Preamble

Research echnologies adionuclide emoval Steel- eaction uring melting Tiezhong Institute Nuclear Energy Technology, Tsinghua University, Beijing, China China National Nuclear Power Co.,Ltd, Beijing, China China Nuclear Industry Construction CO.,LTD., Hebei Yanjiao,101601, China

Abstract

study systematically investigates smelting decontamination technology recycling strategies radioactively contaminated steel. decontamination process involves crystalline molten phases, where molten stage facilitates complexation decontaminants radionuclides elevated temperatures, promoting radionuclide partitioning phase.

Research focuses elucidating mechanisms smelting decontamination, optimizing hybrid smelting processes, analyzing characteristics slag-steel system. findings reveal smelting decontamination fundamentally thermodesorption process, radionuclide partitioning efficiency governed their oxygen affinity. steel contaminated artificial radionuclides, hybrid smelting technology significantly enhances recycling efficiency. dosage decontaminant requires careful optimization, weighing decontamination efficiency against secondary waste generation. resultant exhibits honeycomb-like porous structure, surface morphology closely correlated chemical composition.

Keywords

elting; wo-phase reaction; xygen affinity;

Introduction

Radioactively contaminated steel primarily generated nuclear facilities radioactive sources, radiation devices during their operation decommissioning. types waste contaminated materials formed radionuclides carried media trapped surface steel [3,4] decontamination process radioactive steel predominantly involves distinct phases [5,6] crystalline phase molten state.

Within molten phase, volume reduction reshaping steel achieved, while stage serves primary mechanism decontamination.

Smelting decontamination technology entails batch addition decontaminants smelting furnace, where undergo co-melting radioactively contaminated steel.

Elevated temperatures facilitate complexation reactions between decontaminants contaminating radionuclides, promoting partitioning majority radionuclides phase phase, thereby purifying steel bulk.

After smelting, residual radionuclides within steel exist homogeneous stable state incorporation. efficacy smelting decontamination process significantly influenced physicochemical properties decontaminants.

Transuranic elements, being author. address: Zhao)

highly oxidation-prone, exhibit pronounced enrichment within phase.

Conversely, artificial radioactive nuclides exhibiting -decay, cobalt (Co), nickel (Ni), chromium (Cr), (Zn), manganese (Mn), demonstrate significant limitations decontamination efficiency. study investigates hybrid smelting recycling radioactively contaminated steel, focusing elucidating migration transformation mechanisms [7,8] radionuclides.

Concurrently, evaluates characteristic parameters yield resultant smelting slag.

Experimental Prior furnace ignition, radioactive testing conducted lining medium-frequency furnace. batch material preparation, radioactively contaminated steel suitable smelting feedstock shall selected based control requirements target product. decontaminant shall prepared according process formulation, minimum addition quantity controlled under premise ensuring decontamination efficacy, processed well-dispersed particles particle before During operation, decontaminant shall first added furnace bottom formulation amount. practice extends contact between decontaminant radionuclides enhance decontamination efficiency protects furnace bottom facilitates pre-slagging.

After subsequent addition decontaminant, interval least minutes shall maintained skimming, replenishment fresh decontaminant, other related operations. temperature radioactively contaminated steel furnace rises furnace temperature shall stabilized maintain partially melted state avoid excessive melting molten steel).

Subsequently, low-pressure oxygen shall introduced surface molten steel.

During oxygen-blowing process, pressure strictly controlled prevent overpressure, while ensuring continuous oxygen supply minutes.

After ceasing oxygen blowing, input power medium-frequency furnace shall gradually increased approximately lower radioactively contaminated steel gradually melts, stirring charge shall assist smelting.

Secondary charging shall performed melting charge reaches After radioactively contaminated steel completely melted, molten shall fully stirred, high-efficiency remover shall added, skimming shall conducted. resulting steel shall temporarily stored standard steel drums.

After complete smelting feedstock, furnace temperature shall elevated stabilized Deoxidizers ferrosilicon alloy aluminum shall added according process requirements complete deoxidation process, followed tapping casting.

Characterize steel, slags using Scanning Electron Microscopy (SEM, SU8020) built-in Energy-dispersive X-ray

Analysis

(EDS, HORIBA 350i). Raman spectroscopy performed using Witec system laser wavelength.

