Patent Description:
A pressurized water reactor comprises a primary reactor coolant circuit inside which a primary reactor coolant circulates under high pressure. A Chemical and Volume Control System (CVCS) is fluidically connected to the primary reactor coolant circuit. The CVCS comprises a low-pressure part which usually provides a point of entry or injection for various fluids into the primary reactor coolant. In particular, it may be required to inject hydrogen into the primary reactor coolant, e.g. to bond dissolved oxygen.

Prior art document <CIT> discloses a pressurized water reactor according to the preamble of claim <NUM>. In the disclosed system, a hydrogen feed line discharges into the low-pressure part of the CVCS upstream of the high-pressure charging pump (i.e. on its suction side), hence realizing a low-pressure injection of hydrogen (typical operating pressure: <NUM> bara).

Apart from that, there are wet-chemistry systems for injection of hydrazine or hydrogen peroxide into low-pressure lines.

Drawbacks of these approaches are usually related to a limitation of the hydrogen feed rate. The injection process is temporarily inert, and its effects are time delayed. Furthermore, wet chemistry solutions may be toxic, cancerous, corrosive, and/or generally harmful to the environment and the operating personnel and do not provide free hydrogen.

<CIT> describes a nuclear power station primary coolant high-pressure hydrogenation system.

<CIT> describes a start-up/shutdown, hydrogen injection system for a boiling water reactor.

<CIT> describes a nuclear power plant's hydrogen sypply system.

Therefore, an objective underlying the present invention is to provide a pressurized water reactor with a hydrogenation system and an according method of operation which are suited for efficient and fast hydrogen injection into the primary reactor coolant. The device and the according process shall be reliable and robust with respect to perturbations. A modular and space-saving construction is desirable.

According to the invention, these goals are met by a pressurized water reactor according to claim <NUM>. A related method of operation is specified in claim <NUM>.

Hence, a key feature is that a high-pressure feeding pump is arranged in the hydrogen gas feeding line to provide a feeding pressure higher than the discharge (or: outlet) pressure of the charging pump for the primary reactor coolant, and that the feeding line discharges into the charging line, i.e. at a point of injection downstream the charging pump.

The proposed technical modification of the common approach is the creation of an active high-pressure injection instead of a passive injection in a low-pressure part of the system.

This design is based on the needs to have a hydrogen injection that is able to provide a precise defined amount of hydrogen in a defined time. In current designs there is a variation of amounts of gaseous hydrogen either in the Volume Control Tank (VCT) or the gas separator based on the plant design. In both cases the maximum operating pressure of the related system parts is varying from <NUM> to <NUM> bara (higher in case of transients). For the high-pressure hydrogen injection according to the invention, the injection point is downstream of the high-pressure charging pumps. The advantage in terms of physics for the injection of hydrogen is diffusion with a specific pressure in a specific time via a specific surface while the pressure is approx. <NUM> times higher than for the low-pressure injection.

Instead of passive mechanisms (for example the injection in the hydrogenation station or the spray lance in the VCT) a piston compressor shall preferably be used to raise the gas pressure in the pipe to ensure the injection downstream the high-pressure charging pumps.

The regulation of the piston compressor outlet pressure shall be limited by the operating conditions in the charging line. By this design the diffusion process is largely increased in speed due to the high pressure. Gas bubbles will be injected directly into the charging flow, where the solubility of hydrogen is ~<NUM> times higher than in the low-pressure part of the system. This makes the process of hydrogenation also more easily controllable (i.e. waiting time for an effect of the regulation is shorter).

Another benefit of the design is that nitrogen pollution will not occur as much as it is the case on the low-pressure injection via the hydrogenation station (with VCT connected and flushed with nitrogen). This limits the creation of C14 in the core thus limiting the amount of radioactive deposits.

In terms of time and cost, an important point is the decrease of the time spent during start up waiting for the hydrogen concentration to raise in order to reach <NUM>% power conditions.

The design can also easily be doubled (redundancy) on any point in order to guarantee a high availability.

