Chapter 4
Complicating Factors
A number of complications can thwart the proper functioning of nuclear weapons.
In this chapter we explore several of these. In the Manhattan Project, graphite was
used as a moderating medium in plutonium-production reactors, and in Sect. 4.1 we
analyze the difficulties of operating such a reactor in the face of effects of the neutroncapturing properties of graphite itself and any impurities that might be present. The
other two factors involve the problem that if stray neutrons should be present during
the brief time that one is assembling sub-critical pieces of fissile material to form
a supercritical core, one runs the risk that such neutrons could initiate a premature
detonation. Some of these “background” neutrons arise from spontaneous fissions
within the fissile material itself and are fundamentally uncontrollable, while others
arise from so-called (α, n) reactions due to the presence of light-element impurities
in the fissile material. The issue of spontaneous fission is taken up in Sects. 4.2
and 4.3, where the probability of a spontaneous fission-initiated “fizzle” and the
expected yield of a weapon in such a case are examined. The light-element issue,
where the creation of neutrons can be minimized (albeit with difficulty) is examined in
Sect. 4.4. Also, it is essential to provide a way of triggering a nuclear explosion once a
suitable assembly of fissile material has been achieved. In the Manhattan Project, this
was achieved with so-called “neutron initiators”; ironically, these devices functioned
via the same (α, n) reactions which had otherwise to be avoided. The physics and
production of initiator is explored in Sect. 4.5. Finally, some of the yield of the Trinity
bomb was due to high-energy neutrons inducing fission in the
238 U tamper shell of
that device. This effect is not in any sense a malfunction, but does complicate the
question of estimating a bomb’s yield. This is analyzed in Sect. 4.6.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
B. C. Reed, The Physics of the Manhattan Project,
https://doi.org/10.1007/978-3-030-61373-0_4
145
Complicating Factors
A number of complications can thwart the proper functioning of nuclear weapons.
In this chapter we explore several of these. In the Manhattan Project, graphite was
used as a moderating medium in plutonium-production reactors, and in Sect. 4.1 we
analyze the difficulties of operating such a reactor in the face of effects of the neutroncapturing properties of graphite itself and any impurities that might be present. The
other two factors involve the problem that if stray neutrons should be present during
the brief time that one is assembling sub-critical pieces of fissile material to form
a supercritical core, one runs the risk that such neutrons could initiate a premature
detonation. Some of these “background” neutrons arise from spontaneous fissions
within the fissile material itself and are fundamentally uncontrollable, while others
arise from so-called (α, n) reactions due to the presence of light-element impurities
in the fissile material. The issue of spontaneous fission is taken up in Sects. 4.2
and 4.3, where the probability of a spontaneous fission-initiated “fizzle” and the
expected yield of a weapon in such a case are examined. The light-element issue,
where the creation of neutrons can be minimized (albeit with difficulty) is examined in
Sect. 4.4. Also, it is essential to provide a way of triggering a nuclear explosion once a
suitable assembly of fissile material has been achieved. In the Manhattan Project, this
was achieved with so-called “neutron initiators”; ironically, these devices functioned
via the same (α, n) reactions which had otherwise to be avoided. The physics and
production of initiator is explored in Sect. 4.5. Finally, some of the yield of the Trinity
bomb was due to high-energy neutrons inducing fission in the
238 U tamper shell of
that device. This effect is not in any sense a malfunction, but does complicate the
question of estimating a bomb’s yield. This is analyzed in Sect. 4.6.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
B. C. Reed, The Physics of the Manhattan Project,
https://doi.org/10.1007/978-3-030-61373-0_4
145