Results

iscussion

3.1 Mechanism

of d econtaminant in d econtamination

smelting decontamination radioactively contaminated steel essentially thermodesorption process driven thermal energy, radionuclides other pollutants adsorbed molten steel medium facilitated dissociate volatilize.

Subsequently, based physicochemical properties different elements their radioactive isotopes, these pollutants achieve redistribution among phase, molten steel, exhaust phases. process, decontaminant plays critical role, primarily enhancing pollutant removal efficiency through complexation effect impurities molten steel.

Similar non-metallic inclusions molten steel, radionuclides combine oxygen phase under action flux. liquid state density difference molten steel, effective separation molten steel readily achievable.

Specifically, high-melting-point radionuclides require existence oxide efficiently enter phase undergo complexational extraction flux; whereas low-boiling-point/low-melting-point radionuclides (e.g., either escape exhaust carbon monoxide bubbles during boiling molten steel remain residual furnace lining. removability radionuclides represented uranium molten steel smelting primarily depends their oxygen affinity. oxidation radionuclides mainly relies monoxide oxidizing agent. perspective thermodynamics, whether radionuclide enter phase efficiently depends relationship between Gibbs energy difference reaction equilibrium constant between oxide During smelting, following redox reaction occur between oxides: ....................(1) equilibrium constant

c y ( Fe O) ∙ c x ( M ) = e [ （ 2 Δ G （ M x O y ）－ 2 Δ G （ Fe O ））／ RT ] ............(2)

Formula equilibrium molar fraction, Gibbs energy oxide formation, universal constant,

temperature, Based thermody namic relationships involving enthalpy entropy temperature Gibbs energy serves effective criterion determining direction chemical reaction equilibrium

conclusions

highly consistent oxygen affinity elements. (i.e., reaction equilibrium constant Reaction proceeds forward. case, radionuclides exist oxides complexationally extracted decontaminant phase.

Conversely, Kc<1, Reaction proceeds reverse direction, leaving elemental state complexed decontaminant, resulting decontamination failure.

exhibits stronger oxygen affinity leading significant decontamination efficiency. forms phases covering molten steel surface through complexation existing forms uranyl complexes embed silicate anions (e.g., U-O-Si three-dimensional network. readily generates which reduce mobility through hydrogen bonding electrostatic interactions silicate anions. portion encapsulated within crystal defects immobilized solid solutions precipitates.

Additionally, react uranyl precipitates Isotopes which exhibit properties similar challenging remove conventional smelting.

Their distribution among steel ingots, slag, determined thermodynamic equilibrium.

Although cobalt oxides captured slag, their lower thermodynamic stability compared oxide prevents Reaction proceeding forward, making cobalt oxide formation difficult limiting decontaminant efficacy.

Thus, cobalt decontamination requires two-step approach:

First, high-temperature crystallographic phase temperature gradient decontamination technology achieves phase-specific oxidation [11,12] initial decontamination second, molten-phase complexation decontamination performed.

Cobalt mainly exists three forms: temperatures, decomposes which reacts basic fluxing agents (CaO, residual stable compounds (e.g., CoSiO CaCoO enriching cobalt phase. upward flotation silicate further promotes forward reaction reducing activity.

Non-oxidized cobalt physically entrained removed slag. small amount embeds silicate anion networks (e.g., forming Co-O-Si bonds becoming encapsulated silicate network.

3.2 Analysis

melting ifferent teels During decommissioning million-kilowatt pressurized water reactor (PWR), approximately 7,000 medium- low-level radioactive contaminated steel generated.

Activated corrosion products deposited inner walls primary coolant system related auxiliary pipelines primarily contain nuclides Among these, dominates inner walls system pipelines originating primarily cobalt reactor coolant pressure boundary alloy materials degradation cobalt stellite hardfacing wear-resistant components (e.g., valves shafts). nuclide contributes approximately total dose, simplifying regulatory requirements focus limit.

During large-scale smelting, complete separation steel challenging, potentially leaving small Co-containing steel pellets slag.

results

elevated radioactive levels slag, being primary contributing nuclide.

Generally, surface-contaminated steel cooling system

pipelines, steam generator transfer tubes, pump/valve components [5-9] recycled smelting; however, neutron-activated contaminated steel components reactor pressure vessel, internals, biological shielding often subjected geological disposal.

