Steel for nuclear power

The loop main pipeline is an important barrier to prevent the leakage of nuclear reaction fission products to the containment under normal, abnormal, accident and test conditions of nuclear power plants. Therefore, the nuclear power main pipeline must be resistant to high temperature, high pressure and corrosion.

Some of the main pipelines of early nuclear power plants used low-alloy steel pipes and welded stainless steel inside the pipes. Later, 18-8 austenitic stainless steel was generally used for nuclear power main pipelines, and the composition and production process were continuously optimized on this basis. Stabilized austenitic stainless steel: Titanium (Ti) or niobium (Nb) is added to 18-8 stainless steel to improve the resistance to intergranular corrosion, but its welding performance is not good and it causes too many inclusions, which affects the processing of elbows.

304 and 316 austenitic stainless steel: 304 stainless steel reduces the carbon content on the basis of 18-8 austenitic stainless steel, and 316 steel adds 2% molybdenum (Mo), but they still have a tendency to "sensitize" when they stay for a long time between 480 and 820℃.

Ultra-low carbon 304L and 316L austenitic stainless steel: The carbon content is further reduced on the original steel type, and excellent intergranular corrosion resistance, welding performance and processing performance are obtained, but the biggest problem is insufficient strength.

The primary main pipeline of the second-generation pressurized water reactor nuclear power plant uses cast duplex stainless steel. A small amount of ferrite (12% to 20%) is added to the austenite matrix, which not only improves the strength and thermal cracking resistance of the material, but also inhibits the occurrence of stress corrosion. However, the ferrite content cannot exceed 20%, otherwise severe thermal aging will occur. The primary main pipeline of the third-generation pressurized water reactor AP1000 nuclear power plant uses integrally forged 316LN austenitic stainless steel, which belongs to ultra-low carbon nitrogen-controlled austenitic stainless steel. Nitrogen is added to 316L, which can not only improve the strength of the material, but also maintain a high level of plasticity and toughness.

Reactor pressure vessels operate under harsh conditions such as high temperature, high pressure, fluid erosion and corrosion, and strong neutron irradiation. Their design life is not less than 40 years and cannot be replaced.

Pressure vessel materials must meet the following special requirements: sufficiently high purity, density and uniformity, appropriate strength and good toughness and plasticity, excellent resistance to radiation embrittlement and aging, excellent weldability, hot and cold processing performance and excellent corrosion resistance.

▲ Figure 4 Schematic diagram of nuclear reactor

▲ Figure 5 Cross-section of reactor pressure vessel

▲ Figure 6 Actual appearance of reactor pressure vessel

▲ Figure 7 Actual appearance of reactor pressure vessel

Pressure vessel materials are generally improved on the basis of mature materials in engineering.

The earliest pressure vessel material was carbon (C)-manganese (Mn) steel A212B for boilers (forgings are A105), and then it was changed to Mn-Mo steel A302B (forgings are A336) with better hardenability and high temperature performance.

In the mid-1960s, nickel (Ni) was added to A302B steel to develop A533B (forging material is A508-Ⅱ steel) with better hardenability and toughness.

A508-Ⅲ steel is developed on the basis of A508-Ⅱ steel by reducing the C, chromium (Cr), and Mo content and increasing the Mn content. It is currently the preferred material for large pressurized water reactor pressure vessels.

Steel for steam generators

The function of the steam generator is to continue to transfer the heat brought out of the reactor by the primary coolant to the secondary medium, and convert it into steam to drive the turbine to generate electricity.

▲ Figure 8 Steam generator cross-section

▲ Figure 9 Steam generator appearance

▲ Figure 10 Nuclear power plant generator set diagram

Due to the need to withstand high temperature, high pressure and medium corrosion, abrasion and other effects, steam generator components, especially heat transfer tubes, have very stringent requirements on material properties.

In the early days of nuclear power plants, there were many accidents caused by steam generator failures and shutdowns due to improper material selection or processing technology, such as a 1300MW nuclear power plant in France in 1989, the Troy Nuclear Power Plant and the Zain Nuclear Power Plant in the United States in 1993, and the Indian Point Nuclear Power Plant in the United States in 2000.

(1) The shell of the steam generator (including the upper head, upper cylinder, lower cylinder and cone) is made of ferritic steel plate;

U-shaped heat transfer tubes used to use 18-8 stainless steel, and now Ni-based alloys such as 690 and 800 are widely used;

The tube sheet is forged with high-strength low-alloy steel, and the primary coolant side is a stainless steel cladding layer.

