Patent Publication Number: US-2004042579-A1

Title: Nuclear power plant and method of operating the same

Description:
[0001] This invention relates to the generation of electricity. More particularly it relates to a nuclear power plant. It also relates to a method of regulating the power generated by the plant.  
       [0002] According to one aspect of the invention there is provided a nuclear power plant which includes  
       [0003] a closed loop power generation circuit making use of helium as a working fluid and having at least one compressor;  
       [0004] a recirculation circuit whereby helium can be recirculated around the compressor; and  
       [0005] valve means for regulating the flow of helium in the recirculation circuit.  
       [0006] The power generation circuit may include  
       [0007] a nuclear reactor;  
       [0008] a low pressure compressor;  
       [0009] a high pressure compressor;  
       [0010] drive means for driving the low pressure compressor and the high pressure compressor;  
       [0011] a pre-cooler positioned upstream of the low pressure compressor;  
       [0012] an inter-cooler positioned between the low pressure compressor and the high pressure compressor;  
       [0013] a low pressure recirculation circuit for recirculating helium around the low pressure compressor;  
       [0014] a high pressure recirculation circuit for recirculating helium around the high pressure compressor; and  
       [0015] valve means for regulating the flow of helium in each of the recirculation circuits.  
       [0016] According to another aspect of the invention in a nuclear power plant having a closed loop power generation circuit which uses helium as the working fluid and which has a nuclear reactor, there is provided a method of regulating the power generated by the plant, which includes the step of regulating the flow of helium through the reactor.  
       [0017] When the nuclear power plant is a nuclear power plant as described above, regulating the flow of helium through the reactor may include regulating the flow of helium in the or each recirculation circuit.  
       [0018] The drive means may include, arranged in series, a high pressure turbine, a low pressure turbine and a power turbine drivingly connected, respectively, to the high pressure compressor, the low pressure compressor and an electrical generator, the power generation circuit further including a recuperator having a low pressure side connected between the power turbine and the pre-cooler, and a high pressure side connected between the high pressure compressor and the nuclear reactor, the high pressure recirculation circuit including a high pressure recirculation line in which a recirculation valve is mounted the high pressure recirculation line extending from a point between the high pressure compressor and the high pressure side of the recuperator to a point between the low pressure compressor and the intercooler and the low pressure recirculation circuit including a low pressure recirculation line in which a recirculation valve is mounted, the low pressure recirculation line extending from a point between the low pressure compressor and the intercooler to a point between the recuperator and the pre-cooler.  
       [0019] Regulating the flow of helium in the recirculation circuits, may include controlling the operation of the recirculation valves to regulate the flow of helium in the recirculation circuits.  
       [0020] Regulating the flow of helium through the reactor may include adjusting the helium inventory in the power generation circuit.  
       [0021] To this end the nuclear plant may include a helium inventory control system which is selectively connectable in flow communication with the power generation circuit to permit helium to be introduced into or removed from the power generation circuit.  
       [0022] Adjusting the helium inventory may include connecting the helium inventory control system in flow communication with the power generation circuit selectively to increase or decrease the helium inventory in the power generation circuit and thereby increase or decrease the power generated as required.  
       [0023] The driving force for the transfer of helium between the helium inventory control system and the power generation circuit may be the pressure difference between the helium inventory control system and the power generation circuit.  
       [0024] The helium inventory control system may include a plurality of storage tanks, the pressure in which varies from a low pressure tank to a high pressure tank.  
       [0025] The helium inventory control system may be selectively connectable to the power generation circuit at a relatively high pressure point and a relatively low pressure point of the power generation circuit.  
       [0026] The high pressure point may be downstream of the high pressure compressor.  
       [0027] The low pressure point may be upstream of the low pressure compressor between the low pressure compressor and the power turbine.  
       [0028] In one embodiment of the invention, when the plant is in load following mode and it is desired to increase the power generated, the method may include the step of introducing helium from the helium inventory control system into the power generation circuit.  
       [0029] In this embodiment, helium may be introduced from the helium inventory control system into the power generation circuit at a low pressure point of the power generation circuit. Similarly, helium will typically be extracted from the power generation circuit at a high pressure point and fed to the helium inventory control system.  
