Monitoring Nuclear Weapons and Nuclear-Explosive Materials An Assessment of Methods and Capabilities (2005) / Chapter Skim
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Appendix A Physics and Technology of Nuclear-Explosive Materials
Appendix A Physics and Technology of Nuclear-Explosive Materials
Pages 221-244
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From page 221... ...
(This property is essential to the operation of the thermal-neutron reactors that have accounted for most nuclear electricity generation, nuclear naval propulsion, and weapon plutonium production around the world.) The underlying physics is such that all fissile nuclides are also nuclear explosives, but not all nuclear-explosive nuclides are fissile; for example, the even-numbered isotopes of plutonium -- most importantly Pu-238, Pu-240, and Pu-242 -- are not fissile, but they are nuclear explosives.
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From page 222... ...
For delta-phase plutonium, the critical masses would be about 60 percent larger. In the case of Pu-239, neutron production is 22/kg-sec from spontaneous fission but 630/kgsec from alpha-n reactions.
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From page 223... ...
;1 · the rate of heat generation by radioactive decay; high rates of heat generation can accelerate deterioration and/or inter nal distortion of weapon components if the heat is not re moved by appropriate design. · the rate of neutron production by spontaneous fission and reactions with alpha particles emitted in radioactive decay; the emission of neutrons by these processes may pre initiate a chain reaction earlier in the process of assembling a nuclear weapon than is optimal.
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From page 224... ...
A nuclear weapon contains NEM stored in a subcritical configuration. Detonation of the weapon then requires that the NEM be rapidly assembled into a supercritical configuration, wherein the chain reaction grows almost instantaneously to explosive proportions.
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From page 225... ...
The critical mass can be made smaller than the bare-sphere value by surrounding the nuclear-explosive material with a "tamper" composed of materials that reflect neutrons. Note also that the implosion approach compresses the NEM to higher than normal density, thereby also reducing the critical mass.
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From page 226... ...
Fission weapons of more advanced design have involved a range of yields from a fraction of a kiloton to about 500 kilotons; thermonuclear weapons, combining fission and fusion processes may have yields extending into the multimegaton range. Production Technologies for NEM Here we review briefly what is entailed in producing the two most important classes of NEM, namely, highly enriched uranium and separated plutonium.
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From page 227... ...
Gaseous diffusion requires large factories containing complex piping arrangements and highly specialized membranes (the characteristics of which remain classified) , and utilizes immense 4Uranium also exists in seawater, at the concentration of about 3 parts per billion by weight.
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From page 228... ...
Gas centrifuge plants can be considerably smaller than gaseous diffusion plants, but they still need room for many hundreds or thousands of centrifuges, so concealment poses some challenges. The gaseous diffusion and centrifuge plants operated for civilian nuclear power enrich uranium only to the 3 to 5 percent U-235 level, unsuitable for nuclear weapons.
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From page 229... ...
. But high burnup is undesirable for production of weapon plutonium, both because it leads to greater accumulation of the less desirable even-numbered plutonium isotopes, since much of the desirable Pu-239 product is lost through fission, and because higher burnup means the spent fuel contains larger amounts of radioactive fission products in relation to the quantity of fuel handled, making it more dangerous and difficult to separate out the plutonium.
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From page 230... ...
Hence a 3,000 thermal-megawatt light-water reactor operating at full power for 330 days per year will discharge 220-270 kilograms of plutonium per year in its spent fuel.5 If such a reactor were operated instead for purposes of optimum production of weapon plutonium at much lower burnup, the net amount of weapon-grade plutonium per year produced in a reactor of given thermal power might be comparable to or somewhat larger than the reactor-grade yield in normal commercial operations but electric power production would be reduced. Reprocessing Spent Fuel to Extract Plutonium In order to recover the plutonium produced in a nuclear reactor from the spent fuel it must be chemically separated or reprocessed from the fission products produced, and from the residual U-238.
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From page 231... ...
A fuel assembly from a light-water reactor that had experienced a burnup level appropriate to weapon plutonium production but then cooled for just two years would deliver a dose rate at its surface of nearly 40,000 rem per hour. The approach to reprocessing that has been used virtually universally for military and civilian purposes alike -- called the "PUREX" process -- consists of chopping up the radioactive spent fuel into pieces, dissolving these in nitric acid, and then performing a set of solvent extractions on the resulting solution to separate the plutonium, the uranium, and the fission products into three output streams.