Nuclear-grade valve steel

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Nuclear-grade valves are key accessories in nuclear power equipment, connecting more than 300 subsystems of nuclear power plants. They mainly include gate valves, stop valves, check valves, butterfly valves, safety valves, main steam isolation valves, ball valves, diaphragm valves, pressure reducing valves and control valves. Although nuclear-grade valves account for a small proportion of the construction cost of nuclear power plants, the maintenance cost of nuclear-grade valves accounts for more than 50% of the maintenance cost of all components of nuclear power plants.

▲ Figure 12 Control valve

The materials used for nuclear-grade valves generally need to have good corrosion resistance, radiation resistance, impact resistance and intergranular corrosion resistance. Therefore, low-carbon or even ultra-low-carbon austenitic stainless steel is used as the main material in some main systems, and some alloy materials with high strength, good toughness, high temperature and high pressure resistance, erosion resistance and abrasion resistance are used to make valve stems or sealing surfaces and other parts.

According to the choice of valve body materials, carbon steel valves account for about 41% of the nuclear island, stainless steel valves account for about 55%, and other material valves account for only about 4%.

Steel for reactor internals

Heap internals refer to all structural components in the pressure vessel except for fuel assemblies and related components.

It has many components, complex structures, high precision requirements, and needs to withstand high temperature and high pressure, neutron irradiation, coolant corrosion and other tests. Therefore, the selection principles of reactor internal components are generally: moderately high strength, good plasticity and toughness, impact resistance and fatigue resistance; small neutron absorption interface and neutron capture cross section and induced radioactivity; radiation resistance, corrosion resistance and good compatibility with coolant; small thermal expansion coefficient; good welding and machining process performance.

The main structural material of the second-generation pressurized water reactor nuclear power plant is generally austenitic stainless steel, such as 304L, 304LN, 321, 347, 310, and the bolt material is 316LN, 321H stainless steel. Some special parts use martensitic stainless steel, such as 1Cr13 for compression springs.

The third-generation pressurized water reactor AP1000 nuclear power plant has greater power and longer life, and has stricter requirements on the composition and performance of the reactor internal components. Its main structural material is forged F304 and F304H austenitic stainless steel, and the compression spring is improved 403 martensitic stainless steel.

Eight characteristics of nuclear power steel

The characteristics of nuclear power steel mainly include the following aspects: 1

Nuclear power steel has a complete variety and a wide range. The steel types cover carbon steel, alloy steel, stainless steel and nickel-based materials, and all have relatively strict requirements. Since the steel used for nuclear island equipment works in high temperature and high pressure environments for a long time, it is required to have suitable strength, high toughness and low brittle transition temperature (NDT). 2

Nuclear power steel is difficult to produce, close to the limit level of advanced rolling mills at home and abroad. The main reason is that the steel plate is heavy and large in size, and it is super wide, super thick and super heavy. For example, the steel 18MND5 used for the CPR1000 steam generator cylinder has a single steel plate weight of nearly 40t.

Strict chemical composition requirements. Conventional island equipment steel generally requires P and S content below 0.015%, while nuclear island equipment steel requires P and S content less than 0.010% and 0.0005%. 4 Strict and complex mechanical performance requirements. The number of samples has increased significantly, and it is necessary to conduct longitudinal and transverse inspections at different positions in different states such as high temperature, room temperature and low temperature in the delivery state and after the simulated post-weld heat treatment (SPWHT). For example, 16MND5 steel plates for pressurizers require up to 50 impact specimens for one steel plate. 5 It should have good structural stability, weldability, hot and cold workability and fatigue strength at working temperature, and good sensitivity to radiation embrittlement under reactor irradiation conditions.

It has strict non-destructive testing requirements. Most of the steel used for nuclear power equipment requires 100% ultrasonic inspection, and the surface of the steel plate needs to be magnetic particle inspected, and at the same time, it puts forward higher requirements on the qualifications of the flaw detection operators. 7 Considering the long-term exposure to neutron radiation, the more alloy elements there are, the weaker the overall neutron radiation resistance is. Generally, steel with rare alloy elements and strong radiation resistance is used. At present, Mn-Ni-Mo low-alloy high-strength steel is widely recognized by countries around the world. 8 Steel for nuclear power is mainly divided into two major areas: carbon steel and alloy steel. The more typical nuclear power steels in the world are A508-3 and A533 (B, D) in the United States, BHW35 in Germany, 16MND5 in France, and SFVV3 in Japan.