       [0030] Helium extracted from the power generation circuit is dumped into the storage tank with the highest pressure which has capacity to accommodate the helium. Helium fed from the helium inventory control system to the power generation circuit is taken from the tank with the lowest pressure and which has capacity to supply the helium.  
       [0031] One problem associated with this arrangement is that when, in load following mode, the introduction of helium into the power generation circuit at a low pressure point in response to a request for a power increase, results in a non-minimum phase response of power, which actually results in a dip in the power generated which can be undesirable.  
       [0032] The method may accordingly include introducing helium into the power generation circuit at a low pressure point of the power generation circuit and compensating for a non-minimum phase response by regulating the flow of helium in the or each recirculation circuit.  
       [0033] Although this arrangement results in an overall decrease in the efficiency of the power generation circuit, it permits the power generated by the power generation circuit to be increased in a manner which avoids the non-minimum phase response of power.  
       [0034] In another embodiment of the invention, increasing the power generated when the plant is in load following mode may include introducing helium into the power generation circuit at the high pressure point in the power generating circuit.  
       [0035] The high pressure point of the power generation circuit is typically between the compressor and the reactor and introduction of helium at this point avoids the non-minimum phase response and hence the dip in power.  
       [0036] In this embodiment the method may include, if necessary, regulating the flow of helium through the or each recirculation circuit to avoid a non-minimum phase response.  
       [0037] To this end, the helium inventory control system may include at least one booster tank in which helium is stored at a pressure higher than that of the maximum pressure in the power generation circuit and from which helium can be fed into the power generation circuit at the high pressure point.  
       [0038] The helium inventory control system may include a compressor arrangement for feeding helium to the at least one booster tank at the desired pressure.  
       [0039] The method may include, as the pressure in the booster tank decreases, feeding helium into the power generation circuit from the helium inventory control system at a low pressure point in the power generation circuit and feeding at least some of the helium exiting the compressor to an upstream side of the compressor so that a portion of the helium circulates around the compressor.  
       [0040] Under load following conditions when a portion of the helium in the power generator is recirculated in the or each recirculation circuit,increasing the power generated may include the step of reducing the volume of helium flowing through the or each recirculation circuit.  
       [0041] The plant may include a variable resistor bank which is electrically disconnectably connectable to the generator.  
       [0042] The plant may include a recuperator bypass line which extends from a position upstream of the high pressure side of the recuperator to a position downstream of the high pressure side of the recuperator and a recuperator bypass valve mounted in the recuperator bypass line to regulate the flow of helium therethrough.  
       [0043] The plant may include a gas bypass line in which a gas bypass valve is provided to regulate the flow of helium therethrough, the gas bypass line extending from a position upstream of the high pressure side of the recuperator to a position upstream of the pre-cooler.  
       [0044] In the event of loss of load, the method may include the steps of,  
       [0045] opening the high pressure recirculation valve, the low pressure recirculation valve and the gas bypass valve;  
       [0046] closing the gas bypass valve; and  
       [0047] regulating the operation of the high pressure bypass valve and the low pressure bypass valve to stabilize the power generation circuit.  
       [0048] Typically, when the valves are opened they are displaced to their fully open position.  
       [0049] The gas bypass valve may be opened immediately after the loss of load event is detected and closed after a predetermined time has elapsed.  
       [0050] The method may include, after the process stabilizes, activating the helium inventory control system to bring the plant into a stable, low power operation mode.  
       [0051] The plant may include a variable resistor bank which is disconnectably connectable to the generator, the method including controlling the speed of the power turbine by regulating the load on the generator via the resistor bank.  
       [0052] Introducing helium into the power generation circuit at the high pressure point can be used both when in load following mode, to step-up the power generated and when rapid increases of generated power are required.  
       [0053] When a power step-down is required the method may include the step of opening at least one of the recirculation valves.  
       [0054] Preferably, the method includes opening both of the high pressure and low pressure recirculation valves.  
       [0055] When the plant includes a variable resistor bank which is disconnectably connectable to the generator, the method may include using the variable resistor to compensate for small changes in the power demand. This arrangement avoids unnecessary wear of the valves.  
       [0056] The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings. 
     
    
    
     [0057] In the drawings,  
     [0058]FIG. 1 shows a schematic representation of part of a nuclear power plant in accordance with the invention; and  
     [0059]FIG. 2 shows a schematic representation of a helium inventory control system forming part of the nuclear power plant in accordance with the invention. 
