Power recirculation flow control

The BWR operates at constant pressure and maintains constant steam pressure similar to most fossil boilers. The integration of the turbine pressure regulator and control system in conjunction with the reactor water recirculation flow control system permits automated changes in steam flow to accommodate varying load demands on the turbine. Power changes of up to 25% of rated power can be accomplished automatically by recirculation... 

After passing through the core, the coolant steam-water mixture enters the bank of centrifugal steam separators mounted above the core, where the water is separated out by vortex action and flows down to join the recirculation flow through the annulus. The steam passes upwards into the steam driers in which the moisture is further reduced, and thence to the turbine. The steam leaving the core is at a temperature of 286°C, at a pressure of 1040 psi (73 kg cm The total thermal output from the core is 3833 MWt. Recirculation flow control is used to provide automatic load following power changes of up to 25% of full power can be accommodated in this way. 

The reactor core, the source of nuclear heat, consists of fuel assemblies and control rods contained within the reactor vessel and cooled by the recirculating water system. A 1,220-MWe BWR/6 core consists of 732 fuel assemblies and 177 control rods, forming a core array 16 feet (4.8 meters) in diameter and 14 feet (4.2 meters) high. The power level is maintained or adjusted by positioning control rods up and down within the core. The BW R core power level is further adjustable by changing the recirculation flow rate without changing control rod position, a feature that contributes to excellent load-following capability. 

An electrical resistance heater with more turns at the tube ends (to compensate for heat losses) surrounds each tube. There is a vertical laminar flow hood over the loading area to minimize particle contamination of the wafers being loaded. As we can see, there are temperature controls for the furnace tubes, and a power module to provide the electrical power. When operated as a LPCVD system, a unit including both the gas flow and vacuum systems is positioned on the right side. Such a unit is shown in Figure 8. Here we can see the vacuum pumps on the left, and the mass flow controllers on the right. The vacuum pump oil recirculation systems are shown in the slide out drawers. As can be seen in Figure 9, this system, as well as most current similar systems, operate under computer control. 

In BWR plants, the reactor power is easily controlled by means of the recirculation pump flow rate. Normally, an upper level of reactor power is established by means of control rod manoeuvring until a certain control rod pattern in the core has been attained, and then adjustments of the recirculation flow rate are utilized to control the power level. A BWR is characterized by... 

Control rods are manually withdrawn according to a predetermined schedule to achieve criticality of the reactor. They are further withdrawn to approximately 32% of rated power with the reactor water flow control valves fully open and the recirculation pumps operating at low speed (25%). The rate at which power level is raised is usually limited by conditions of thermal expansion of the reactor vessel. 

At approximately 32% of rated power, the reactor water flow control valves are closed and the recirculation pump transferred to auxiliary power and operated at rated speed. 

From approximately 30% to approximately 40% of rated power, the control of power level is by manual confrol of recirculation flow by changes in control valve position from minimum position. 

After the generator is synchronized to the electrical system and is producing a substantial output, the power output is adjusted to meet the system requirements by manual adjustment of control rods, manual or automatic adjustment of reactor recirculation flow, or a combination of fhese two methods. 

The BWR is unique in that reactor power output can be varied over a power range of approximately 25% of the operating power level by adjustment of the reactor recirculation flow without any movement of control rods. This is the normal method used for load following and maneuvering the reactor and allows for load following at rates of up to 1% of... 

Interlocks are used on the intermediate range neutron monitors to ensure that all units are operating properly and on the proper range. Control rod withdrawal is blocked if the ratio of reactor power to recirculation flow exceeds a predefermined value. 

Control of water level must overcome some anomalies which have their parallels In conventional water tube boilers. Thus, a rise in reactor power will increase channel voidage and this causes a slight reduction in recirculation flow also as the new voidage Is established in the channel and riser, water is displaced causing an Initial rise In drum water level. But an Increase In reactor power should call for an Increase In feed flow and this raises the water level further. A corresponding fall In drum water level follows a reduction In power. Hence It Is seen that control of drum level is not a simple function and leaves no additional freedom for feed flow as an operational control. 

Because the SCWR has a once-through steam cycle, in which steam from the core outlet is directly supplied to the HP turbines, it has many similarities with BWRs. However, on a closer look there is a basic difference in the coolant flow path inside of the reactor that causes a difference of the steam cycle control. In a BWR the feed-water pump is controlling the liquid level in the reactor pressure vessel, the steam pressure is controlled by the turbine governor valve, and the core power is either controlled by the control rods or by the speed of the recirculation pumps. The SCWR concepts do not include any recirculation loop. The feed-water pump can control either the steam temperature at the core outlet, if the core power is controlled by the control rods, or it can control the core power if the steam outlet temperature is controlled by the control rods. Again, the steam pressure is controlled by the turbine governor valve in both cases. 

This is a typical flow increasing transient. The demand of the main coolant flow rate is assumed to rise stepwise up to 138% of the rated flow as is assumed in the feedwater control system failure of Japanese ABWRs. Since increase in the core coolant flow rate is mild in ABWRs due to the large recirculation flow, the feed-water flow rate is assumed to increase stepwise. This assumption is too conservative for the Super LWR. The main coolant flow rate is gradually increased by the control system in the safety analysis. The calculation results are shown in Fig. 6.31. The reactor power increases with the flow rate due to water density feedback. A scram signal is released when the reactor power reaches 120% of the rated power. The maximum power is 124% while the criterion is 182%. The increase in the pressure is small. The sensitivity analysis is summarized in Table 6.15. 

Inability to control gas recirculation flow, as it relates to the specified quantity of fuel, which in turn sets the power system,... 

The rate of addition of solid fuel is controlled from the metering bin. A hydraulic system drives the metering bin conveyor. When the metering bin conveyor is stationary the electrically driven pump on the hydraulic power unit recirculates the hydraulic fluid to the reservoir. To start the metering bin conveyor a solenoid valve is opened allowing hydraulic fluid to turn the hydraulic motor. The flow and pressure of fluid to the hydraulic motor and ultimately the conveyor speed is controlled by a 12 position regulator valve on the hydraulic power unit. 

The relatively simple, lumped-parameter system model described above has been tested against and used in earnest to analyse the behaviour of the boiler recirculation loops of a number of power stations. It has been found to give excellent quantitative predictions of all the variables whose trends are important for control engineering purposes, namely steam drum pressure and temperature, feedwater flow, steam production, downcomer flow and, very important, drum water level. 

Fig. 16.3 gives the flow sheet of a cooling water system commonly used in power generation that is also used in the process industries. Chemicals such as acids, biocides, scale and corrosion inhibitors and dispersants are added to control problems of fouling in the recirculating system (described elsewhere in this book). The technique is generally applicable where the make-up water is generally of low hardness and silica and low concentration factors are employed. 

The internal recirculation pumps are provided with more than 10% excess, flow rate capacity, which allows xenon override, and the fine motion control rod drives and the grey-tipped control blades allow control rod movements at full power. The excess pump capacity is utilized for hydraulic spectral shift operation the core coolant flow is increased towards the end of the operating cycle. The built-in "redundanc> " also implies that the reactor can be operated at full power even if one pump should fail. 

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