INTRODUCTION
Before addressing main topics of the present textbook, i.e. ecological and nuclear non-proliferation problems of nuclear fuel cycles, it is necessary for readers to get some acquaintance with basic conceptions of nuclear power industry.
Peaceful as well as military applications of nuclear energy are based on various nuclear technologies dealing with nuclear materials (NM). Nuclear materials are those substances without which it is impossible to actuate the following two self-sustaining nuclear reactions accompanied by release of huge energy amounts:
1. Chain fission reaction of heavy nuclei.
For example, neutron-induced fission of isotope 235U results in production of two (in very rare cases, three) fission products (FP), in emission of 2.5 fission neutrons (in average) which can continue the chain fission reaction, and in intense generation of thermal energy (about 200 MeV per one fission).
235U + n → FP1 + FP2 + (2-3)n + 200 MeV.
That is why nuclear materials include all uranium and thorium isotopes (natural NM) and isotopes of artificial transuranium isotopes (mainly isotopes of plutonium, neptunium, americium and curium). Also, nuclear materials include highly radioactive artificial uranium isotope 233U (half-life T1/2 = 1,6·105 years), which can be produced by neutron irradiation of natural thorium.
2. Thermonuclear fusion reaction of light nuclei.
For example, fusion reaction of light hydrogen isotopes, namely reaction of deuterium with tritium is able to produce stable helium, highenergy neutrons and about 21 MeV of thermal energy:
D + T → 4He + n + 21 MeV.
That is why nuclear materials include two hydrogen isotopes: deuterium and tritium. Abundance of stable deuterium in natural hydrogen is
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about 0,015%. Natural hydrogen does not contain its heavier isotope (tritium) because of its rapid radioactive decay (Т1/2 = 12,3 years). Lith-
ium is also regarded as a nuclear material because its light isotope 6Li can be used for intense production of tritium through 6Li(n,α)T reaction. Micro cross-section of 6Li(n,α)T reaction in thermal point (En =
0,025 eV) is sufficiently large (about 940 barns). Natural lithium contains 7,5% 6Li.
Thus, the following NM categories are under consideration now:
1.Initial NM – natural uranium and natural thorium, d epleted uranium, i.e. uranium with reduced content of 235U.
2.Special NM – enriched uranium, i.e. uranium with in creased content of 235U, plutonium with any isotope composition and artificial uranium isotope 233U.
3.Transuranium elements (Np, Am, Cm, Bk, Cf).
4.Deuterium, tritium, lithium and heavy water.
The first three NM categories are related with nuclear power based on fission reactions of heavy isotopes while the fourth NM category is related with fusion reactions of light isotopes. As thermonuclear power facilities are not built and put in operation yet, main attention in the textbook is given to nuclear technologies dealing with the first three NM categories.
Nuclear technologies include the procedures intended for NM production, storing, applications, transportation, reprocessing for repeat usage of secondary NM or ultimate disposal of technological wastes.
For a long time the world public expresses deep concerns about links between nuclear technologies and safe vital activity of the humankind. Therefore, the textbook gives the largest attention to analysis of these links. The term “safety” should be interpreted here in a wide sense including radiation safety, nuclear safety, non-proliferation safety (or security) and ecological safety.
The term ”radiation safety” means a sufficient protection against the striking effects caused by direct exposure to any type of ionizing radiations.
The term ”nuclear safety” means an inadmissibility for the selfsustaining uncontrolled chain fission reaction to initiate and propagate. Serious violations of the nuclear safety requirements can lead to a nu-
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clear explosion, thermal explosion or, at least, to the flash of ionizing radiation and over-exposure of operation staff members.
The term ”non-proliferation safety (or security)” means a sufficient NM protection against their thefts or diversion for manufacturing of nuclear explosive devices or radiological weapons. Presently, the IAEA experts propose to use the term “nuclear secur ity” for designation of this nuclear non-proliferation aspect that differs in principle from the aforementioned term “nuclear safety”.
The term “ecological safety” means an inadmissibility of unacceptably strong negative impact of nuclear technologies on the environment including radiation, chemical and thermal effects. One else negative effect is related with a necessity to alienate large territories for safe operation of some nuclear facilities.
Nuclear fuel is a nuclear material containing nuclides which can be split (fissioned) by neutrons. The following NM can be regarded as fissionable nuclides:
1.Natural uranium and thorium isotopes.
2.Artificial plutonium isotopes (products of consecutive neutron captures beginning from 238U).
3.Isotopes of artificial transuranium elements (Np, Am, Cm and so on).
4.Artificial uranium isotope 233U (product of neutron capture by 232Th).
As a rule, uranium, thorium and plutonium isotopes with even mass numbers (“even” nuclides 238U, 232Th, 240Pu, 242Pu) can be fissioned only
by high-energy neutrons (energy thresholds for neutron-induced fission reactions of these nuclides cover the range from 1 MeV to 1.5 MeV).
