Registered nurses at UMass Memorial Medical Center in Worcester intend to strike for a day later this month, after more than a year of unsuccessful negotiations for a union contract. The Massachusetts Nurses Association, which represents the nurses, said the strike is slated from 6 a.m. on Thursday, May 23, to 6 a.m. the next day. A sticking point has been nursing staffing levels, the union said. “Our members are tired of working in an environment that they know is unsafe for our patients,” nurse Margaret McLoughlin, part of the union’s bargaining unit, said in a statement. A hospital spokesman, Rob Brogna, said in a statement a staffing agency had been contracted to fill-in for the striking workers. He called the decision to strike “highly disappointing.”Joshua Miller can be reached at firstname.lastname@example.org. Follow him on Twitter @jm_bos.