Allegations of shoddy workmanship and drug and alcohol abuse at the Seabrook nuclear power plant in New Hampshire either were unsubstantiated or remedied by plant management, the Nuclear Regulatory Commission reported. The report followed an NRC investigation into charges by the Massachusetts-based Employees Legal Project of substance abuse among some construction workers at the $5-billion plant, which is completed but not operating. An investigation by the House subcommittee on energy, conservation and power "shows a markedly different picture," said Rep. Edward J. Markey (D-Mass.), subcommittee chairman. He gave no specifics.