WO2003046929A2

WO2003046929A2 - System for and method of controlling a nuclear power plant

Info

Publication number: WO2003046929A2
Application number: PCT/IB2002/004988
Authority: WO
Inventors: Michael Christiaan Nieuwoudt; Hendrik Albertus Grobbelaar
Original assignee: Pebble Bed Modular Reactor (Proprietary) Limited
Priority date: 2001-11-30
Filing date: 2002-11-28
Publication date: 2003-06-05
Also published as: AU2002351036A1; AU2002351036A8; WO2003046929A3

Abstract

The invention provides a method and a control system for controlling a nuclear power plant. The system includes a plurality of sensors for monitoring variables relating to the operation of the nuclear power plant. A feedback control system is used to maintain plant stability in the or each mode of operation. When required, a logic control system is used for controlling a transition from one mode of operation to another mode of operation.

Description

SYSTEM FOR AND METHOD OF CONTROLLING A NUCLEAR POWER PLANT

THIS INVENTION relates to a control system for and a method of controlling a nuclear power plant.

In a nuclear power plant it is necessary to control the operation of the plant when in a particular mode of operation and to control the transition of the plant from one mode of operation to another.

According to one aspect of the invention there is provided a method of controlling a nuclear power plant, which method includes monitoring the operation of the power plant via sensing means located throughout the plant and collecting data relating to the operation of the plant thereby to determine a plant status, which is indicative of a mode of operation of the plant; using feedback control to maintain plant stability in the or each mode of operation; and when required controlling a transition from one mode of operation to another mode of operation.

According to another aspect of the invention there is provided a control system for a nuclear power plant having a closed loop power generation circuit which uses helium as the working fluid and which includes a nuclear reactor, a high pressure turbine, a low pressure turbine, a power turbine and generator, a recuperator, a pre-cooler, an intercooler, a low pressure compressor and a high pressure compressor, the control system including sensing means for monitoring a plurality of variables relating to the operation of the nuclear power plant, thereby to determine a state of the plant ; at least one feedback control system, associated with at least one subsystem of the plant for controlling the operation of the subsystem when the plant is in a particular mode of operation; and a logic control system for selectively controlling the transition of the nuclear power plant from one mode of operation to another.

The method may include, when required, selectively controlling a transition from one mode of operation to another mode of operation as a result of an operator instruction.

The method may include controlling the transition from one mode of operation to another mode of operation in the event of a transient or unexpected occurrence , due to factors either internal or external to the plant.

The plant may have a closed loop power generation circuit which includes a nuclear reactor, a high pressure turbine, a low pressure turbine, a power turbine and a generator, a recuperator, a pre-cooler, an intercooler and a high pressure compressor, and the modes of operation in which the plant is operable may be selected from: a first mode of operation, in which the reactor is in de-fuelled maintenance mode; a second mode of operation, in which the plant is in fuelled maintenance mode; a third mode of operation, in which the plant is in reactor shut- down mode; a fourth mode of operation in which the plant is in standby mode; a fifth mode of operation, in which the plant is in power conversion unit (PCU) operational mode; and a sixth mode of operation, in which the plant is in power operational mode.

The method may include, when the plant is in the second or fuelled maintenance mode, using feedback to control the temperature of the plant via a temperature controller and a core conditioning system (CCS).

Accordingly, the system may include a reactor temperature controller.

The method may include, when the plant is in the third or reactor shut-down mode of operation, controlling the speed of a start-up blower system, thereby to control the mass flow rate of coolant through the core, and hence through the pre-cooler, thereby removing heat generated in the reactor core due to decay, from the core.

The plant may include a start-up blower system and the reactor temperature controller controls the speed of the start-up blower system thereby to control the mass flow of working fluid in the plant.

The method may further include, when the plant is in the third mode of operation, controlling an inlet temperature of the reactor by partially bypassing the recuperator, via control of a recuperator bypass valve. Accordingly, the method may include monitoring the inlet temperature to the start-up blower system to prevent unacceptably high start-up blower system temperatures and measuring the reactor outlet temperature and comparing it to the reactor inlet temperature to prevent an unacceptably high temperature differential across the reactor.

In doing this, the start-up blower system outlet temperature will also change.

The control system may include a recuperator inlet temperature controller for controlling a coolant valve or valves, thereby to control the recuperator inlet temperature.

The nuclear power plant typically includes a power conversion unit, in which thermal energy generated by the reactor is converted into electrical energy. In a preferred embodiment of the invention the thermodynamic conversion cycle is based on a Brayton cycle.

The method may include, when the plant is in the fourth mode of operation, maintaining the nuclear plant in one of a reactor ready sub-mode or main power system ready sub-mode using a conditioning controller.

Accordingly, the system may include a conditioning controller which includes a temperature controllerfor controlling the reactor outlet temperature; a recuperator inlet temperature controller, for controlling a coolant valve or valves, thereby to control the recuperator inlet temperature; a reactor inlet temperature controller; a power turbine and generator speed controller; a startup blower system controller; and a compressor guide vane position controller for maintaining a surge margin at a maximum value.

The method may include, when the plant is in the fifth mode of operation, operating the plant with a stable Brayton cycle, the method further including controlling a reactor outlet temperature, controlling the power to a given set point and regulating a generator frequency if the plant is disconnected from an electrical grid.

The system accordingly includes a temperature controller for controlling a reactor outlet temperature, a base load controllerfor controlling the power to a given set point and a power turbine generator (PTG) speed controller to regulate the generator frequency if the plant is disconnected from the electrical grid.

When in the sixth mode of operation, the plant may be in a reduced capability sub-mode or a normal power operation sub-mode.

In the reduced capability operation sub-mode, the base-load power controller will be operational.

The system may include the following controllers which may be operational in the sixth or power operational mode of operation, namely: governing controller for primary frequency support, also known as governing; generation controller for automatic generation control (AGC) also known as regulation or secondary frequency support; and/or base load controller for load following. Accordingly, the system may include a temperature controller for controlling a normal reactor outlet temperature and a floating reactor outlet temperature; and a power controller which includes a base load controller, for controlling the plant during periods of constant demand and a bypass reserve controller for controlling rapid power request changes by controlling helium injection and a high pressure injection controller.

