The phrase “nuclear energy” conjures images of large steaming towers or Tony Stark’s arc reactor from the iconic “Iron Man” movies. But two Seattle-based startups are designing nuclear technologies small enough to pick up and carry that, thanks in part to buy-in from the Defense Department, they hope will fuel a new generation of spaceships. Seattle’s Avalanche Energy and Ultra Safe Nuclear Corporation received undisclosed amounts of funding from the Pentagon’s Defense Innovation Unit in May to further develop two different approaches to small-scale nuclear power. Avalanche is pushing the boun…