In summary, the present invention uses a-high pressure feeding pump, preferably a piston compressor or a membrane compressor, to inject hydrogen at any suitable position of the CVCS downstream the charging pump (which can be of any suitable type). The concept may also be summarized as a backpressure independent hydrogenation of primary reactor coolant due to the general lower pressure of the primary reactor coolant circuit compared to the discharge pressure of the feeding pump and charging pump.

To recap, the now proposed high-pressure injection with an injection point downstream of the charging pump (typical discharge pressure: <NUM> bar) provides, among others, the following advantages:.

Drawbacks are the need for a high-pressure feeding pump in the gas feeding line and the high hydrogen pressure prevailing in the feeding line.

In a preferred embodiment an isolation valve is arranged in a section of the feeding line between the feeding pump and the point of injection into the charging line.

Preferably, the gas feeding line comprises a double walled pipe with an evacuated interspace between an inner wall and an outer wall, wherein a leakage detection system is designed to monitor pressure within the interspace, and preferably to isolate a leaking line section.

In another advantageous embodiment, the hydrogenation system comprises a control system with a hydrogen sensor which measures the hydrogen contents of the primary reactor coolant in the charging line downstream the point of injection, wherein the control system is designed to close the isolation valve when said measured hydrogen contents does not match the set hydrogen feed rate of the hydrogenation system.

In yet another advantageous embodiment, the hydrogenation system comprises a control system with a hydrogen sensor which measures the hydrogen contents of the primary reactor coolant in the letdown line, wherein the control system is designed for controlling the hydrogen feed rate by setting the power of the feeding pump on the basis of a difference between said measured hydrogen contents and a given setpoint for said hydrogen contents.

Exemplary embodiments of the invention are subsequently discussed with reference to the accompanying drawings.

According to <FIG>, a Pressurized Water Reactor (PWR) <NUM> comprises a primary reactor coolant circuit <NUM> carrying a primary reactor coolant. The primary reactor coolant circuit <NUM> comprises a Reactor Pressure Vessel (RPV) <NUM>, a pressurizer <NUM>, a steam generator <NUM>, and a Reactor Coolant Pump (RCP) <NUM>. The steam generator <NUM> provides a thermal connection to a secondary reactor coolant circuit. The volume, the chemical composition, and other physical properties of the circulating primary reactor coolant can be controlled by a Reactor Chemical and Volume control system (CVCS) <NUM> which is fluidically connected to the primary reactor coolant circuit <NUM>. This is shown schematically in <FIG>.

<FIG> shows a simplified Piping and Instrumentation Diagram (P&ID) of a hydrogenation system <NUM> coupled to a CVCS <NUM> of a PWR <NUM> in a Nuclear Power Plant (NPP). As explained above, the CVCS <NUM> is fluidically coupled to a primary reactor coolant circuit <NUM> of the PWR <NUM> in order to continuously extract a stream of primary reactor coolant from the primary reactor coolant circuit <NUM>, to treat it chemically and/or physically, and to re-charge it into the primary reactor coolant circuit <NUM> after said treatment. The treatment is usually accomplished at a low pressure (e.g. <NUM> barg) compared to the high pressure in the primary reactor coolant circuit <NUM> (e.g. <NUM> barg) during operation.

A letdown line <NUM>, in short letdown, carries a stream of low-pressure primary reactor coolant after pressure reduction by a pressure reducer (not shown here). A heat exchanger <NUM>, flown through by a cooling medium (see next paragraph), is arranged in the letdown line <NUM> in order to remove heat from the stream of primary reactor coolant. Downstream the heat exchanger <NUM> the low-pressure, low temperature primary reactor coolant is led through a main line <NUM> of the CVCS <NUM> and may be subjected to chemical and/or physical treatment. The main line <NUM> may be regarded as a downstream section of the letdown line <NUM>. For example, a boric acid supply line and/or a demineralized water supply line (not shown here) may be connected to the main line <NUM> in order to inject boric acid and/or demineralized water into the stream of primary reactor coolant, if required. Furthermore, there is a Volume Control Tank (VCT) <NUM> fluidically connected to the main line <NUM> of the CVCS <NUM>, intended to act as a compensation reservoir. Furthermore, there may be a fluid discharge line <NUM> attached to the VCT <NUM>, for example to facilitate removal of gaseous waste or generally for the purpose of degasification.

