Patent Number: 043436820
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring to the FIGURE, there is illustrated a plant having a nuclear steam supply unit 1 including a nuclear reactor 2, a steam generator 3 and a coolant pump 4. Pump 4 circulates coolant through the reactor where it is heated, then the steam generator, where the heat of the coolant is transferred to feed water to change the feed water into steam. The cooled coolant is then circulated back to the reactor by the pump 4 where it is again heated. Feed water is fed into the steam generator through a feed water delivery line 5 having a feed water control valve 5' therein. Steam generated in the steam generator 3 is delivered by a main steam delivery line 6 to a high pressure turbine 7 through a turbine control valve 7'. Steam from the high pressure turbine 7 is discharged into a steam reheater 8 and then into a low pressure turbine 9 from which it is then exhausted into condenser 10. As shown, the feed water line 5 extends between the condenser 10 and the steam generator 3 and has therein a feed water pump 11. A plurality of high pressure heaters 12 and 13 are included in the line 5 between the pump 11 and the steam generator 3 forming part of the steam supply unit 1. High pressure heaters 12 and 13 are connected by high pressure heater steam delivery lines 14 and 15 to extraction points 16 and 17 respectively on the turbine 7. Line 14 has a heater operation valve 12' therein while line 15 has a heater operation valve 13' therein. Heater 13 is connected by a drain 18 having a drain control valve 18' therein to the heater 12. Heater 12 in turn is connected by a drain 19 which in turn connects with line 20 having a drain control valve 20' and which leads to condenser 10. Drain 19 also connects with line 21 which extends to a low pressure feed water heater 23. Line 21 has a control valve 21' therein for regulating flow from drain 19 into the low pressure heater 23. While only two high pressure heaters 12 and 13 are shown, more could be included wherein the drains of heaters further from the feed water pump would be connected in series to heaters connected closer to the feed water pump. In addition to low pressure heater 23, further low pressure heaters 24, 25 and 26 are included in the feed water line between the condenser 10 and the feed water pump 11. Low pressure heaters 23-26 are connected by lines 27-30 respectively to extraction points on the low pressure turbine 9. Steam is admitted into steam reheater 8 from the main steam delivery line 6 through reheater control valve 8' and then passes through line 31 having a vlave 31' therein to the high pressure heater 12. The structure described above with the exception of the drain 20 and valve 20' is typical of a plant having a pressurized water reactor cycle. In order to provide for heating of the feed water during start up of the plant, we propose to add start up lines 40 and 41 connecting the high pressure heaters 12 and 13 with the main steam delivery line 6. Start up steam control valves 40' and 41' provided with pressure controllers are positioned in the lines 40 and 41 respectively to regulate the pressure and temperature rise in the heaters 12 and 13 during start up of the plant. The operation of the plant illustrated in the drawing is as follows. Prior to start up, the turbine control valve 7', the steam reheater valve 8' and feed water control valve are all closed and the feed water pump 11 is idle. During start up, it is necessary to add feed water to the steam generator 3 and according to the invention, this feed water is heated prior to entry into the steam generator. This is accomplished by turning the feed water pump 11 on, opening feed water control valve 5' and opening start up control valve 40' to allow steam from the steam generator to pass through lines 6 and 40 to be admitted gradually into the high pressure heater 12. Drain control valve 21' is closed while start up drain control valve 20' is opened such that the only net heat loss is in the heater drain. Any feed water make up that is required is admitted into the condenser 10 through a feed water make up line. When the temperature rise in the heater 12 approaches its design limit, valve 41' is then gradually opened to admit steam gradually into heater 13 to further increase feed water temperature. Valve 18' is opened to open the drain between heaters 13 and 12. Once the desired temperature rise is reached in each heater, its start up valve can be placed on automatic pressure control. This mode of feed water heating is continued until turbine 7 is started by opening valves 7' and 9' simultaneously and is partially loaded. The start up valves 40' and 41' are then gradually closed until the pressures in the heaters 12 and 13 are reduced to the pressures in the turbine at the extraction points 16 and 17. Valves 12' and 13' are then opened while at the same time the start up valves 40' and 41' are closed thus transferring steam supply to the heaters from the extraction points. Steam reheater valve 8' and valve 31' are opened allowing flow of steam through the reheater 8. Valve 20' may then be closed and valve 21' opened to connect a drain from the heater 12 into the low pressure heater 23. The same procedure explained above utilized in start up of the plant may be also utilized in the reverse order when making a controlled shut down of the nuclear steam supply unit 1. When a reactor plant trips from power, auxiliary feed water is conventionally supplied automatically in place of normal feed water where the pump 11 is shut down. The procedure described above can be used to replace the normally cold auxiliary feed water by restarting the main feed water pump 11 and admitting main steam to the heaters 12 and 13 by opening valves 40' and 41'. While the nuclear steam supply unit 1 is shown as comprising a pressurized water cycle system having a steam generator 3 positioned exteriorily of the reactor 2, the unit 1 could take the form of a boiling water reactor which does not have a separate steam generator or coolant pump 4 and which would have feed water directly admitted into the reactor. A plant utilizing the feed water heating means as described above has smaller thermal gradients applied to the line 5 where it enters the steam generator since the temperature of the feed water is raised to more closely approach the temperatures existing in the steam generator. Thermal stresses of the feed water pipes and nozzles where the feed water enters into the steam generator are thus reduced. Further, since heated feed water rather than cooled feed water is admitted into the steam generator, there is less likelihood of inadvertent overfilling of the system because of contraction of water already in the system due to cold feed water being fed into the system.