Patent Number: 040173585
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, FIG. 2 shows the storage capacity for borate ions on the resin for two different temperature levels as effected by the boric acid concentration of the liquid which surrounds the resin and the resins capacity for storage of boron. The curve of FIG. 2 represents conditions only where the resin is saturated with boron at the temperature represented on either the T-hot curve or the T-cold curve. Sketch W of FIG. 3 simulates the amount of boron stored on the resin during storage until saturation has been achieved. The effluent concentration equals the in-flow concentration under these conditions. Now assuming that the in-flow concentration, which is the reactor coolant concentration, corresponds to point A on the T-cold curve of FIG. 2. Boron will be released from the resin at hot in-flow conditions and yield a high effluent concentration as indicated by point B on the T-hot curve of FIG. 2. If the boration process is stopped before the reactor coolant system concentration reaches point C on the T-hot curve of FIG. 2, the concentration which would be achieved if the resin were completely equilibrated with the conditions of the incoming flow, then the boron on the resin would be distributed over the resin column as indicated in sketch X of FIG. 3. This results from the tendency of the resin bed to release boron first from the resins at the inlet side of the resin column. The boron concentration of the solution will then increase over the length of the resin column until it reaches a level corresponding to the equilibrium condition at that temperature. As indicated above, the outlet side of the resin column does not store or release boron during the initial phase of any particular change in flow condition directed toward changing the concentration of boron within the reactor. Sketch Y of FIG. 3 illustrates what would then happen if boron were subsequently stored on the resin before all of the boron had been released from the resin as for example as shown in sketch X of FIG. 3. The incoming flow stores boron first on the in-flow side of the resin column, as indicated above. The concentration of liquid decreases over the first part of the column. However, because the effluent side of the boron column has not released boron during the previous boration period, it will not be able to store boron at this in-flow condition. The flow which has decreased in concentration over the first part of the resin column will again increase over the last part of the column and will finally leave the resin column at a condition which is in equilibrium with the condition at which the resin was left after complete equilibrium which corresponds to point A of FIG. 2. The result is that no dilution of the reactor coolant water will occur during the first stage of the storage operation. This is of course extremely undesirable where rapid dilution is required for a subsequent step in reactor operations. The concentration of the effluent from the boron column will eventually decrease because the concentration of the liquid at the in-flow side decreases and removes boron which was stored at the effluent side. The amount of boron stored on the resin at the effluent side therefore decreases resulting eventually in a decrease in the effluent concentration. Thus, the result of leaving the column in a non-equilibrated state is that a substantial length of time is required to reach a desired effluent concentration from the boron column. A sketch Z of FIG. 3 illustrates the case where boron is stored on the resin after the resin column has been completely rinsed at hot conditions. The effluent concentration during boron storage will then correspond to point D on the T-cold curve of FIG. 2. This low effluent concentration would cause an immediate dilution of the reactor collant water. The explanation above indicates that in a single directional flow resin column the resin must become equilibrated at the new temperature level in order to give any immediate response if the temperature of the resin bed is changed. A severe operational problem results since partial changes in the reactor coolant concentration of boron with respect to the storage capacity of the boron column make subsequent changes both somewhat unpredictable and always time consuming. This operational problem can be minimized by the use of the resin bed which allows flow in both directions. FIG. 2 illustrates that if the flow direction would have been reversed after the conditions shown on sketch X of FIG. 3 then a low concentration corresponding to point D on the T-cold curve would have been reached immediately. Accordingly, the resin bed should always be used such that hot flow which removes boron from the resin will always enter at one side of the bed and cold flow which stores boron on the resin will always enter at the other side of the bed. This will, after reversal in flow condition from a hot condition to a cold condition, immediately result in a low effluent concentration at cold operating conditions and a high effluent concentration at hot operating conditions. The resins used in the regeneration system disclosed herein are hydroxyl based anion resins which normally are saturated with borate ions prior to being placed in the system tanks. Typical resins available for use are Rohm and Haas IRN-78, Duolite ARA-336W, Lewatit M-500 and Ionac-A935. These resins are well known in the nuclear reactor manufacturing field and all appear to have essentially the same characteristics. They deteriorate somewhat with use but pronounced deterioration takes place above about 160.degree. F and this factor determines the upper temperature level at which the thermally regenerable ion exchangers having the resin beds may be operated. The upper temperature level at which coolant may therefore be introduced into the ion exchangers ranges between 140.degree. and 160.degree. F while the lower temperature level is 40.degree. to 60.degree. F. The lower level of 40.degree. F is chosen to maintain the coolant at just above the freezing point to eliminate the necessity for using an anti-freeze solution in the coolant. Desirably, the range between the upper and lower temperature levels should be kept as wide as possible to promote efficiency in the system, i.e., to store boron on the resin and remove boron from the resin in the maximum amount and at the best rate for the temperatures chosen for system operation. The system of this invention operates at 50.degree. and 140.degree. F. It will be obvious to those skilled in the art that as improved resins become commercially available, the upper temperature level may be raised from the present 140.degree. - 160.degree. F, the object being to achieve the greatest range between the points B and D on the curve of FIG. 2. FIG. 1 shows an example of a resin tank which is suitable for practicing the process of this invention in that it permits dual-directional flow. Tank 10 is bisected by a vertical divider plate 12 which extends through a horizontal screen 14. Tank 10 further includes bottom mounted flow conduits 16 and 18. Flow which enters through conduit 16 under the resin screen 14 at one side of the divider plate 12 is forced upwards over the divider plate 12 and exits via the conduit 18 on the other side of the divider plate 12. The reverse flow path is followed after flow-reversal. The resin located in the upward flow path will be lifted a few inches. However, the divider plate 12 assures that the resins are not intermixed after flow reversal. Tank 10 employs two different tank lids designated by the numerals 20 and 22 in order to properly adjust the resin level within the tank. Resin lid 20 is employed during initial filling. The properties of the employed resin are such that during storage of borate ions, during which time relatively low temperature coolant is passing through the resin column, the resin volume shrinks approximately 10%. This 10% is added employing resin tank lid 22 which is subsequently replaced by resin lid 20 to insure that during operation there is always space in the tank available for expansion and contraction of the resins. An advantage of the design exemplified is that flow which enters the resin tank at one side will precool (if the previous operation was release of boron at relatively high temperatures) or preheat (if the previous operation was storage of boron at relatively low temperatures) the liquid which leaves the column on the other side of the divider plate 12. This will cause a faster response than would be possible with other arrangements. The use of dual-directional flow process for boron storage and release in conjunction with a resin tank 10 specifically designed to practice this process enables a wide variety of load follow operations to be practiced in a minimum time. The system of this invention should be contrasted with previous systems which only allowed fixed changes to take place during acceptable time periods.