealed manner to the conduits associated with the cartridge; and while the second set of conduits is coupled to the conduits associated with the cartridge, effecting encapsulation of the waste material by operating the fluid flow control means for blocking the first outlet conduit, and supplying encapsulating material via the inlet conduit until the waste storage region is filled with encapsulating material, while removing air from the waste storage region via the second outlet conduit. The method and apparatus according to the present invention serves to produce a dewatered dispersion of reactor coolant system corrosion products or steam generator blowdown sludge in a water-impervious organic resin-steel wool matrix, encapsulated in a plastic cylinder which can be automatically loaded, immediately after the encapsulation operation, into a standard shipping container whose top is subsequently sealed in a subsequent conventional operation. Operation of this system according to the present invention provides a convenient, economical and efficient technique for preparing backflush slurry or sludge for disposal at minimal risk of significant radiation exposure to personnel. The invention is particularly applicable to the storage of particulate matter which has been transported through a reactor coolant system and its auxiliaries. Such particulate matter consists largely of corrosion products having small particle sizes ranging from a few microns to colloidal size, as well as a certain amount of ion exchanger resin fines and, occasionally, bits of debris from various sources. All of the material is radioactive since it has been subjected to the neutron flux in the reactor. The coolant system auxiliaries include cartridge filters installed to remove particulate matter having sizes in the range of several microns. Some of the smaller particles either agglomerate or adhere to larger particles and are removed therewith. The auxiliaries may also include a CVCS letdown demineralizer which filters larger particles and absorbs some colloidal material which then is removed when the resins are replaced. The internal surface of the reactor coolant system, and especially the core, are the overwhelming competitors of the waste particles since the rate of filtration through the auxiliaries is a small fraction of the reactor coolant flow (less than 0.05%). The waste particles depositing on these surfaces account for 85% of the occupational radiation exposure experienced by plant personnel. When the steam generators of such reactor systems exhibit primary-to-secondary leakage, the blowdown sludge from these generators must be handled as radioactive waste. This sludge can also be removed by collecting it in backflushable filters forming part of the reactor system, and then transporting it to cartridges for permanent storage in accordance with the present invention. The system according to the invention for premanently storing such particulate waste material in cartridges takes advantage of the magnetic properties of the reactor coolant particulates and the steam generator sludge to effect a quantitative dewatering of the waste material, together with efficient packaging for burial. In order to utilize the magnetic properties of the waste particles, introduction of the waste material into the cartridge, as well as subsequent introduction of encapsulating material, are carried out while the cartridge is within a magnetic field. According to techniques known in the art, this can be achieved by introducing the cartridge into an axial passage enclosed by an annular solenoid generating the requisite magnetic field level. To avoid damaging the cartridge or in case of failure to achieve proper engagement between the cartridge and the cartridge filling means, appropriate interlocks can be provided to prevent attempted engagement of the cartridge filling means with the cartridge when the two are not correctly positioned relative to one another. The system according to the present invention would be well suited for handling waste products derived from high temperature large flow rate filtration of reactor coolant. The system according to the invention could also be used for disposal of radioactive material produced during decontamination of steam generators. For example, it is known to decontaminate the primary side channel heads of such steam generators by means of a slurry of boron oxide in water which is applied in a manner to "grit blast" the channel head surfaces. Other grits which have been considered are magnetite and aluminum oxide. The purpose of such decontamination is to remove the highly radioactive primary corrosion deposit, i.e. nickel ferrite, so that the radiation source strength will be lower and result in a lower radiation does to operating personnel. A process of this type employing boron oxide could produce of the order of 20,000 gallons of 4-5% boric acid solution with a high concentration of nickel ferrite particles. The system according to the present invention could be employed to dispose of the waste (or crud), enabling the boric acid solution itself to be reused as a carrier for boron oxide for other steam generators, or to be disposed of. If magnetite were used as the abrasive, it, along with the nickel ferrite "crud" , could be collected and permanently stored according to the invention leaving essentially clean water for reuse or disposal. The system according to the present invention could, if desired, be mounted on a trailer and carried to the place where it would be used, such use being carried out either in connection with existing tankage, or temporarily constructed tankage. The system according to the invention could also be constructed with a plurality of operating stations disposed along a conveyor line, so that a plurality of cartridges could be simultaneously loaded with waste material .