The Pressurized Water Reactor

As an illustration of the design aspects of a modern pressurized water reactor, we may consider the two-unit Sequoyah plant of total net capacity 2280 MWe, which is being built by Westinghouse at a site near Chattanooga, Tennessee. Full power operation of the first of the two reactors commenced in 1981. 

The steam generators are divided into two sections, the evaporator below and the steam drum above. The evaporator consists of a U-tube heat exchanger. Water of the primary coolant, at a temperature of 321 C, flows through the tubes, which are made from Inconel. The flow in the secondary circuit is upward through the tube bundle, the steam-water mixture passing 

The single pressurizer serves all four coolant loops. It serves to hold the primary circuit pressure constant and to allow for expansion and contraction of the coolant volume caused by changes of load. An electric immersion heater of 1800-kW capacity is used to raise the pressure to, and maintain it at, the required value during negative pressure surges caused by an increase in load demand. In the event of a positive surge, caused by a reduction in load demand, a spray system, fed from the cold leg of one of the reactor coolant loops, condenses steam in the pressurizer to avoid tripping the pressure relief valves with which it is fitted. 

The fuel assembly for the Sequoyah reactor is shown in Fig. 9.4. The fuel 

The fuel is in the form of pellets of slightly enriched UO2 contained in tubes of cold-worked zircaloy-4. The pellets, of diameter 8.2 mm, are dished at the ends to allow for the differential thermal expansion due to the temperature profile across the pellet. Following the technique pioneered by Westinghouse, the rods are pre-pressurized with helium to minimize the compressive clad stresses and creep induced by the coolant pressure. Adequate void volume is provided to take up the differential expansion between fuel and cladding and to allow for accumulation of fission products. There is no differential enrichment of fuel within an individual assembly, but different enrichments are used for the three radial regions into which the core is divided to improve the radial form factor. 

---

A variety of nuclear reactor designs is possible using different combinations of components and process features for different purposes (see Nuclear REACTORS, reactor types). Two versions of the lightwater reactors were favored the pressurized water reactor (PWR) and the boiling water reactor (BWR). Each requites enrichment of uranium in U. To assure safety, careful control of coolant conditions is requited (see Nuclearreactors, water CHEMISTRY OF LIGHTWATER REACTORS NuCLEAR REACTORS, SAFETY IN NUCLEAR FACILITIES). 

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. 

The key feature of the pressurized water reactor is that the reactor vessel is maintained above the saturation pressure for water and thus the coolant-moderator does not bod. At a vessel pressure of 15.5 MPa (2250 psia), high water temperatures averaging above 300°C can be achieved, leading to acceptable thermal efficiencies of approximately 0.33. 

Pressurized vs. boiling LWRs The pressurized water reactor (PWR) transfers its energy from the fuel to an intermediate heat exchanger to generate the steam that... 

Another type of reactor is the pressurized water reactor (PWR). In a PWR, coolant water surrounding the reactor core is kept under high pressure, preventing it from boiling. This water is piped out of the reactor vessel into a second building where it is used to heat a secondary set of pipes also containing ordinary water. The water in the secondary system is allowed to boil, and the steam formed is then transferred to a turbine and generator, as in the BWR. 

The Pressurized Water Reactor (PWR) reload core optimization problem, though easily stated, is far from easily solved. The designer s task is to identify the arrangement of fresh and partially burnt fuel (fissile material) and burnable poisons (BPs) (control material) within the core which optimizes the performance of the reactor over that operating cycle (until it again requires refueling), while ensuring that various operational (safety) constraints are always satisfied. 

We illustrate the general principles of thermal reactors by a short description of the two most inqx)itant power reactor types the pressurized water reactor (PWR) and the boiling water reactor (BWR). They are further discussed in Chapter 20. 

The pressurized water reactor is generally preferred for propulsion purposes (military surface vessels and submarines), partly because it can react faster on changes in power demand than many other types of thermal reactors. 

Two types of light water reactors, namely, the boiling water reactor (BWR) and the pressurized water reactor (PWR) are in use in the United States of America. The fuel for these reactors consists of long bundles of 2-4 wt% of enriched uranium dioxide fuel pellets stacked in zirconium-alloy cladding tubes. 

The principal parameter for decontamination of liquid effluent from the pressurized water reactor by RO and UF is the decontamination factor (DF), which is the ratio of the amount of nuclides in the inlet stream (specified in terms of the concentration or activity of radioactive materials) to the amount of nuclides in the effluent stream following RO or UF treatment. According to the Gaseous... 

Except where explicitly indicated, the text refers to the pressurized water reactor. Extrapolation to other kinds of plants is, however, possible. 

As it appears firom the numbers given above, the pressurized water reactor has become the most frequently used reactor type for electric power stations. A schematic sketch is shown in 0 Fig. 57.8. 

After this more detailed discussion of the pressurized water reactor, some other types of reactors will also be treated briefly. 

Critical Experiments on Enriched Uranium Stainless Steel Water Moderated Lattices, L. M. Welshans and K, M. Johnson (MARTIN). A series of 12 cold, clean critical cores has been studied under the Army Nuclear Power Program in conjunction with the Pressurized Water Reactor Code Development task. The purpose of the experimental program Is to Supply data on highly enriched uranium, water moderated systems using stainless steel as cladding material. 

This section includes a brief early history of the development of nuclear power, primarily in the United States. Individual chapters cover the pressurized water reactor (PVVR), boiling water reactor (BWR), and the CANDU Reactor. These three reactor types are used in nuclear power stations in North America, and represent more than 90% of reactors worldwide. Further, this section includes a chapter describing the gas cooled reactor, liquid metal cooled fast reactor, the molten salt reactor, and small modular reactors, and concludes with a discussion of the next generation of reactors, known as "Gen IV."... 

The purpose of this chapter is to provide a general insight into the manufacture of fuels used in nuclear reactors. The primary focus will be on uranium dioxide (UO2) fuels for light water reactors (LWRs), including both the pressurized water reactor (PWR) and the boiling water reactor (BWR). Many of the details relating to the fuel for these reactors are also presented in Sections 1.2 and 1.3 of this handbook. Some of the information from those chapters will be repeated for clarity. 

In the pressurized water reactors designed by Westinghouse, Framatome and other manufacturers, the steam generator tubes are made from Inconel 600, an alloy with high Ni content (about 72%). In newer plants this material has been replaced by Inconel 690, an alloy showing higher stability against selective corrosion attack. 

---