Contract negotiations between Entergy and the union representing workers at the Pilgrim nuclear plant reached an impasse Wednesday, with the company sending almost half the plant’s workforce home and workers picketing in response.The previous contract with members of the Utility Workers Union of America Local 369 expired Tuesday, and negotiations broke down in the early morning the following day. Plant workers deemed non-essential were then sent home or turned away, which the union characterized as a lockout but that Entergy said was a paid day off as negotiations continue.Union members told New England Cable News that negotiations stalled on $1.2 million in healthcare costs and on safety issues. Entergy told local media in a statement it had asked for an extension to the existing contract, which was declined. Entergy also said workers reserved the right to strike at any time, which the utility found unacceptable.Earlier this month, the union had voted to approve a strike by its 380 workers at the plant. While a strike remained on the table, one had not been initiated as of Wednesday afternoon. The Boston Herald reported that the union planned to vote on the last contract offer by Entergy next week.