The company said reactor 4 was shut down last Friday but the second reactor will remain online supplying electricity to over 500,000 homes.
The reactor outage, which occurs every three years, began on 14 October.
EDF director, Mike Harrison, said work will be done "to the highest possible standards of safety and quality".
He said: "This is a very important time for Hinkley Point B and is the culmination of around two years of planning.
"As well as being important for the power station itself, the work is also great news for the local economy as it results in millions of pounds going to local and regional contractors, businesses and accommodation providers."
The "planned work", according to the company, is costing Â£35m and will involve inspections of the reactor core and the "replacement of four large gas circulators" in the gas-cooled reactors.
One thousand additional contractors will also be on site, working alongside the station's 550-strong workforce.
Hinkley Point B came into service in 1976 but its working life has been extended to 2016.
EDF Energy has signalled it plans to keep the complex going for five years beyond that date.
EDF Energy has shut one of two reactors at its Hinkley Point B nuclear power station in Somerset as part of planned maintenance.