Activated steel, characterized radioactivity uniform nuclide distribution, cannot recycling requirements through simple smelting alone. co-smelting decontaminated low-level contaminated steel, synergistic recycling surface-contaminated steel activated steel achieved.

Ultimately, specific activity charged steel reverse-calculated based product requirements ensure falls below permitted limit percentage composition novel decontaminant Smelting decontamination agent reduces contamination levels Tab.1 Mixed smelting activated steel pollution steel Weight ctivate Steel Specific ctivity ctivated (Bq/g) Weight Specific ctivity (Bq/g) Weight harged urnace Specific ctivity harged aterial (Bq/g) Specific ctivity apped aterial (Bq/g) Weight apped urnace Decontam ination fficiency Recycling 24.08% 94.90% 31.39% 94.71% 20.87% 93.06% shown steel contain non-metallic impurities desulfurizer entrapped materials before smelting weight steel furnace reduced compared steel furnace, falls under decontrolled contaminated steel.

Through smelting, homogenization contaminated nuclides material reshaping achieved without requiring additional decontaminant addition. cases where activated steel incorporated, achieving decontrol necessitates addition activated steel.

Experiment

addition amount activated steel increased specific activity tapped material decreased Bq/g, complying limit requirement specified "Notice Strengthening Radiation Safety Supervision Enterprises Engaged Recycling Smelting Scrap Metal" issued former Ministry Environmental Protection China, which states materials specific activity below recycled reuse under

supervision local environmental protection departments.

3.4 Analysis

melting teel- presents images post-smelting stainless steel surface element distribution. observed, steel surface exhibits microcracks, residual radionuclides displaying uniform distribution characteristics within steel ingot matrix.

Notably, activity reaches Bq/g, which satisfies radioactive decontrol criteria. lists elemental composition ratios post-smelting steel.

Compared typical stainless steel, post-smelting steel demonstrates significantly higher fractions elements These elevated elemental contents

result

increased hardness, reduced impact toughness, diminished fatigue resistance post-smelting steel, rendering unsuitable high-strength structural material.

Nevertheless, metallic properties still fully utilized, prioritizing applications products steel drums, steel boxes, steel shots. image surface element distribution post-smelting stainless steel Post- melting omposition Element Elemental omposition melting Elemental composition typical steel During smelting process, radioactive floats surface molten steel cooling forms high-hardness agglomerates. surface exhibits typical morphologies: smooth regions rough porous regions.

Figure presents transmission image after crushing rough grinding. surface rough regions displays uneven arrangement interspersed pores micrometers size, while smooth regions appear grayish-white.

Figure shows radioactive collected filter screen, which initially exists black powdery micrometer-sized particles.

filtration shown 3(a), overall surface radioactive appears honeycomb-like grayish-black solid.

Figure reveals local surface exhibits colorless glassy state, primarily composed silicate compounds (e.g., region, atoms connected covalent bonds silicon-oxygen framework.

Fe-O-Si bonds encapsulate within silicate network, creating non-reactive zones

result

lower decontamination efficiency decontaminant area.

Image Radioactive Specifically, inherently white, while exists colorless crystal; composite compounds CaSiO within slag, overall color shifts grayish-white. contains higher concentrations impurities manifests surface morphology categorized three distinct regional types:

Region (grayish-white): Exhibits relatively smooth surface,

analysis

indicating content elements. Region (outer grayish-black stone-like structure):

Displays smooth surface characterized content based results.

Region3 (outermost grayish-black region): Features concave-convex grooved texture surface internal grid-like skeletal structure.

Region1, disrupt silicon-oxygen network, forming tetrahedra. disruption increases internal porosity slag, reduces viscosity, enhances oxygen transport capacity.

Under high-temperature conditions, decomposes readily reacts basic components (e.g., stable compounds CoSiO CaCoO thereby enriching radionuclides within phase.

Additionally, small amount embeds three-dimensional network structure formed ions, binding silicon-oxygen network Co-O-Si bonds becoming encapsulated.

Region non-reactive where dense three-dimensional networks formed ions. bound region exhibits oxygen potential, encapsulating effect silicon-oxygen network significantly hinders contact between molten steel interface, deteriorating kinetic conditions oxidation reactions.

