The Avonmouth factory, which produces concrete segments for Balfour’s tunneling contract at Hinkley Point C, was closed late last month after the outbreak.
The contractor has been working closely with Public Health England and deployed the NHS’s Test and Trace procedure.
Additional precautions have been installed at the plant and no-one has been infected at the main Hinkley nuclear construction site.
A spokesperson for Balfour Beatty said: “The Health safety and wellbeing of all our workforce and the general public remains our priority.
“We have worked closely with the public health authorities to reduce the potential spread of Covid-19 at the Avonmouth manufacturing facility and to identify all those potentially impacted.
“We closed the site on Friday 24 July 2020 and have deployed the NHS’s Test and Trace procedure.
“Having discussed our approach with Public Health England and the Health and Safety Executive we are implementing a carefully managed, phased return of our workforce.
“To support the phased re-opening we have implemented a number of additional measures, such as segregated entrance and exit points around the site, as well as supporting those who must self-isolate when displaying any symptoms.
“The NHS’s Test and Trace procedure continues to show that no one has been affected on the Hinkley Point C site and there has been no impact on its main construction programme of works.
“We would like to take this opportunity to thank our workforce, Public Health England and the Health and Safety Executive for their continued support.”