    
    
     [0060] With reference, particularly to FIG. 1 of the drawings, reference numeral  10  refers generally to part of a nuclear plant in accordance with the invention. The nuclear power plant  10  includes a closed loop power generation circuit, generally indicated by reference numeral  12 . The power generation circuit  12  includes a nuclear reactor  14 , a high pressure turbine  16 , a low pressure turbine  18 , a power turbine  20 , a recuperator  22 , a pre-cooler  24 , a low pressure compressor  26 , an intercooler  28  and a high pressure compressor  30 .  
     [0061] The reactor  14  is a pebble bed reactor making use of spherical fuel elements. The reactor  14  has a working fluid inlet  14 . 1  and a working fluid outlet  14 . 2 .  
     [0062] The high pressure turbine  16  is drivingly connected to the high pressure compressor  30  and has an upstream side or inlet  16 . 1  and a downstream side or outlet  16 . 2 , the. inlet  16 . 1  being connected to the outlet  14 . 2  of the reactor  14 .  
     [0063] The low pressure turbine  18  is drivingly connected to the low pressure compressor  26  and has an upstream side or inlet  18 . 1  and a downstream side or outlet  18 . 2 . The inlet  18 . 1  is connected to the outlet  16 . 2  of the high pressure turbine  16 .  
     [0064] The nuclear power plant  10  includes a generator, generally indicated by reference numeral  32  to which the power turbine  20  is drivingly connected. The power turbine  20  includes an upstream side or inlet  20 . 1  and a downstream side or outlet  20 . 2 . The inlet  20 . 1  of the power turbine  20  is connected to the outlet  18 . 2  of the low pressure turbine  18 . The plant  10  includes a variable resistor bank  33  which is electrically disconnectably connectable to the generator  32 .  
     [0065] The recuperator  22  has a hot or low pressure side  34  and a cold or high pressure side  36 . The low pressure side of the recuperator  34  has an inlet  34 . 1  and an outlet  34 . 2 . The inlet  34 . 1  of the low pressure side is connected to the outlet  20 . 2  of the power turbine  20 .  
     [0066] The pre-cooler  24  is a helium to water heat exchanger and includes a helium inlet  24 . 1  and a helium outlet  24 . 2 . The inlet  24 . 1  of the pre-cooler  24  is connected to the outlet  34 . 2  of the low pressure side  34  of the recuperator  22 .  
     [0067] The low pressure compressor  26  has an upstream side or inlet  26 . 1  and a downstream side or outlet  26 . 2 . The inlet  26 . 1  of the low pressure compressor  26  is connected to the helium outlet  24 . 2  of the pre-cooler  24 .  
     [0068] The inter-cooler  28  is a helium to water heat exchanger and includes a helium inlet  28 . 1  and a helium outlet  28 . 2 . The helium inlet  28 . 1  is connected to the outlet  26 . 2  of the low pressure compressor  26 .  
     [0069] The high pressure compressor  30  includes an upstream side or inlet  30 . 1  and a downstream side or outlet  30 . 2 . The inlet  30 . 1  of the high pressure compressor  30  is connected to the helium outlet  28 . 2  of the inter-cooler  28 . The outlet  30 . 2  of the high pressure compressor  30  is connected to an inlet  36 . 1  of the high pressure side of the recuperator  22 . An outlet  36 . 2  of the high pressure side of the recuperator  22  is connected to the inlet  1   4 . 1  of the reactor  14 .  
     [0070] The nuclear power plant  10  includes a start-up blower system  5  generally indicated by reference numeral  38  connected between the outlet  34 . 2  of the low pressure side  34  of the recuperator  22  and the inlet  24 . 1  of the pre-cooler  24 .  
     [0071] The start-up blower system  38  includes a normally open start-up blower system in-line valve  40  which is connected in-line between the outlet  34 . 2  of the low pressure side of the recuperator and the inlet  24 . 1  of the pre-cooler  24 . Two blowers  42  are connected in parallel with the start-up blower system in-line valve  40  and a normally closed isolation valve  44  is associated with and connected in series with each blower  42 .  
     [0072] A low pressure compressor recirculation line  46  extends from a position between the outlet or downstream side  26 . 2  of the low pressure compressor  26  and the inlet  28 . 1  of the inter-cooler  28  to a position between the start-up blower system  38  and the inlet  24 . 1  of the pre-cooler  24 . A low pressure recirculation valve  48  is mounted in the low pressure compressor recirculation line  46 .  
