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( 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 ductilitydip 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 offthe-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
TIG rods ( GTAW ).
• 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 .
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