Spent Fuel Characteristics

In this module we are going to learn about
spent nuclear fuel safeguards. So then the first question is “what is spent nuclear fuel?”
The U.S. Nuclear Regulatory Commission (NRC) defines spent nuclear fuel as: “nuclear fuel
elements that have been used at nuclear reactors, but that are no longer capable of economically
sustaining a nuclear reaction.” However a more general or common definition is: “any
nuclear fuel that has been irradiated in a nuclear reactor and subsequently removed.”
Alright so let’s start with some general physical characteristics of nuclear fuel assemblies.
We will start with a pressurized water reactor (PWR) fuel assembly. On the bottom here we
see an illustration of PWR assembly with all the sub components that are in it. On the
very bottom in the right corner we start with a fuel pellet, and that is placed in a fuel
rod, then the fuel rods are arranged in a square lattice that goes into the fuel assembly.
You also see in the center there the control rods that are centrally located within the
fuel assembly that is actually a control rod assembly cluster. The manifold on the top
that slides down the center of the different positions of the fuel assembly. In the upper
corner there, you have an actual picture of a fuel assembly (PWR). Next we will take a look at a boiling water
reactor (BWR) fuel assembly. Here we have an illustration with some of the central rods
moved out so you can see within. You see several grid plates in the fuel rods moving down.
There are no control or guide rods within these BWR assemblies, control in a BWR is
done with an external control rod or control assembly that goes between the fuel assemblies.
The next image is a Russian fuel assembly, known as the VVER (water water energetic reaction)
that has to do with water moderation and water cooling of a nuclear reactor. Here you can
see the hexagonal fuel design, with the fuel pins with the guide tubes in the central area
there. So now let’s take a look at more of the specific
physical characteristics of nuclear fuel assemblies. In the table on the left we have, first column
is the physical characteristic of interest. The next columns is for a PWR, a BWR, and
a fast breeder reactor (not pictured in previous slides). The table lists properties of the
cladding, the initial enrichment, fuel type, as well as the lattice arrangement and how
many fuel rods are in each assembly. On the right we have illustrations of these lattice
arrays. On top is a PWR, you can see it is about 21.5 cm on each side. It also lists
the fuel rod positions and the guide tube locations. In the BWR below that illustration,
you can see that a BWR is about 13.5 cm on each side. Then for the fast breeder reactor,
with the hexagonal lattice array, and that is about 10.8 cm across. So next let us talk about spent nuclear fuel
operation characteristics. What we will go through is the irradiation history, spent
fuel burnup, initial enrichment, and cooling time. All of these factor into the operation
characteristics of a spent nuclear fuel assembly. So what happens to a fuel assembly during
reactor operation? What we have in front of us is a grid layout of a nuclear reactor core.
To begin reactor operation, fuel assemblies are loaded at each one of these square positions
within the core, and then the reactor is started. You’ll notice on this map, that as the reactor
is operated the different colors will change from blue to red (red indicates more power
and more neutrons). So during the course of reactor operation, the reactor is turned on
and the assembly produces power and neutrons. Then it is turned off after a cycle of irradiation.
The fuel assemblies are lifted out, rearranged to optimize performance, and put back in to
start irradiation again for another cycle. Then perhaps that cycle ends, the fuel assemblies
come out of the reactor again, rearranged one more time, and get another cycle of irradiation.
You’ll notice that during the operation, different fuel assembly positions experience greater
power then others. The power experienced by a fuel assembly, is not necessarily constant
between different cycles. It can go up, some assemblies get a much higher irradiation with
higher power (more neutrons), while others experience less power produced during the
course of operation. So the operational parameters that we are going to discuss, describe these
different irradiation histories that a fuel assembly experiences. So irradiation history is a record of the
specific power or energy released from the nuclear fuel as a function of time. So on
this slide we have two figures describing different irradiation histories. On the left
we have a single cycle with constant power, that has a specific power of 35 watts per
gram with irradiation for 1000 days. The second figure shows a three cycle constant power
(per cycle) irradiation history. The first and third cycles both have a specific power
of 70 watts per gram, while the second cycle has a specific power of 17.5 watts per gram.
We have also inserted two, 20 day shutdowns between cycles. Now these are just examples,
irradiation histories for actual nuclear fuel assemblies may be more complicated than this.
They may increase or not be constant during the individual cycle, so these are just examples
that illustrate the concept of irradiation history. Next concept that we will talk about is burnup.
Irradiation history and burnup are related. Burnup is the total energy extracted from
the nuclear fuel. So this is simply the integral of the irradiation history, which is the area
under the curve here. Of course, since that curve is just a simple rectangle, we know
that that is just the length of the rectangle (1000 days) times the height (30 watts per
gram), which equals 35000 MWD/MT uranium, which is the standard unit of measure for
burnup. If we take a look at a second irradiation history, we have 1200 days and 20 watts per
gram, so multiply those together and we get a burnup of 60000 MWD/MT uranium. So one thing
we should point out is that burnup is not necessarily unique to a specific irradiation
history. On the left we have two figures here containing different irradiation histories.
The first case in the blue is from before (35 watts per gram, 1000 day irradiation).
The green is twice that, at 70 watts per gram for 500 days. The pink there is half of case
one’s specific power, at 17.5 watts per gram for 2000 days. On the bottom figure we have
a forth case that operates at 35 watts per gram for total irradiation of 1000 days, but
we also inserted two 20 day shutdowns in-between there. What you see though when you look at
the burnup, is that the burnup for these different irradiation histories are all the same. We’ll
point out that the units on this burnup are different, they are now GWD/MT. You can see
burnup reported as both, MWD/MT and GWD/MT as shown in the previous figure. Again, burnup
is not necessarily unique for these four irradiation histories which are all different (same burnup). Another thing we want to consider for operational
characteristics is the initial enrichment. Now we also talked about initial enrichment
during the physical characteristics of fresh fuel assemblies, but it also impacts the operation.
Specifically it impacts the potential final burnup. The more fissile content, whether
that is 235U or 239Pu (MOX fuel assemble, meaning both uranium and plutonium are in
the fuel assembly to begin), the higher burnup you can operate to and the more energy you
can get out of the fuel. So initial enrichment is an important thing to consider for the
operational characteristics of a spent fuel assembly. Finally, we want to talk about the cooling
time. Cooling time is simply the amount of time after the final irradiation of the spent
fuel assembly. So here we have a figure of the irradiation history. We see that it what
operated at 35 watts per gram, with two shutdowns during operation, and then after the final
irradiation ends the cooling time (noted by the blue line). Typically, the spent fuel
sits in the spent fuel pool where it is allowed to cool, but this may also include time the
spent fuel assembly has been placed into dry storage.

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