Controlling the transition between the third and fourth modes may include switching from a decay heat temperature controller to a normal temperature controller.

The method may also include controlling a transition between intermediate shutdown load and reactor ready mode. The method may also include controlling the reverse transition.

The method may include controlling a transition between partial shutdown mode and reactor ready mode.

The method may further include controlling a transition between reactor ready mode and main power system (MPS) ready mode.

The method may further include controlling a transition from MPS ready mode to power operation mode.

Furthermore, the method may include controlling a transition from

PCU operational mode to power operation mode. The method may include controlling a transition from PCU operational mode to MPS ready mode.

The method may also include controlling transitions from: reduced capability operation to normal power operation and vice versa;

MPS ready to partial shut-down; reactor ready to partial shut-down; reduced capability operation to MPS ready; and reduced capability operation to PCU operational.

The method may further include controlling a loss of load transient detected in the power operational mode.

More particularly, the method may include , when the plant is in power operational mode and a loss of load condition is detected, restricting the speed of the generator.

The method may include sensing the power turbine frequency and triggering a loss of load condition ifthefrequencyand/orthe rate of increase of the frequency exceed predetermined limits. In an embodiment of the invention, if the frequency exceeds 52,5 Hz or the rate of increase of the frequency exceeds 2,5 Hz/s then the loss of load condition is triggered.

Restricting the speed of the generator may include opening a gas cycle bypass valve to dissipate power turbine fluidic power.

The method may include monitoring the generator and/or power turbine and closing the gas bypass valve when the rotational acceleration of the generator and/or power turbine becomes negative to keep the Brayton cycle self-sustaining.

Restricting the speed of the generator may include opening at least one of a high pressure recirculation valve, a low pressure recirculation valve and a high pressure coolant valve.

Restricting the speed of the generator may include applying an electrical load to the generator by means of a resistor bank.

Furthermore, the method may include controlling a PCU trip transient.

More particularly, the method may include when the plant is in power operational mode, monitoring the plant and effecting a power conversion unit trip if a predetermined condition is detected.

Effecting a power conversion unit trip may include opening a gas cycle bypass valve to dissipate power turbine fluidic power, and shut down the Brayton cycle; applying an electrical load to the generator to decelerate the power turbine and the generator; and activating a start-up blower system to maintain flow through the reactor.

The predetermined condition may include, inter alia, the detection of excessive vibration or an excessive loss of coolant, an excessive deviation of the surge margin, excessive manifold pressure, excessive temperatures, over speed, generator system fault, electrical protection signal and the like. The method may include controlling a control rod reverse and initiating a reactor scram and control rod scram.

Furthermore, the system may be configured to control the plant in the event of the detection of a fault mode.

The control system may include a base load controller for controlling the power generated by the plant to a given set point and a power turbine generator speed controller to regulate the generator frequency if the plant is disconnected from the electrical grid.

According to another aspect of the invention in a nuclear power plant which has a closed loop power generation circuit which includes a power turbine which is drivingly connected to a generator, there is provided a method of controlling the plant which includes, in the event of a loss of load condition being detected, restricting the speed of the generator.

The method may include sensing the power turbine frequency and triggering a loss of load condition if the frequency and/or the rate of increase of the frequency exceed predetermined limits. In an embodiment of the invention, if the frequency exceeds 52,5 Hz or the rate of increase of the frequency exceeds 2,5 Hz/s then the loss of load condition is triggered.

The method may include monitoring the generator and/or power turbine and closing the gas bypass valve when the rotational acceleration of the generator and/or power turbine becomes negative. Restricting the speed of the generator may include opening at least one of a high pressure recirculation valve, a low pressure recirculation valve and a high pressure coolant valve.

According to yet another aspect of the invention, in a nuclear power plant which has a closed loop power generation circuit which includes a power turbine which is drivingly connected to a generator, there is provided a method of controlling the plant, which includes, monitoring the plant and effecting a power conversion unit trip if a predetermined condition is detected when the plant is in a power operation mode.

Effecting a power conversion unit trip may include opening a gas cycle bypass valve to dissipate fluidic power; applying an electrical load to the generator to decelerate the power turbine and the generator; and activating a start-up blower system to maintain flow through the reactor.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings,

Figure 1 shows a schematic representation of part of a nuclear power plant;

Figure 2 shows a schematic representation of a helium inventory control system forming part of the nuclear power plant of Figure 1 ; and Figure 3 shows a flow diagram of the primary modes of operation of the plant of Figure 1.

In Figure 1 of the drawings, reference numeral 10 refers generally to part of a nuclear power 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, and a power conversion unit (PCU) which includes 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 inter- cooler 28 and a high pressure compressor 30.

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.

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.

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.

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.

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.

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.

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.

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.

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 intercooler 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

14.1 of the reactor 14.

The nuclear power plant 10 includes a start-up blower system 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.

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.

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.

A high pressure compressor recirculation line 50 extends from a position between the outlet or downstream side 30.2 of the high pressure compressorand 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 compressor recirculation line 50. 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.

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.

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 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.

Referring now to Figure 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.

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 (Figure 1 ).

Referring now to Figure 3 of the drawings, the plant has six primary modes of operation, namely, de-fuelled maintenance mode (0), fuelled maintenance mode (1 ), shutdown mode (2), standby mode (3), power conversion unit (PCU) operational mode (4) and power operational mode (5).

In the reactor de-fuelled mode (O), the reactor 14 is de-fuelled and the power generation circuit is de-pressurised with no major sub-systems operational.

In this mode planned and unplanned maintenance activities requiring a de-fuelled reactor can be performed. Air can be allowed into the core internals and primary pressure boundary. The system is waiting for permission to fuel the reactor.

