From the man who created the largest, smallest, and most efficient pure fission bombs produced:

I copied the following from ArmsControlWonk.com

Finally, from the man who created the largest, smallest, and most efficient pure fission bombs produced:

Utility of Reactor Grade Plutonium in Nuclear Weapons
Theodore B. Taylor
Visiting FellowCenter for Energy and Environmental Studies
Princeton University, Princeton NJ 08544
June 10, 1998

Contrary to opinions expressed by many nuclear engineers that are not familiar with the still secret intimate details of nuclear weapon design and operation, plutonium extracted from all types of spent fuel removed from nuclear power plants or research reactors can be used for making modern fission or thermonuclear weapons that are reliabily predictable in performance, over a very wide range of yields, from fractions of a kiloton to megatons of high explosive equivalent. This has been true for decades, and confirmed by numerous nuclear weapon tests.

It is true that the first generation of implosion type fission bombs, such as the one that destroyed Nagasaki in 1945, could “fizzle” and produce much lower than the design yields if the plutonium they contained were of “reactor grade.” This could be the result of premature initiation of a fission chain reaction by spontaneous fission neutrons emitted by Pu-240 or other isotopes that are more abundant in reactor grade than weapon grade plutonium. But ways to avoid this problem, by use of plutonium in different designs that could be reliably used for fission and thermonuclear weapons were developed and demonstrated before the end of the 1950s. The performance of these weapons is not significantly degraded by using reactor grade plutonium instead of weapon grade plutonium.

And, here is what has been reported regarding Taylor's skills:
"He was famous in the community of bomb experts as the most creative and imaginative of the designers," said Freeman Dyson, a physicist, author and retired Princeton University professor who was a friend of Dr. Taylor's. "His bomb designs were the smallest, the most elegant and the most efficient. He was able to draw his designs freehand, without elaborate calculations. When they were built and tested, they worked."