What do you mean exactly? They could build something that goes boom, they could build first try a 100% yield fission bomb...? Just because someone builds an explosive device that incorporates fissile material into the design doesn't mean they've cracked the problem. I bet I could build a "viable bomb" if you give me the resources, I just can't say with any certainty it won't fizzle or it won't be a dirty bomb. Can you do your deterrence with a warhead filled with C4 strapped to uranium ore, while I use the money saved to go on vacation?