Patent Number: 053352521
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawing FIG. 1, a schematic arrangement of a heat removal system in accordance with the present invention for transferring heat from a reactor gas coolant to a secondary fluid medium is indicated generally. Although the heat removal system finds particular application as a heat removal system in a high temperature gas cooled reactor, utilizing water and steam as the secondary fluid medium, it will become apparent herein that the inventive concept may be employed in other applications with other fluid media. In the illustrated embodiment the non-nuclear portion of the heat removal system is shown as being placed in a lower pit area 10 in close proximity to the nuclear pressure vessel 12 which contains the nuclear portions of the heat removal system, the reactor core (not shown) and other nuclear components (not shown). The nuclear pressure vessel 12 may be of steel or prestressed concrete construction. More particularly, the nuclear portion of the heat removal system is housed within a nuclear steam generator cavity 14 defined internally of the nuclear pressure vessel 12. Turning now to a more detailed description of the heat removal system in acordance with the present invention, and referring to FIG. 1 the nuclear portion of the heat removal system is relatively compact and thus enables the nuclear steam generator cavity 14 within the nuclear pressure vessel 12 to be located below a transverse reactor gas coolant inlet duct 16 which conventionally communicates with the lower end of the core cavity (not shown) for removing reactor gas coolant therefrom. The nuclear steam generator cavity 14 is a generally cylindrical configuration and has a suitable metallic shield liner 18 establishing the outer peripheral surface of the cavity 14 and to which is suitably attached a thermal barrier 20 in a known manner. Within the nuclear steam generator cavity 14 of the nuclear pressure vessel 12 the steam generator is comprised of the reheater tube bundle 22 and the main steam tube bundle 24 which are arranged within a metallic shroud 26 the upper end of which serves as a flow guide 26a to direct reactor gas coolant to the inlet of the reheater tube bundle 22, while the lower portion of the shroud 26b immediately surrounding the reheater tube bundle 22 and the main steam tube bundle 24 is of double wall construction to reduce heat transfer through the shroud 26b. The reheater tube bundle 22 is above the main steam tube bundle 24 which is comprised of the initial superheater stage 24a above and the economizer/evaporator stage 24b below. The reheater tube bundle is comprised of a plurality of heat transfer tubes arranged such that internal steam flows in parallel tube circuits which are connected by reheater lead-in tubes 28 to the reheater inlet penetration 30 in the nuclear pressure vessel 12, and by reheater lead-out tubes 32 connected to the reheater outlet penetration 34 in the nuclear pressure vessel 12. The main steam tube bundle 24 within the nuclear pressure vessel 12 is also comprised of a plurality of heat transfer tubes which are arranged such that internal water and steam flows in parallel tube circuits connected by economizer/evaporator lead-in tubes 36 to the economizer/evaporator inlet penetration 38 in the nuclear pressure vessel 12, and by initial superheater lead-out tubes 40 to the initial superheater outlet penetration 42 in the nuclear pressure vessel 12. Outside of the nuclear pressure vessel 12 the non-nuclear portions of the heat removal system are located in the lower pit area 10. The finishing superheater tube bundle 44 is contained in the finishing superheater pressure vessel 46 which is located adjacent to the nuclear pressure vessel 12 such that the length of the shell side inlet pipe 48, which carries maximum temperature shell side steam from the reheater tube bundle outlet penetration 34 in the nuclear pressure vessel 12 to the finishing superheater shell side inlet penetration 50 is minimized. Shell side steam flows from the finishing superheater shell side inlet penetration 50 through the finishing superheater tube bundle 44 and exits from the finishing superheater pressure vessel 46 through the finishing superheater shell side outlet penetration 52. Shell side steam flow continues through the shell side connecting pipe 54 and enters the intermediate superheater pressure vessel 56 through the intermediate superheater shell side inlet penetration 58, flows through the intermediate superheater tube bundle 60, to