Spent Nuclear Fuel Safeguards: Spent Fuel Signatures

In this section we are going to take a look
at Spent Nuclear Fuel Signatures. There are several signatures that we are going
to go through. The first set is physical signatures, then
gamma radiation signatures, Cherenkov radiation signatures, neutron radiation signatures,
and then combined radiation signatures. So the first thing we will talk about is the
difference between signals and signatures, and what we mean by that. First in our hypothetical situation here we’ll
have an event or phenomena that occurs, and that event emits several signals. In reality, it might be several signals or
one signal depending on what the event is. Here we have three signals: signal A, B, and
C. if we collect these signals, and do some intelligent processing of the signals, we
can then put them together and get a signature. In this case, the signature developed from
signals A, B, and C is the signature for the event shape. So we get one piece of information that we
learned about the event from our signature. So the difference between signals are just
the raw data or whatever is emitted by the event, and the signature is taking those signals
and putting them together (whether it is one or several signals) and using the signal to
learn information about the actual event. So we are going to start with some physical
signatures. The first one here is just the I.D. code,
very simple starting with the simple stuff. Here we have two pictures of BWR fuel assemblies,
and you will see on the top (the handle type piece there in the assembly) there is an engraved
identity code. That is very common, and an inspector can
come and take a look at the identity code and say, “Yes, this is the fuel assembly that
was supposed to be here.” So that I.D. code is a physical signature
for spent fuel identity. The next physical signature is: corrosion
and deformation of a spent fuel assembly. These are physical signatures to prove that
the assembly was irradiated in a nuclear reactor. When you start with a fresh fuel assembly
(in this image we have here), its shiny, its straight, precision engineered piece of equipment,
and it looks very nice. When you put it into a nuclear reactor, all
sorts of bad things start happening to the fresh fuel assembly. Reactor environments with neutron irradiation
and high pressure and evaporating / boiling water in a BWR produces some significant changes. The first image we will see here is crud deposition
on spent fuel. Crud is just a colloquial term, there is some
belief that is was an acronym for chalk river unidentified deposits. The acronym actually came after the use of
the word crud just to describe sort of a gunky build up that happened on spent fuel rods. What that is, is corrosion, its rust. Nickle and a few other things can start building
up on the fuel rods that is leached out from steel from somewhere in the process and just
start building up and produce corrosion wear on the fuel assemblies. Here is another image, we have on the left
spent fuel with crud. Then this spent fuel assembly was cleaned
and so you can see the spent fuel after cleaning and the difference between with crud and without
crud. So this crud deposition is a physical signature. Another product of the harsh environment and
the high pressure and the neutron irradiation and the changes that are occurring in the
fuel, is that the fuel rods can bow. This is a result of several things, the fuel
pellets that are inside fuel rods undergo several structural changes. They can expand, they can condense, and that
will shift around the insides of the fuel assembly. Then there is, in a PWR, an immense pressure
on the outside of the fuel assemble. The moderator, the water, is flowing through
at very high speeds. The fuel assemblies are shaken around and
vibrating around as a result of this high pressure and high speed flow. So it can really bow and kind of warp the
fuel rods and fuel assemblies. Here is another image of a fast reactor fuel
assembly with the rods twisted from the irradiation induced swelling and creep: two of the terms
used to describe the processes that occur in fuel rods and fuel assemblies. So if you see a fuel rod that looks worn and
rusty and kind of beat upped and banged up, those are physical signatures of fuel irradiation. If it looks like your fresh fuel assembly,
then it hasn’t been irradiated. So next we will look at the radiation signatures,
and we start with total gamma-ray activity, which is if you just take a gross count of
the gamma radiation that comes off of spent fuel. First it is a signature for irradiation, you
are not going to get gamma radiation without the irradiation in the reactor to begin with. There is a ton of gamma ray activity that