In the fuelled maintenance mode (1 ), the reactor 14 is sub-critical. Further, the Brayton cycle is not self-sustaining. The reactor temperature is monitored by means of sensors. A decay heat controller is operational and maintains the reactor temperature below 400°C. This mode of operation is associated with maintenance activities performed while the reactor is fuelled. A core cooling system will be operational. To avoid water ingress into the system, the start-up blower system 38 cannot be operational in this mode or state. The reason for this is that it is important that the helium pressure in the coolers 24, 28 always exceed the water pressure. Should a leak then occur in the coolers the direction of the leak will be from the helium into the water thereby preventing the ingress of water which could adversely affect fuel integrity. Hence, when the startup blower system is operational, the helium inventory must be at a sufficiently high level that the helium pressure in the coolers exceeds the water pressure. The plant makes use of an active cooling system (ACS) which consists of an inner loop or cascade using water as coolant, that removes heat from the helium in the main power system by means of helium-to-water heat exchangers 24, 28. The heat thus transferred to the water is removed from this closed circuit or inner loop and subsequently dumped into an open circuit water cooling line via a water-to- water heat exchanger. Active cooling system feed pumps supply cooling waterfroman ultimate heat sink, which might be a body of water, eg the sea, a cooling tower etc. to the water-to-water heat exchangers in the water-to- water heat transfer interface.

The fuelled maintenance mode (1 ) has two sub-modes, namely, open maintenance mode (1a) and closed maintenance mode (1 b).

In open maintenance (1a) air ingress into the main power system is allowed with the exception of core internals. The power conversion unit is isolated from the reactor unit by inserting maintenance valves in the gas pipes connecting the power conversion unit and the reactor unit. This is typically scheduled maintenance done on a six-yearly basis, and includes maintenance on turbo machinery, coolers, power turbine, recuperator, gas cycle valves and the start-up blower system. The maintenance valves are typically of the gate-valve type. The shut-off disc or gate is a circular disc of the same diameter as the pipe in which it is utilised for sealing. The disc/gate is located outside the pipe when the valve is in an open position. When the valve is closed, the disc or gate is lowered into the pipe, thus blocking the flow path. This configuration results in reasonable sealing integrity, but cannot be used in high pressure applications. The maintenance valves will therefor only be used during maintenance conditions when pressure in the system has been lowered by removing some of the helium inventory.

In the closed maintenance sub-mode (1 b) unscheduled maintenance is carried out on a number of specific sub-systems described below. The main power system is filled with helium at a pressure just above atmospheric.

In closed maintenance sub-mode (1 b) maintenance on the core conditioning system (CCS) and the reactor pressure vessel conditioning system (RPVCS) will be conducted. However, in this case the isolation of the CCS or RPVCS from the reactor unit (RU) is achieved by the installation of maintenance shut-off discs in the gas pipes between the CCS and the RU.

In addition, maintenance on the gas cycle valves can be carried out.

Maintenance operation on the gas cycle valves will only commence once the system pressure has been removed. Removal and replacement shall take place with the primary pressure boundary (PPB) under slightly above atmospheric conditions (30 Pa/0.3mbar).

In addition, during closed maintenance (1 b), maintenance operations on the reactivity control system (RCS) and reserve shutdown system (RSS) can be performed. Maintenance on these systems will only commence once the system pressure has been removed . Removal and replacement of motor drives, magnets, rods and small absorber spheres (SAS) shall take place with the PPB under slightly above atmospheric conditions (30 Pa/0.3 mbar) and the reactor still fuelled.

In the shutdown mode (2), the reactor 14 is sub-critical. Three reactivity control devices can keep the reactor sub-critical, namely, the RSS, the shutdown rods and the control rods. The functionality of the sets of shutdown rods and control rods is similar, and both sets are used to control reactivity by absorbing neutrons. The major difference between the sets of rods is in the control thereof, and thus also the drive mechanism. The control rods are used to control reactivity during normal plant operation. These rods can be manipulated to various positions in predetermined incremental distances to assist in power control. The shutdown rods will, however, have two positions, namely, inserted or withdrawn. The shutdown rods are thus normally in their withdrawn positions and are inserted when it is desired to shut the reactor down. The control rods are also capable of shutting the reactor down at high temperatures. However, as the temperature decreases the fuel becomes more reactive and the possibility exists that the control rods on their own do not have enough absorbence capacity to shut down the reactor. Accordingly, the temperature at which the reactor must be shut down will dictate whether or not the shutdown rods are required. In this mode and all sub-modes the start-up blower system ( SBS ) 38 is operational. By controlling the speed of the SBS (and therefor the mass flow rate through the core) the reactor temperature is controlled. Heat generated in the reactor in this mode is due to decay heat only. This mode has three sub-modes, namely, full shutdown (2a), intermediate shutdown (2b) and partial shutdown (2c). The full shutdown mode (2a) is a mode or state which occurs before or after maintenance. The RSS, the shutdown rods and the control rods are fully inserted and the reactor outlet temperature is less than 550°C.

The intermediate shutdown sub-mode (2b) is a mode or state which would occur after a reactor SCRAM transient or after an insert shutdown rods transition. The shutdown and the controls rods are inserted. The outlet temperature of the reactor is not allowed to go below a predetermined temperature, typically 550°C. At this temperature and with both sets of rods inserted, the reactor would be sub-critical even at zero xenon level.

The partial shutdown sub-mode (2c) is a mode or state which would occur after a control rod SCRAM transient or after Insert control rods transition. The control rods are inserted. The outlet temperature of the reactor is not allowed to go below a predetermined temperature, typically 750°. At this temperature and with the control rods inserted, the reactor will be sub-critical even at zero xenon level.

The standby mode (3) has two sub-modes, namely, reactor ready mode (3a) and MPS ready mode (3b).

