* Tricastin nuclear reactors sit below canal water level
* ASN says flooding could lead to nuclear fuel meltdown
* EDF says reinforcement works in coming six months (Adds EDF comment, details)
PARIS, July 10 (Reuters) - French regulator ASN has ordered state-controlled utility EDF to reinforce the dikes of the cooling water canal at its Tricastin nuclear plant by the end of 2022 at the latest.
The regulator did not order the plant to be closed again but said it had ordered EDF to increase surveillance of the canal and to increase availability of staff and emergency equipment to be ready to deal with an earthquake.
In 2017, ASN ordered the closure of Tricastin's four reactors from September to December while EDF strengthened the dikes to prevent them from collapsing in case of an earthquake.
"Studies show that flooding resulting from the breakage of the dike could lead to an accident involving the meltdown of the four reactors' nuclear fuel, while the rescue operation would be particularly difficult," ASN said in a statement.
The 3,600 megawatt Tricastin nuclear plant - part of one of France's largest nuclear sites - is located several meters below the DonzEere-Mondragon canal, which diverts water from the Rhone river to cool the reactors.
Anti-nuclear activists say this is a major design flaw as it exposes the plant to flooding due to earthquakes or a possible breakage of the hydro dam higher up the canal.
An EDF spokesman said reinforcement works on the northern part of the embankment will be carried out over the next six months. Reinforcement work closer to the plant was done in 2017.
The Tricastin 1 reactor is offline from June to Nov. 28 for major maintenance work.
Commissioned in 1980, it is the first of EDF's 58 reactors to undergo a fourth 10-yearly overhaul, a thorough review to ensure its integrity beyond 40 years of operation, and the ability to function for another 10 years. (Reporting by Geert De Clercq and Bate Felix; Editing by Alexander Smith)