     [0073] A high pressure compressor recirculation line  50  extends from a position between the outlet or downstream side  30 . 2  of the high pressure compressor and the inlet  36 . 1  of the high pressure side  36  of the recuperator  22  to a position between the outlet or downstream side  26 . 2  of the low pressure compressor  26  and the inlet  28 . 1  of the inter-cooler  28 . A high pressure recirculation valve  51  is mounted in the high pressure recirculation line  50 .  
     [0074] A recuperator bypass line  52  extends from a position upstream of the inlet  36 . 1  of the high pressure side  36  of the recuperator  22  to a position downstream of the outlet  36 . 2  of the high pressure side  36  of the recuperator  22 . A normally closed recuperator bypass valve  54  is mounted in the recuperator bypass line  52 .  
     [0075] The plant  10  includes a high pressure coolant valve  56  and a low pressure coolant valve  58 . The high pressure coolant valve  56  is configured, when open, to provide a bypass of helium from the high pressure side or outlet  30 . 2  of the high pressure compressor  30  to the inlet or low pressure side  18 . 1  of the low pressure turbine  18 . The low pressure coolant valve  58  is configured, when open, to provide a bypass of helium from the high pressure side or outlet  30 . 2  of the high pressure compressor  30  to the inlet  20 . 1  of the power turbine  20 .  
     [0076] The plant  10  includes a gas bypass line  70  in which a gas bypass valve  72  is provided to regulate the flow of helium therethrough. The gas bypass line  70  extends from a position upstream of the inlet  36 . 1  of the high pressure side of the recuperator  22  to a position upstream of the inlet  24 . 1  of the pre-cooler  24 .  
     [0077] Referring now to FIG. 2 of the drawings, the nuclear power plant  10  further includes a helium inventory control system, generally indicated by reference numeral  80 . The helium inventory control system  80  includes eight storage tanks  82 ,  84 ,  86 ,  88 ,  90 ,  92 ,  94 ,  96  and a booster tank  98 .  
     [0078] The pressure in the storage tanks  82  to  96  varies from a high pressure tank  96  to a low pressure tank  82 . The pressure of helium within the booster tank  98  is higher than that within the power generation circuit  12 . To this end, a compressor arrangement, generally indicated by reference numeral  100  is provided to feed helium at a sufficiently high pressure to the booster tank  98  and/or storage tanks  82  to  96 . The helium inventory control system  80  is selectively connectable to the power generation circuit to permit the flow of helium therebetween at a low pressure point  102  and a high pressure point  104  (FIG. 1).  
     [0079] In use, it is necessary that the power output of the nuclear power plant can be adjusted continuously to the power demand or requirement. As described in more detail herebelow, the helium inventory control system can be used to increase and reduce the power generated in the nuclear power plant.  
     [0080] When in load following mode, the generator output is adjusted to the power demand of the grid to which the plant is connected at all times. Typically this will require that the plant be capable of following a sequence of from 100% to 40% to 100% of the maximum continuous power rating without any external compressor. The rate of increase or decrease will typically not exceed 10% of the maximum continuous power rating per minute.  
     [0081] In order to decrease the power generated, helium is extracted from the power generation circuit  12  at the high pressure point and dumped into the storage tank with the highest pressure and spare capacity for receiving the helium.  
     [0082] Several options are available to increase the power generated.  
     [0083] One option includes feeding helium from the helium inventory control system to the power generation circuit at the low pressure point after a request for a power increase. Although this will eventually lead to an increase in power, initially it results in a non-minimum phase response of the power, which results in a dip in the power generated. This dip disturbs the smooth control of the power output of the system.  
     [0084] A second option of increasing the power which avoids the non-minimum phase response of the low pressure injection is by compensating with the compressor recirculation valves  48 ,  51 . This will require that the recirculation valves  48 ,  51  are, under normal circumstances, partially open when the nuclear power plant is in load following mode. If the grid requires a power increase, helium is injected from the helium inventory control system  80  to the power generation circuit at the low pressure point. Simultaneously, one or both of the recirculation valves  48 ,  51  is displaced towards its closed condition which results in an accurately controlled increase in the power generated. The advantage with this arrangement is that the response does not show the non-minimum phase response behaviour and the power increase is easy to control. A disadvantage with this arrangement is that it is necessary to operate the nuclear power plant with the recirculation valves  48 ,  51  partially open so that there is reserve power to cancel the non-minimum phase effect of low pressure injection. Running the nuclear power plant with the compressor recirculation valves  48 ,  51 , partially open, will decrease the overall efficiency of the plant.  