Downstream the tee connection <NUM> to the VCT <NUM> there is a high-pressure charging pump <NUM> switched into the main line <NUM> to bring the pressure of the flowing primary reactor coolant back to the level associated with the primary reactor coolant circuit <NUM> (e.g. <NUM> barg) and to re-inject or charge it into said circuit or loop via the subsequent charging line <NUM>. More precisely, in the shown example there are two high-pressure charging pumps <NUM> in parallel-flow configuration for reasons of redundancy. Furthermore, there is a heat exchanger <NUM>, flown through by a heating medium, arranged in the charging line <NUM> to increase the temperature of the primary reactor coolant before injection into the primary reactor coolant circuit <NUM>. Advantageously, the hot incoming primary reactor coolant running through the letdown line <NUM> acts as the heating medium, such that recuperative heating and cooling is achieved.

To facilitate injection of hydrogen (H<NUM>) into the primary reactor coolant running through the CVCS <NUM>, there is a hydrogenation system <NUM> fluidically connected to the CVCS <NUM>. According to the invention, the hydrogenation system <NUM> is designed for high-pressure hydrogen injection into the high-pressure stream of primary reactor coolant running through the charging line <NUM>. To this end, there is a hydrogen supply <NUM> or hydrogen source, in particular an electrolytic cell or a hydrogen cylinder providing hydrogen at low or medium pressure, for example <NUM> barg. A hydrogen (gas) feeding line <NUM> leads from the hydrogen supply <NUM> to the charging line <NUM>, the injection point being downstream the charging pump(s) <NUM>. A high-pressure feeding pump <NUM> is arranged in the feeding line <NUM> to provide a feeding pressure insignificantly higher than the discharge pressure of the charging pump(s) <NUM>, and thus higher than the pressure prevailing in the charging line <NUM>. In a preferred embodiment there is an optional overflow line bypassing the feeding pump <NUM> and preferably leading into an exhaust system in case the isolation valve <NUM> is a solenoid valve and the membrane compressor of the feeding pump <NUM> needs some time for the runout in order not to pressurize the gas feeding line <NUM> more than necessary and/or not to induce unnecessary pressure transients on the feeding pump <NUM>.

According to the invention, the connection between the feeding line <NUM> and the charging line <NUM>, preferably a simple tee connection <NUM> or a nozzle, is located in a section of the charging line <NUM> between the charging pump(s) <NUM> and the heat exchanger <NUM>. An isolation valve <NUM>, arranged in the feeding line <NUM> between the feeding pump <NUM> and the tee connection <NUM>, allows for closing the feeding line <NUM> regardless of the state of the feeding pump <NUM>, thereby decoupling the hydrogenation system <NUM> from the CVCS <NUM> and shutting down the hydrogen stream into the charging line <NUM>, if desired or required.

The feeding pump <NUM> is preferably a piston compressor or a membrane compressor, in short compressor <NUM>, preferably with adjustable motor speed. This means that the pumping power and hence the hydrogen feed rate are adjustable. The hydrogen feed rate is controlled via the pumping power by an according control system <NUM> (see <FIG> and the description further below). Alternatively, based on the compressor technology, a solution comprising of a control valve in the feeding line <NUM> and additional instrumentation can be foreseen for the control. The control system <NUM> also controls or sets the isolation valve <NUM>. Therefore, the feeding pump <NUM> and the isolation valve <NUM> can be considered as actors of the hydrogenation system <NUM>. A suitable control scheme will be described in more detail further below.

Preferably, the connection from low pressure hydrogen distribution to the feeding pump <NUM> will be done nearby the tee connection <NUM> to the charging line <NUM> in order to shorten the pipe length of high-pressure piping. The connection is preferably foreseen to be located outside the reactor building in order to ensure the possibility for maintenance and to lower the qualification needs. In existing systems, there are no additional changes on the charging line <NUM> necessary except the tee connection <NUM> to connect the gas feeding line <NUM>.