In the reactor ready sub-mode (3a) the reactor is critical and the reactor outlet temperature is controlled with the control rods. Adjusting the reactor outlet temperature, reactor neutronic and fluidic power and power turbine power will be possible in this mode by using the RCS, SBS, the RCS and gas cycle valves. The Brayton cycle is not self-sustaining. This mode or state typically would occur after reactor start-up transitions or not conditioned transition. In the MPS ready sub-mode (3b) the reactor is critical, however, the Brayton cycle is not self-sustaining and the reactor temperature is kept to between 750°C and 900°C. This is a mode or state from which the main power system ( MPS) start-up and synchronisation transition can be initiated, provided that the plant is within the start-up margins (conditioned). The plant will also enter this mode/state after a PCU trip transient. If all the plant parameters remain within the "conditioned" margins, the plant will remain in this mode or state. The plant operator will be able to select the MPS start-up and synchronisation transition or select the insert control rods transition. It should be noted that the MPS start-up and synchronisation transition is directly from this mode/state to power operation mode (5), bypassing PCU operational mode (4), as synchronisation can take place before the Brayton cycle is fully self-sustaining.

In the PCU operational mode (4) the reactor is critical, the Brayton cycle is self-sustaining and the reactor outlet temperature will be kept between 750°C and 900°C. This mode/state is entered via the loss of load transient or the controlled grid separation transition. In this mode/state, the plant operates with a stable Brayton cycle at low generator power, but always at high helium inventories, between 40% and 100% of the maximum continuous rating (MCR) inventory. No power is exported to the grid because the plant is not connected to the grid.

Two main operational states are associated with this mode, namely the generator breaker closed where the house load is supplied internally (HV and the external supply breaker open) and generator breaker open where house load is supplied externally. The power operational mode (5) consists of two sub-modes, namely, reduced capability operational (5a) and normal power operational (5b). In this mode the plant grid power can be automatically adjusted to a power set point from a remote grid operator. In this mode/state, the helium inventory level of the MPS will be greater than 40% of the helium inventory level associated with MCR.

In reduced capability operational sub-mode (5a) the reactor is critical and the reactor outlet temperature is kept between 750°C and 900°C. This mode/state is entered via the MPS start-up and synchronisation transition or the synchronisation transition. After these transitions, the reactor outlet temperature will be below or equal to 900°C and bypass valves will be operational.

This mode can also be entered from normal power operational sub- mode (5b). In reduced capability operational sub-mode (5a) no automatic generation control (AGC) or primary frequency control will be possible. The base-load power controller will be operational. The base-load power controller is used to control power to an operator initiated set point. The operator changes the set point locally and is hence in complete control of the plant power output.

The reactor temperature controller will operate in one of the following two sub-modes, namely, normal reactor temperature controller mode or floating reactor temperature controller mode.

In normal reactor temperature controller mode the reactor outlet temperature request may be adjusted at any given rate. In floating reactor temperature controller mode, if the plant was operated at high power levels before a power reduction, the xenon concentration increases. The increased xenon concentration level may imply that sustained critical operation of the plant at 900°C is not feasible. This will occur when the amount of reactivity that the control rods can compensate for at 900°C is less than the negative transient reactivity associated with the high levels of xenon. That is, the control rods are nearly completely withdrawn. Underthese conditions. the reactormayenterthefloating reactor temperature controller mode. This controller will allow the average core temperature to float down (in the allowable range 750°C to 900°C), thus keeping the reactor critical. This reactor sub-mode will be triggered when the rods are fully withdrawn.

In the normal power operational sub-mode (5b), the reactor outlet temperature will be controlled to 900°C.

In this mode/state of operation, the plant is capable of providing power to the grid, as well as supporting ancillary services. The ancillary services supported in this mode/state of operation are: Load following;

Automatic generation control (AGC) also known as regulation or secondary frequency support; and

Primary frequency support, also known as governing.

In load following, the power supplied to the grid can automatically be adjusted to match the signal dispatched by the system operator within a scheduling period. In automatic generation control, the power supplied to the grid is adjusted in accordance with the control signals issued by the system operator. The grid power of the plant can be adjusted between 40 and 100% of MCR by using the helium inventory control system ( HICS) to control MPS helium inventory levels up to 10% of MCRI per minute.

In primary frequency support, the power supplied to the grid is adjusted by the plant in response to the rotational frequency of the power turbine generator (PTG). This is characterized by the "droop" function when operating outside a specified frequency dead-band.

Runbacks are pre-programmed transitions within a specific mode/state to ensure sustainability of operation given a sub-system capacity reduction. This results in a degraded mode of operation where all the control capabilities for the plant still exist and are operational, but subject to additional limitations. A typical example would be the operation of plant when only half the active cooling system (ACS) feed pumps were in operation. All the control software, controlled and mode/state logic is active and operational, but the allowable power that can be delivered to the grid is limited. It is limited physically by the heat removal capacity of the heat exchangers as well as being limited in the controller software by changing the maximum allowable power request.

The plant has seventeen primary mode/state transitions, namely, insert maintenance valves, remove maintenance valves, pressurize PPB, de- pressurise PPB transition, reactor start-up (A), reactor start-up (B), reactor start-up (C), insert control rod transition, insert shut-down rods transition, insert RSS, conditioning transition, PCU start-up and synchronisation transition, controlled grid separation transition, synchronisation transition, close-down, increase capability transition and reduced capability transition.

The Insert Maintenance valves transition is a transition from closed maintenance (1 b) to open maintenance 1 (a). In this transition maintenance valves are positioned in the piping between the PCU and reactor unit (RU). The function of these valves is to limit the probability of air entering the fuelled reactor.

The Remove Maintenance valves transition is a transition from Open maintenance (1 a) to closed maintenance (1 b). This transition involves, after all PPB lids have been replaced and bolted down, the dry air in the PCU being removed by drawing a vacuum in the PCU. The extracted air and helium mixture is removed via the primary loop initial clean-up system (PLICS) filters. During this activity the helium fed into the RU is shut-off and the maintenance valves remain in position. The helium can be allowed to leak from the RU into the PCU volume until a vacuum is drawn over the total volume of both the RU and PCU. Once a vacuum has been established, helium is re-introduced into the RU and a controlled leak of helium into the PCU is again established. This cycle of drawing a vacuum in the PCU and re-introducing helium into the RU is repeated until the allowable level of impurities is achieved in the PCU. It is expected that not more than 2 cycles are required.