     [0085] A third option of increasing the power which avoids the non-minimum phase response of the low pressure injection is by compensating for the non-minimum phase response by the simultaneous injection of helium at the high pressure point.  
     [0086] A fourth option to increase the power generated in load following mode, is to feed helium from the booster tank  98  of the helium inventory control system  80  to the power generation circuit  12  at the high pressure point of the power generation circuit. This leads to an increase in the power generated without the non-minimum phase response behaviour. As the pressure in the booster tank  98  decreases, additional power is required, the compressor recirculation valves  48 ,  51  will open in order to permit the power generated to be increased by closing the valves  48 ,  51  in the manner described above and thereby avoiding the non-minimum phase response. This process can be optimised in a way that the amount of recirculation around the compressors is at a minimum thereby maximising the efficiency of the power generation plant.  
     [0087] In the event of loss of load, it is important that the speed of the power turbine  20  and the generator  32  not exceed a predetermined maximum speed. In addition, it is preferred that the Brayton cycle remains functioning at very low load conditions, referred to as house load. This process to keep the energy conversion cycle running at house load conditions is called “load rejection”.  
     [0088] In the case of loss of load the low pressure recirculation valve  48 , high pressure recirculation valve  51  and the gas recirculation valve  72  are fully opened. A predetermined time period after the initiating event, the gas bypass valve  72  is closed and the high pressure recirculation valve  51  and low pressure recirculation valve  48  are displaced towards their closed conditions. After the process stabilizes, the helium inventory control system  80  is activated to bring the plant into a stable, low power operation mode and the low pressure recirculation valve  48  and high pressure recirculation valve  51  may be closed if required.  
     [0089] The resistor bank  33 , as part of a power turbine speed controller, may be used to control the speed of the power turbine.  
     [0090] The plant  10  is typically configured to make use of a modified Brayton cycle as the thermodynamic conversion cycle. In the event of an emergency stop of the Brayton cycle, only the gas bypass valve  72  is opened and remains open until the Brayton cycle stops.  
     [0091] In existing power plants of which the Inventor is aware, the load rejection process and also an emergency stop is achieved by bypassing the turbines for at least part of the working fluid. In the present application, however, this solution would result in the introduction of high pressure (of the order of 85 bar) and high temperature (of the order of 900° C.) control valves which are potentially expensive and unreliable. In contrast, with the present invention, load rejection is achieved by operating the valves  48 ,  51 ,  72  which operate at a significantly lower temperature.  
     [0092] When it is desired to step up the power produced by the plant  10  more rapidly than in the load following mode, use can be made of the booster tank to inject helium into the power generation circuit at the high pressure point. Typically, the volume of the booster tank will be selected to permit the power to be stepped up at a rate of at least 20% of the maximum continuous rating per minute for a period of at least 30 seconds and an occurrence frequency of less than once per hour.  
     [0093] To this end, when the power plant has a reactor outlet pressure of approximately 85 bar and a power capacity of approximately 128 MW, the booster tank  98  will typically have a volume of approximately 100 m 3  and the helium will be stored at a pressure of approximately 100 bar.  
     [0094] As mentioned above, in order to reduce the power generated by the plant  10 , helium can be extracted from the power generation circuit and fed to the helium inventory control system. Although this adequately permits the power to be reduced when in load following mode, when it is required for a power step down the process is too slow. Accordingly, in order to have a power step-down, one or both of the recirculation valves  48 ,  51  is opened which results in the mass flow of helium through the reactor  14  decreasing and less power is transferred to the helium. This in turn results in less power being generated in the power turbine. Typically, the plant is capable of operating with a power step down of at least 20% of the maximum continuous rating per minute decrease for a duration of 30 seconds and an occurrence frequency of less than once per hour.  
     [0095] The Inventors believe that a nuclear power plant in accordance with the invention will permit close control of the power generated by the nuclear power plant.