156
14 Difficulties of Radiological and Nuclear Interdiction
ray-induced reaction that releases a neutron. According to Ogawa [8] a 4-kg mass
of weapons-grade plutonium (containing no more than 6% Pu-240 by weight) will
emit about 400,000 neutrons per second and will have a detection distance of about
12 m when depending on gamma detection and 25 m when identifying the device
using neutrons. Fetter et al. [4] also note that highly enriched uranium emits far fewer
neutrons than Pu-239; 12 kg of HEU will emit about 1400 neutrons per second with
a correspondingly lower detection radius (about 1.5–2 m). Fetter et al. also look at
photon emissions, concluding that gamma emissions will be indistinguishable from
background at a distance of about 3-6 m from 4 kg of plutonium and as far as 8 m
from 12 kg of HEU. Given that a working weapon will require a higher mass of either
of these nuclides the gamma and neutron emissions will be higher and the detection
radius correspondingly larger, but likely no more than double the distances noted
here.
Another challenge to interdicting an IND involves identifying the presence of
special nuclear materials (i.e. weapons-grade uranium and plutonium). In particular,
the identification of U-235 and Pu-239, while never easy, can be made more difficult
by the presence of other radionuclides. The most common interference comes from
Ra-226, which has a gamma energy nearly identical to that of U-235—close enough,
in fact, that it is not uncommon for any brand of RIID to mistakenly identify Ra-226
as “SNM” or as U-235.
While this sort of mistaken identity can be annoying, there are other means by
which isotopes can be purposely masked in order to hide their identify. In a technical document, for example, the US Government Accountability Office notes that
the presence of SNM can be concealed using “masking” radionuclides, making the
detection and identification of highly enriched uranium, weapons grade plutonium,
and other nuclides of interest more difficult [9]. However, it should also be noted
that, while it is possible to mask the presence of nuclear materials, doing so requires
the use of sufficient quantities of licensed radioactive materials. Thus, a group trying
to mask a nuclear device would have to do so by either stealing or obtaining a license
to purchase other radionuclides. This might be an effective stratagem, but it also adds
time, complexity, and the chance of being caught to a plot that is likely already fairly
complex.
14.3 Instruments and Their Maintenance
Radiation instruments are a crucial part of any interdiction efforts, but they are even
more important in nuclear interdiction than for radiological. One reason for this is the
low radiation “signature” mentioned earlier in this chapter; detecting and identifying
a weak radiation signal is far more difficult than doing so for a strong one. Adding
to the difficulty is the relatively low energy of the majority of the gamma radiation
emitted by uranium and plutonium and the difficulty of surveying for both these
gammas as well as challenges associated with neutron detection.
14 Difficulties of Radiological and Nuclear Interdiction
ray-induced reaction that releases a neutron. According to Ogawa [8] a 4-kg mass
of weapons-grade plutonium (containing no more than 6% Pu-240 by weight) will
emit about 400,000 neutrons per second and will have a detection distance of about
12 m when depending on gamma detection and 25 m when identifying the device
using neutrons. Fetter et al. [4] also note that highly enriched uranium emits far fewer
neutrons than Pu-239; 12 kg of HEU will emit about 1400 neutrons per second with
a correspondingly lower detection radius (about 1.5–2 m). Fetter et al. also look at
photon emissions, concluding that gamma emissions will be indistinguishable from
background at a distance of about 3-6 m from 4 kg of plutonium and as far as 8 m
from 12 kg of HEU. Given that a working weapon will require a higher mass of either
of these nuclides the gamma and neutron emissions will be higher and the detection
radius correspondingly larger, but likely no more than double the distances noted
here.
Another challenge to interdicting an IND involves identifying the presence of
special nuclear materials (i.e. weapons-grade uranium and plutonium). In particular,
the identification of U-235 and Pu-239, while never easy, can be made more difficult
by the presence of other radionuclides. The most common interference comes from
Ra-226, which has a gamma energy nearly identical to that of U-235—close enough,
in fact, that it is not uncommon for any brand of RIID to mistakenly identify Ra-226
as “SNM” or as U-235.
While this sort of mistaken identity can be annoying, there are other means by
which isotopes can be purposely masked in order to hide their identify. In a technical document, for example, the US Government Accountability Office notes that
the presence of SNM can be concealed using “masking” radionuclides, making the
detection and identification of highly enriched uranium, weapons grade plutonium,
and other nuclides of interest more difficult [9]. However, it should also be noted
that, while it is possible to mask the presence of nuclear materials, doing so requires
the use of sufficient quantities of licensed radioactive materials. Thus, a group trying
to mask a nuclear device would have to do so by either stealing or obtaining a license
to purchase other radionuclides. This might be an effective stratagem, but it also adds
time, complexity, and the chance of being caught to a plot that is likely already fairly
complex.
14.3 Instruments and Their Maintenance
Radiation instruments are a crucial part of any interdiction efforts, but they are even
more important in nuclear interdiction than for radiological. One reason for this is the
low radiation “signature” mentioned earlier in this chapter; detecting and identifying
a weak radiation signal is far more difficult than doing so for a strong one. Adding
to the difficulty is the relatively low energy of the majority of the gamma radiation
emitted by uranium and plutonium and the difficulty of surveying for both these
gammas as well as challenges associated with neutron detection.