Alloy 600 Vacuum Vessels for the Atomic Weapons Establishment (AWE)
The Client
GC Logicals was drafted in by Goodman Metal Works (goodmanmetalworks.co.uk), a specialist fabricator holding ISO 19443 nuclear supply chain certification, who were manufacturing on behalf of a design house working directly for the Atomic Weapons Establishment (AWE). The scope required the manufacture of two vacuum vessels in Alloy 600 — a nickel-chromium alloy selected for its corrosion resistance and performance in demanding service environments — to tolerances that pushed the boundaries of what conventional fabrication can achieve.
The Challenge
This project presented a combination of technical, dimensional, and cleanliness challenges that collectively made it one of the most demanding fabrication scopes GC Logicals has been involved in.
Dimensional Tolerances Beyond Normal Fabrication Capabilities
The vessel shell was specified at 10mm wall thickness, with an internal roundness tolerance of 406.5mm +/- 0.5mm. This tolerance is unachievable on a conventionally rolled and welded fabrication — the inherent variability in plate rolling and longitudinal seam welding makes it impossible to guarantee internal roundness to this degree of precision.
The solution was a deliberate and considered departure from the nominal specification: vessels were fabricated from 12.7mm plate, deliberately oversized, with the inner bore subsequently machined by a specialist gun barrel and honing contractor to achieve the required internal diameter and roundness tolerance. This decision required early engagement between GC Logicals, the fabricator, and the machining specialists to ensure the approach was fully documented, agreed, and reflected in the manufacturing path before work commenced.
Machining to Ra 0.2 on All Sealing Faces
As vacuum vessels, all sealing faces and O-ring grooves were subject to a surface finish requirement of Ra 0.2 — an exceptionally fine tolerance demanding specialist tooling, verified technique, and documented proof of compliance. A dedicated machining quality plan was drafted, discussed with the machine shop, and formally issued before any machining commenced. A surface roughness checker was procured to verify compliance at each stage.
Nuclear Cleanliness Requirements
As an AWE nuclear programme, contamination control was non-negotiable. A full machine clean-down of all equipment was required prior to machining commencing — preventing any risk of contamination from previous operations. All fabrication was carried out in a dedicated stainless steel and nickel alloy shop, physically segregated from carbon steel operations to eliminate any risk of carbon contamination of the Alloy 600 material.
Complex Joint Configurations – Three New PQR Required
A full weld mapping, PQR and WQR gap analysis was carried out at the outset. The review identified that three new Procedure Qualification Records were required, driven by a number of unique joint configurations not covered by existing qualifications:
- Thin gauge tube welded directly into heavy walled sections — a significant thickness transition demanding precise heat input control
- Swagelok VCR fittings welded to 1.6mm instrument tubing — exceptionally thin gauge work requiring a dedicated procedure and welders with the dexterity and discipline to operate consistently at this scale
The GC Logicals Approach
Manufacturing Path Development
A detailed manufacturing path was developed incorporating formal Hold Points and Witness Points at every critical stage — fabrication, machining, NDT, and testing. This gave the AWE design house full visibility and control at every stage without disrupting production flow.
Collaboration with Goodman Metal Works Machine Shop
The majority of critical tolerance machining — with the exception of the specialist barrel honing — was carried out by the Goodman Metal Works in-house machine shop. The collaboration between GC Logicals, the machine shop foreman, and the shop floor team throughout this project was outstanding. Problems were discussed openly, solutions were found quickly, and the standard of work delivered was a genuine credit to UK manufacturing at its best.
NDT Sourcing & Control
All non-destructive testing was sourced, controlled, and managed by GC Logicals:
- Dye Penetrant Inspection (DPI) — carried out in-house
- Radiographic Testing (RT) — applied to all vessel seams
- Ultrasonic Testing (UT) — applied to thick section welds
Works Procedures
A complete suite of works procedures was generated and issued for client approval, covering DPI, RT, UT, Hydrostatic Testing, Visual Inspection, Vacuum Leak Testing, and Weighing.
All procedures were returned Code A — Approved with ITP Comments Incorporated. Every comment raised during the ITP review process was addressed and incorporated before fabrication commenced.
Testing & Verification
- Hydrostatic testing carried out on all water jackets and both vessels
- Vacuum Leak Testing demonstrated a Maximum Leak Rate of 7×10⁻³ mbar l/s — meeting the specified AWE requirement
- All sealing face and O-ring groove surface finishes verified against the Ra 0.2 requirement prior to final inspection
Vacuum Leak Testing was carried out by a specialist in Vacuum Technology
The Outcome
Both Alloy 600 vacuum vessels were manufactured, tested, documented, and delivered on time and within budget to the satisfaction of the AWE design house client.
Every weld was traceable. Every tolerance was verified. Every cleanliness requirement was met. The project required engineering judgement at the outset — the decision to overbuild and machine back to tolerance rather than attempt the impossible with conventional fabrication — and that judgement proved correct.
It also demonstrated something that statistics and certificates alone cannot capture: what a highly skilled, motivated, and collaborative UK manufacturing team can achieve when everyone from the welding engineer to the machine shop floor is pulling in the same direction.
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