IN THE NUCLEAR INDUSTRY, ‘THICK section’ components can be welded using various processes that are cost-effective. But the presence of residual magnetism in the materials has hindered the effective application of these processes. For many years the aim has been to find a process that can be used more widely across the nuclear industry. Even though output within nuclear is low, the safety critical nature of these components demands a solution.
Typically, thick-section nuclear components such as pressure vessels were welded using arc-welding techniques, which require multiple weld passes with inter-stage non-destructive examination (NDE) and pre-heating of the component to reduce the risk of hydrogen cracking.
For a nuclear plant, the joining of components is currently used through the use of the tungsten inert gas (TIG) process. TIG-welding of thick-section items such as the reactor pressure vessel is an expensive and time-consuming practice involving extensive pre-work including fixtures, tooling, pre-heating of the components and multiple weld passes. Another drawback in using the TIG process is that it can only penetrate to a certain depth, so thick-section welding is executed by filling the weld groove with several passes. This involves up to 100 runs of weld for a typical pressure vessel section of 140mm or greater.
Consequently, there are a few disadvantages of using the process: multiple runs requiring preheating, inter-pass temperature control and inter-stage inspection by NDE throughout the whole process. The welding, inspection and completion of a pressure vessel therefore takes many weeks – even months – accounting for a vast proportion of the fabrication cost and component lead-time.
There have been many attempts to deploy electron beam welding (EB) with local vacuum pumping, but most were hampered by the need to work at high vacuum. Trade organisation The Welding Institute (TWI) demonstrated that operating the EB process in pressures of 0.1-10m bar – so-called ‘reduced pressure’ – in preference to high vacuum (10-3mbar) offers more reliable deployment of local sealing and pumping for electron beam welding on a large structure.
In the late 1990s, TWI developed a high power (60kW) EB welding system for girth welding of long offshore oil and gas transmission pipelines. Excellent weld quality was achieved consistently with rudimentary pumping and flexible rubber seals and the process showed that there was a good tolerance to material cleanliness, fit-up, surface condition and working distance with potential to fully girth-weld pipe sections of 40mm wall thickness and 711mm diameter in less than five minutes.
More recent developments of electron beam welding technology offer the opportunity to weld ‘thick-section’ components in a single pass and negate the need for NDE, so there is a significant saving in time and cost. Furthermore, the preheat step can be eliminated, since the EB process is carried out in a vacuum.
Compared with other welding processes there are many advantages of using electron beam welding in the nuclear industry. It can offer significant savings in cost and time for thick-section fabrication, but due to the physical size and geometry of nuclear pressure vessels traditional vacuum chambers would be prohibitively expensive, given the low number of nuclear projects.
Cambridge Vacuum Engineering has launched a local vacuum EB technology called EBFlow aimed at cost- effective manufacture of large scale power generation infrastructure. The EBMan Power project, a collaboration between CVE, TWI, U-Battery and Cammell Laird, will implement and validate the first EBFlow system in a large-scale fabrication facility.
The EBFlow technology will focus on reducing the cost of thick-section steel structures for both nuclear and offshore wind structures.
The aim of this particular project is to manufacture components for nuclear power plants. Similar processes have been successfully applied in other industrial sectors, but this is the first time this approach has been applied within the power sector.
The demand for thick-section steel structures in power generation is already strong and will continue to grow over the years. Currently, to produce a typical 100m long monopile (100mm thick) it can take more than six thousand hours of ‘arc-on’ welding time. However, the EbFlow system, based on high productivity electron beam welding, can reduce the welding time to less than 200 hours, equivalent to a reduction in cost of over 85%.
Due to complete in 2021, the EBMan Power project aims to resolve many years of trying to deploy electron beam welding.
