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Graded-Z Radiation Shielding for Fusion using Cold Spray

Introduction

The ColdShield project develops advanced radiation shielding solutions for fusion reactors using Cold Spray Additive Manufacturing (CSAM). By combining low atomic number (low-Z) neutron absorbers (e.g. B₄C) with high-Z gamma attenuators (e.g. W, WC) in a single graded structure, the project aims to reduce external dose, minimise activation, and enhance component survivability in next-generation fusion systems. The project aims to achieve Manufacturing Readiness Level 3 (proof-of-concept) for graded-Z fusion shielding components.

Methods

A functionally graded material (FGM) architecture (W → B4C → Eurofer97) was developed using CSAM, supported by neutronics analysis. Powder blends were engineered with suitable binders to enable deposition of metal matrix composites (MMCs) incorporating shielding materials. Process optimisation focused on balancing deposition efficiency, shielding material retention, and interfacial integrity. Demonstrators were produced in planar and tubular geometries and characterised using detailed microscopy, hardness, and dilatometry. Figure 1 shows the Titomic TKF-1000 CSAM cell at TWI that was used for this study.

Results

Dense, defect-free FGM structures were produced using CSAM, with consistent deposition across all MMC layers and strong adhesion at graded interfaces. Microstructural analysis confirmed near-fully-dense microstructures (<2% porosity) throughout the architecture.

Optimised processing achieved >55% overall deposition efficiency, with shielding material retention exceeding 40% across all MMC compositions, including >98% W retention in Al-W systems. Powder feed and binder selection enabled well-controlled compositional gradients.

Mechanical characterisation showed consistent hardness distributions, with minor reductions (~20%) after heat treatment, while dilatometry confirmed stable, temperature-dependent thermal expansion and improved high-temperature stability.

Small-scale planar (120×75×20mm) and tubular (Ø178×300mm, ~38mm wall) demonstrators validated reproducible processing and integration of ceramics, refractory metals, and steels into a single monolithic graded-Z architecture.

Figure 2 presents a representative micrograph of the as-deposited CSAM small-scale graded-Z shielding demonstrator, illustrating the layer sequence from the substrate to the outer surface. Figure 3 shows representative SEM images of the FGM interfaces, while Figure 4 presents a cross-sectional micrograph of a cold sprayed MMC exhibiting >98% W retention. Figure 5 shows the CSAM small-scale graded-Z shielding demonstrator following post-processing.

Overall, the project progressed beyond proof-of-concept, with MMC and FGM systems achieving MRL 3–4, demonstrating CSAM as a viable route for complex, multi-material fusion shielding components.

Figure 1. TWI’s Titomic TKF-1000 Cold Spray Additive Manufacturing (CSAM) cell, featuring a dual powder feeder suitable for co-deposition of powders, enabling functional grading or multi-material deposition
Figure 1. TWI’s Titomic TKF-1000 Cold Spray Additive Manufacturing (CSAM) cell, featuring a dual powder feeder suitable for co-deposition of powders, enabling functional grading or multi-material deposition
Figure 2. Representative stitched images of the cross-section of the as-deposited CSAM small-scale graded-Z shielding demonstrator, showing the layer sequence from substrate to outer surface
Figure 2. Representative stitched images of the cross-section of the as-deposited CSAM small-scale graded-Z shielding demonstrator, showing the layer sequence from substrate to outer surface
Figure 3. Example SEM images showing FGM interfaces: (a) Al-B4C-W to Al-W, (b) Al-W to Al-W-Cu
Figure 3. Example SEM images showing FGM interfaces: (a) Al-B4C-W to Al-W, (b) Al-W to Al-W-Cu
Figure 4. Example cross-sectional optical micrograph of a cold sprayed MMC with >98% W retention, deposited onto an Al substrate
Figure 4. Example cross-sectional optical micrograph of a cold sprayed MMC with >98% W retention, deposited onto an Al substrate

Conclusions

ColdShield demonstrates that Cold Spray Additive Manufacturing is a viable route for producing complex, multi-material graded-Z shielding structures. Planar and tubular demonstrators validated full-architecture integration and exceeded initial targets. The project has progressed beyond proof-of-concept, achieving MRL 3–4 for MMC and FGM systems. Future work will focus on process scale-up, irradiation testing, thermal performance validation, and regulatory qualification, paving the way for potential deployment in STEP and EU DEMO fusion power systems.

Acknowledgement

This project has been supported by UK Atomic Energy Authority (UKAEA) through the Fusion Industry Programme (FIP) Cycle 6 Phase 1. The FIP is stimulating the growth of the UK fusion ecosystem and preparing it for future global fusion powerplant market.

Figure 5. CSAM small-scale graded-Z shielding demonstrator (cylindrical geometry) after post-processing: 89mm internal diameter, 178.6 mm outer diameter, and 38.5 mm wall thickness. Layer sequence (from substrate to outer surface): Al substrate → Eurofer97 → Eurofer97-Al-B4C → Al-B4C → Al-B4C-W → Al-W → Al-W-Cu → W-Cu
Figure 5. CSAM small-scale graded-Z shielding demonstrator (cylindrical geometry) after post-processing: 89mm internal diameter, 178.6 mm outer diameter, and 38.5 mm wall thickness. Layer sequence (from substrate to outer surface): Al substrate → Eurofer97 → Eurofer97-Al-B4C → Al-B4C → Al-B4C-W → Al-W → Al-W-Cu → W-Cu
Avatar Dibakor Boruah Senior Engineer, Metallic Materials and Integrity

Dr Dibakor Boruah is a Senior Engineer at TWI Ltd, Cambridge, UK, specialising in materials and advanced manufacturing methods, with applications in surface engineering, remanufacturing, and additive manufacturing. His association with TWI began in 2017 through the NSIRC programme, where he completed a PhD on the structural integrity of cold spray additively manufactured Ti-6Al-4V, sponsored by the Lloyd's Register Foundation. Between 2020 and 2022, he further developed his research expertise as a Postdoctoral Researcher at Ghent University, leading collaborative projects focused on wire arc additive manufacturing (WAAM) of steel and copper alloy systems. Returning to TWI in 2022, Dr Boruah has since built a substantial portfolio of research and innovation activities, securing competitive funding and working with public-sector and industry partners. Dr Boruah served as Technical Manager of the EU Horizon 2020 FORGE project, a multinational consortium developing advanced compositionally complex coatings for energy-intensive industries. He has also led several projects funded through multiple cycles of UKAEA’s Fusion Industry Programme (FIP), including the ColdShield project, and has managed projects funded by the Henry Royce Institute’s Industrial Collaboration Programme (ICP). He currently leads a Joint Industry Project focused on the industrialisation of cold spray repair technologies for critical applications across the nuclear, defence, and oil and gas sectors.

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