Nuclear forensic characterization enables
the gathering of information about nuclear
materials and their origin.
This information includes the type of nuclear
power plant the material came from, its history
and age.
To better understand this connection between
isotopic ratios of the fuel and the reactor
design, let’s recall some of the main features
of a nuclear power plant and the differences
between the various reactor types.
Depending on their design, nuclear power plants
differ in the degree of fuel enrichment, choice
of coolant and neutron moderator.
The general principle of power generation
is the same for all types of nuclear power
plants.
In the reactor, a controlled nuclear chain
reaction is maintained by neutron-induced
fission of atomic nuclei.
By energy transfer the fission and decay products
heat the coolant, which in turn drives a steam
generator.
A steam turbine converts the heat contained
in the steam into mechanical energy and a
generator converts this into electrical energy.
In order to maintain the nuclear chain reaction
in the reactor, each fission has to cause
on average one or more subsequent fissions.
For example, the neutron-induced fission of
uranium-235 into barium and krypton releases
in average 2.3 neutrons, which are then available
for further reactions.
Nuclear fuel is made of natural uranium that
is composed of the isotopes U-234, 235 and
238.
Of these, only U-235 whose abundance is around
0.7% is fissile by thermal neutrons.
Therefore, for use in most reactor types,
natural uranium must be enriched prior to
fuel fabrication.
The degree of fuel enrichment differs for
different reactor types.
Other nuclear reactions contribute to the
energy production too, for example when a
uranium-238 nucleus absorbs a thermal neutron,
an n-gamma reaction takes place and uranium-238
is converted into uranium-239.
Due to two subsequent β- decays the fissile
plutonium-239 is produced.
This phenomenon is called breeding.
Fission of uranium-235 or plutonium-239 generates
fast neutrons with a kinetic energy of several
MeV.
They have to be cooled down to thermal energies,
to continue the nuclear chain reaction, because
the cross section for neutron-induced fission
of uranium-235 is three orders of magnitude
higher for thermal neutrons than for fast
ones.
Fast neutrons are cooled down due to a transfer
of their kinetic energy by elastic collision
with other nuclei.
The energy transfer is most efficient for
nuclei of equal mass, therefore materials
with a high density of light nuclei, such
as water, are chosen as the moderator.
Reactor designs differ according to their
moderator and coolant.
In civil energy production, mainly graphite
moderated reactors, light water and heavy
water reactors are used.
The most prominent design of graphite-moderated
reactors is the “Pressurized-tube boiling
reactor” also known as RBMK, which was developed
and built by the former Soviet Union.
A fundamental difference in the design of
RBMK reactors compared to western reactors
is that there is no physical containment surrounding
the reactor core in an RBMK.
RBMK reactors are operated with uranium dioxide
fuel with a U-235 enrichment grade of 2.0-2.8%.
The fuel is contained in zircaloy pressure
tubes, which are embedded in the graphite
moderator block.
Ordinary water is used as the coolant.
The RBMK reactor sadly became famous due to
the Chernobyl accident.
After the accident, additional safety precautions
were applied to the remaining RBMK reactors
in order to improve operating safety.
An example of heavy-water reactors that use
deuterated water for both coolant and moderator
are the CANDU-reactors, which were developed
by Canadian companies.
The fuel in CANDU-reactors is also contained
in individual pressurized tubes which are
flowed by the coolant.
The advantage of this design is that the production
of smaller pressure tubes is more straightforward
than the production of a large pressure vessel.
The absorption cross-section for thermal neutrons
of deuterons is three orders of magnitude
smaller as compared to protons, making heavy
water a superior moderator.
As a result, fewer neutrons are absorbed by
the moderator and therefore heavy water reactors
can be operated with natural uranium.
In the context of non-proliferation pressurized-tube
reactors like CANDU and RBMK reactors have
to be considered critically, since these reactors
are designed in order to allow the replacement
of fuel elements during power generation.
Thus, it is possible to replace the fuel after
a short irradiation time and thereby obtain
weapon-grade plutonium-239.
Today, about 85% of the world’s nuclear
energy is produced by light-water reactors.
There are two major types of light water reactors:
pressurized water reactors and boiling water
reactors.
In order to reach criticality in a light water
reactor, the fuel has to be enriched to about
3% uranium-235 to compensate the neutron deficit
due to the larger capture cross-section of
hydrogen compared to deuterium.
Pressurized water reactors are the most widely
used reactors in the world.
In contrast to boiling water reactors, they
have two separate water loops.
The water in the primary loop flows through
the reactor core and serves simultaneously
as moderator and coolant.
This water is pumped into the steam generator,
where the heat is transferred to the secondary
loop that drives the steam turbine.
The advantage of two separate water loops
is that the secondary loop is free of radioactivity,
thus simplifying the maintenance of the steam
turbine.
As this brief overview of three different
reactor designs shows, if a fuel pellet is
illicitly trafficked and seized, its degree
of enrichment enables identification of the
type of reactor it came from.
Moreover, the type of reactor can be further
limited by the geometry and other physical
characteristics, since the fuel elements are
specific for each reactor type.
This table summarizes the basic technical
characteristics of the reactor designs just
presented.
For example, the fuel pellets of CANDU and
RBMK reactors are significantly larger than
those of pressurized water reactors.
However, these two can be easily distinguished
by their degree of enrichment, since CANDU
reactors are operated with natural uranium
in contrast to RBMKs.
If a fuel pellet that has already been irradiated
in a reactor is seized, the irradiation time
can be estimated by its elemental composition
and the different isotope ratios.
In reactors loaded with low enriched uranium
(LEU), after the burn-up, the initial uranium-235
abundance decreases from 3% to about 0.8%.
Due to the already mentioned neutron capture
of uranium, long-lived plutonium isotopes
are produced and their concentration increases
to an abundance of about 0.9% and that of
other fission products to about 3.4%.
The ratio of plutonium-240 to plutionum-239
is also of major interest to nuclear forensic
scientists.
Indeed, this ratio increases the longer the
fuel is irradiated in the reactor, because
Pu-239 can not only be fissioned by neutrons,
but can also react to Pu-240 by an n-gamma-reaction.
The ratio also depends on reactor characteristics
like the neutron flux that passes the reactor
core, which permits an assignment of the ratio
to a specific reactor design.
The radiochemists can support nuclear forensic
investigations by collecting data concerning
the chemical and isotopic composition of nuclear
materials to provide information about their
intended use and origin.