In an alternative method, instead of drawing a vacuum on the PPB, the PCU can beflushed by inserting helium at the highest points on the PCU, drawing off air from the bottom of the lowest points on the PCU aηd maintaining a slightly higher pressure on the RU during this process. The layout of the PCU regarding cavities where large quantities of air can get trapped needs to be taken into consideration. A combination of flushing and drawing a vacuum should also be considered.

Once the acceptable level of impurities is achieved in the PCU, the maintenance valves are removed. These are removed/opened by manipulating hoisting mechanisms from outside the PPB via small maintenance openings. Once the valves are removed/opened, the PPB is ready for pressurized operations.

The Pressurize PPB transition is from fuelled maintenance mode (1 ) to shutdown mode (2). Helium is injected into the PPB from the helium inventory tanks. During this process the SBS is started and the CCS stopped after the helium pressure exceeds the water pressure in the coolers since cooling can then be achieved using the active cooling system.

The steps involved in this transition are: Fill PPB with helium to 40% MCRI (maximum continuous rating inventory);

Switch on main closed circuit pumps that supply cooling water to the pre- and intercoolers.

Switch on SBS and decay heat controllers; and Switch off CCS.

The de-pressure PPB transition is from shutdown mode (2) to fuelled maintenance mode (1 ). Helium is extracted from the PPB into the helium inventory tanks or may be vented to atmosphere. During this process the CCS is started and the SBS stopped because the helium pressure is less than the water pressure in the coolers. The stepwise sequence is as follows:

The start-up blower system (SBS) is on and run at not lower than 40% inventory until a reactor outlet temperature (ROT) of 200°C is reached. This condition is expected to be reached in less than 2 days;

Once the 200°C ROT is reached the core conditioning system (CCS) is switched on and checked for functionality. If problems are experienced on the CCS, maintenance is carried out on the CCS before maintenance on the PCU commences;

The pressure in the primary pressure boundary (PPB) is lowered to about 12% of inventory by pumping the helium to the inventory control system (ICS) storage tanks. From 12% inventory the helium is released via the ICS to the HVAC filters and then to atmosphere. The HVAC filters are filters in the heating, ventilation and air-conditioning system responsible for cleaning ventilation air before being released into the stack and atmosphere. These filters will not see process gas (ie helium) under normal conditions, but only during abnormal releases. The temperature limitation on the filters may require the helium to be released directly through the stack. The pressure is released to just above atmospheric (30 Pa/0.3mbar); Switch off SBS; Switch off main closed circuit pumps that supply cooling water to the pre- and intercoolers.

The helium content of the PCU is flushed with dry air. This helium

(approximately 30kg) is released via the ICS and HVAC filters to atmosphere. During this activity, the RU is maintained at slightly higher pressure than the PCU. With the low-pressure differential over the maintenance valves, a small helium leakage is expected from the RU into the PCU and will be taken into consideration.

The reactor start-up (a) transition is from full shutdown mode (2a) to reactor ready mode (3a). The reactor is made critical during the transition. The SAS and the shutdown rods will be removed during transition. The control rods and reactor temperature will be used for reactivity adjustment.

The SBS must be operational to ensure that accurate temperature measurements and control are possible. In this transition the reactor outlet temperature controller is switched from the ROT decay heat controller to the ROT reactivity controller using the control rods.

Stepwise procedure:

Withdraw the control rods to a pre-determined position. The shutdown rods and the RSS remain inserted;

Empty the SAS channels one at a time in a pre-determined sequence; When the neutron count rate takes more than 60 seconds to stabilize, withdraw the control rods slowly to make the reactor critical and switch to the ROT reactivity controller;

The next SAS channel is emptied and the control rods are inserted while keeping the reactor critical (1 ); Repeat (1 ) until the control rods approach a pre-determined lower limit;

Increase the ROT with nuclear heating using the control rods. The ROT must remain below 550°C.

Repeat (1 ) until all SAS channels have been emptied; Remove the shutdown rods while inserting the control rods without exceeding a predetermined lower limit on the control rods; Increase the ROT with nuclear heating using the control rods until the shutdown rods are fully withdrawn .

Nuclear heating must not exceed 20 MWt before all the SAS channels have been emptied (this value is determined from the allowable heat generation in the SAS channels).

The reactor start-up (b) transition is from intermediate shutdown mode (2b) to reactor ready mode (3a). The reactor is made critical during the transition. The shutdown rods will be removed during the transition. The control rods and reactor temperature will be used for reactivity adjustment.

The SBS must be operational to ensure that accurate temperature measurements and control are possible. In this transition the reactor controller is switched from the ROT decay heat controller to the ROT reactivity controller.

This transition consists of the following steps: Withdraw the control rods to a pre-determined position. The shutdown rods remain inserted;

Withdraw the shutdown rods to make the reactor critical and switch to the reactor outlet temperature (ROT) reactivity controller;

Withdraw the shutdown rods while inserting the control rods without exceeding a predetermined lower limit on the control rods;

When necessary, increase the ROT with nuclear heating until the shutdown rods are fully withdrawn.

The reactor start-up (c) transition is from partial shutdown mode (2c) to reactor ready mode (3a). The reactor is made critical during the transition. The control rods will be partially withdrawn during the transition until the reactor becomes critical.

The SBS must be operational to ensure that accurate temperature measurements and control are possible. In this transition the reactor controller is switched from the ROT decay heat controller to the ROT reactivity controller. During this transition, the reactor flux measurement is switched from the start-up channels to the power channels.

The insert control rods transition is from standby mode (3) to partial shutdown mode (2c).

In this transition the operator will initiate the transition. During this transition the shutdown rods are inserted.

The insert shut-down rods transition is from partial shutdown mode (2c) to intermediate shutdown mode (2b). During this transition the shutdown rods are inserted.

Stepwise procedure:

The operator will initiate the transition.

During this transition the shutdown rods are inserted.

With the insert RSS transition, the operator will initiate the transition. Before the ROT of 550 °C is reached the operator must ensure that the RSS is fully inserted and shall then bypass the interlocks that disallow the operation of both the SBS and CCS at low ROT. The conditioning transition is from reactor ready mode (3a) to MPS ready mode (3b). The main goal of this transition is to get the MPS to specified temperature levels. This is necessary to reduce temperature differentials when the Brayton cycle is started.

