Spent Fuel Radiation


So in this section we are going to talk about
spent fuel radiation. There are three types of spent fuel radiation: gamma radiation,
Cerenkov radiation, and neutron radiation. So let’s start with gamma radiation and take
a look at a spent fuel gamma radiation spectrum. This comes from a 32 GWD/MT burnup assemble
with nine month cooling time. So you see various different fission product activities and structural
activation products. We will point out the structural activation products here because
they are short lived and those will go away within one or two years. Then lets focus on
the 137Cs gamma ray, and we’ll note that the 137Cs gamma ray activity is about 10^7 times
greater than 235U or 239Pu gamma ray activities. What this means is that you are never going
to see the 235U or 239Pu gamma rays on the spectrum. The uranium and plutonium gamma
ray activities are completely masked by the fission product gammas. So now let’s take
a look at the axial profile of gamma ray activities, and we will focus on specifically on 137Cs
gamma ray activity. In this figure you have the 137Cs peak intensity on the vertical axis
as a function of the vertical detector position alongside the spent fuel assembly (from bottom
to top) on the horizontal axis. You will see the curve shape there which shows that the
greatest intensity comes from the center of the fuel assembly. That makes sense because
the most fissions occur during reactor operation at the center of the fuel assembly, and there
are less fissions that occur at the ends. So now let’s take a look at Cerenkov radiation,
and a great question is, “What is Cerenkov radiation?” We see an image here of a spent
fuel assembly and Cerenkov radiation is that blue characteristic glow that you see from
spent fuel. But what specifically is Cerenkov radiation? The definition is light emitted
when a charged particle exceeds the speed of light in a dielectric medium (like water)
that the particle is passing through. You can’t exceed the speed of light in a vacuum,
but you can exceed it in different media. So if a particle is moving too fast, it has
to radiate out energy, and it does that through Cerenkov radiation. So there is several sources
of Cerenkov radiation in spent fuel. The first in beta radiation, and betas are just electrons
that are emitted, and so they produce Cerenkov radiation directly. Other indirect sources
of Cerenkov radiation include: neutrons radiation and gamma radiation. As it turns out the most
significant source of Cerenkov radiation comes from the gammas indirectly. So let’s take
a look at that process specifically. So if we start with a spent fuel rod or spent fuel
assembly, a gamma ray is emitted and it has an interaction with an electron belonging
to water or a structural material atom. The interaction that it has is a Compton scatter,
and the gamma scatters off continuing with a new energy and wavelength. The electron
is ejected, and as I said before, it is ejected with too much energy and is moving too quickly
through the water so it has radiate the extra energy. As it does that, as it moves through
the water, the Cerenkov light is emitted. It actually happens like a shockwave, like
if a supersonic jet exceeding the speed of sound in air producing a sonic boom, this
is kind of like a light boom. So the important thing to take away though is that Cerenkov
radiation is proportional to gamma radiation. So next let’s take a look at the spent fuel
neutron radiation. We have a figure here showing the neutron radiation of the most active neutron
emitters in spent fuel. On the vertical axis we have the relative neutron emission rate
(log scale) and the horizontal axis is cooling time. So as time progresses, these radioactive
neutron emitters are decaying. So we see five principal neutron emitters: two isotopes of
curium (244 and 242) and three isotopes of plutonium (240, 238, and 242). What you see
here is that the neutron emissions are totally dominated by the curium. At the beginning,
it is dominated by both 244Cm and 242Cm, but then the 242Cm dies away pretty quickly (short
half-life). So that drops out and basically the total neutron emission activity is completely
represented by the 244Cm. Again what we see here is that the plutonium signal is completely
dominated by the higher actinide curium. So then if we take a look at the axial distribution
of neutron activity, again you see the same trend as the gamma ray activity. In this figure
we have neutron counts on the vertical axis and vertical position (from bottom to top)
on the horizontal axis. You see this curve with the peak intensity in the middle, which
indicates that the most fissions occurred in the middle of the fuel (which is the way
it works). So let’s look at gamma radiation vs. neutron
radiation. We will take a look at a couple things here. First point that we will make
here, is the source of the radiation within the fuel assembly. The gamma radiation that
you see outside the fuel assembly comes primarily from the fuel rods at the edge of the assembly.
The attenuation within the fuel assembly is high for gamma rays, so if they are emitted
in the center of the fuel assembly they are not going to make it to the outside. So when
you take a look at the gamma radiation signal, what you are really looking at is just the
outer fuel rods of the fuel assembly and you are not getting information from the inner
fuel rods of the assembly. Neutrons, however, travel much farther through the fuel assembly
than the gamma rays. They are attenuated much less, so you will see neutrons that come from
central fuel rods all the way through the outside of the fuel assembly. The next thing we will take a look at is the
source material of the radiation. We mentioned before that gamma rays come from both the
fuel and the cladding / other structural materials within the fuel assembly. So you are getting
radiation from two sources there: the fuel and the structural materials. The structural
materials radiation is short lived but it is still there at the beginning. The neutrons
are coming only from the fuel. So these two factors, the assembly source location and
source material of the radiation, provide several advantages and disadvantages that
we will take a look at. So for the neutrons, you can measure neutron radiation almost immediately
after spent fuel discharge. With the gamma radiation, you will get a lot of the structural
activation products radiation that is not useful for looking at examining the spent
fuel (which we will talk about later). Also, neutrons have the advantage that the signal
is from all the fuel rods. It’s from the central fuel rods and from the outer fuel rods, the
gamma radiation is just from the outer fuel rods. However there are some disadvantages
from neutrons. Neutron detectors are sensitive to the gamma rays, which you have to take
into account when you are doing interrogation of the fuel assembly (at least with non-fission
chamber neutron detectors). There is some other advantages and disadvantages that depend
on radiation detectors that we will take a look at a little later. One other thing I’d like to mention here is
neutron radiation with active interrogation. Passive interrogation just means observing
spent fuel with a radiation detector. So you just put a radiation detector beside the fuel
assembly and see what you read. For neutron radiation, that passive interrogation will
give a signal mainly from curium isotopes and the spontaneous fission within those curium
isotopes. With active interrogation, you introduce an external neutron source and that external
source basically shoots neutrons into the fuel assembly. You then get fissions from
the 235U and 239Pu. So you get many more neutrons (1) and you also get a neutron signal from
235U and 239Pu with active interrogation that you don’t get from passive interrogation.

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2 Responses

  1. developercm says:

    Excellent presentation.

  2. Jagoda urban says:

    This is the best presentation – all of the series, I could find online.

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