As the name implies, LWRs use light water as the coolant and the moderator, and in many cases as the reflector material. These are typical thermal reactors as they utilize thermalized neutrons to cause nuclear fission reaction of the U235 atoms. The thermal efficiency of these reactors hover around 30%. Two main types of LWR are PWR and BWR. These two types are created mainly because of the differ­ence in approaches of the steam generating process (good quality steam should not contain more than 0.2% of condensed water). LWRs have routinely been designed with 1000 MWe capacity.

Pressurized Water Reactor PWRs were designed and implemented commercially much sooner than the BWRs due to the earlier notion that the pressurized liquid water would somehow be much safer to handle than the steam in the reactor core and would add to the stability of the core during the operation. That is why the first commercial reactor in Shippingport was a PWR. PWRs are designed and installed by companies such as Westinghouse and Areva.

A schematic design of a typical PWR plant is shown in Figure 1.9a. A PWR plant consists of two separate light water (coolant) loops, primary and secondary. The PWR core is located inside a reactor pressure vessel (RPV) made of a low-alloy fer­ritic steel (SA533 Gr. B) shell (typical dimensions: outside diameter ~5 m, height ~12 m, and wall thickness 30 cm), which is internally lined by a reactor cladding of 308-type stainless steel or Inconel 617 to provide adequate corrosion resistance against coolant in contact with the RPV. The PWR primary loop works at an average pressure of 15-16 MPa with the help of a set of pressurizers so that the water does not boil even at temperatures of 320-350 °C. The PWR core contains an array of fuel elements with stacks of a slightly enriched (2.5-4%) UO2 fuel pellets clad in Zircaloy-4 alloy (new alloy is Zirlo or M5). Individual cladding tubes are generally about ~10mm in outer diameter and ~0.7 mm in thickness. The fuel cladding tube stacked with fuel pellets inside and sealed from outside is called a fuel rod or fuel pin. About 200 of such fuel rods are bundled together to form a fuel element. Then, about 180 of such fuel elements are grouped together to form an array to create the reactor core of various shapes — square, cylindrical, hexagonal, and so on (Figure 1.9b). The reactor core is mounted on a core-support structure inside the RPV. Depending on the specific design, the above-mentioned dimensions of the various reactors may vary.

The control rod used is typically an Ag-In-Cd alloy or a B4C compound, which is used for rapid control (start-up or shutdown). Boric acid is also added to the primary loop water to control both the water chemistry acting as “poi­son” and the long-term reactivity changes. This primary loop water is trans­ported to the steam generator where the heat is transferred to the secondary loop system forming steam. The steam generator is basically a heat exchanger containing thousands of tubes made from a nickel-bearing alloy (Incoloy 800) or nickel-based superalloy (e. g., Inconel 600) supported by carbon steel plates (SA515 Gr.60).

Boiling Water Reactor BWR design embodies a direct cycle system of cooling, that is, only one water loop, and hence no steam generator (Figure 1.10a). Early boiling water experiments (BORAX I, II, III, etc.) and development of experi­mental boiling water reactor (EBWR) at the Argonne National Laboratory were the basis of the future commercial BWR power plants. Dresden Power Station (200 MWe), located at the south of Chicago, IL, was a BWR power plant that started operating in 1960. It is of note that this was a Generation-I BWR reactor. However, most BWRs operating today are of Generation-II type and most significant features are discussed below. The reactors (Fukushima Daii — chi) that underwent core melting following the unprecedented earthquake and tsunami in Japan during 2011 were all of BWR type.

Figure 1.9 (a) A view of a typical PWR plant. Courtesy: US Nuclear Regulatory Commission (b) A view of various PWR components. Courtesy: Westinghouse Electric Corporation.

The BWR reactor core is located near the bottom end of the reactor pressure ves­sel. Details of various components in a typical BWR are shown in Figure 1.10b. The BWR RPV is more or less similar to the PWR one. The BWR core is made of fuel assembly consisting of slightly enriched UO2 fuels clad with recrystallized Zircaloy-

BWR/6

REACTOR ASSEMBLY

1. VENT AND HEAD SPRAY

2. STEAM DRYER LIFTING LUG

3. STEAM DRYER ASSEMBLY 4 STEAM OUTLET

5. CORE SPRAY INLET

6. STEAM SEPARATOR ASSEMBLY

7. FEEDWATER INLET

8. FEEDWATER SPARGER

9 LOW PRESSURE COOLANT INJECTION INLET

10. CORE SPRAY LINE

11. CORE SPRAY SPARGER

12. TOP GUIDE

13. JET PUMP ASSEMBLY

14. CORE SHROUD

15. FUEL ASSEMBLIES

16. CONTROL BLADE

17. CORE PLATE

18. JET PUMP/RECIRCULATION WATER INLET

19. RECIRCULATION WATER OUTLET

20. VESSEL SUPPORT SKIRT

21. SHIELD WALL

22. CONTROL ROD DRIVES

23. CONTROL ROD DRIVE HYDRAULIC LINES

24. IN CORE FLUX MONITOR GENERAL ELECTRIC

Table 1.2 PWR versus BWR. PWR BWR Principle of steam generation RPV pressure ^15 MPa RPV pressure ~7 MPa RPV temperature ~326 °C RPV temperature ~290 °C Steam generated in steam generator (via Steam generated in RPV (with secondary loop) separator and dryer) No bulk boiling in RPV Bulk boiling allowed in RPV Major components RPV RPV with separator and dryer Two-four steam generators No steam generator One pressurizer No pressurizer Top entry control rod clusters Bottom entry control rod drives Zircaloy-4 fuel cladding tubes Zircaloy-2 fuel cladding tubes

2 cladding tubes (about 12.5 mm in outer diameter). For a BWR core of 8 x 8 type, each fuel assembly contains about 62 fuel rods and 2 water rods, which are sealed in a Zircaloy-2 channel box. The control material is in the shape of blades arranged through the fuel assembly in the form of a cruciform and is generally made of B4C dispersed in 304-type stainless steel matrix or hafnium, or a combination of both. Water is passed through the reactor core producing high-quality steam and dried at the top of the reactor vessel. The BWR operates at a pressure of about 7 MPa and the normal steam temperature is 290-330 ° C.