is coming off these fuel assemblies, they are very hot. A fresh fuel assembly may have a little gamma
radiation from the uranium, just a little bit. Nothing of the sort that comes off of spent
fuel assemblies. Next there is a correlation of gamma ray activity
to burnup and cooling time, so it can be a signature for both the burnup and the cooling
time. Here we have a figure of total gamma ray activity
over burnup on the vertical axis, and cooling time on the horizontal axis. Of course these are radioactive elements and
isotopes that are producing these gamma rays, so you have to account for the cooling time
but there is also this relation to burnup. So you can use the total gamma ray activity
as a signature for burnup and adjust it for cooling time. Here the equation shows the functional relationship:
activity (A) over burnup (BU) is equal to some function of cooling time. Next let’s take a look at the fission product
gamma-ray activity signatures. First of course, it’s a signature for irradiation
in a reactor. Here we have a figure showing the fission
process, and here are our gamma ray emitting products. Of course the thing that starts this all is
neutron irradiation in a nuclear reactor. If you take that away, then you lose everything
else. You just have uranium without the irradiation. If you see the fission products, and the gamma
rays emitted by the fission products, then you know that you definitely seen neutron
irradiation. So for the fission product gamma-ray activity
there are three main fission products that you are mostly going to be concerned with. Those are two isotopes of cesium, 134Cs and
137Cs, and 154Eu. On the right we have a figure of a gamma-spec
of a fuel assembly with 47 GWD/MT burnup with a 12 year cooling time. We have the main fission peaks highlighted
there. The fission product buildup chain is what
effects the signature products and quickly we will go ahead and take a look at the fission
product buildup chains. We will start with the 137Cs and you see the
chain starting with 136Te and 137Te, and those are produced directly through fission and
then those decay down to iodine and then to xenon. There is some absorption of 136Xe to 137Xe,
not too much there, it is probably offset by the absorption of 137Cs itself. You see both the absorption cross sections
are noted there, on the figure its 0.26 barns for 136Xe and 0.25 barns for the 137Xe. So any additional production that comes in
from 136Xe is basically taken out by the absorption of 137Xe itself during irradiation. So that is really a pretty linear production
rate just directly from fission, there are not too many things going on there that you
see from 137Cs. 134Cs is a little bit different, the production
depends on absorption and 133Cs neutron absorption during neutron irradiation in a nuclear reactor. There is an absorption resonance that I do
not want to get into too much detail there, but basically neutrons at a specific energy,
133Cs really likes to absorb a lot. So you do get some significant production
of 134Cs and the 133Cs production is fairly linear except the neutron absorption of 133Xe
is somewhat dependent on irradiation there (100 barns). So it is mostly linear production, we will
take a look at it in a second, but there are some dependencies on reactor parameters that
will produce a few differences that we will take a look at in a second. 154Eu is very complex and there are lots of
things that can impact its production but because there are different things that effect
its production that means it can be a signature for those different reactor operation properties
that may occur. So now let’s go ahead and take a look at the
specific signatures. First is burnup and cooling time and here
we have a figure of the 137Cs concentration on the vertical axis. As I have mentioned before the production
is directly proportional to the amount of fissions that occur, so it is directly proportional
to burnup. So we see here in this figure that was produced
from a simulation from PWR fuel, the 137Cs concentration is directly correlated to burnup. So it is a completely linear relationship
but you do need to account for the cooling time. So if we slide to the right on the adjoining
figure, we have the decay of different concentrations of 137Cs corresponding different final burnups:
20, 25, 30, and 35 GWD/MT. So the 137Cs activity can be used as a signature
for both burnup and cooling time. The next thing we will look at is a fission
product signature for fissioning isotopes. Here we are using the 154Eu fission product,
as I have mentioned before it is sensitive to several different things and one of those
is the fissioning isotope. So on this curve in the figure, we have the
amount of 154Eu in grams per ton of heavy metal which is mostly uranium against on the