The SBS is used to control reactor fluidic heat by adjusting the mass flow rate through the reactor. The ROT reactivity controller will control the reactor outlet temperature. The heat removed from the reactor is used to heat the components. The hot gas coming from the reactor is mixed with cold gas from the manifold using the high pressure coolant valve (HCV) 56. In this manner the recuperator low pressure inlet temperature is controlled. The plant is conditioned for start-up when the MPS is within the start-up limits.

The PCU start-up and synchronisation transition is from MPS ready mode (3b) to reduced-capability operation mode (5a). The Brayton cycle becomes self-sustaining in this transition. This is achieved by using the SBS and the gas cycle valves. Speed control of the PTG is achieved using the resistor bank and by-pass valves.

This transition involves the following steps: ROT reactivity controller will control the reactor outlet temperature: For optimal starting conditions the SBS operates at maximum possible delivery (limited by either SBS maximum torque or SBS maximum allowable speed);

A power request of 1 MW for the generator is set together with a speed request of 30 Hz. The low pressure recirculation valve (LPB ) 48 and high pressure recirculation valve (HPB ) 51 together with the resistor bank will ensure that the generator is stable in this state; The spin-up sequence is done in a controlled manner such that the PTG will pass through the critical frequencies rapidly enough not to exceed undesirable vibration of force limits at the critical frequencies;

The control system will adjust the PTG speed to a value just below the grid frequency;

The synchronisation sequence is entered and completed and will be similar to the sequence used in the synchronisation transition;

If the Brayton cycle is not yet self-sustaining, the LPB and HPB valves are closed until the Brayton cycle is self-sustaining. This increases the mass flow through the MPS and the ROT reactivity controller will ensure that the reactor power increase is kept below allowable limits. The HCV valve will still be controlled by the temperature limit on the recuperator;

As soon as the Brayton cycle becomes self-sustaining the SBS is withdrawn from service; and The base load controller is initialized.

In the controlled grid separation transition the PTG power is reduced and breakers required to separate the PTG from the grid are opened.

This transition involves the following steps: The operator will choose whether the house-load will be supplied from, the generator or externally;

The operator adjusts the base-load controller set point to the power level required; and

Once the required power level has been reached, the operator will open the relevant breaker. The HV breaker will be opened in the case where power if supplied by the PTG and the generator breaker will be opened in the case where power is supplied externally. The synchronisation transition is the transition from the PCU operational mode (4) to reduced-capability operation mode (5a). In this transition, the generator frequency, voltage and phase of the voltage is synchronized with the external grid.

The stepwise procedure is as follows:

Control the speed of the generator to close to the grid frequency; Switch to generator synchronisation control relevant to the specific breaker;

Slave to generator synchronisation controller's speed request; When the generator has synchronized, the synchronisation controller will issue "synchronized completed" and the relevant breaker will be closed; and

Increase power to the grid by removing the resistor load.

The Close Down transition is the transition from the PCU operational mode (4) to MPS ready mode (3b). There are two different close-down transitions that may be followed and they are dependent on the starting conditions in PCU operational mode (4).

In the case of Close-down with generator breaker open, power externally supplied, the generator breaker was open in PCU operational mode (4). Thus the plant is receiving power from an external source. This is also true for MPS ready mode (3b) and thus the transition is relatively simple.

The Brayton cycle is still operational at the start of the transition and by using the speed controller the speed of the PTG can be reduced in a controlled manner. However, once the PTG has attained the required speed, the HPB and LPB can be opened fully. The Brayton cycle will stop functioning. As soon as it is safe to operate the SBS, the blower will be started and the "conditioning" controller initialized.

In the case of Close-down with generator breaker closed, with synchronisation, the generator breaker was closed in PCU operational mode (4). This is the more likely of the events. Thus the plant is supplying its own power. Thus, if the close-down transition is necessary and the plant is to end in MPS ready mode (3b), then the plant must be switched from an internal to an external supply. This is possible only if synchronisation occurs, or, the plant is shut down completely.

The plant capabilities can be selectively reduced and increased by using the inventory control system (ICS), compressor by-pass valves and booster tank.

The plant has four prime mode/state transients, namely, loss of load transient, PCU trip transient, control rod scram/reverse transient and reactor scram transient.

The plant will enter the loss of load transient when an over-speed is detected in power operational mode (5). A loss of load transient is caused by loss of load on the generator. From a safety viewpoint, only the gas cycle bypass valve (GBP) 72 is required to prevent over-speed of the generator.

The stepwise procedure is as follows: A loss of load condition is detected; The gas cycle bypass valve (GBP) 72 will open within 0.3 seconds with no feedback control. Opening the valve will ensure the PTG does not over-speed;

The GBP valve will close as soon as the rotational acceleration is negative. This will ensure that the Brayton cycle remains self-sustaining;

Simultaneously the HPB, LPB and HCV are opened and the resistor bank load is set to maximum. The low- and high-pressure recirculation valves and resistor bank are used to control the PTG rotational frequency to

50 Hz. The HCV will ensure that the recuperator low pressure inlet temperature does not exceed 600 °C;

The ROT reactivity controller will control the reactor outlet temperature; and

The HV breaker will be opened when the following condition is true:

Generator breaker closed. AND. speed > 52.5 Hz.

Although referred to in the singular above, the gas cycle bypass valve

72 typically includes a plurality of valves, typically a set of eight. These valves can be controlled independently or together to provide greater flexibility. The gas cycle bypass valves connect the outlet of the high pressure compressor with the inlet of the pre-cooler. Opening the gas cycle bypass valves causes a flow from the high pressure compressor outlet to the pre-cooler inlet. This result in a large recirculation flow through both the compressors, and subsequently reduce flow through the reactor and turbine. The reduced flow through the reactor results in less heat being removed from the reactor, and thus less energy is transmitted to the turbine. Hence, the nett effect of opening the gas cycle bypass valves is reduced power out from the system, up to a point where the Brayton cycle actually ceases to be self- sustaining. A loss of load condition is detected by sensing the power turbine frequency. Ifthefrequency exceeds a certain limit, typically 52.5 Hz, and/or the rate of increase of the frequency exceeds a predetermined limit, typically 2.5 Hz/s, a loss of load condition is tiggered.

