Nuclear Proliferation

Turning uranium oxide,  or yellow cake,  into weapons grade material requires large facilities and materials.  It requires a uranium enriching plant or a reprocessing facility and these are large, highly technological and expensive.  They separate the uranium and plutonium from the waste product in uranium fuel that has been cycled through a reactor.  This process is also very dangerous for the people involved in the reprocessing as they are dealing with highly radioactive waste.

Furthermore,  the type of plutonium produced in a nuclear power plant is plutonium-240 which is very poor weapons grade material.  That is why no nation has ever produced nuclear weapons from plutonium generated by a nuclear power plant but use plutonium-239 which is made in a special production reactor.  

Globally there are 37 countries that have the infrastructure and capability to build nuclear weapons if they wanted.   However, only nine of these countries have nuclear weapons which are the United States,  United Kingdom, France, Russia, China, India, Pakistan, Israel and North Korea.

Rather than adding to nuclear weapons capabilities, nuclear power plants have helped reduce the number by using the material from dismantled nuclear warheads and bombs for fuel.  The agreement between the United States and Russia to recycle the uranium from nuclear warheads for commercial use has recycled over 450 tonnes of uranium.  By 2013 it would have recycled 500 tonnes which is the equivlant of 20,000 nuclear warheads. The electricity produced from the reprocessed uranium from Soviet nuclear bombs is consumed by nearly every tenth house of the United States.

If terrorists use dirty bombs the most likely result and intent wouldn't be massive deaths but massive chaos. An example of this is an incident that occurred in 1987 in Goiania, Brazil. Scavengers broke into an abandoned hospital and pried open a canister from a discarded radiation therapy machine that contained powdered cesium-137, a hard gamma emitter. It glowed in the dark. They took it home and people played with it, rubbed it on their bodies, ate sandwiches with the powder on their hands and shared the blue, luminous substance with others.  It caused 4 deaths and contaminated 250 other people who were treated with a drug that reduces the internal dose of cesium. The amount of people that rushed to hospital because of panic was 130,000.

There is far more danger of terrorist obtaining chemicals you can buy at your local hardware store and strapped explosives to that.  Stopping nuclear power plants isn't going to stop the possiblility of nuclear weapons being manufactured,  the only thing that will do this is going back in time and erasing all knowledge of nuclear physics,  and that isn't going to happen.  It would be easier and quicker for rogue nations to steal an already completed nuclear weapon than create one from scratch by stealing spent fuel from nuclear reactors.

India announced in November 2011 that they are building a thorium reactor which should be working by 2020 and the spent fuel from this,  which is a lot less than Uranium,  is near impossible to be of any use in nuclear weapons.