**2. Description and operation of recirculating steam generators**

#### **2.1. Design configuration and flow**

A recirculating steam generator consists of a bottom head of a hemispherical shape, the central cylindrical part where the heat exchange occurs and the upper part, the steam dome. The lower head is divided into inlet and outlet plenums by a vertically oriented plate. The primary coolant, after leaving the reactor, enters the lower plenum and then flows upwards through the tube bundle area, the riser, in the cylindrical part of the SG. Water evaporation takes place on the secondary side of the riser. The tube bundle is the interface between the primary and secondary circuits. It is composed of U-shaped tubes with approximate height of 11 m. Water in the reactor coolant system thus flows first upwards and then downwards through the tube sheet, transferring the heat to the secondary fluid. The cooled down water is then poured into the other half of the header tank before flowing back to the reactor. Primary coolant enters the steam generator at 588–603 K on the hot-leg side and leaves at about 560 K on the cold-leg side. The steam generator is shown in **Figure 1** and the schematic drawing of the flow paths inside the RSG in **Figure 2**. The main RSG characteristics and parameters are shown in **Table 1**.

**Figure 1.** Recirculating steam generator (taken from Ref. [2]).

barriers. The priority of a nuclear safety philosophy is to maintain a sufficient water level in the SGs to avoid the damage of the tubes and release of radioactive fluid from the NPP. Thus, there are two functional requirements placed on the steam generators. The first is to act as a heat sink for the reactor core to prevent any core damage. The second functional requirement is to generate the flow rate of steam from the feedwater supply at the temperature, pressure and enthalpy conditions necessary to efficiently drive the steam turbine/electric generating system.

There are few steam generator designs depending on the type of the nuclear power plant [1]. Here, it is necessary to mention that the discussion inside the chapter is focused solely on heat exchangers in the so-called pressurized water reactor (PWR) NPPs where both the primary and secondary fluids are water. There are some other NPP types with gases, salts, or liquid metals used as reactor coolants, but they are not going to be presented. There are also light water reactor power plants where water boils directly inside the reactor core, boiling water reactors (BWRs), which do not have the steam generators at all, because the steam is produced in the reactor itself. Nonetheless, PWRs are the most common type of NPPs making up to 63%

The most common design is the SG with the inverted U-tube heat exchanger bundle, where steam separation equipment is located inside the top shell of the SG [2]. The primary water flows upwards through the tubes first, making a bend, and then flows downwards back into the reactor coolant system (RCS) piping. On the secondary side, roughly a quarter of water, injected through the feedwater system, evaporates, and the remaining water is recirculated back into the boiling region; therefore, that type of SG is called recirculating steam generator (RSG). The American company Babcock and Wilcox (B&G) developed the once-through steam generator (OTSG), a vertical shell counterflow straight-tube design which directly generates superheated steam as the feedwater flows through the SG in a single pass. The primary-side water enters at the steam generator at the top, flows through the generator in unbent tubes and exits at the bottom. There is also a horizontal type of the SG which has the horizontal cylindrical housing and horizontal coils. The steam is dried at the top of the housing by gravitational separation. That type of SG is used mostly in Russian types of PWR reactors called vodo vodijanoj energetičeskij reactor (VVER). The last important design, to be later analyzed in the chapter, is the vertical SG with helical tubes, which has the intention to be used in the future advanced modular and high-temperature reactor systems [3]. The helical-coil design offers compactness and increased heat transfer. At the SG outlet, the steam is superheated,

of the total number worldwide, while BWRs make up 18% of the total.

**2. Description and operation of recirculating steam generators**

A recirculating steam generator consists of a bottom head of a hemispherical shape, the central cylindrical part where the heat exchange occurs and the upper part, the steam dome. The lower head is divided into inlet and outlet plenums by a vertically oriented plate. The primary coolant, after leaving the reactor, enters the lower plenum and then flows upwards through

which results in higher thermal efficiency.

168 Heat Exchangers– Advanced Features and Applications

**2.1. Design configuration and flow**

On the secondary side, feedwater enters the steam generator in the upper shell through a nozzle via a feed ring and is mixed with water draining from the moisture-separating equipment. Then, the water flows downwards through the downcomer to the tube sheet, a vertical plate separating the lower header and riser sections. The downcomer is situated between the tube bundle shroud and the SG outer shell. After reaching the tube sheet, water goes up, flowing below the shroud wall, in the central riser section where it is heated by the primary water flowing inside the U-tubes. Since the fluid in the secondary side, outside the tubes, is saturated, secondary-side water evaporates and a two-phase flow is established near the top of the tube bundle. The buoyancy force caused by a difference in densities of water inside the downcomer and the two-phase mixture in the tube bundle section ensures fluid circulation in the SG. As the secondary fluid flows upwards the riser, the steam quality increases up to 30%. This is not sufficient for a safe turbine operation as the droplets of water in the steam could damage the turbine blades. Thus, the steam that exits the tube bundle goes first into steam-moisture separators, composed of swirl vanes, and steam dryers, in the form of chevron separators, before it goes out of the steam generator through a nozzle at the top of the SG dome. The steam lines direct the steam flow into the turbine. The water collected by the separation devices falls down to the riser or is, for the most part, directed to the downcomer. After the drying process, the steam is saturated with a residual humidity, the moisture carryover, of less than 0.0025. Dry and saturated steam leaves the steam generator vessel and enters the steam lines.

**Figure 2.** Simplified RSG flow paths.


**Table 1.** Typical RSG dimensions and operating parameters.

About 25% of the secondary coolant is converted to steam on each pass through the generator, and the remainder is recirculated. That amount of recirculated coolant is described by the important design parameter called circulation ratio which is defined as the ratio between the total flow rate through the tube bundle and the flow rate of steam exiting the steam generator.

