Philippe Wullens, a SUD union spokesman, said: "We are not surprised by these announcements. It has all been said before. We do not want these new operations. We are refiners."
Some workers will move to Total refineries and other plants elsewhere in France and about 20 will be offered early retirement, a company spokesman told the AFP news agency, as unions and management held talks on Monday.
The Dunkirk plant has been shut down by a strike since mid-January.
Last month, Total's five other refineries joined the stoppage, causing fuel pumps to run dry and prompting the government to pressure the company to safeguard jobs.
Unions have demanded that the plant maintain its refining operations, despite the company's insistence that it must shift away to more profitable activities as the market responds to demand for cleaner fuels.
"An adjustment of refining operations in Europe is inevitable" since demand for such products has been falling "for more than 10 years," said Michel Benezit, Total's head of refining.
'Fire and blood'
Earlier on Monday, Total also announced plans to create at least 50 jobs by building a gas works in Dunkirk with energy distributor EDF.
The SUD union shrugged this off as a mere "public relations announcement" that would not help the Total workers it represents.
Another key union, the CGT, threatened "fire and blood" if management did not offer more.
Charles Foulard, a CGT spokesman, said: "The company's plan announced today is a true provocation. It is unacceptable."
The Dunkirk plant also indirectly employs about 450 contractors.
Benezit said their situation would not be discussed at Monday's talks but tackled later in co-operation with local authorities.