horizontal axis the burnup. You see you have two different curves based
on the fissioning isotope or the type of fuel; whether that is LEU which fissions mainly
from 235U, or MOX which is a blend of 239Pu and 235U. MOX stands for mixed oxide. So the difference there between the two is
a signature for the fissioning isotope. Fission product gamma ray activity can also
be used as a signature for initial enrichment. Here we are taking a look at 134Cs and on
the vertical axis we have the concentration of 134Cs and on the horizontal axis we have
burnup. Again, you see two different curves for two
different enrichment of 235U, both being LEU fuels. The top curve is enriched to 3 % and the bottom
curve is enriched to 4.5 %. Another thing that we haven’t really talked about yet is
void fraction and the 154Eu fission product gamma ray activity can serve as a signature
for void fraction. Void fraction is the fraction of moderator
that is gas basically, it only applies to BWR reactors. In BWRs there is boiling that occurs in the
moderator while it’s cooling the fuel and moderating the neutrons and so the amount
the void or gas from boiling that occurs impacts the moderation that occurs within the reactor
core and shifts the neutron spectrum to a fast or harder spectrum. The complexity of the europium production
chain impacts the amount of 154Eu produced. So you have two different curves here, the
concentration of 154Eu is on the vertical axis against the burnup on the horizontal
axis and the upper dashed curve is 77 % void and the lower curve is 44 % void. So you can distinguish that and use the 154Eu
activity as a signature for void fraction. So the next thing we will take a look at is
fission product gamma ray activity ratios. So the question is then why use activity ratios. Activity ratios actually make things a bit
simpler. The ratio cancel out detection biases and
biases in the efficiency, and you only need the relative detection efficiency to fully
calculate the activity values so they make things easier. There are other efficiencies that will come
into play if you are trying to measure a single fission product, but when you measure two
and take the ratio, the other types of efficiencies and biases just cancel out and make your life
a bit simpler. So fission product gamma ray activity ratios
can be used as a signature for (1) irradiation, again you don’t get fission products without
irradiation. Again you can use it as a signature for burnup
and cooling time. Here we will take a look at this figure, on
the vertical axis we have the 134Cs/137Cs ratio on the vertical axis and on the horizontal
axis we have burnup with the ratio values are simulated for PWR fuel. We see this nice semi linear curve, it is
not perfect, we notice these three discontinuities that come from reactor shutdowns when there
is some decay of both of these fission products. For the most part it builds up fairly linearly
and we can calculate using simulations these discontinuities and use this for an effective
signature for burnup and cooling time. So next let’s take a look at the ratio between
134Cs and 154Eu. We can use that as a signature for both fissioning
isotopes and initial enrichment. In this figure we have the ratio of 134Cs/154Eu
on the vertical axis and burnup on the horizontal axis. We have four different curves or data plots:
two for LEU and two for MOX with different enrichments for both LEU and MOX. You can see some separation at the higher
burnups between not just the fissioning isotope (LEU vs. MOX) and also the initial enrichment
(within those fuel types). Next we will take a look at the Cherenkov
radiation signatures that we can get from Cherenkov radiation. First, we will just say Cherenkov radiation
presence; this signature is for simply radiation. If you don’t stick it into a nuclear reactor
you are not going to get the blue glow. We have an image of spent fuel vs. fresh fuel,
and with spent fuel you see the blue glow. Then if you look at Cherenkov radiation intensity
and you measure this very precisely and use a digital measurement, you can use Cherenkov
radiation as a signature for burnup and cooling time. What we have here is an image, top view of
a spent fuel assembly using a digital Cherenkov viewing device. What you see actually, is the blue holes that
are occurring there are actually the water holes (moderator) between the fuel pins. The dark areas are actually where the fuel
pins are because the Cherenkov radiation occurs in the water that surrounds the fuel pins,
and not in the fuel pins themselves. The dark red holes there are the guide tube
holes, so lots of Cherenkov radiation can occur there and so it is very intense and
