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.