Region flowing gradually infiltrates within three-dimensional network, cooling, solidifies hard, honeycomb-like structure.

Region acting transitional zone, exhibits semi-reactive characteristics.

After flowing fully infiltrates densifies three-dimensional network, ultimately forms compact rigid solid. adioactive

3.5 Analysis

eneration waste generated during smelting decontamination process exhibit concentrations radionuclides, concomitant elevated disposal costs. objective smelting decontamination enhance decontamination efficiency increasing radioactive dosage while maintaining consistent specific activity contaminated steel, simultaneous minimizing reduce disposal costs. composition dosage decontaminant determined based contamination level radionuclides.

Generally, lower decontaminant quantities required smelting low-level contaminated radioactive steel compared high-level contaminated steel.

Specifically, contaminant radionuclides exhibit strong oxidation potential, decontaminant demonstrates higher efficiency, broader component selectivity, reduced dosage requirements; conversely, oxidation potential contaminant radionuclides weaker steel matrix, decontaminant efficiency decreases.

Although increasing decontaminant dosage improve decontamination effectiveness, approach leads substantial increase waste volume, necessitating integration high-efficiency pretreatment techniques collaborative decontamination.

Rational control contributes enhanced decontamination outcomes; however, merely increasing volume fails effectively promote radionuclide removal exacerbates generation secondary waste.

Therefore, selection high-efficiency decontaminants represents superior solution. quantity secondary waste generated effectiveness decontamination critical considerations smelting decontamination.

Under identical process control conditions, performance decontaminant serves decisive factor. contaminated steel Figure illustrates relationship between decontaminant dosage (expressed percentage contaminated steel mass) decontamination efficiency generation amount dosages 0.25%, 0.5%, 1.5%, shown Figure decontaminant dosage exhibits significant positive correlation generation amount, indicating reducing decontaminant usage effectively mitigate secondary waste production during smelting decontamination.

Regarding trend decontamination efficiency, first increases decreases rising decontaminant dosage, though overall reduction remains relatively minor, demonstrating excellent decontamination performance decontaminant toward natural radionuclides.

Notably, decontaminant dosage decontamination efficiency reaches generation amount accounting approximately contaminated steel mass. dosage represents optimal addition ratio balances effective decontamination secondary waste control.

Relationship etween econtaminant osages, econtamination fficiency, eneration mount uring econtamination artificially contaminated steel Figure illustrates relationship between decontaminant dosage (expressed percentage contaminated steel mass) decontamination efficiency generation amount dosages 0.5%, 4.5%, shown Figure decontamination efficiency generation amount exhibit positive correlation decontaminant dosage: beyond dosage increase decontamination efficiency slows significantly, whereas generation amount rises rapidly.

Therefore, decontaminant dosage yielding decontamination efficiency generation amount approximately identified preferable option balances decontamination effectiveness secondary waste control.

Relationship etween econtaminant osage, econtamination fficiency, eneration mount uring econtamination Notably, radioactively contaminated steel underwent high-temperature crystallographic phase temperature gradient decontamination pretreatment prior smelting, which significantly enhanced decontamination efficiency.

However, lower thermodynamic stability compared decontamination performance still faces certain limitations.

During smelting decontamination, minimizing generation critical objective. decontaminant selection balance improvements steel quality.

During melting phase, promoting uniform distribution radionuclides within molten steel effectively reduces formation localized high-dose spots," thereby lowering overall radiation dose.

4. Conclusion

study conducts systematic investigation smelting decontamination technology radioactively contaminated steel.

Through experimental verification theoretical analysis, reveals mechanisms underlying smelting decontamination steel, validates feasibility co-smelting technology, identifies parameters optimizing decontaminants.

These findings provide theoretical support technical

references

radioactive waste management. Future research develop novel composite decontaminants targeting low-oxygen-affinity nuclides (e.g., construct nuclide migration prediction models, explore novel processes smelting-electrolysis coupling.

These advancements enhance decontamination efficiency, reduce secondary waste generation, promote resource utilization radioactively contaminated steel development circular economy.

Acknowledgments

project supported "Research Engineering Demonstration Project Collaborative Decontamination Control Technology Equipment Radioactively Contaminated Metal Materials" (2019YFC1907704), funded National Research Development Program.

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