The plant will default to the PCU Trip transient when a PCU trip signal is generated. Typical trip signals which will activate this transient include, inter alia, excessive vibration, total loss of ACS cooling, surge margin being less than 5%, manifold pressure being excessively high, typically greater than 91.5 bar, recuperator low pressure inlet temperature being excessively high, typically greater than 650°C for 30 seconds, over speed of the PTG, eg higher than 55 Hz, a generator system fault signal, an electrical protection signal or the like. This transient will default to MPS ready mode/state (3b) after the Brayton cycle has stopped functioning.

The plant will take the following actions when the PCU trip mode/state is triggered:

The generator breaker should trip on reverse power conditions;

The gas cycle bypass valve (GBP) will be opened to dissipate the power turbine fluidic power. The gas cycle bypass valve (GBP) will remain open until the Brayton cycle has shut down completely; The resistor bank will be utilised to load the PTG until it has decelerated to an acceptable level; and

During the transient the SBS will be activated to keep flow through the reactor.

The control rod scram / reverse transient is initiated with the "control rod SCRAM / reverse transient" signal. Excessive reactor outlet temperatures and neutron flux levels and rate of neutron flux increase are typically the parameters that could initiate reverse or a SCRAM. This transient will end in the partial shutdown mode (2c) after the Brayton cycle has shut down. The reactor will be sub-critical.

The plant will take the following actions when this transient is triggered:

The SBS/CCS inhibit for low ROT temperatures is automatically activated when the SCRAM transient is initiated or in the case of the reverse transient, when two control rods have reached their fully inserted positions; A Control rod SCRAM / Reverse transient is accomplished by dropping the control rods into the reactor in the case of the SCRAM condition or fully inserting them in a controlled manner in the case of the Reverse condition;

A controlled shutdown of the PCU will take place with the operation of the LPB and HPB valves. This causes the speeds of the turbo units and the power being delivered by the PTG to be reduced. The generator breaker will trip on reverse power conditions;

The resistor bank will be utilised to load the PTG until it has decelerated to an acceptable level; and

The SBS will be started and the ROT control will be passed from the ROT reactivity controller to the ROT decay heat controller.

The reactor scram transient is initiated with the Reactor SCRAM transient signal. This SCRAM will only occur if the previously initiated control rod SCRAM has not had the required result. This transient will end in the Intermediate shutdown mode (2B) after the Brayton cycle has shut down. The reactor will be sub-critical. The plant will take the following actions when this transient is triggered:

The SBS/CCS inhibit for low ROT temperatures is automatically activated when the transient is initiated; The shutdown and control rods are dropped into the reactor;

The gas cycle bypass valve (GBP) will be opened to dissipate the power turbine fluidic power. The gas cycle bypass valve (GBP) will remain open until the Brayton cycle has shut down completely. The resistor bank will be utilised to load the PTG until it has decelerated to an acceptable level; The operation of the GBP causes the speeds of the turbo unit and the power being delivered by the PTG to be reduced. The generator breaker will trip on reverse power conditions;

The resistor bank will be utilised to load the PTG until it has decelerated to an acceptable level; and The SBS will be started and the ROT control will be passed from the

ROT reactivity controller to the ROT decay heat controller.

The fault mode/states are usually more complex than the operation mode/states. In general, if a fault condition arises, the plant will operate in a specific fault mode/state until the fault is cleared. Then it will move back to a specific operation mode/state.

Note that the control system will try to keep the plant within the operational mode/states by running back to degraded mode/states (via so- called "run-backs"), until it becomes necessary to make the transition to a fault mode/state.

The plant will have various possible fault modes. These include loss of ACS cooling, helium leaks from the pressure boundaries, ACS helium to water leaks, generator helium to water leaks, decay heat removal using CCS (as a result of SBS failure), rotor vibration, electromagnetic bearing failures and degraded ACS cooling.

Claims

1. A method of controlling a nuclear power plant, which method includes monitoring the operation of the power plant via sensing means located throughout the plant and collecting data relating to the operation of the plant thereby to determine a plant status, which is indicative of a mode of operation of the plant; using feedback control to maintain plant stability in the or each mode of operation; and when required controlling a transition from one mode of operation to another mode of operation.

2. A method as claimed in claim 1 , which includes when required selectively controlling a transition from one mode of operation to another mode of operation as a result of an operator instruction.

3. A method as claimed in claim 1 or claim 2, which includes controlling the transition from one mode of operation to another mode of operation in the event of a transient or unexpected occurrence .

4. A method as claimed in any one of the preceding claims, in which the plant has a closed loop power generation circuit which includes a nuclear reactor, a high pressure turbine, a low pressure turbine, a power turbine and a generator, a recuperator, a pre-cooler, an intercooler and a high pressure compressor, and in which the modes of operation in which the plant is operable are selected from: a first mode of operation, in which the reactor is in de-fuelled maintenance mode; a second mode of operation, in which the plant is in fuelled maintenance mode; a third mode of operation, in which the plant is in reactor shutdown mode; a fourth mode of operation in which the plant is in standby mode; a fifth mode of operation, in which the plant is in power conversion unit (PCU) operational mode; and a sixth mode of operation, in which the plant is in power operational mode.

5. A method as claimed in claim 4, which, when the plant is in the second or fuelled maintenance mode, includes using feedback to control the temperature of the plant via a temperature controller and a core conditioning system (CCS).

6. A method as claimed in claim 4 or claim 5, which, when the plant is in the third or reactor shut-down mode of operation, includes controlling the speed of a start-up blower system, thereby to control the mass flow rate of coolant through the core, and hence through the pre- cooler, thereby removing heat generated in the reactor core due to decay, from the core.

7. A method as claimed in claim 6, which, when the plant is in the third mode of operation, further includes controlling an inlet temperature of the reactor by partially bypassing the recuperator, via control of a recuperator bypass valve.

