Port Talbot is one of three European plants where one of the blast furnaces is to be shut down
Union leaders have appealed for Corus not to make any "knee jerk reaction" after the steel giant announced plans for a 30% cut in production in Europe.
Three blast furnaces, at Port Talbot and Scunthorpe in Britain, and one in the Netherlands, will shut temporarily.
Officials of the Community union, which represents 3,000 workers in Port Talbot, say they want reassurances over jobs beyond the end of the year.
Corus blamed the general economic downturn for the production cut.
"We are hopeful that Corus will look to retain capacity to meet long-term demand rather than make a knee-jerk reaction to short-term trends," said Michael Leahy, general secretary of the Community union.
Robert Edwards, regional director of Community, which represents 3,000 workers at Port Talbot, said there was hope that the company could overcome short-term difficulties.
"Certainly this can't have come at a worse time in the run up to Christmas with people concerned about their finances and this means our members are walking a path of uncertainty," he said.
'Lot of energy'
The cut is greater than expected. Last month, Corus said it would cut production between October and December by a million tonnes of crude steel - about 20% of its output.
In a statement the firm said it had now decided to extend these production cuts beyond December.
Mr Edwards said: "At the moment we have a promise from Corus that there won't be any redundancies, this will run out at the end of December and they've said this [production cut] will go on into March, so we will have to talk with the company, both nationally and locally, about what that means for our members.
"There are things which they could consider in the meantime - a re-lining is due for one of the furnaces at Port Talbot, so maybe they should take advantage of this downturn until there is a change in the market."
One blast furnace at Scunthorpe, Port Talbot, and IJmuiden in the Netherlands, will be temporarily shut down, the firm said.
Mr Edwards said he could understand the company's approach in the temporary shutdowns of the furnaces.
"I'm an ex-blast furnaceman myself, blast furnaces take up a lot of energy, they gobble it up to be honest, and they do use a lot of coal so it makes sense."
Corus chief executive, Philippe Varin, said Corus had to adapt "to the changing environment with maximum speed".
The firm, which employs more than 24,000 workers in the UK, also announced 400 job losses in its distribution business, on 6 November.