Standstill times can be decreased by a second (redundant) compressor allowing maintenance on the second injection train during full power operation of the plant. While the VCT <NUM> could easily be used with any atmosphere by now, it could also be used for degasification means, without impacting the hydrogenation.

Sensory input to the control system <NUM> is provided by a number of hydrogen concentration sensors, in short hydrogen sensors or H2 sensors.

A first hydrogen sensor <NUM> is arranged for measuring the hydrogen content or concentration in the low pressure, low temperature stream of primary reactor coolant in the letdown line <NUM> or the subsequent main line <NUM> of the CVCS <NUM>. This is also called the letdown sensor or 'H2 letdown'. In the shown example the measuring point is downstream the heat exchanger <NUM> and upstream the tee connection <NUM> to the VCT <NUM>. For practical reasons, the measurement is preferably inbound into a bypass from the main flow. In other words, there is a short branch line <NUM> arranged in parallel flow configuration with respect to the main line <NUM>, wherein the inlet <NUM> into the bypass and the outlet <NUM> are realized by tee connections, such that a branch flow of primary reactor coolant is diverted from the main flow and then reunited with it. The hydrogen sensor <NUM> is arranged either directly within said branch line <NUM> or in a secondary branch.

A second hydrogen sensor <NUM> is arranged for measuring the hydrogen content or concentration in the high-pressure part, but preferably low temperature stream of primary reactor coolant within the charging line <NUM>, downstream the injection point of the hydrogenation system <NUM> (i.e. downstream the tee connection <NUM>). This is also called the charging sensor or 'H2 charging'. In the shown example the measuring point is upstream the heat exchanger <NUM>. Just like the first hydrogen sensor <NUM>, the second hydrogen sensor <NUM> may be arranged in bypass of the main flow. That is, there may be a branch line <NUM> diverting from the charging line <NUM>, such that the second hydrogen sensor <NUM> is arranged within said branch line <NUM> or in a secondary branch. As shown in <FIG>, the branch line <NUM> may lead to the low-pressure section of the main line <NUM> or to the branch line <NUM> of the first hydrogen sensor <NUM>, preferably discharging downstream the first hydrogen sensor <NUM>. This way, a sampling backflow from the high-pressure section to the low-pressure section of the CVCS <NUM> is realized. A pressure reducing valve (not explicitly shown here) in the backflow line <NUM> compensates for the different pressure levels.

The Instrumentation and Control (I&C) concept visualized in <FIG> shows the simplified logic to be integrated for the control of the hydrogenation system <NUM>. The equipment (in particular sensors, valves, compressors) may also be integrated in the plant's own I&C or may be supplied with a standalone black box I&C. The input for the concept comprises a H2 concentration setpoint and as many component feedback signals as requested by the plant operator. The I&C itself will be based on standard I&C components, thus it is easy to be implemented into any existing I&C structure. Additional components, for instance an electrolyzer as hydrogen source can also be implemented in the black box if requested.

As discussed above, there are preferably two hydrogen online measurements, one is connected to the letdown line <NUM> (including the subsequent main line <NUM>) and one to the charging line <NUM>. The letdown line measurement is located in the low pressure/temperature part of the CVCS <NUM> in order to facilitate the interface to the online measurement physically. The measurements will preferably be inbound into a bypass from the main flow. For availability reasons at each location in the charging line <NUM> and/or letdown line <NUM>, the hydrogen sensors <NUM>, <NUM> can be implemented two or three times with a simple voting logic providing e.g. a <NUM>-out-of-<NUM> or <NUM>-out-of-<NUM> signal.

As discussed above, there are essentially two actuators foreseen in the hydrogenation station design. A piston or membrane compressor <NUM> that is used as a feeding pump <NUM> to inject hydrogen into the main charging line <NUM> of the CVCS <NUM> and an isolation valve <NUM> downstream the compressor <NUM> that is dedicated to isolation functions for either normal operation cases or limitation functions. Based on the compressor technology, an additional control valve can be necessary or beneficial in the gas feeding line <NUM>.