exit from the intermediate superheater pressure vessel 56 through the intermediate superheater shell side outlet penetration 62. Shell side steam flow continues through the reheat turbine inlet pipe 64 to deliver power to the reheat turbine 66, and then continues through the reheat turbine outlet pipe 29 to the plant condenser (not shown). Tube side steam from the initial superheater outlet penetration 42 in the nuclear pressure vessel 12 flows through the tube side inlet penetration 70 in the intermediate superheater pressure vessel 56, continues through the intermediate superheater tube bundle 60, and exits from the intermediate superheater pressure vessel 56, through the intermediate superheater tube side outlet penetration 72. Tube side steam then flows through tube side connecting pipe 74 to enter the finishing superheater pressure vessel 46 through the finishing superheater tube side inlet penetration 76, flows through the finishing superheater tube bundle 44, to exit from the finishing superheater pressure vessel 46 through the finishing superheater tube side outlet penetration 78. Steam flow continues through the main steam turbine inlet pipe 80 to the main steam turbine 82, to which it delivers power, and returns through the main steam turbine outlet pipe 98 to the reheater tube bundle inlet penetration 30 in the nuclear pressure vessel 12 for reheating. A water recirculation system is provided to produce satisfactory water velocities in the economizer/evaporator tube bundle stage 24b during low load and start-up operation. In this system water is received from the tube side inlet pipe 68 at the intermediate superheater pressure vessel 56 tube side inlet, passed through heat exchanger 17 to condense excess steam and reduce water temperature to meet recirculation pump 84 requirements, is then circulated by the recirculation pump 84 to mixing tee 86 where recirculated water is mixed with feedwater flow from the plant feedpump (not shown) coming through feedwater pipe 31. Mixed flow continues from the mixing tee 86 to the economizer/evaporator inlet penetration 38 in the nuclear pressure vessel 12. A bypass system is also provided to divert excess flow from the tube side inlet pipe 68 at the intermediate superheater pressure vessel 56 tube side inlet, through the bypass pipe 96 and the pressure reducing valve 94, to the flash tank 92. Excess flow occurs because minimum water velocity requirements in the main steam tube bundle 24 may result in main steam flow above that required to operate the main steam turbine 82 during low load and start-up operation of the plant. Water and steam are separated in the flash tank 92, water being drained through flash tank drain pipe 26 to the plant condenser (not shown), and low pressure steam being diverted through the flash tank steam outlet pipe 23 and the flash tank steam outlet valve 25 to the main steam turbine outlet pipe 98 for return to the reheater tube bundle 22 during plant start-up. Low pressure steam from the flash tank 92 may also be used for hot restarts and other start-up purposes. The by-pass system also serves as a pressure relief system. A main steam diverting pipe 19 is also provided to deliver low temperature superheated steam from the tube side inlet pipe 68 at the intermediate superheater pressure vessel 56 inlet to plant feedwater heaters (not shown). Diverting a portion of the main steam flow to feedwater heaters during continuous plant operation increases the ratio of reheat steam flow to main steam flow through the intermediate superheater tube bundle 60 and the finishing superheater tube bundle 44, thereby reducing the maximum steam temperature requirement at the reheater tube bundle 22 outlet. In briefly reviewing the operation of the steam generator of the present invention, hot reactor gas coolant which during maximum continuous operation may be up to 1600 degrees F. ,at a pressure of approximately 700 psia. and flow rate of between 3 and 6 lb./sec.sq.ft., enters the nuclear steam generator cavity 14 from the reactor gas coolant inlet duct 16, passes into the open top of the flow guide portion of the shroud 26a, flows downwardly through the reheater tube bundle 22, then through the main steam tube bundle 24, and radially through the space between the bottom face of the main steam tube bundle 24 and the thermal barrier 20 on the lower surface of the nuclear pressure vessel 12. The reactor gas coolant, now at substantially lower temperature then passes upwardly within a generally annular flow area between the outer surface of the