Apparatus and Method for Depressurizing, Degassing and Affording Decay of the Radioactivity of Weakly Radioactive Condensates in Nuclear Power Plants

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of the steam and water circulation system of a boiling water reactor, of the invention.

FIG. 2 is a schematic circuit diagram of means for guiding the secondary condensates.

FIG. 3 is a cross section through a condenser system showing the condenser having a lower condensate collecting tank, taken along line III--III of FIG. 4.

FIG. 4 is a longitudinal section through the condenser assembly lower part taken along line IV--IV of FIG. 3, and

FIG. 5 is a horizontal section through the condensate collecting tank, taken along line V--V of FIG. 4.

FIG. 1 shows the circuit diagram for a boiling water reactor. From the boiling water reactor 100, the generated saturated steam flows first through the line 101, to the saturated steam turbine 102 and, therefrom, through lines 103, into the steam desiccator 104 and through the intermediate overheater 106 which is heated by fresh steam, via bypass line 105. From this intermediate overheater, the steam arrives through line 107, in the low pressure turbine 1. The turbines 102 and 1 are seated on a common shaft and drive the generator 108. The exhaust steam of the low pressure part of turbine 1 is guided through the line 2, into the condenser and, here, following the condensation and collection in the condensate collecting tank 4, returned through the condensate pump 109, into the cycle, in form of feed-water. The feed-water then flows in the line 110, first in the heat exchanger 111 wherein the drive steam of the steam jet vacuum pump 112 gives off its heat to the feed-water. Thereafter, the feed-water passes through the low pressure preheaters 113 and 114 which are heated via bleeder steam lines 115 and 116, with bleeder steam from the low pressure turbine 1. The feed-water pipe 117, proper, is followed by the high pressure preheaters 118 and 119 which, by bleeder steam lines 120, 121 are heated with bleeder steam from the saturated steam turbine 102. A line 122 is connected into the last high pressure preheater 119 from the intermediate overheater 106 drain or discharge.

The condensates of the entire preheater path are gradually guided back through lines 123, 124 and 125, from preheater 119 to preheater 113. From the first low pressure preheater 113, the condensate is then guided through the condensate cooler 126 and the line 6, into the pressure relief chamber 5.

This so-called secondary condensate is utilized now, in accordance with the invention, for the degassing of the main condensate. The pressure relief chamber 5 which is shown as a vertical standpipe absorbs all constantly collecting secondary condensates such as, for example, the condensate from the preheater 111 which is guided through line 6' to the chamber 5. These secondary condensates collect at a higher pressure than the main condensate in the condenser 3. The chamber 5 is connected, by line 127, with the condenser 3, through a throttle or choke 7, installed into the line 127. The steam being collected in the chamber 5 is thereby guided directly back into the condenser 3 and is condensed therein. Through the choke 7, a slightly higher pressure than prevails in the condenser 3, is established in the chamber 5. Thus, for example, the condenser pressure may amount to 0.04 atmospheres at 28.degree. C, while the secondary condensate in the decompressor 5 is under a pressure of 0.06 atmospheres at 35.degree.. The water is therefore approximately at the boiling point.

Referring to FIG. 2, the remaining expansion of the secondary condensates and the simultaneous degassing of the main condensate is now effected in the degassing channels 15, 16, 17 and 18, which are schematically illustrated in FIG. 2 and in cross-section in FIG. 3. Depressurizing chambers 9, 10 and 10a, are connected through suitable valves and connectors 20 - 23 to channels 15 - 18.

FIG. 3 shows a cross-section through the entire condenser 3 with the condensate collecting tank 4 underneath. The exhaust steam from the low-pressure turbine 1 flows through the steam dome 130 towards the piping or baffles 131 of the condenser 3. This piping 131 which is cooled is divided into individual packages whose peripheries are shown in the drawn cross-section.

