WASHINGTON — The growing confrontation between Washington and Moscow over Ukraine has derailed a recent accord that promised one of the most expansive collaborations ever between the countries’ nuclear scientists, including reciprocal visits to atomic sites to work on projects ranging from energy to planetary defense.

It was only 11 months ago that the American energy secretary — Ernest J. Moniz, a former M.I.T. professor who has championed scientific programs that would bury the Cold War competitions between the United States and Russia — went to Vienna to sign the agreement, an indication of how recently the Obama administration believed it had a chance of building on a quarter-century of gradual integration of Russia with the West.

Handshakes and congratulations exchanged with Mr. Moniz’s Russian counterpart, Sergey V. Kirienko, sealed an arrangement that would let Russian scientists into, among other places, the heart of the American nuclear complex at Los Alamos National Laboratory in New Mexico, where the first atomic bomb was constructed 70 years ago, and a dozen sister laboratories devoted to the making of the American nuclear arsenal. In return, American scientists would be allowed deep into Russian nuclear facilities, including the birthplace of the Soviet bomb.

The Energy Department’s announcement of the deal also highlighted its potential for “defense from asteroids,” shorthand for a proposal to recycle a city-busting warhead that could be aimed at an incoming earth-destroyer — a plot Hollywood had imagined 15 years before in two far-fetched thrillers, “Armageddon” and “Deep Impact,” in which Bruce Willis and Ben Affleck, among others, saved humanity.