8. A method as claimed in claim 7, which includes monitoring the inlet temperature to the start-up blower system to prevent unacceptably high start-up blower system temperatures and measuring the reactor outlet temperature and comparing it to the reactor inlet temperature to prevent an unacceptably high temperature differential across the reactor.

9. A method as claimed in any one of claims 4 to 8 inclusive, which, when the plant is in the fourth mode of operation, includes maintaining the nuclear plant in one of a reactor ready sub-mode or main power system ready sub-mode using a conditioning controller.

10. A method as claimed in any one of claims 4 to 9 inclusive, which when the plant is in the fifth mode of operation, includes operating the plant with a stable Brayton cycle, the method further including controlling a reactor outlet temperature, controlling the power to a given set point and regulating a generator frequency if the plant is disconnected from an electrical grid.

11. A method as claimed in any one of claims 4 to 10, inclusive, in which controlling a transition between the third and fourth modes includes switching from a decay heat temperature controller to a normal temperature controller.

12. A method as claimed in any one of claims 4 to 11 , inclusive, which, when the plant is in power operational mode and a loss of load condition is detected, includes restricting the speed of the generator.

13. A method as claimed in claim 12, which includes sensing the power turbine frequency and triggering a loss of load condition if the frequency and/or the rate of increase of the frequency exceed predetermined limits.

14. A method as claimed in claim 12 or claim 13, in which restricting the speed of the generator includes opening a gas cycle bypass valve to dissipate power turbine fluidic power.

15. A method as claimed in claim 14, which includes monitoring the generator and/or power turbine and closing the gas bypass valve when the rotational acceleration of the generator and/or power turbine becomes negative.

16. A method as claimed in any one of claims 12 to 15, inclusive, in which restricting the speed of the generator includes opening at least one of a high pressure recirculation valve, a low pressure recirculation valve and a high pressure coolant valve.

17. A method as claimed in any one of claims 12 to 16, inclusive, in which restricting the speed of the generator includes applying an electrical load to the generator by means of a resistor bank.

18. A method as claimed in any one of claims 4 to 17, inclusive, which includes when the plant is in power operational mode, monitoring the plant and effecting a power conversion unit trip if a predetermined condition is detected.

19. A method as claimed in claim 18, in which effecting a power conversion unit trip includes opening a gas cycle bypass valve to dissipate power turbine fluidic power, and shut down the Brayton cycle; applying an electrical load to the generator to decelerate the power turbine and the generator; and activating a start-up blower system to maintain flow through the reactor.

20. A control system for a nuclear power plant having a closed loop power generation circuit which uses helium as the working fluid and which includes a nuclear reactor, a high pressure turbine, a low pressure turbine, a power turbine and generator, a recuperator, a pre-cooler, an intercooler, a low pressure compressor and a high pressure compressor, the control system including sensing means for monitoring a plurality of variables relating to the operation of the nuclear power plant, thereby to determine a state of the plant ; at least one feedback control system, associated with at least one subsystem of the plant for controlling the operation of the subsystem when the plant is in a particular mode of operation; and a logic control system for selectively controlling the transition of the nuclear power plant from one mode of operation to another.

21. A control system as claimed in claim 20, which includes a reactor temperature controller.

22. A control system as claimed in claim 21 , in which the plant includes a start-up blower system and the reactor temperature controller controls the speed of the start-up blower system thereby to control the mass flow of working fluid in the plant.

23. A control system as claimed in any one of claims 20 to 22, inclusive, which includes a recuperator inlet temperature controller for controlling a coolant valve or valves, thereby to control the recuperator inlet temperature.

24. A control system as claimed in any one of claims 20 to 23, inclusive, which includes a conditioning controller comprising a temperature controller for controlling the reactor outlet temperature; a recuperator inlet temperature controller; a reactor inlet temperature controller; a power turbine and generator speed controller; a startup blower system controller; and a compressor guide vane position controller for maintaining a surge margin at a maximum value.

25. A control system as claimed in any one of claims 20 to 24, inclusive, which includes a base load controller for controlling the power generated by the plant to a given set point and a power turbine generator speed controller to regulate the generator frequency if the plant is disconnected from the electrical grid.

26. A system as claimed in any one of claims 20 to 25, inclusive, which includes the following controllers which are operable in a power operational mode of the plant; governing controller for primary frequency support; generation controller for automatic generation control; and/or base load controller for load following.

27. In a nuclear power plant which has a closed loop power generation circuit which includes a power turbine which is drivingly connected to a generator, there is provided a method of controlling the plant which includes, in the event of a loss of load condition being detected, restricting the speed of the generator.

28. A method as claimed in claim 27, in which restricting the speed of the generator includes opening a gas cycle bypass valve to dissipate power turbine fluidic power.

29. A method as claimed in claim 28, which includes monitoring the generator and/or power turbine and closing the gas bypass valve when the rotational acceleration of the generator and/or powerturbine becomes negative.

30. A method as claimed in any one of claims 27 to 29, inclusive, in which restricting the speed of the generator includes opening at least one of a high pressure recirculation valve, a low pressure recirculation valve and a high pressure coolant valve.

31. A method as claimed in any one of claims 27 to 30, inclusive, in which restricting the speed of the generator includes applying an electrical load to the generator by means of a resistor bank.

32 In a nuclear power plant which has a closed loop power generation circuit which includes a power turbine which is drivingly connected to a generator, there is provided a method of controlling the plant, which includes, monitoring the plant and effecting a power conversion unit trip if a predetermined condition is detected when the plant is in a power operation mode.

33. A method as claimed in claim 32, in which effecting a power conversion unit trip includes opening a gas cycle bypass valve to dissipate the fluidic power; applying an electrical load to the generator to decelerate the power turbine and the generator; and activating a start-up blower system to maintain flow through the reactor.

34. A method of controlling a nuclear power plant as claimed in claim 1 , claim 27, claim 32 substantially as described and illustrated herein.

35. A control system for a nuclear power plant as claimed in claim 20 substantially as described and illustrated herein.

36. A new method or a new system substantially as described herein.