Also, the condenser has in its central lower portion an air cooler unit 132 where oxygen-rich condensate is precipitated. These air cooler units are disposed within a separate housing 133, open on the bottom. The degassing channels 15, 16 and 17, in which the condensate flows have on a longitudinal wall, an overflow edge 28, so that a condensate level A is created in these channels. Beneath the air cooler units 132, the guide baffles 25 and 26 are arranged so that the oxygen-rich condensate flows from these air cooler units 132, separately, toward the channels 16 and 17. The remaining condensate is caught on the right side by the baffles 24a and 24b and, on the left side, by the baffles 27 and 27b and are guided past channels 16 and 17 having the oxygen-rich condensate, into the channels 15 and 18. The condensate is then guided through baffles 24 and 27, from the external region into the channels 15 and 18.

In these channels 15 to 18, are arranged horizontal feed pipes 11, 12, 13 and 14, below the overflow edge 28, so that these feed pipes are disposed completely under the resulting condensate level. These feed pipes 11 to 14 are in contact by lines 8, (FIGS. 1, 2 and 4) with the depressurizing chamber 5, and pass the secondary condensate from the chamber 5, into these feed pipes 11 to 14. These pipes have discharge openings 30 on their undersides through which the secondary condensate, as previously indicated is at higher pressure and at higher temperature than the main condensate, can flow out. The pressure of the secondary condensate in the feed pipes 11 to 14 must be high enough that this secondary condensate will reliably bubble out downwardly from the openings 30 and overcome the pressure of the thereabove situated water column of the main condensate. The pressure of the secondary condensate in the chamber 5 is so adjusted that the pressure drop through the throttle or choke 7 and the water column of the main condensate disposed above the discharge openings 30, just about maintains a balance.

Following the discharge of the secondary condensate from the openings 30, it bubbles out and evaporates immediately due to the low pressure of the main condensate and thereby provides the degassing of the main condensate. The condensate running off from the overflows 28 is caught by a slightly inclined runoff sheet 31 and fed to an outlet discharge nozzle 32, by the inclined outlet 33 on the outside of the run-off sheet 31. Therefore, a mixing of the accumulating oxygen-rich condensate from the air cooler unit 132 and the remaining condensate, is effected on the run-off sheet 31.

In such a boiling water plant not only the constant secondary condensates are accumulated but, also, additional secondary condensates occurring in the event of a disturbance, if, for example, the load at generator 108 is suddenly disconnected or if other interferences occur. During such a disruption, additional secondary condensates accumulate in great amounts and are collected in separated pressure relief chambers 9, 10 and 10a (FIG. 2). These secondary condensates may arrive for example, via line 218, from the steam overheating run-off 106 or may arrive via emergency run-offs 129, from the lowest preheater 113 when such a preheater stops working and is bridged. Other, not specifically illustrated, disturbances which also produce a considerable accumulation of secondary condensates are, for example, the stoppage of the steam generator for the turbine or of the decompressor for the feed pump. These secondary condensates which collect during an interference, are collected in pressure relief standpipes 9 and 10 and are expanded to the pressure of the condenser 3.

In order to mix the secondary condensates occurring during a disturbance, also with the main condensate, the arrangement as shown in FIG. 4 was provided. Here, the inlet pipes 11, 12, 13 and 14 extend only approximately about three-quarters of the total length of the degassing channels 15 to 18. The read ends of these channels 15 to 18 are separated by vertical baffles 34. In this area, separated by the baffles 34, the secondary condensates collect during a disturbance from pressure relief standpipes 9 and 10, FIGS. 1 and 2, are directly fed by pipe line 19 and the inlets 23 which open into the channel 18. Due to the greater amount accumulating during a disturbance, the degassing process which occurs during operation, is no longer possible and is not required.

By thus dividing the condenser into two halves, where each half is fully operable by itself, one-half of the condenser may be shut down in case of a leak in order to prevent cooling water from getting into the circuit. As will be seen in FIG. 3, the steam space above the run-off sheet 31 is divided into two halves by a partition 31a which extends transversely to the walls 36 but above the collecting space 35. At the outer ends, decay chamber 35 has condensate feed connections 44 and 45, and 46 and 47, respectively, which are connected with the outlets 32 for the condensate running off from the channels 15 to 18 via reversing lines, not shown.

