A portion of the homeland security laws designed to keep explosives away from terrorists is tying the hands of some ski resorts. Many resorts use seasonal workers from Australia and New Zealand to blast jutting snow and ice to help prevent avalanches. Jeff Walters is the vice president of Mountain Operations at Kirkwood Ski Resort in California. He says about one-fifth of his avalanche control workers are from overseas, and says they are not a threat to security. Under changes effective Friday, nonresident aliens are not allowed to possess explosives on the job. Other provisions kick in May 24 that require background checks and permits for any employee using explosives on the job. Safety officials at several resorts and for national organizations emphasized that the avalanche-control work will still be done and skier safety will not be affected.