double wall portion of the shroud 26b and the thermal barrier 20 on the inner surface of the nuclear pressure vessel 12, then outwardly through the annular space 15 for return to the reactor core (not shown), it being understood that flow of reactor gas coolant is effected by a gas circulator (not shown). As the reactor gas coolant passes downwardly within the shroud 26, reheat steam enters reheater inlet penetration 30 in the nuclear pressure vessel 12 simultaneously with feedwater entering the economizer/evaporator inlet penetration 38 in the nuclear pressure vessel 12 during continuous operation of the plant. The reheat steam, which is coming from the main steam turbine outlet pipe 98, is at a temperature of approximately 575 degrees F.,flow rate of approximately 300 lb./sec.sq.ft. and pressure of approximately 700 psia. The feedwater is at an inlet temperature of approximately 350 degrees F., flow rate of approximately 400 lb./sec.sq.ft. and pressure of between 2800 and 4000 psia. The entering reheat steam passes upwardly in series through the reheater lead-in tubes 28 and the reheater tube bundle 22, while feedwater passes upwardly in series through the economizer/evaporator lead-in tubes 36, the economizer/evaporator tube bundle stage 26b and the initial superheater tube bundle stage 24a. During such upward passage within the reheater tube bundle 22, which is arranged in counterflow with respect to the reactor gas coolant flow, reheat steam increases in temperature to between 1300 and 1500 degrees F. by heat transfer from the downwardly flowing reactor gas coolant, the temperature of which is reduced to approximately 850 degrees F. upon reaching the lower end of the reheater tube bundle 22, continuing downwardly to enter the main steam tube bundle 24. Simultaneously during similar upward passage of feedwater within the economizer/evaporator tube bundle stage 24b and the initial superheater tube bundle stage 24a, which are arranged in counter flow with respect to the reactor gas coolant flow, the feedwater undergoes a phase change to superheated steam emerging at the top of the main steam tube bundle 24 at a temperature of approximately 750 degrees F. The phase change and temperature increase is effected by heat transfer from the downwardly flowing reactor gas coolant which is emerging from the reheater tube bundle 22. The temperature of the reactor gas coolant is reduced to approximately 500 degrees F. upon reaching the lower end of the main steam tube bundle 24. Reheat steam exiting from the reheater tube bundle 22 passes downwardly through the reheater lead-out tubes 32 and through the reheater outlet penetration 34 in the nuclear pressure vessel 12 where tube to tube differences in temperature are dissipated by mixing. Similarly superheated steam which is exiting from the initial superheater tube bundle stage 24a passes downwardly through the initial superheater lead-out tubes 40 and through the initial superheater outlet penetration 42 in the nuclear pressure vessel 12 where tube to tube differences in temperature are dissipated by mixing. The intermediate superheater pressure vessel 56 and the finishing superheater pressure vessel 46 which ape located outside of the nuclear pressure vessel 12 contain respectively, the intermediate superheater tube bundle 60 and the finishing superheater tube bundle 44, which produce an increase in temperature of superheated steam emerging from the initial superheater tube bundle stage 24a, by regenerative heat transfer from high temperature excess heat which is available in the shell side reheat steam flow. The reheat steam exiting from the nuclear pressure vessel 12, which during maximum continuous operation is at a temperature of between 1300 and 1500 degrees F., a flow Pate of approximately 300 lb./sec.sq.ft. and a pressure of approximately 700 psia flows downwardly in shell side inlet pipe 48 where tube to tube temperature differences which developed in the reheater tube bundle 22 are dissipated by mixing, to enter the finishing superheater pressure vessel 46 shell side through the finishing superheater shell side inlet penetration 50. Shell side reheat steam then flows transversely across the finishing superheater tube bundle 44, which is arranged in counterflow with respect to shell side reheat steam flow and tube side main steam flow, and exits from the finishing superheater pressure vessel 46 through the finishing superheater shell side outlet penetration 52 where it enters the shell side connecting pipe 54 from which shell side reheat steam flow