Spent Fuel Safeguards Goals

In this section, we are going to look at IAEA
safeguards goals and objectives for spent nuclear fuel. First we will take a look at
the safeguarded materials in spent fuel, then look at the overall safeguards goals, and
then some more specific verification objectives for safeguards and some general methods that
are used there. So we have our spent fuel and there are several material types, the
IAEA has specific definitions for nuclear materials and several of those are in spent
fuel. The first being plutonium, and we also have LEU (for uranium oxide fuels) and specifically
we are concerned about the 235U in LEU. For thorium fuels, we would have 233U and thorium.
Now these materials have specific designations for the IAEA. The plutonium, low-enriched
uranium, and 233U are all referred to as special fissionable material. Whereas the thorium
is referred to as a source material. Of course all of these materials (special fissionable
materials and source materials) need nuclear safeguards. The next question is, “what are the specific
safeguards goals for the nuclear material contained in spent nuclear fuel?” To develop
the goals for safeguards, the IAEA has created several material categories for the different
material types based on the suitability of the material to be converted into a nuclear
weapon and the time it might take to convert the material into a nuclear weapon. So for
each of these materials in the nuclear fuel, we have a categorization. For the plutonium,
it is categorized as an irradiated direct use material. It has been in a nuclear reactor
(irradiated) but the plutonium can be used directly to make a nuclear weapon. LEU is
an indirect use material. LEU fuel can be used to create plutonium, but the LEU itself
unless it is further enriched to HEU can’t be used directly to make a nuclear weapon.
So LEU needs further, either enrichment or reactor irradiation (further processing),
to be used to create a nuclear weapon. The 233U coming out of a thorium reactor is an
irradiated direct use material. 233U can be used directly, however it has been irradiated
so that might add a little more handling time to the consideration as opposed to an non-irradiated
material. The thorium is also an indirect material, it can be irradiated in a reactor
to create 233U but the thorium itself cannot be used directly to produce a nuclear weapon.
So then our IAEA spent fuel safeguard goals are (1), to detect the diversion of spent
fuel containing 8 kg of plutonium within three months of possible diversion, 8 kg of 233U
within three months of possible diversion, 75 kg of 235U in LEU fuel within one year
of possible diversion, and 20 tons of thorium within one year of possible diversion (thorium
fuel). We also want to detect the possible misuse of spent fuel handling facility for
undeclared nuclear activities, whether a nuclear reactor or a spent fuel storage facility or
a reprocessing facility. We want to detect any misuse of the facility and the spent fuel
that it contains. So from those goals, we have several verification
objectives and we can divide those into three categories: nuclear material accountancy (NMA),
containment and surveillance (C/S) objectives, and design verification objectives. So focusing
on our nuclear material accountancy objectives, the first objective is to verify the presence
or absence of a spent nuclear fuel assembly or object. We refer to this as the detection
of gross defects. Gross defects are when an entire item is missing from the accountancy.
So if there is a spent fuel assembly and you take a look at it and its not there, or there’s
a spent fuel assembly where it’s not supposed to be, then this is something that needs to
be resolved. The second objective is to verify the identity of the spent nuclear fuel object.
Here we want to ensure that the spent nuclear fuel assembly or item is in face the assembly
declared by the facility operator. If it’s not the assembly declared by the operator,
then you have two questions. One, where did the assembly go that was supposed to be there,
and two, where did this other assembly come from that is now in its place. So those are
issues that need to be resolved. So the next verification object, is to verify the integrity
of the spent nuclear fuel object. We refer to this as detection of partial defects. Partial
defects are when, for example, missing fuel rods from a nuclear fuel assembly. They could
have been replaced with dummy fuel rods, or were just taken, but this is an important
verification objective. We want to ensure that not only is the item present, and it’s
the one that we expect it to be, but that the item integrity has been completely maintained.
The fourth objective is to verify the nuclear material content of the spent nuclear fuel
object: how much uranium, how much plutonium or thorium is contained within the object.