#### **2.2. Regulation and control**

ing inside the U-tubes. Since the fluid in the secondary side, outside the tubes, is saturated, secondary-side water evaporates and a two-phase flow is established near the top of the tube bundle. The buoyancy force caused by a difference in densities of water inside the downcomer and the two-phase mixture in the tube bundle section ensures fluid circulation in the SG. As the secondary fluid flows upwards the riser, the steam quality increases up to 30%. This is not sufficient for a safe turbine operation as the droplets of water in the steam could damage the turbine blades. Thus, the steam that exits the tube bundle goes first into steam-moisture separators, composed of swirl vanes, and steam dryers, in the form of chevron separators, before it goes out of the steam generator through a nozzle at the top of the SG dome. The steam lines direct the steam flow into the turbine. The water collected by the separation devices falls down to the riser or is, for the most part, directed to the downcomer. After the drying process, the steam is saturated with a residual humidity, the moisture carryover, of less than 0.0025. Dry

170 Heat Exchangers– Advanced Features and Applications

and saturated steam leaves the steam generator vessel and enters the steam lines.

**Figure 2.** Simplified RSG flow paths.

In order to achieve the normal steam generator operation, a variety of parameters are subjected to regulation. Steam and feedwater flows need to be balanced; otherwise, the SG would overfill (if the feedwater flow is larger than the steam flow) or dry out (in the case the steam flow is larger than the feedwater flow). The amount of steam leaving the steam generator depends on the electrical load demand by the consumers. The turbine power is a linear function of the reactor coolant average temperature. The reactor temperature regulation system maintains the temperature by adjusting the position of the reactor control rods taking into account the signal of the turbine power. For the fast load changes, the excess steam is directed directly into the turbine condenser, thus bypassing the turbine. In that way, the steam generator pressure is being kept below the safety limits. Additionally, for the SG pressure control, poweroperated relief and safety valves, mounted on the steam lines, are used. The relief valves are motor-operated valves, while safety valves are passive components.

The most important SG parameter subjected to regulation is the SG level. If the level is too low, the insufficient heat removal by the secondary side may cause evaporation of the reactor coolant, thus overheating of the reactor core. On the other hand, if the level is too high, the steam exiting the steam generator would carry water droplets (the void fraction would be higher than zero) which can be damaging to the turbine. The SG level is maintained by the feedwater flow by means of controller which continuously compares measured feedwater flow with steam flow and a compensated steam generator downcomer water level signal with a water level set point. A functional diagram of the steam generator level control system is shown in **Figure 3**.

The measured steam generator level is compensated by a lag controller (1/(1+τ<sup>1</sup> s)) and subtracted from a desired reference SG level. That signal is then corrected by a proportional-integral (PI) controller (K1 (1+1/τ<sup>2</sup> s)) and added to a difference between steam flow and feedwater flow signals. The resulting signal goes through a final PI correction (K<sup>2</sup> (1+1/τ<sup>3</sup> s)) before being used for feedwater flow control. Parameters K<sup>1</sup> and K2 are scaling factors and τ<sup>1</sup> , τ<sup>2</sup> and τ<sup>3</sup> time constants depending on the design of nuclear power plant control system.

According to **Figure 3**, in the case the reference level signal is larger than the measured level or the steam flow is larger than the feedwater flow, the feedwater flow will be increased by increasing the feedwater control valve area. In the opposite case, the control valve flow area will be decreased.

**Figure 3.** Functional diagram of the SG level control system.

Two types of water levels are measured inside the steam generator: the narrow range (NR) level and the wide range (WR) level. The term "water level" should not be taken literally since no free water surface in the SG secondary side can be established. The fluid is in a state of boiling, and in the area above the tube bundle, steam quality steadily rises from the top of the U-tubes to the inlet in the steam separators. Thus, the level is deduced from the pressure difference, pressures being measured at two different heights. The level is affected by variations in the fluid density as well as residual pressure drops.

In general, the SG level is a measure of a pressure difference inside the steam generator compared to a pressure difference between the liquid and gas phases. It is calculated by the expression:

$$\text{SGL } \mathsf{VL}[\%] = 100 \cdot \frac{\Lambda \mathsf{p} \cdot \Lambda \mathsf{p}\_{\mathsf{v} \mathsf{u}}}{\Lambda \mathsf{p}\_{\mathsf{v} \mathsf{u} \mathsf{v}} - \Lambda \mathsf{p}\_{\mathsf{v} \mathsf{u}}} \tag{1}$$

where Δp0% = ρ<sup>g</sup> gh and Δp100% = ρ<sup>l</sup> gh.

The height in the expressions for the pressure difference is the distance between the measurement taps. For both the narrow and wide range measurements, the upper tap is in the separator area. The lower tap for the NR level measurement is just below the U-tube bend area, near the top of the downcomer. For the WR level measurement, the lower tap is at the bottom of the downcomer. The total height for the NR level measurement is about 5 m and for the wide range 15 m.

The narrow range level is used to control the feedwater flow rate. Except in the case of extreme events, such as very fast transients caused by accidental depressurization, the narrow range level is maintained constant by the control system. The feedwater flow rate and temperature, and thermal hydraulic SG conditions, have much larger influence on the wide range level. Therefore, the WR level is only used as a level indicator for the NPP operators during slow running transients or during the plant shutdown and start-up operation modes after an outage. Dependence of the WR level on the dynamic SG pressure prevents it to be used for the control of the NPP performance [4].
