Israel is balking at a request from Norway to inspect 20 tons of heavy water the Scandinavian country sold in 1959 and which may have been used in the production of nuclear weapons, Norwegian and U.S. sources said. U.S. government officials declined to comment for the record on the Norwegian request. But one Administration source said Israel consistently refuses to allow international inspection of the Dimona nuclear reactor, where Israel is believed to manufacture weapons-grade plutonium. Israel has not acknowledged that it has nuclear weapons. Norway last inspected the heavy water in 1961.