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Materials and Processes for Light Water Reactors

Jean-Paul MASSOUD1 and Eric MOLINIÉ2

1 EDF/DIPNN/DT, France

2 EDF/DIRES/R&D, France

1.1. Acronyms

1.2. Introduction with general description of the systems

1.2.1. Pressurized water reactor (Kaercher 2002; Cattant 2014; Hutin 2017a)

From the end of the 1960s, light water reactors (LWRs) have been used extensively in many countries around the world for electricity production.

The predominant reactor design worldwide is the pressurized water reactor (PWR), accounting for about two-thirds of the installed capacity, followed by boiling water reactors (BWRs) at about 20%.

Figure 1.1 shows an overview of a typical modern PWR. The condenser is generally cooled by sea, brackish or river water. However, in the latter situation, when the river flow is too low, cooling towers are used, as shown in the picture.

Figure 1.1. Sketch of a PWR presenting the main buildings and facilities

(courtesy of EDF).

The flow of primary water is maintained by the pressurizer and flows through the reactor coolant system (RCS) by reactor coolant pumps. The typical RCS water conditions are as follows:

• high temperature: between 285°C (cold leg, reactor pressure vessel inlet) and 325°C (hot leg, reactor pressure vessel outlet);
• high pressure: around 155 bars;
• reducing environment for water radiolysis mitigation by hydrogen injection (typically: 30 cc/kg);
• presence of boric acid for the neutronic reaction control. The primary water pH300°C is typically adjusted to 7.2 by way of lithium hydroxide injection (a few ppm maximum).

The RCS heat energy is exchanged with the secondary circuit through the steam generator tubes. The secondary water is heated to steam that powers the high- and low-pressure turbines, which turn the main generator for electricity production.

The steam is then condensed to water and flows back to the steam generators. For thermodynamic considerations (cycle efficiency), the water is steam-heated before entering the steam generator and the steam is dried and reheated between the high- and low-pressure turbines in the moisture separator-reheaters (MSRs). At the steam generator outlet, the typical steam conditions are as follows:

• temperature: 286°C;
• pressure: 70 bars.

A schematic of the primary and secondary circuits and materials of construction is presented in Figures 1.2 and 1.3 for PWR and BWR, respectively.

There are hundreds of various systems in a PWR; some related to the primary circuit are indicated.

Although optimized, the water and steam environments are still harsh enough to pose a threat to the systems materials, at times leading to corrosion failures. Mechanical loads can also result in mechanical failures, sometimes in conjunction with corrosion, like in the cases of stress corrosion cracking (SCC) and corrosion fatigue.

Figure 1.2. Principle of PWR: main components and materials. In PWR, the water heated in the RPV is used to heat a secondary circuit.

Figure 1.3. Principle of BWR: main components and materials. In BWR, the water heated in the RPV directly feeds the turbine.

1.2.2. BWR (Boiron 2011; Cattant 2014)

Figure 1.4 presents the BWR power cycle. Basically, the BWR has a lower reactor pressure vessel (RPV) pressure and a simplified steam cycle compared to PWR. The RPV pressure is around 7 MPa (1,020 psig). The temperature in the RPV reaches 288°C (550°F).

As the steam is generated in the RPV, bulk boiling is allowed in the BWR core, which involves limitations regarding possible water/steam chemical conditioning (i.e. no boric acid or lithium hydroxide injection).

One particular component found in BWRs is jet pumps, which provide flow to control reactor power, which enables a higher power level without increasing the RPV size. Jet pumps also provide part of the boundary required to maintain two-thirds of the core height following a recirculation line break event.

Figure 1.4. Advanced BWR power cycle

(Cattant (2014), courtesy of GE).

Figure 1.5 presents a typical BWR lower plenum, with the presence of:

• control rod drive guide tubes;
• control rod blades;
• control rod drive housings;
• stub tubes;
• in-core housings;
• guide tubes;
• flux monitor dry tubes.

Another major component of BWR core internals is the core shroud, which is a large austenitic stainless steel cylinder surrounding the core. It separates upward flow through the core from downward flow in the downcomer annulus. It also provides a two-thirds core height floodable volume.

Another big piece of equipment sitting at the top of BWR RPV internals is the steam dryer. The steam dryer provides a 99.9% steam flow to the main turbine. In the dryer, wet steam is forced through the dryer panels horizontally:

• the steam is forced to make a series of rapid changes in direction;
• the moisture is thrown to the outside.

Note that initial power uprate plants experienced flow induced vibrations, which have been minimized by design improvements.

Figure 1.5. BWR typical lower plenum

(Cattant (2014), courtesy of GE).

1.3. Requirements for materials (Meyzaud and Vieillard-Baron 1998; Champigny 2005; Lemaignan 2010; Zinkle and Was 2013)

The selection of materials must take into account some drivers that are specific to LWR, as summarized below:

• large size components (with homogeneous properties), such as the reactor vessel, the primary pump, the pressurizer or the steam generators;
• good corrosion resistance (primary and secondary circuits);
• knowledge of materials...