The control of the hydrogenation system <NUM> is based on the hydrogen measurement in the letdown line <NUM>. The hydrogen concentration in the letdown line <NUM> is essentially the same as the one in the primary reactor coolant circuit <NUM>, provided that the main primary pumps (i.e. Reactor Coolant Pumps <NUM>) are in operation and the main primary reactor coolant circuit <NUM> is therefore in a homogenized condition.

The setpoint (labelled 'Setpoint' in the diagram) of the hydrogenation system <NUM> is set by the operator as constant value and together with the H2 concentration in the letdown line <NUM> (H2 letdown), the control deviation is derived by subtraction of the H2 concentration in the letdown line <NUM> from the setpoint (naturally the setpoint is the target value in the main primary circuit). The PID controller <NUM> regulates the reactivity of the control by a variable GAIN control based on the difference between the setpoint and the hydrogen concentration measured in the letdown line <NUM>. The higher the deviation between the setpoint and the H2 concentration in the letdown line <NUM>, the more reactive the controller will be due to the GAIN that is increased proportional to the deviation.

The (speed regulated) compressor <NUM> of the feeding pump <NUM> is adjusted via the PID controller <NUM>. It is shut off on a couple of different signals, like the closed isolation valve <NUM>, sensor errors, low Reactor Coolant Pump (RCP) pressure or H2 max signal. The isolation valve <NUM> is operated in a similar manner.

The signal from the PID controller <NUM> to the compressor <NUM> is delayed for the startup of the regulation compared to the signal opening the isolation valve <NUM>. In order not to have a continuous active control loop, the isolation valve <NUM> is only opened in case of a defined minimum control deviation. If the regulation shows that this step is not necessary, the limit can be set to <NUM>.

Basically, there is a 'MAX concentration reached' signal and a 'MIN concentration reached' signal, both are generated from the measured hydrogen concentration in the letdown line <NUM>. In order to make the concept more robust in case parts of the CVCS <NUM> are not in operation, it is also possible to have an external signal from a nuclear sampling system.

The hydrogen sensor <NUM> in the letdown line <NUM> acts as an operational safety device in order to start up the hydrogenation in case of load follow operation or other types of perturbations of hydrogen concentration in the main primary system or hydrogenation shut down as soon as technical specified limits of concentration are reached.

The hydrogen sensor <NUM> in the charging line <NUM> is used in order to check the injection capability. If the concentration in the charging line <NUM> does not rise in a predefined time despite the compressor <NUM> operating, the compressor <NUM> is shut down (for example in case of feeding line isolation).

Claim 1:
Pressurized water reactor (<NUM>), comprising a primary reactor coolant circuit (<NUM>) flown through by a primary reactor coolant during operation, and comprising a chemical and volume control system (<NUM>) for the primary reactor coolant, the chemical and volume control system (<NUM>) comprising, along the direction of flow of the primary reactor coolant, a letdown line (<NUM>), a high-pressure charging pump (<NUM>) with a given discharge pressure, and a charging line (<NUM>) leading to the primary reactor coolant circuit (<NUM>), the chemical and volume control system (<NUM>) further comprising a hydrogenation system (<NUM>) with a hydrogen supply (<NUM>) and a hydrogen feeding line (<NUM>),
wherein a high-pressure feeding pump (<NUM>) is arranged in the hydrogen feeding line (<NUM>) to provide a gas pressure higher than the discharge pressure of the charging pump (<NUM>), and wherein the hydrogen feeding line (<NUM>) discharges into the charging line (<NUM>),
wherein a heat exchanger (<NUM>), configured to be flown through by a heating medium, is arranged in the charging line (<NUM>) to increase the temperature of the primary reactor coolant before injection into the primary reactor coolant circuit (<NUM>),
characterized in that the connection between the hydrogen feeding line (<NUM>) and the charging line (<NUM>) is located in a section of the charging line (<NUM>) between the charging pump (<NUM>) and the heat exchanger (<NUM>).