Wednesday, July 27, 2016

Reprocessing of Nuclear Wastes

I was recently asked a question about taking spent fuel rods from a nuclear power plant and converting them into “glass logs” fort storage.  Unfortunately, the answer to this is very complicated and requires a little bit of basic nuclear physics.

What uranium (U) is mined out of the ground, it is processed into uranium ore that contains 99.3% U238 (atomic weight), and 0.7% U235, which is the fissionable component used in reactors.  In order to produce fuel rods, the uranium must be “enriched” via centrifuge technology, separating out the heavier atoms from the lighter ones.  Commercial nuclear fuel is enriched to 3% U235 leaving the remaining U238 at 97%, while military fuel is enriched to a higher level.  This is a complex and expensive step in the nuclear fuel cycle, and has caused much concern with the Iran and North Korea nuclear programs.

Once inside the reactor, under the right conditions, several major things happen.  First, a U235 atom is split apart by a neutron (fission), releasing energy (heat), 2-3 new neutrons, and creating two new, smaller atoms (there are some 35 possible “daughter” product combinations) which are generally very radioactive.  The new neutrons can go on to fission more U235, creating a chain reaction.  However, some of them are captured by non-fissionable U238 atoms, becoming Plutonium (Pu239).  In about a year of operation in a reactor, the fuel becomes “spent” with a makeup of 96% U238, 1% Pu, about 2.5% radioactive fission (daughter) products, and about 0.5% unused U235.  This fuel rod is considered High Level Waste because of its high radioactivity from these fission products, and it must be cooled and shielded.  A spent fuel pool is generally used, since water is a good coolant and radioactivity moderator.  After about 5-6 years, most of the highly active products have sufficiently decayed, so the rods can be placed in heavily shielded lead/concrete casks (dry casks), and left out in the open air to dissipate their waste heat.  This is how Trojan, Humboldt Bay, as well as other plants are storing or planning on storing their High Level Waste.

However, Plutonium is a very valuable atom (it does fission under the right conditions) if you want to build a nuclear weapon, so Hanford was built in the 40’s with the sole purpose of producing spent fuel from which Pu could be extracted. Several reactors and huge structures (the infamous “canyons”) were built, shielding workers and the environment, where the fuel rods were dissolved in acids, and the Pu was separated out (Purex process).  The U238, as well as the small amounts of U235, was also extracted for possible re-use, leaving behind a soupy, corrosive liquid containing the high-level fission products.  These wastes were dumped into unlined tanks, which after 30-40 years began to leak, leading to the enormous task of remediation today.  The basic idea is to combine the radioactive sludge with sand, and heat (2100oF) it so it all melts together into a “glass log.” The purposed is to stabilize the fission products into a manageable form so it can be placed in dry storage or in a repository.  This process is called vitrification, and France has done some of it with their liquid wastes, and the Savannah River complex in South Carolina has a small plant experimenting with their wastes.  The plant being built by Bechtel at Hanford is about half built today, started in 2000, and over budget from $1.5 billion to more than $12b.  It may never go into operation because of the magnitude of technical and financial issues it faces.  This does not get rid of the high level wastes; it just makes turns the liquids into a stable form, which will have to be monitored and stored for tens of thousands of years.

Enter another interesting piece to the story…Reprocessing, as the Pu extraction process is also called, has been pushed by the nuclear industry as a way to deal with the High Level waste issue.  Pu can be used as a fuel in special “breeder” reactors, because under special controlled conditions, it can fission, releasing energy, and more neutrons, which can then be absorbed by U238 creating more Pu…essentially creating (breeding) more of its own fuel.  Though on paper this looks like a great idea, it has been proven technically, economically, and realistically unfeasible.  And it also produces radioactive fission products, which must be dealt with.  We abandoned our program in the late “80”s, with France, Japan, and the UK abandoning their breeder programs after spending countless billions of dollars, and are now faced with the huge task of cleaning up large volumes of highly radioactive liquids and sludges.  Reprocessing, recycling, whatever you want to call it, doesn’t solve the High Level waste problem; it just transforms one problem into a huge new problem.  It does not get rid of the waste.  We would still need dry cask storage, and/or a repository.  And it would require a whole new generation of unproven reactor technology, as well as a new “Hanford” technology to manage the front end and the back end of the fuel cycle.  Very expensive! Very dangerous!  Hell of a way to boil water!

For years, the industry has dreamed of a Plutonium economy.  I still hold on to the dream of a renewable Hydrogen economy.

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