This is fairly difficult to do, and usually if you are doing item accountancy (which for
spent nuclear fuel assemblies we are), the first three verification are sufficient (without
verifying the actual material content). So if we have a nuclear fuel assembly, its present
(where we want it to be), we confirm the identify of it, it is in fact the nuclear fuel assembly
we want, and it has full integrity, then it’s not critical how much nuclear material or
fissile content is in the fuel assembly. It can be one significant quantity or ten significant
quantities of plutonium within the spent fuel assembly, but you still have it so you know
there was nothing diverted. With advanced techniques that are being developed, it may
be possible to determine this directly. When you get to a reprocessing facility, where
you are chopping up the fuel assemblies and dissolving the fuel contained in them, and
going item accountancy to bulk accountancy, the nuclear material of the spent nuclear
fuel assembly becomes very important (at that stage). For the most part, when you are just
doing item accountancy the first three verifications for nuclear material accountancy are sufficient. So then we have several containment and surveillance
objectives. The first being to verify the continuity of knowledge over spent nuclear
fuel assemblies and other objects. We also want to verify that there is no use or production
of undeclared nuclear materials. We do this through containment and surveillance by placing
and verifying seals on spent nuclear fuel transfer points. Whether that is on the reactor
vessel itself, when you close that after refueling to make sure that there is no unauthorized
opening and diversion of spent fuel from the reactor core; or at some transfer point, transfer
channel from the reactor to the spent fuel pool. You can place a seal over that and the
IAEA can verify has not been broken, so there has been no spent fuel diverted down that
channel. Also another verification is to maintain and review the surveillance videos of SNF
transfer paths that the SNF might follow. If you can look at the video and say “Hey,
there is no spent nuclear fuel that has travelled down this path,” then you can be confident
that no spent nuclear fuel was diverted along that path. The next thing we should mention is design
verification. Here we want to verify the facility design and mainly to ensure no new un-safeguarded
spent nuclear fuel transfer paths have been added between the last design verification.
We do this through a special inspection called a design inspection verification. Of course,
it should not be any mystery how spent fuel is verified. We spent a lot of time developing
and going through the spent nuclear fuel signatures in the previous modules. So we will use those
spent nuclear fuel signatures for verifications. Here we have an image of a fork detector being
used to survey a nuclear fuel assembly and from that we can develop a spent nuclear fuel
signature. We can use that and other signatures to verify the identity, integrity, and presence
of a (hopefully if you are using a fork detector) spent nuclear fuel assembly in front of you.
You can complete all of your verification objectives and verify the spent fuel assembly
for safeguards. Now we are not going to go into the specific detection methods and signature
quantification methods. That is detailed in the written portion, so take a look there.
We’ll talk about the fork detector, we’ll talk about different types of detectors in
the written portion. Here we are just going to go through two general verification procedures
that are used for spent fuel signatures. The first being independent spent fuel parameter
verification, and the second being spent fuel parameter consistency check. So both of these
procedures make use of the facility operator’s report. Ideally, you’d want to do independent
spent fuel parameter verification in all cases. Which you start with, is measuring the nuclear
fuel signature and you get your measured signature value. You can then convert that to an independent
measured parameter value. Then you take a look at the same parameter value (i.e. burnup,
operational history characteristic, etc.) from the facility operator’s report and then
you ask, “Is this reported parameter value correct?” Does it match the measured parameter
value within a certain error tolerance? If it is not, then you need to do some further
investigation. It doesn’t necessarily mean that there is anything wrong, it just needs
further investigation at that moment. However, this is not always possible with the available
signatures. As we talked during the discussion on the spent nuclear fuel signatures, a lot
of the signatures are not independent. They might depend on several parameters, from the
operational history or from the nuclear fuel assembly. So if you have something that is
not independent, dependent on several values, than you can do a spent fuel parameter consistency
check. Here you start with a facilities operator report, and you get the reported parameter
value. Then you look at your measured signature value and you check to see if they are consistent.
You say, “Could I get this measured signature based on the operational history and spent
fuel characteristics that were present?” Could that lead to the signature value that I am
measuring? If it could, then you declare that is consistent, and complete your verification.
As I said before, detailed descriptions of spent fuel safeguards verification techniques,
methods, and detection systems are given in the written portion. We are not going to go
through this during our discussion, but take a look at the written portion and that will
be all.

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