WASHINGTON, June 5 (Reuters) - U.S. President ‌Donald Trump's special envoy Steve Witkoff and son-in-law ​Jared Kushner traveled to the Oak Ridge National Laboratory in Tennessee on Thursday to consult with experts that could play a role in nuclear negotiations with Iran, a source familiar with the trip said on Friday. The source, confirming an Axios report, did not provide additional details. President Donald Trump is adamant that any deal to end the war with Iran include a provision that Tehran will not develop a nuclear weapon.