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From page 232... ...
None of these countries is thought to be producing HEU at this time. In the past, these countries produced enriched uranium at a range of enrichment above 20 percent not only for nuclear weapons but also for use in nuclear reactors for propulsion of submarines, other warships, and icebreakers; in research reactors; and in experimental power reactors.
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From page 233... ...
monitoring to assure it is not diverted to the weapons program. Except for these three Russian reactors, none of the military plutonium production reactors in the de jure nuclear weapon states is now operational, although some are mothballed and presumably could be reactivated after some delay.
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From page 234... ...
The operation of the world's civilian power reactors leads to the discharge of about 80 tons per year of plutonium embedded in 8,000 tons of spent nuclear fuel. This spent fuel, in which the plutonium is intimately mixed with intensely radioactive fission products and unfissioned U-235 and U-238, goes directly to spent fuel storage pools at the reactor sites.
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From page 235... ...
Plutonium stored in this form amounted to about 1,250 tons at the end of 2002. Although this material cannot be used in nuclear weapons unless it is first separated in a reprocessing plant, the stocks that exist in this form nonetheless need to be monitored in addition to monitoring reprocessing plants, to ensure that reprocessing of civilian spent fuel for weapon purposes is not occurring in undeclared facilities.
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From page 236... ...
(The remainder is 0.006 percent U-234, which can be neglected for our purposes here.) Enrichment levels for typical LEU power reactor fuels are 3-5 percent U-235 fuels, and the weapon-grade HEU preferred by bomb makers is 93 percent U235.
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From page 237... ...
The electric power requirements for uranium enrichment plants range from 100-150 kilowatt-hours per SWU in a centrifuge plants to 2,000-3,000 kilowatt-hours per SWU in gaseous diffusion plants to something like 4,000 kilowatt-hours per SWU for the noz
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From page 238... ...
electricity costs of 7 cents per kilowatt-hour, this is $2 million worth of electricity. This electricity requirement likewise means that a gaseous diffusion complex big enough to enrich the uranium for, say, a dozen of these hypothetical gun-type HEU bombs per year would require the full annual output of a 50 megawatt power plant (which is a size adequate to meet the needs of a town of 50,000 people)
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From page 239... ...
Nearly all of the reactors that have been built to date for electric power generation, as well as most of those that have been built for producing weapon material, rely primarily on the fissile uranium isotope U235 to sustain their fission chain reaction; and most of them do so by exploiting the especially high fission probability of U-235 when exposed to slow neutrons (i.e., those whose speeds are not much higher than those of neutrons in thermal equilibrium with their surroundings)
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From page 240... ...
About 85 percent of the world's power reactors are so-called lightwater reactors, in which ordinary water plays both roles. These require uranium fuel enriched to 3 to 5 percent in U-235 or similar concentrations of U-233 or Pu-239.
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From page 241... ...
The possibility of producing more plutonium than does a thermalneutron reactor arises because fissions induced by fast neutrons release, on the average, more neutrons per fission than fissions induced by slow neutrons, and these extra neutrons are potentially available for plutonium-producing absorption by U-238. Gas and liquid metals are the main possibilities for cooling fast reactors.
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From page 242... ...
The fission of one gram of uranium or plutonium leads to the deposition in the reactor of about 82 billion joules of fission energy, which corresponds to about 0.95 of a megawatt-day. Rounding this off to one megawatt-day of thermal energy release per gram of heavy nuclei fissioned gives a rule of thumb that is often used for making estimates of nuclear fuel consumption rates in reactors, based on their rated capacity and the fraction of the time that they achieve it.
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From page 243... ...
Using the rule of thumb of one gram of heavy nuclei fissioned per megawatt-day of thermal output indicates that a large power or production reactor rated at 3,000 thermal megawatts will fission about 3 kilograms of heavy nuclei per full-power day of operation. (Since the mass of the radioactive fission products is very nearly the same as the mass of the nuclei whose fission produced them, such a reactor generates about 3 kilograms per full-power day of radioactive fission products.)
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From page 244... ...
India: 2 heavy-water-moderated production reactors near Bombay. North Korea: a graphite-moderated, gas-cooled production reactor at Yonbyon.
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