continues into the intermediate superheater pressure vessel 56 through the intermediate superheater pressure vessel shell side inlet penetration 58. Shell side reheat steam flow then flows transversely across the intermediate superheater tube bundle 60 which is arranged in counterflow with respect to shell side reheat steam flow and tube side main steam flow before exiting from the intermediate superheater pressure vessel 56 through the intermediate superheater shell side outlet penetration 62 at a temperature of approximately 950 degrees F. As reheat steam flows through the shell side of the intermediate superheater tube bundle 60 and the finishing superheater tube bundle 44, main steam flow from the initial superheater tube bundle stage 24a passes downwardly through the initial superheater outlet penetration 42 in the nuclear pressure vessel 12, and downwardly through the intermediate superheater pressure vessel tube side inlet pipe 68 where tube to tube temperature differences which developed in the main steam tube bundle are dissipated by mixing, to split into two flow streams in which a flow of approximately 450 lb./sec.sq.ft. at a temperature of approximately 750 degrees F. and pressure of between 2800 and 4000 psia, continues into the intermediate superheater pressure vessel 56 through the intermediate superheater tube side inlet penetration 70, while a flow of approximately 50 lb./sec.sq.ft. at a temperature of approximately 750 degrees F. and pressure of between 2800 and 4000 psia. enters the main steam diverting pipe 11 continuing on to plant feedwater heaters (not shown). Main steam flow of approximately 450 lb./sec.sq.ft. continues through the tube side of the intermediate superheater tube bundle 60 which is arranged in counterflow with respect to shell side reheat steam flow and tube side main steam flow, and exits from the intermediate superheater pressure vessel 56 through the intermediate superheater tube side outlet penetration 72 where main steam flow enters the tube side connecting pipe 74 in which tube to tube temperature differences which developed in the intermediate superheater tube bundle 60 are dissipated by mixing. Main steam flow then enters the finishing superheater pressure vessel 46 through the finishing superheater tube side inlet penetration 76, flows through the finishing superheater tube bundle 44, and exits from the finishing superheater pressure vessel 46 through finishing superheater pressure vessel tube side outlet penetration 78. During passage through the intermediate superheater tube bundle 60 and through the finishing superheater tube bundle 44 main steam flow attains a temperature of approximately 950 degrees F. and tube to tube temperature differences which developed in the finishing superheater tube bundle 44 are dissipated by mixing in the main steam turbine inlet pipe 80, before reaching the main steam turbine 82. Upon delivering power to the main steam turbine 82, main steam flow at reduced temperature and pressure returns through main steam turbine outlet pipe 98, to the reheat inlet penetration 30 in the nuclear pressure vessel 12. During start-up and low load operation of the plant the recirculation system and the bypass system are in operation to maintain minimum required water velocities, and thereby produce positive upward flow in all of the parallel tube circuits, in the economizer/evaporator tube bundle stage 24b, and the initial superheater tube bundle stage 24a. The recirculation system is operated by opening the inlet valve 88 and the outlet valve 90 while the recirculation pump 84 is running. The recirculation pump inlet heat exchanger 17, inlet valve 88 and outlet valve 90 are adjusted to provide a minimum flow rate of 100 lb./sec.sq.ft. at a temperature of approximately 350 degrees F. and a pressure of between 2800 and 4000 psia to the economizer/evaporator inlet penetration 38 in the nuclear pressure vessel 12. Flow in excess of approximately 133 lb./sec.sq.ft. at the initial superheater outlet penetration 42 in the nuclear pressure vessel 12 is diverted to the bypass flash tank 92 during start-up and low load operation of the plant, to maintain feedwater flow between one-quarter and one-third of maximum continuous flow. While a preferred embodiment of the present invention has been illustrated and described it will be understood to those skilled in the art the changes and modifications that may be made therein without departing from the invention in its broader aspects. Various features of the invention are defined in the following claims.