130
3 Producing Fissile Material
= 180 MeV. Given that there are some 100 commercial reactors in operation in the
United States, we can infer the annual production of plutonium to be on the order of
20,000 kg, enough for more than 2000 Nagasaki-type bombs. However, commercialreactor fuel rods in the United States are not reprocessed, so the Pu created remains
locked up in them. Ironically,
239 Pu α-decays back to
235 U with a half-life of about
24,000 years; our distant descendants will find a fresh supply of “enriched” fuel rods
awaiting them! A reactor which produces one gigawatt of electricity at η = 0.3 but
fueled with natural uranium (F = 0.0072, such as used in the Canadian CANDU
system) will produce some 920 kg of Pu per year.
Fuel rods typically remain in commercial reactors for months or years. A result of
this is that some of the
239 Pu nuclei that are formed have time to capture neutrons to
become nuclei of
240 Pu. As we explore in Sect. 4.2, this isotope is characterized by
an extremely high spontaneous fission rate, a situation which presents a dangerous
challenge for anyone who seeks to construct a nuclear weapon from such spent fuel.
An excellent treatment of issues in civilian nuclear power generation appears in
Garwin and Charpak (2001).
For the present purposes, our interest is with the Hanford reactors, which were
fueled with natural uranium and operated at a thermal power P t = 250 MW. For
these figures, (3.24) gives a production rate of about 0.76 grams per MW per day, or
190 gr day
−1 . Three reactors operating at this power would produce 570 gr day
−1 .
To synthesize enough Pu to construct a bomb core of 6 kg would therefore require
about 11 days of steady-state operation. Fuel slugs were left in the Hanford reactors
for typically 100 days of neutron bombardment; after being withdrawn, they had to
be cooled, and time allowed for dangerous short-lived fission products to decay. A
discussion of the design of these reactors appears in Weinberg (2002).
The reaction rate Eq. (3.20), cast in terms of the mass of fuel in a reactor, will be
used again in Sect. 4.5 to estimate the rate of production of polonium in a reactor via
neutron bombardment of bismuth. Polonium was used in Manhattan Project bombs
in neutron-emitting “initiators” used to trigger the nuclear chain reactions in the Little
Boy and Fat Man bombs.
3.4 Electromagnetic Separation of Isotopes
The Manhattan Project’s Oak Ridge, Tennessee, facility was devoted to enriching
uranium for use in the Little Boy bomb. Three separate techniques were involved
in this effort: (i) Electromagnetic separation; (ii) Gaseous (barrier) diffusion, and
(iii) Liquid thermal diffusion. The first two of these can be examined on the basis
of undergraduate-level physics, and are so treated in this and the following sections.
The physics of liquid thermal diffusion is extremely complex, however, so we do
not consider that process further. Readers interested in the technical details of liquid
thermal diffusion are urged to consult the classic paper “The Separation of Isotopes
by Thermal Diffusion” by Jones and Furry (1946). A qualitative description of the
use of thermal diffusion in the Manhattan Project can be found in Reed (2011).
3 Producing Fissile Material
= 180 MeV. Given that there are some 100 commercial reactors in operation in the
United States, we can infer the annual production of plutonium to be on the order of
20,000 kg, enough for more than 2000 Nagasaki-type bombs. However, commercialreactor fuel rods in the United States are not reprocessed, so the Pu created remains
locked up in them. Ironically,
239 Pu α-decays back to
235 U with a half-life of about
24,000 years; our distant descendants will find a fresh supply of “enriched” fuel rods
awaiting them! A reactor which produces one gigawatt of electricity at η = 0.3 but
fueled with natural uranium (F = 0.0072, such as used in the Canadian CANDU
system) will produce some 920 kg of Pu per year.
Fuel rods typically remain in commercial reactors for months or years. A result of
this is that some of the
239 Pu nuclei that are formed have time to capture neutrons to
become nuclei of
240 Pu. As we explore in Sect. 4.2, this isotope is characterized by
an extremely high spontaneous fission rate, a situation which presents a dangerous
challenge for anyone who seeks to construct a nuclear weapon from such spent fuel.
An excellent treatment of issues in civilian nuclear power generation appears in
Garwin and Charpak (2001).
For the present purposes, our interest is with the Hanford reactors, which were
fueled with natural uranium and operated at a thermal power P t = 250 MW. For
these figures, (3.24) gives a production rate of about 0.76 grams per MW per day, or
190 gr day
−1 . Three reactors operating at this power would produce 570 gr day
−1 .
To synthesize enough Pu to construct a bomb core of 6 kg would therefore require
about 11 days of steady-state operation. Fuel slugs were left in the Hanford reactors
for typically 100 days of neutron bombardment; after being withdrawn, they had to
be cooled, and time allowed for dangerous short-lived fission products to decay. A
discussion of the design of these reactors appears in Weinberg (2002).
The reaction rate Eq. (3.20), cast in terms of the mass of fuel in a reactor, will be
used again in Sect. 4.5 to estimate the rate of production of polonium in a reactor via
neutron bombardment of bismuth. Polonium was used in Manhattan Project bombs
in neutron-emitting “initiators” used to trigger the nuclear chain reactions in the Little
Boy and Fat Man bombs.
3.4 Electromagnetic Separation of Isotopes
The Manhattan Project’s Oak Ridge, Tennessee, facility was devoted to enriching
uranium for use in the Little Boy bomb. Three separate techniques were involved
in this effort: (i) Electromagnetic separation; (ii) Gaseous (barrier) diffusion, and
(iii) Liquid thermal diffusion. The first two of these can be examined on the basis
of undergraduate-level physics, and are so treated in this and the following sections.
The physics of liquid thermal diffusion is extremely complex, however, so we do
not consider that process further. Readers interested in the technical details of liquid
thermal diffusion are urged to consult the classic paper “The Separation of Isotopes
by Thermal Diffusion” by Jones and Furry (1946). A qualitative description of the
use of thermal diffusion in the Manhattan Project can be found in Reed (2011).