that is what the red signals represent. These Cherenkov viewing devices are similar
to night vision goggles which enhance the light. So now let’s take a look at the neutron activity. Neutron activity, not surprisingly, can be
used as a signature for burnup and cooling time. On this figure we have the relative neutron
activity on the vertical axis which is on a log scale, and we have that versus burnup
on the horizontal axis also on a log scale. We see it’s not quite a linear relationship,
it’s actually this power relationship that is given in the equation on the figure. So it’s slightly indirect, but still useful
as a signature nonetheless. Neutron activity is also useful as a signature
for initial enrichment. Again we can plot neutron activity on the
vertical axis and burnup on the horizontal axis. We see three different curves here; the top
curve is an initial enrichment of 2.2 % 235U, the middle curve is 3 %, and the bottom curve
is 4 % 235U. So these differences can be exploited to use
neutron activity as a signature for initial enrichment. We can also use it for a signature for irradiation
history shutdowns. Again, in the figure we have neutron activity
on the vertical axis and burnup on the horizontal axis, but we have two different sets of data
here. The dark circles are for continuous irradiation
and the open circles are for non-continuous irradiation, and you can the reactor shutdowns
where the spikes and discontinuities occur. After the shutdown the neutron activity actually
bumps up, most likely due to decays from americium into curium that spikes that signal, which
is higher than the continuous irradiation neutron activity. So you can use that as a signature for irradiation
history shutdowns. Neutron activity can also be used as a signature
for assembly integrity and partial defects. What I mean by that are missing fuel rods
from the assembly. If an entire assembly is missing, that is
referred to as a gross defect. So a partial defect is when individual rods
from within the assemble are missing or replaced with dummy fuel rods. To use this signature though, you must already
know the burnup either from the facility operators report or from another burnup signature measurement. In the figure we have 244Cm neutron counts
plotted on the vertical axis (majority of the neutrons coming from the fuel are from
244Cm) and then along the horizontal axis, burnup in MWD/kg is plotted. MWD/kg is actually equal to GWD/MT. So we have several sets of data points that
were measured from fuel assemblies, there were two facilities that were where fuel assemblies
were measured. The first one I am not going to try to pronounce,
starts with an O, and the second one is the acronym CLAB. The first facility, the data points are circles;
data points from CLAB with partial defects are diamonds; and complete assemblies measured
at CLAB are the triangles. On the figure, you see the data points all
clustered along the correlated saw line there and then there are dashed lines above and
below. The dashed lines represent the error corridor
that the measured data points should fall within. So if there is a data point that ends up being
below the error corridor, then it’s not producing as many neutrons as should be expected based
on the burnup. That would be a result of missing fuel rods
from the assembly. So if we look at the figure, we actually see
one data point at 18 MWD/kg that actually has 35 % of its fuel rods were missing. So that demonstrates can work if you trust
the value of burnup that you have for the assembly. One note on this is that there is a lower
limit in partial defects for this signature, and that is about 20 % of the fuel rods. If you get below 20 % of the fuel rods missing,
say you only have 10 %, this neutron activity signature is probably not going to pick it
up. So now let’s take a look at one combined radiation
signature, and that is the neutron counts to total gamma ratio. Here we have a figure where the neutron to
total gamma ratio is plotted on the vertical axis (log scale) and it’s normalized to the
first data points at 50 GWD/MT. You can see two sets of data for 15, 30, and
45 GWD/MT, and those two sets of data are 30 days cooling time (blue) and three years
of cooling time (orange). What you should see when you look at this
figure is that there is very little difference between the different cooling times for each
of the data points. So the conclusion is that the neutron to total
gamma ratio can be used as a signature for burnup that is totally isolated (mostly) from
cooling time. So just about all of our previous signatures
for burnup depended on cooling time, and here we have one that can remove the dependence
on cooling time and have a signature just on burnup.

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