Chapter 3
Producing Fissile Material
The vast majority of the manpower and funding involved in the Manhattan Project
were devoted to producing fissile material. Uranium-235 had to be separated from
natural uranium, and plutonium had to be synthesized in nuclear reactors. In this
chapter we examine some of the physics behind these processes. Historically, the
first major step along these lines occurred when Enrico Fermi and his collaborators
achieved the first operation of a self-sustaining chain-reaction on December 2, 1942,
with their CP-1 (“Critical Pile 1” or “Chicago Pile 1”) reactor. This proved that a
chain-reaction could be created and controlled, and opened the door to the design and
development of large-scale plutonium-production reactors located at Hanford, WA.
We thus look first at issues of reactor criticality (Sects. 3.1 and 3.2), and then examine
plutonium production (Sect. 3.3). Sections 3.4 and 3.5 are devoted to analyzing
techniques for enriching uranium.
3.1 Reactor Criticality
The key quantifier in achieving a self-sustaining chain reaction is what is known as
the “criticality factor” or “reproduction factor,” designated as k. This dimensionless
number is defined in such a way that if k > 1, then a reaction will be self-sustaining,
whereas if k < 1, the reaction will eventually die out. In fact, if k > 1, the reaction
rate will grow exponentially; reactors are equipped with control mechanisms that
can be adjusted to maintain k = 1. k is analogous to the secondary neutron number
ν that was involved in the discussion of critical mass and efficiency in the preceding
chapter. k is also known as the “reproduction constant”, although this terminology
is somewhat of a misnomer.
Achieving a chain reaction with uranium of natural isotopic composition involves
several competing factors. The small fraction of
235 U present is inherently extremely
fissile when bombarded by slow neutrons, and, for each neutron consumed in
© 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_3
119
Producing Fissile Material
The vast majority of the manpower and funding involved in the Manhattan Project
were devoted to producing fissile material. Uranium-235 had to be separated from
natural uranium, and plutonium had to be synthesized in nuclear reactors. In this
chapter we examine some of the physics behind these processes. Historically, the
first major step along these lines occurred when Enrico Fermi and his collaborators
achieved the first operation of a self-sustaining chain-reaction on December 2, 1942,
with their CP-1 (“Critical Pile 1” or “Chicago Pile 1”) reactor. This proved that a
chain-reaction could be created and controlled, and opened the door to the design and
development of large-scale plutonium-production reactors located at Hanford, WA.
We thus look first at issues of reactor criticality (Sects. 3.1 and 3.2), and then examine
plutonium production (Sect. 3.3). Sections 3.4 and 3.5 are devoted to analyzing
techniques for enriching uranium.
3.1 Reactor Criticality
The key quantifier in achieving a self-sustaining chain reaction is what is known as
the “criticality factor” or “reproduction factor,” designated as k. This dimensionless
number is defined in such a way that if k > 1, then a reaction will be self-sustaining,
whereas if k < 1, the reaction will eventually die out. In fact, if k > 1, the reaction
rate will grow exponentially; reactors are equipped with control mechanisms that
can be adjusted to maintain k = 1. k is analogous to the secondary neutron number
ν that was involved in the discussion of critical mass and efficiency in the preceding
chapter. k is also known as the “reproduction constant”, although this terminology
is somewhat of a misnomer.
Achieving a chain reaction with uranium of natural isotopic composition involves
several competing factors. The small fraction of
235 U present is inherently extremely
fissile when bombarded by slow neutrons, and, for each neutron consumed in
© 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_3
119