Secondary loop

Most nuclear reactors use a heat exchanger to transfer heat from a primary coolant loop through the reactor core to a secondary loop that suppHes steam (qv) to a turbine (see HeaT-EXCHANGETECHNOLOGy). The pressurized water reactor is the most common example. The boiling water reactor, however, generates steam in the core. 

In control situations with more then one measured variable but only one manipulated variable, it is advantageous to use control loops for each measured variable in a master-slave relationship. In this, the output of the primary controller is usually used as a set point for the slave or secondary loop. 

A cascade control system can be designed to handle fuel gas disturbance more effectively (Fig. 10.1). In this case, a secondary loop (also called the slave loop) is used to adjust the regulating valve and thus manipulate the fuel gas flow rate. The temperature controller (the master or primary controller) sends its signal, in terms of the desired flow rate, to the secondary flow control loop—in essence, the signal is the set point of the secondary flow controller (FC). 

In the secondary loop, the flow controller compares the desired fuel gas flow rate with the measured flow rate from the flow transducer (FT), and adjusts the regulating valve accordingly. 

Disturbance, such as upstream pressure, which specifically leads to changes in the fuel gas flow rate is now drawn to be part of the secondary flow control loop. (A disturbance such as change in the process stream inlet temperature, which is not part of the secondary loop, would still be drawn in its usual location as in Section 5.2 on page 5-7.)... 

So far, we know that the secondary loop helps to reduce disturbance in the manipulated variable. If we design the control loop properly, we should also accomplish a faster response in the actuating element the regulating valve. To go one step further, cascade control can even help to make the entire system more stable. These points may not be intuitive. We ll use a simple example to illustrate these features. 

If we remove the secondary loop, this characteristic equation should reduce to that of a... 

FC) in a slave loop. This secondary loop remains the same as the G v function in (10-1), where the secondary transfer function is denoted by GC2. 

The dosedloop characteristic equation for this S3rstem is not the same as that given in Eq. (11.1). To derive what it is, let us first look at the secondary loop by itself. From the analysis presented in Chap. 10, the equation that describes this dosedloop system is... 

It is useful to compare these values with those found for conventional control = 19.8 and oi. = 1.61. We can see that cascade control results in higher controller gain and smaller dosedloop time constant (the reciprocal of the frequency). Figure 11.26 gives a root locus plot for the primary controller with the secondary controller gain set at Two of the loci start at the complex poles s = rj which Come from the dosedloop secondary loop. The other curve... 

If, however, a cascade control system is used, as sketched in Fig. ll.3b, the closedloop characteristic equation is not that given in Eq, (11.21). To derive it, let us start with the secondary loop. 

Now we solve for the closedloop transfer function for the primary loop with the secondary loop on automatic. Figure 11.3c shows the simplified block diagrana. By inspection we can see that the closedloop characteristic equation is... 

J) Determine the value of Xj that gives a closedloop damping coefficient of 0.5 in the secondary loop. 

For the cold points of use, a secondary loop is used. The velocity of the water must follow the recommended values in each loop (primary and secondary). A regulation valve shall be provided to balance the loss of pressure between primary and secondary. 

When requested by the user located at the point of use, the first exchanger on the secondary loop starts to cool the water. This one is automatically heated at minimum 85°C before the return in the primary loop. When the temperature asked for the cold point of use is reached, the sampling value is automatically opened. The users ask to stop the sampling. A safety timer must be installed to prevent the draining of the installation. Safety timing is adjustable (maximum 1 h) temperature of cold point 30°C 5°C. 

In the other design, PWRs have two closed loops of water circulating in the plant plus a third, external loop to remove the waste heat. Water is pumped through the reactor core in the primary coolant loop to moderate the neutrons and to remove the heat from the core as in the BWR. However, the reactor vessel is pressurized so that the water does not boil. Steam is necessary to run the turbines, so the primary loop transfers the heat to a secondary loop. The water in the secondary loop is allowed to boil, producing steam that is isolated from both the core and the outside. The water in the primary loop usually contains boron (as boric acid H3BO3 0.025 M) to control the reactivity of the reactor. The steam in the secondary loop is allowed to expand and cool through a set of turbines as in the BWR the cold steam condenses and is returned to the primary heat exchanger. A third loop of water is used to maintain the low-temperature end of the expansion near room temperature and remove the waste heat. 

A limit on the efficiency of the electrical energy conduction can be obtained by applying the second law of thermodynamics to the secondary loop. The maximum thermal efficiency, s,, is given in terms of the input and output heats ... 

Usually the dynamics of the secondary loop are sufficiently faster than those of the primary loop for G / (s) to be approximated by its steady-state gain. For the same reason it is possible to tune the cascade system by tuning first the inner loop and then the outer loop. 

In the secondary loop, a feed water pump circulates water through a heat exchanger where the primary and secondary loops exchange heat. The water in the secondary loop is turned to steam here and feeds a turbine, where electricity is generated. In the boiling-water reactor, there is only one loop, and as a result, the overall efficiencies are higher at the added expense... 

The first one is related to the maximum temperature of the cycle. As stated above, the maximum chemical fluid temperature used in our flow sheet (in the S03 decomposition reactor) is around 850°C. Quite interestingly, increasing this maximum temperature does not lead to reduced heat requirements (Buckingham, 2009). It is therefore not necessary to have the hottest possible heat source, which is good in terms of feasibility. The V/HTR operation temperature just needs to be compatible with a 850°C fluid temperature, which implies an outlet temperature of about 950°C if 50°C pinches are assumed (between the primary coolant and the secondary loop, and between the secondary loop and the chemical fluids). 

The first alternative simplifies the heat integration of the chemical process, and the second simplifies the interface between the chemical process and the secondary loop. CEA (Leybros, 2009) has designed their S-I process for the first alternative, as the requirements for the reactor expected to be used are most suitable for it. GA has developed flow sheets for both alternatives. Sandia National Laboratories (SNL) is a partner of both CEA and GA in the operation of a demonstration loop for the S-I cycle. SNL is charged with design and operation of the sulphuric acid decomposition section. They have developed a bayonet-heater design for the decomposer which incorporates internal heat recovery. As a result, the outlet temperature of the bayonet heat modules is too low to use in the HI decomposition section. Thus, helium is utilised in the HI decomposition section, as in the CEA flow sheets. 

The connection scheme developed by the European Projects HYTHEC and RAPHAEL (Le Duigou, 2007) represents a self-sustainable plant concept in which, in addition to the heat supply to the S-I cycle, the electrical demand of the internal consumers is provided by the nuclear reactor. The high temperature flow exiting the nuclear reactor transfers its heat via an IHX to a secondary loop which interacts with the components of the S-I cycle components. The high temperature heat flow is split and partially directed to the chemical part of the cycle and another part to a Brayton cycle for electricity production needed to power pumps, compressors, heat pumps and other auxiliaries. 

An addition to the noted advantages is that the set point of the secondary controller can be limited. In addition, by speeding up the overall cascade loop response, the sensitivity of the primary process variable to process upsets is also reduced, whereas the secondary loop can reduce the effect of control valve sticking or actuator nonlinearity. The primary or outer control loop of a cascade system is usually a PI or PID controller. A properly selected secondary will reduce the proportional band of the primary controller. 

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