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The facts required to judge the utility of reactor-grade
plutonium (R-Pu) for use in nuclear weapons were first made
widely available in 1993 by J. Carson Mark.(2) The isotopic
composition of reactor-grade plutonium as compared with
weapon-grade Pu (W-Pu), results in four differences between
R-Pu and W-Pu:

1. The "bare sphere" critical mass for R-Pu is about 13 kg,
vs. 10 kg for W-Pu (both alpha-phase metal of density 19.6
g/cc).

As regards the usability of R-Pu to make nuclear
weapons, the larger critical mass for R-Pu means that
about 30% more R-Pu metal is needed than W-Pu to build
a weapon.

2. The alpha-particle radioactivity of R-Pu contributes 10.5
watts of heat per kg R-Pu, vs. 2.3 W/kg for W-Pu.

The greater heat evolution (68 watts for half a
bare-sphere critical mass of R-Pu, vs. 11 watts for
half a bare-sphere critical mass of W-Pu) means that
the thick high-explosive that surrounds the plutonium
and any additional metal shells in a simple implosion
weapon will overheat if R-Pu is substituted for W-Pu.
Mark estimates the amount of aluminum heat conductor
that would suffice to cool the R-Pu. So-called
In-Flight Insertion devices that were used in early
nuclear weapons would allow adequate cooling of the
plutonium until it is inserted into the high explosive
a few minutes before detonation.

3. The continuing neutron emission from spontaneous fission
of Pu-240 contributes 360 neutrons per second per g of R-Pu,
vs. about 66 neutrons per second per g of W-Pu.

According to Mark, as the fissionable material is being
compressed so that it becomes critical, a neutron
injected at the worst possible time would cause the
earliest model of implosion weapon to have an explosive
yield between 1 and 2 kilotons (that is, between 1000
tons and 2000 tons of high explosive such as TNT)
rather than the full yield of some 20 kilotons when
neutron injection is optimally timed to occur near the
time of maximum criticality. In contrast, in 1972 the
U.S. Government officially revealed that the U.S.
possessed more advanced nuclear weapons whose yield
would not be diminished by the injection of a neutron
at no matter what instant of time. With this type of
design, the spontaneous neutrons from R-Pu would in no
way diminish the reliability or the expected yield.

4. A mass of R-Pu provides greater radiation exposure to a
person than does W-Pu. At a distance of 1 meter from an
unshielded 6-kg mass of each, the radiation field is 30
millirem per hour for R-Pu, vs. 5 millirem per hour for
W-Pu.

The greater external radiation from a weapon component
of R-Pu compared with that of W-Pu means that the dose
of 5 rem long deemed acceptable for a radiation worker
would be received in 160 hours one meter from a bare
core of R-Pu, vs. 1000 hours for a core of W-Pu.