Filler Metals used in the Nuclear Industry

A: Apart from the life of the fuel, the safe and economical operation of any nuclear power system relies on its construction materials remaining sound for up to 60 years. They need to maintain mechanical properties while constantly subject to sources that can cause construction materials to degrade, including high temperatures and pressures in a corrosive environment and bombardment with high energy particles released during fission.
The following table summarises the expected environments for two types of nuclear power systems. The pressurised water reactor (PWR) is older technology yet constitutes the majority of the world’s plants. With recent decisions to extend the operating life of nuclear facilities, this old design will remain highly relevant for welding operations.
Modern sodium-cooled fast reactor systems operate at higher temperatures, pressures and radiation doses. Russia, Japan, India, China, France and the USA are investing in the technology.

Requirements of filler metal grades

Welding is considered the most critical operation in constructing a nuclear power plant. Therefore, special attention must be paid to the welding procedure and the production and delivery of welding consumables. The requirements of any materials going into a high-risk environment where failure is life-threatening are the same for every application. Nuclear applications have an added risk due to the materials being bombarded with high-energy particles at an atomic level; consequently, materials behave differently. The filler metals must conform to the required composition specification as defined in the standards. In addition, they must be free from impurities. Typically, the grades are designed to have extremely high resistance to hot cracking (solidification, liquation and re-heat cracking) due to their low impurity levels, and low melting point grain boundary segregates . Nuclear-grade filler metals also offer resistance against ductility-dip cracking (DDC) due to effective, intentionally designed grain boundary pinning. For gas-shielded welding modes, the pinning is commonly achieved by the addition of small quantities of titanium and aluminium. In contrast, in a slag-producing welding mode such as submerged arc welding (SAW) or ‘stick’ welding (SMAW), the pinning is achieved with the addition of niobium.

What grades are used?

What determines the types of materials used in nuclear power systems is dependent on where in the system it is to be located. A pressurised water reactor (PWR) can be divided into three zones: primary, secondary, and tertiary.

• The primary zone has the reactor pressure vessel where the fuel rods are housed, the water pressuriser and the steam generator, so this where nickel-based and zirconium alloys will be required.
• The secondary zone is where the turbine generates the power and the cooling water condenser are located; here there are a variety of low alloy steel, stainless and nickel-based alloys being used.
• The tertiary zone houses the coolant tower, fed from the secondary circuit; as its name suggests, this is basically a water cooler, so the typical materials of choice are stainless steels.

Most common grades

The chemical composition of the filler metals designated as ‘nuclear grades’ generally fall within standards of off-the-shelf grades but will usually be manufactured to tighter tolerances (as well as have the increased quality assurance steps noted in the previous column). There may be an additional requirement to have a very low content of certain elements, such as boron. Some of the most common grades are:

• Nickel based grades: EQNiCrFe-14, EQNiCrFe-7, ENiCrFe-7, ERNiCr-3
• Low alloy grades: ER100S-1, ER90S-D2, E8018-G, E9018-G, E7018, ER70-S
• Stainless steel grades: EQ309L, EQ308L, E309L-16, E308L-16, ER316L, ER385, ER347

The next issue of this column will delve into the metallurgy and mechanical properties of these stainless grades, as well as focus on joining advice.