On the contrary, uranium and plutonium isotopes with odd mass numbers (”odd” nuclides 233U, 235U, 239Pu, 241Pu) can be fissioned by neu-
trons with any energy values including thermal neutrons. Moreover, the lower neutron energy, the more intense fission reaction can occur.
Energy spectrum of fission neutrons is a fast neutron spectrum with mean energy about 2.1 MeV. Besides, these fast neutrons undergo intense slowing down, and their energies sharply drop down below the threshold levels for fission reactions of even nuclides. This means that it is very difficult to maintain the chain fission reaction by even nuclides only because a small fraction of fission neutrons has the energies high enough to overcome the threshold levels. At the same time, it is desirable and quite possible to slow down fission neutrons to thermal
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energies and, thus, provide the best conditions for initiation and propagation of the chain fission reaction on odd uranium and plutonium nuclides.
Nuclear fuel containing only natural fissionable nuclides (235U, 238U, 232Th) is named as a primary nuclear fuel. Nuclear fuel containing artificial fissionable nuclides (233U, 239Pu, 241Pu) is named as a secondary
nuclear fuel.
Natural fissionable nuclides 238U and 232Th are of little use as a nuclear fuel because they can be fissioned by fast neutrons only. However, these nuclides can be used to produce artificial well-fissionable (or fissile) nuclides 239Pu and 233U, respectively, i.e. for reproduction (or breeding) of secondary nuclear fuel. That is why these nuclides are often named fertile nuclides.
The present nuclear energy systems are based upon the use of natural uranium containing the following three isotopes:
1. 238U; natural abundance – 99,28%; half-life Т1/2 = 4,5×109 years; 2. 235U; natural abundance – 0,71%; half-life Т1/2 = 7,1×108 years; 3. 234U; natural abundance – 0,0054%; half-life Т1/2 = 2,5×105 years.
By the way, the Earth’s age (approximately 10 billion years) is comparable with 238U half-life.
It is interesting to note here that 234U is a member of 238U decay family: 234U is produced by a-decay of 238U and two consecutive, relatively rapid b-decays of intermediate nuclides:
238U(a,Т1/2=4,5×109 years)234Th(b,Т1/2=24 d)234Pa(b,Т1/2=6,7 h)234U
All uranium isotopes are radioactive materials. They can emit a- particles whose energies cover the range 4,5÷4,8 Me V and undergo spontaneous fission followed by neutron emission: for example, 238U emits ~13 n/(s·kg).
Uranium isotope 235U is the only natural nuclear material which can be fissioned by neutrons of any energy including thermal neutrons (the lower neutron energy, the better fissionability of 235U) with emission of excessive fast neutrons. Just thanks to these fission neutrons it becomes possible for the chain fission reaction to initiate. Unfortunately, natural uranium contains a rather small fraction of 235U (~0,71%). The overwhelming majority of nuclear power reactors in operation now applies
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enriched uranium, i.e. uranium containing 2-5% 235U instead of 0,71% 235U in natural uranium. Some research reactors still use uranium enriched with 235U up to 90% and above. Currently, the IAEA insistently recommends the states-participants to arrange gradual transfer of their research reactors on the use of uranium fuel containing below 20% 235U. Critical mass of 20%-uranium is equal to ~830 kg. Successful theft of so large uranium mass and manufacturing of a primitive but transportable nuclear explosive device is quite unlikely feasible.
Enriched uranium contains relatively larger 235U quantity than 235U abundance in natural uranium. There are the following categories of enriched uranium depending on 235U content (X5):
1.Low-enriched uranium with X5 below 5%.
2.Middle-enriched uranium with X5 from 5% to 20%.
3.Highly-enriched uranium with X5 from 20% to 90%.
4.Weapon-grade uranium with X5 above 90%.
Depleted uranium (X5 < 0,71%) is a by-product of the uranium enriching process. Contemporary technologies of uranium isotope enrichment can produce depleted uranium with 235U content at the level of 0,2- 0,3%.
When capturing neutron, main uranium isotope 238U transforms into secondary nuclear fuel, namely fissile plutonium isotope 239Pu, after two consecutive β-decays of intermediate nuclides:
238U(n,γ)239U(β,Т1/2=23,5´)239Np(β,Т1/2=2,3 d)239Pu.
Similarly, fissile uranium isotope 233U can be produced by neutron irradiation of natural thorium. When capturing neutron, the only longlived thorium isotope 232Th transforms into secondary nuclear fuel, namely fissile uranium isotope 233U, after two consecutive β-decays of intermediate nuclides:
232Th(n,γ)233Th(β,Т1/2=23,3´)233Pa(β,Т1/2=27,4 d)233U.
However, these conversions of natural fertile isotopes (238U, 232Th) into secondary nuclear fuel isotopes (239Pu, 233U) require that primary nuclear fuel, i.e. fissile uranium isotope 235U, must be placed into the reactor core in such a quantity which makes it possible to initiate the
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