An ammonia hydroxide spill of less than a gallon in a stockroom at the Seabrook nuclear power plant has required the plant to declare an ‘‘unusual event,’’ the lowest of four emergency categories. Plant spokesman Al Griffith said the spill occurred about 1:45 p.m. Wednesday inside the administration building. He said there were no injuries nor any danger to the public. It is not clear how the spill happened. Cleaning materials are kept in the stockroom. The building is in a protected area. Plant operations were not affected.