Note Tables 1.2 and 1.3 contain relevant information on BWRs.	PWRs and various	types of
Table 1.3 Operating parameters and design features of BWRs.
Parameter/feature	BWR (Browns Ferry 3)	ABWR	ESBWRa)
Power (MWt/MWe)	3293/1098	3926/1350	4500/1590
Vessel height/diameter (m)	21.9/6.4	21.7/7.1	27.6/7.1
Fuel bundles (number)	764	872	1132
Active fuel height (m)	3.7	3.7	3.0
Recirculation pumps (number)	2 (Large)	10	Zero
Number of control drive rods	185	205	269
Safety diesel generator	2	3	Zero
Safety system pumps	9	18	Zero
Safety building volume	120	180	135

Courtesy: GE Global Research. a) ESBWR — economic simplified boiling water reactor — of Generation-HI+ category, developed by the GE.

(a)

Figure 1.П (a) A simplified schematic view of a CANDU reactor. Courtesy: Canadian Nuclear Association. (b) The configuration of fuel bundles in the fuel channel. Courtesy: www. cameco. com.

Pressurized Heavy Water Reactor The PHWR reactors were mainly developed by a partnership between the Atomic Energy of Canada Limited (AECL) and Hydro­Electric Power Commission of Ontario in 1960s. The reactors were of Generation — II type. Notably, these reactors are also called CANDU reactors (Figure 1.11a). They are so named because they use heavy water (deuterium oxide) as the moderator and natural uranium as the fuel. These reactors are located mainly in Canada, India, China, and few other countries. The CANDU reactor design does not require a reactor pressure vessel as in LWRs, and hence not a single CANDU reactor oper­ates in the United States since the nuclear safety regulations of the US Nuclear Regulatory Commission specifically call for an RPV in a compliant reactor design.

Unlike LWRs, the natural uranium (0.7% U235) oxide fuel clad in zirconium alloy tubes (known as pressure tubes, made of Zr-2.5Nb) is used in this reactor. These hundreds of pressure tubes are kept inside a calandria shell made of an austenitic stainless steel and reinforced by outer stiffening rings. The shell also keeps channels for the pressurized coolant (hot heavy water or light water) and moderator (heavy water). If light water is used to moderate the neutrons, it would adversely affect the neutron economy due to the absorp­tion of neutrons. That is why cold heavy water is used as the moderator. The pressure tubes along with moderator and cooling tubes are arranged in a hori­zontal fashion (not vertical as in LWRs) and the fuels can be replaced and reloaded without shutting down the whole reactor. Note the fuel and associated structural configuration in Figure 1.11b. The pressurized coolant stays typically at about 290 °C. This reactor system requires a steam generator to produce steam as does a conventional PWR. The control rods are dropped vertically into the fuel zones in case total shutdown or controlling of reactivity is neces­sary. Gadolinium nitrate is added in the moderator system that acts as a secondary shutdown system.

Liquid Metal Fast Breeder Reactor Liquid metal (generally sodium) is used in liquid metal fast breeder reactors (LMFBRs) to transport the heat generated in the core. These reactors are called “breeder” because more new fuels are pro­duced than consumed during its operation. The reactor can convert fertile material (containing U238 and Th232) into fissile materials (Pu239 and U233), respectively. The concept of this reactor type is very practical from the fuel utilization viewpoint. The natural uranium contains only about 0.7% of fissile U235. The majority of the natural uranium contains U238 isotope. These reactors are characterized by high power density (i. e., power per unit volume) due to the lack of a moderator (i. e., much more improved neutron economy). The reactor cores are typically small because of the high power density requirements. The temperature attained in this type of reactors is higher and thus leads to higher efficiency of electric power generation (~42% in LMFBRs versus ~30% in LWRs). The use of Th232 in LMFBRs is particularly advanta­geous for countries like India that do not have a large deposit of uranium, but has plenty of thorium. It should be noted that sodium used in this type of reactor transfers heat to the steam generators. The system containing sodium should be leak-proof since sodium reacts with oxygen and water vapor very fast. Furthermore, it becomes radioactive as the coolant passes through the reactor core. The whole primary coolant system should be put in the shielded protection to keep the operating personnel safe.

The first prototype LMFBR reactor named EBR-I (Experimental Breeder Reactor) was built at the present-day site of the Idaho National Laboratory near Idaho Falls, ID. This was the first reactor to demonstrate that the electricity can be generated using the nuclear energy. Also, it was the first “fast breeder” reactor. It used sodium-potassium (NaK) as coolant. It started its operation in 1951, and was decommissioned by 1964. By this definition, it was a Generation-I fast reactor

Figurel.12 A view of EBR-II reactor.

design. Following the decommissioning of EBR-I, another fast breeder reactor (EBR-II) was installed and started operation in 1963. EBR-II (Figure 1.12) was oper­ated very successfully before it was shut down in 1994. The fuel used was a mixture of 48-51% of U235, 45-48% U238, and the rest a mixture of fissium metals (Mo, Zr, Ru, etc.) and plutonium.