Nuclear energy is undergoing a decisive revival across Europe and North America. As governments commit to net-zero carbon targets, nuclear power's unrivalled ability to deliver large-scale, dispatchable, carbon-free baseload electricity has moved it back to the centre of energy policy. France—home to 56 operating reactors generating nearly 70% of its national electricity—is constructing six new EPR2 units and has committed to an additional eight beyond those. The United Kingdom has revived its civil nuclear programme with the Hinkley Point C and Sizewell C projects, and Poland, Finland, the Netherlands, and Sweden are all advancing new-build or life-extension programmes. Across the Atlantic, the United States—which operates the world's largest reactor fleet—has extended the operating licences of dozens of plants beyond 60 years and is funding advanced reactor designs through the Department of Energy's Civil Nuclear Credit Program.

This global expansion places extraordinary demands on material suppliers. Every major component inside a nuclear power plant—from the reactor pressure vessel and steam generators to control rod drive mechanisms (CRDM), primary coolant piping, and pressurizer heater sleeves—must satisfy exacting metallurgical standards set by design codes such as ASME Section III (United States) and RCC-M (France and most of continental Europe). These codes specify not only chemical composition and mechanical properties but also the mandatory melting route, inclusion cleanliness, traceability from ingot to finished part, and independent third-party material certification.

Fushun Special Steel has invested systematically in the vacuum-melting and secondary-refining infrastructure required to manufacture nuclear-grade alloys to the highest internationally recognised standards. Our capability spans vacuum induction melting (VIM), vacuum arc remelting (VAR), electroslag remelting (ESR), and forge-quality billet, bar, ring, and plate production. The sections below introduce our principal product families for the nuclear power sector and provide a detailed account of our core VIM + VAR double-vacuum melting process.

The primary coolant environment inside a pressurised water reactor is among the most aggressive in all of engineering: water at 315–325 °C, pressure up to 157 bar, dissolved hydrogen and boric acid chemistry, high-velocity flow, and cumulative neutron fluence sustained over a 60-year design life. Materials performing reliably in this environment must be manufactured to the highest possible cleanliness standards—meaning ultra-low sulfur, phosphorus, and oxygen levels, along with strict control of tramp elements such as lead, bismuth, and thallium that can cause liquid-metal embrittlement or accelerate intergranular SCC. These requirements are precisely why the double-vacuum melting route (VIM followed by VAR) is mandated or strongly preferred by all major reactor design codes for primary-circuit nickel alloy components.

Stage 1 — Vacuum Induction Melting (VIM)

In the VIM furnace, carefully selected virgin raw materials—electrolytic nickel, high-purity chromium, molybdenum, niobium, and other alloying additions—are charged into a magnesia or alumina crucible surrounded by an induction coil, all enclosed within an evacuated chamber maintained at pressures typically below 1 Pa. The induction heating rapidly melts the charge, and the controlled vacuum environment performs several critical metallurgical functions simultaneously.

First, dissolved gases are removed: hydrogen, nitrogen, and oxygen partial pressures are dramatically reduced, driving these elements out of solution and suppressing the formation of harmful oxide and nitride inclusions. Second, volatile tramp elements—lead, bismuth, tellurium, selenium, and thallium—which severely degrade hot workability and can promote liquid-metal embrittlement or accelerate intergranular SCC in service, are selectively evaporated under vacuum and removed from the melt. Third, precise chemistry control is achieved through sequential addition of alloying elements, with each addition verified by rapid optical emission spectrometry before the melt is tapped. For precipitation-hardening alloys such as Alloy 718, the exact Nb-to-Ti ratio and aluminium content must be controlled to narrow tolerances that directly govern the volume fraction and morphology of strengthening precipitates in the finished product.

Stage 2 — Vacuum Arc Remelting (VAR)

The VIM electrode is transferred to the VAR furnace, where it serves as the consumable electrode in a direct-current vacuum arc. The arc is struck between the electrode tip and the growing ingot in a water-cooled copper mold, melting the electrode at a precisely controlled rate and allowing clean metal to solidify upward in a shallow, near-flat molten pool. The entire remelting operation occurs under vacuum at pressures typically below 0.1 Pa, with continuous automatic control of arc current, voltage, electrode gap, and melt rate throughout the process cycle.

The VAR process delivers three transformative metallurgical benefits. First, macrosegregation is eliminated: the controlled directional solidification prevents solute redistribution, shrinkage pipe, and freckle formation that occur in conventionally poured ingots. Second, inclusions are dramatically reduced: remaining oxide or nitride particles float to the electrode top and are removed in the discard crop, ensuring the usable ingot body is essentially inclusion-free. Third, compositional homogeneity is maximised: the controlled solidification cycle progressively reduces micro-segregation at grain boundaries and within dendrites to levels unachievable by conventional melting, resulting in more consistent mechanical properties and improved fatigue performance throughout the entire product cross-section.

Why VIM + VAR Is Non-Negotiable for Nuclear Service

Nuclear operators and their regulators require mandatory documentation of the melting route as a procurement condition, retained for the lifetime of the component—often exceeding 60 years. ASME SB-163, SB-564, SB-166, and the corresponding RCC-M M-series material specifications all explicitly require or strongly prefer VIM + VAR double-vacuum melting for primary-circuit nickel alloy components. No alternative melting route can consistently achieve the required combination of low dissolved gas content, freedom from tramp elements, absence of large inclusions, and compositional homogeneity that nuclear service demands across a 60-year design life.

At Fushun Special Steel, our VIM + VAR production facility has been continuously upgraded to support nuclear-scale ingot production. Our nuclear quality management system operates under ISO 9001:2015 with nuclear-specific supplements, encompassing full first-article qualification, material test reports (MTRs), and certified material test certificates (CMTCs) traceable to individual heats and remelt sequences. For the most demanding applications, Fushun also offers triple-melt sequences (VIM + ESR + VAR or VIM + VAR + VAR) as specified by end users or design codes.