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Re: [cdn-nucl-l] EOleen's comment about reprocessing
In a message dated 5/28/04 2:42:02 AM, EOleen@earthlink.net writes:
Okay, now at least one of the Greens is coming to his senses. The next step
to be taken, as I see it, is to think RATIONALLY about reprocessing spent
It is my understanding that a fuel rod becomes "spent" when the fission
products build up to the extent that little useful power can be obtained
from it: that is, the neutrons produced by fission are, to a large extent,
"eaten" by the decay products. Is this correct? If not, then what causes a
rod to be classified as "spent"? I doubt that "spent" means that all but a
small fraction of the U in the rod has been fissed.
When fuel rods are removed from service, they still contain approximately 95-97 % of the initial uranium. The problem that lead to their removal is not a build up of fission product poisons - essentially all fission products and their decay daughters have neutron rich nuclei and do not have large neutron cross sections. The problem is a depletion of the fissile isotope - U-235.
It is not only possible, but proven technology, that it is possible to arrange even light water reactor cores to more effectively absorb neutrons into fertile material - U-238 or Th-232. When you do that, you convert what is now considered to be a waste product into fissile material that is ready to fission - either Pu-239 or U-233.
Our commercial core designs are not focused on this kind of effective use of the extra neutrons that each fission produces. We casually allow those neutrons to leak out into shielding material that is not fertile and we even purposely load our cores with neutron absorbing materials like boron.
When I speak to non nuclear trained people, I use a campfire analogy to explain the above. Our current plants seem to have been built by campfire builders that look into a forest full of limbs and even entire tree trunks but only see tiny twigs (kindling) that can be lit by a match as fuel. They do not seem to think about long term production; they do not put in the effort needed to carefully form the fire and conserve its heat so that the logs turn into fuel.
The chemical reprocessing cycles that have been developed have some problems that are a result of their initial focus. When those cycles were developed, the goal was to eliminate fission products from the irradiated fuel and produce as pure a fissile material as possible. The main reason was not because the fission products absorbed neutrons and interfered with fission, it was because some of them spontaneously fission and release neutrons and some of them decay with alpha emissions which can interact with other materials to produce neutron emissions. When the product that you want to build from irradiated fuel is meant to be a controlled explosive, those extra neutrons at somewhat unpredictable times are inconvenient. When the goal is to be able to sustain fission in a power reactor, extra neutrons are valuable byproducts that can help you meet your goal.
Bold faced plug - there are some articles on our web site that discuss in reasonable detail the concepts of reusing spent fuel and extending the life of reactors by either high conversion ratios or actual breeding.
Plutonium fuel cycle - http://www.atomicinsights.com/oct95/Pu_oct95.html
Liquid metal breeder - http://www.atomicinsights.com/oct95/LMFBR_oct95.html
Light water breeder - http://www.atomicinsights.com/oct95/LWBR_oct95.html
Fuel recycling - http://www.atomicinsights.com/jun95/recycling.html
Waste to energy - http://www.atomicinsights.com/jun95/value.of.waste.html
Opposition to reprocessing - http://www.atomicinsights.com/jun95/opposition.html