Just a bit about MOX:
"Popular" Isotopes:
With Uranium, the two isotopes of concern are Caesium-137 and Iodine-131. The latter having a half-life of 8 days, so is less of a risk now, unless fuel is still critical somewhere (unlikely). The Caesium is especially worrisome as it is not only quite volatile, it also forms from Xenon, a gas, so is likely to be an aerosol when it is formed - it is also further down the decay path than Iodine and is produced for many tens of days after criticality, and has a longer half-life of 30 years - i.e. it can easily spread and will then persist. Caesium has extreme chemical reactivity, and it will find its way into biological material fairly easily, just as Iodine can. Xenon, on the other hand, is chemically inert and the other Iodine isotope, I-129, has a half-life of 15 million years and so has very little radio-activity.
Note that none of these isotopes occur in the natural decay paths for
Uranium-238 or
Uranium-235, and that Plutonium-239 technically precedes Uranium-235 in its
natural decay path. In a nuclear reactor, there is the possibility of new and exciting isotopes being formed from (thermal)
neutron capture, which is really the whole point. This makes natural decay paths of the pure fuel partially redundant (depending on a little thing called "
neutron activation cross section" of each isotope) as long as the reactor is critical.
Activation and Decay Products:
A graph is compiled for
Uranium-235 and
Plutonium-239.
These show the statistical spread of fission products from these fuels during critical operation, and include the effects of neutron capture probabilities and decay pathways, as well as decay rates.
Remember that Pu-239 technically heads the U-235 decay tree and is formed from neutron capture in U-238, hence the similarity of the graphs - although there may well be significant differences in certain species that is not immediately apparent.
The species present in the spent fuel will depend on how long the fuel was critical for as well as how long since that criticality ended. There are more graphs available, such as the
neutron capture cross-section mentioned earlier and the
neutron fission cross-section. These are effectively a measure of the "probability" of such events occurring for a given species, and are incomplete. Neutron-induced fission (i.e. nuclear fission) is actually the main goal of nuclear reactors, but the other stuff happens too - including to any containment material, which is why their selection is so critical. Note that Zirconium-96 is particularly resistant to neutron capture and that Pu-239 is about 3 times more likely to "split" than to simply assimilate a thermal neutron - that same ratio is 6 for U-235,
but Pu-239 absorbs 3 times more neutrons than U-235, and will produce 50% more fission events per neutron than U-235 (and marginally more neutrons per fission) making it sustain criticality more readily and may also require changes to control rods / their controls.
For reference,
Z is proton number (the number that determines the name of the element and its chemical properties) and
N is the neutron number. Neutron capture in an element will yield the element directly to its right in the charts linked above. This may then
decay via any one of many means, depending on the species.
Heat Concerns:
Be aware that reactors will tend not to use 100% MOX rods - only 1/3rd is typical. Add to that the typical concentration of Plutonium-239 in the MOX rods at 7%, the rest being Uranium. Compare that to U-235 at 3-5% in Low Enriched Uranium (typical of Uranium-fuelled reactors) and recall the above ratios of neutron-induced fission to neutron-capture. Also remember the fact that Pu-239 can be formed from neutron capture in U-238, and precedes U-235 in its natural decay chain.
The
real problem, I imagine, is the lower thermal conductivity of Plutonium Oxide fuel versus Uranium Dioxide and the lower melting point (2400°C vs. 2865°C.) Note that "
Zircaloy" is not one grade, and melting points vary around 1850 ± 20°C, and more depending on heating rate.
I don't know the exact difference in thermal conductivity, but it will result in a higher core operating temperature for a given coolant temperature (and void fraction) and thermal load on the fuel itself. This means that the Plutonium Dioxide will begin to melt the casing before Uranium Dioxide in the same environment - coupled with the lower melting point (and boiling point - 2800°C !) the
risk of an exposure is higher with Plutonium Dioxide, and with it the
possible dispersal of radioactive material. Note that these effects are dispersed amongst the fuel itself, so the
bulk behaviour is only marginally different to standard Uranium fuel.
As for gamma rays, I couldn't comment. It depends entirely on what species are present which can vary more from the fuel's usage history than from its type, at least as regards Uranium and Plutonium.
NOTE: I compiled this from information found all over the web. I encourage you all to do your own research to support your own knowledge.
EDIT: Don't want this going unnoticed:
There will be no chain reaction in the spent fuel pools, but:
Interview with nuclear physicist Frank N. von Hippel, Former assistant director for national security in the White House Office of Science and Technology, Rachel Maddow, March 15, 2011 (Begins at 10:05)
No, you reckon?! ... Which brings me onto this:
👍
is that from the earthquake in Japan?