Because in a boiling-water reactor the condensates are weakly radioactive, it is necessary to provide additionally for these condensates a decay section in which the condensates must remain for a definite time before they are returned to circulation via the condensate pumps. Accordingly, dwelling sections are obtained by the provision of a specially designed collecting space 35 disposed underneath the run-off sheets 31 in the lower part of the condenser 4.

As shown in FIG. 5, this collecting space 35 is subdivided by vertical partitions 36 into individual lanes. These partitions, however, have cutouts 37 and 38 which are staggered relative to one another, so that the condensates entering via inlets 44 to 47 are directed through the condenser collecting space 35 along winding or tortuous paths as indicated by the dashed lines 41 in FIG. 5. By providing this flow path for the condensate, a definite decay time for the radioactivity of the condensate is therefore obtained because of the longer flow time which is required. After flowing through this decay path, the condensate is fed to the main condensate pump via connections 42.

By the method and the apparatus according to the present invention, degassing of the condensate is, therefore, effected directly in the condenser, and an appropriate subdivision of the cooling pipes makes it possible to degas the oxygen-rich condensate which comes from the air cooler units separately from the rest of the condensate. In addition, a true separation of degassing and storage spaces has been obtained where the storage spaces themselves, due to appropriate design, additionally serve as the decay section for the radioactivity of the condensate. The arrangement described provides a very low structural height.

In operation in the turbine system as shown in FIG. 1 of the application, the main condensate of the exahust steam comes directly from turbine 1 collects in the condenser 3. In addition, the turbine system includes so-called secondary condensates which flow from the individual preheater stages or from steam-jet ejectors. Since, for the preheating of the feed water, bleeder steam is taken from higher pressure stages of the turbine as the heating medium, the condensate of the bleeder steam, after flowing from the individual preheaters, exhibits a higher pressure and higher temperature than the main condensate in the condenser 3. Both condensates consist of water. The secondary condensates coming from the preheater stages are fed through the lines 6 to the members 5. The pressure relieving standpipes 5 are connected with the main condenser 3 by means of a conduit provided with a choke 7. Since, in the standpipes 5, due to the higher pressure of the secondary condensates, a higher pressure prevails than in the condenser 3, suitable adjustment of the choke 7 can result in a lower pressure in the standpipes 5. Therefore, a depressurization of the secondary condensates takes place. In addition to the standpipes 5, there are also provided standpipes 9, 10 and 10a, wherein secondary condensates are collected which are produced in the event of a disturbance. Such a disturbance, may, for example arise when a preheater is defective and is disconnected and must be bridged by a bypass line. Condensate is produced also in this case, particularly from so-called mixing preheaters. This condensate produced in the event of a disturbance should be collected separately.

The condensate flowing from condenser 3 is now collected in degassing channels 15 to 18. The secondary condensate from standpipe 5 is conducted into the individual pipes 11 to 14 through line 8. These pipes are provided with openings at their underside, through which the secondary condensate flows into the main condensate. However, since the secondary condensate exhibits a higher pressure and higher temperature than the main condensate, it will immediately vaporize upon leaving pipes 11 to 14 and rise to the surface in the form of steam bubbles. Since the secondary condensate has a higher temperature than the main condensate, the main condensate is thereby heated to the point where it practically begins to boil, so that in this way an additional deaeration of the main condensate is assured.

Regarding the feeding of the various condensates, that is, the oxygen-enriched condensate from the air coolers and the remaining main condensate, it must be noted that this is not done selectively but these two condensates are produced continuously and in each case at the same location. The oxygen-enriched condensate drips onto the run-off baffles 25 and 26 and thus is fed to the channels 16 and 17, whereas the remaining main condensate falls upon the remaining surface, that is, baffles 27a and 27b as well as 27, and is conducted from there into the degassing channels 15 and 18. Conduction of the condensate from the run-off surfaces 25 and 26 onto the baffle 27 is not provided for. Rather, the condensates collect only after running over the overflow baffles 28 onto the run-off sheet 31 where they are mixed. During this running off, a further degassing takes place, to a small extent. All condensates caught by the run-off baffles 26, 27, 27a and 27b, come from the main condenser 3. Only the secondary condensates come from the line 8 and are fed therefrom through the pipes 11 - 14 from which the secondary condensates emerge.