TOPICS
2.5 Nuclear fission
The fission of uranium-235
From the energy scale in Figure 2.1 it is clear that large amounts of energy are released upon the fission of very heavy nuclei. The action of thermal neutrons on23592U results in a reaction of the general type shown in equation 2.15 where the fission process is variable; Figure 2.5 shows a schematic representation of the process. Reaction 2.16 gives a typical example; once formed, yttrium-95 and iodine-138
decay by b-particle emission with half-lives of 10.3 min and 6.5 s respectively.
235 92Uþ 10n
slow
"fission productsþ x10n
fast
þ energy ð2:15Þ
235
92Uþ10n"9539Yþ13853Iþ 310n ð2:16Þ
A particular reaction path during nuclear fission is called a reaction channel, and the yields of different nuclei in the fission of 23592U indicate that it is more favourable to form two isotopes lying in the approximate mass ranges 100 to 90 and 134 to 144, than two nuclides with masses <90 and >144, or >100 and <134. Equation 2.16 illustrates the general point that the sum of the mass numbers of the two fission pro-ducts plus the neutrons must equal 236. The average number of neutrons released per nucleus undergoing fission is 2.5 and the energy liberated (2 1010kJ mol1of23592U) is about two million times that obtained by burning an equal mass of coal. Since each neutron can initiate another nuclear reac-tion, a branching chain reaction (Figure 2.6) is possible. If this involves a quantity of23592U larger than a certain critical mass, a violent explosion occurs, liberating enormous amounts of energy. This is the principle behind fission-type nuclear Fig. 2.5 A schematic representation of the collision of a thermal neutron with a heavy nuclide leading to fission into two
nuclides of lower mass numbers and the release of (in this case) three neutrons. The fission is accompanied by the release of large amounts of energy. [Redrawn from P. Fenwick (1990) Reprocessing and the Environment, Hobsons, Cambridge.]
Fig. 2.6 A representation of a branched chain reaction in which each step of the reaction produces two neutrons, each of which can initiate the fission of a23592U nuclide. If left uncontrolled, such a chain reaction would lead to a violently explosive situation.
bombs and illustrates that extreme precautions are required when handling23592U on an industrial scale.
Worked example 2.3 Balancing nuclear equations
Identify the second nuclide formed in the fission reaction:
235
92Uþ10n"10342Moþ ? þ 210n
The reaction must proceed with conservation of mass number and of charge. The mass numbers are denoted by the superscripts, and the charges by the subscripts (i.e. the number of protons).
Let the unknown product beAZE.
Z¼ 92 42 ¼ 50
A¼ 235 þ 1 103 2 ¼ 131
The value of Z identifies the element as Sn (see the periodic table inside the front cover of the book).
The nuclide is13150Sn.
Self-study exercises
1. Identify the second nuclide formed in the reaction:
235
92Uþ10n"9236Krþ ? þ 210n [Ans:14256Ba ]
APPLICATIONS
Box 2.1 Electricity from nuclear power
Nuclear power is now used in a number of countries as a source of electrical power. The fuel in all commercial nuclear reactors is uranium, but of naturally occurring uranium only 0.7% is 23592U, the radionuclide required for the fission process. Enrichment of the uranium is usually carried out but, even then,23592U constitutes only a few per cent of the
uranium used as the fuel source. Using nuclear power on a commercial basis is a controversial issue; the public is made very aware of the problems involved in disposing of nuclear waste. The chart below shows the world production of nuclear electric power in 2000.
[Data: Department of Energy, Energy Information Administration, International Energy Annual 2000.]
Chapter 2 . Nuclear fission 59
2. Identify the second nuclide formed in the reaction:
235
92Uþ10n"14155Csþ ? þ 210n [Ans:9337Rb ]
The production of energy by nuclear fission
Nuclear fission can be successfully harnessed for the produc-tion of nuclear energy (see Box 2.1). This source of energy does not contribute to atmospheric pollution in the sense that there are no emissions of the gaseous oxides of carbon, nitrogen and sulfur associated with fossil fuels. Disadvantages of nuclear power include the problems of disposing of radio-active isotopes generated as fission products, and the risks involved if a nuclear reactor ‘goes critical’.
The production of energy by nuclear fission in a nuclear reactor must be a controlled process. Neutrons released from the fission of 23592U lose most of their kinetic energy by passage through a moderator (graphite or D2O). They then undergo one of two nuclear reactions. The first is capture by 23592U leading to further fission; the second is capture by 23892U (scheme 2.17). Such isotope production is called breeding.
238
92Uþ10n"23992Uþ g
239
92Ub "
239
93Npb "
239 94Pu
)
ð2:17Þ
The occurrence of a potentially catastrophic branching chain reaction is prevented by controlling the neutron concentra-tion in the nuclear reactor by inserting boron-containing
RESOURCES, ENVIRONMENTAL AND BIOLOGICAL Box 2.2 The disaster at Chernobyl
The name of Chernobyl (near Kiev, Ukraine) became known throughout the world on 26 April 1986 when reactor number 4 exploded. The power in the nuclear reactor is estimated to have increased from 200 MW to 3800 MW (MW ¼ megawatt) in 2.5 s, and it took only another 1.5 s for the power to reach 120 its normal value. Energy well in excess of that required to melt the fuel in the reactor was generated within a mere 20 s. In the ensuing explosion, the reactor lid weighing 106kg was blown off, allowing radioactive material to escape into the atmosphere, where prevailing winds carried it to Scandinavia within a couple of days, and eastwards towards Japan over the following week. The release of radioactive material was exacerbated by graphite fires that started in the reactor and continued to burn for several days. It was about two weeks before the radiation levels from the reactor had been reduced to less dangerous levels.
Estimates of the total radiation released from the Chernobyl disaster vary but it may have been as great as
178 MCi; 1 Ci is roughly equal to the activity of 1 g of radium. Thirty-one people died on the night of the explosion from radiation or burns, and there were 200 known casualties from radiation sickness. In the longer term, Chernobyl has left the world with a number of long-lived radioisotopes distributed in the atmosphere. The main health risks come from13153I (t1
2¼ 8:02 days),13455Cs (t1
2¼ 2:06 yr) and
137 55Cs (t1
2¼ 30:2 yr). While the half-life of13153I is much shorter than those of13455Cs or13755Cs, it is easily taken up by the thyroid gland and may cause cancer. Exposure to13153I by people and animals in the few days after the disaster was unavoidable, and the graph below indicates how incidences of thyroid cancer in children in the Ukraine increased following the Chernobyl accident. The final death toll from Chernobyl remains an unknown statistic; one estimate is32 000, while other estimates are lower. In 1995 the World Health Organiza-tion (WHO) called for further research into the radiaOrganiza-tion effects to be carried out.
[Data from: I.A. Likhtarev et al. (1995), Nature, vol. 375, p. 365.]
Further reading
C.H. Atwood (1988) Journal of Chemical Education, vol. 65, p. 1037 – ‘Chernobyl: What happened?’
I.A. Likhtarev et al. (1995) Nature, vol. 375, p. 365 –
‘Thyroid cancer in the Ukraine’.
steel, boron carbide or cadmium control rods. The choice of material follows from the high cross-section for neutron capture exhibited by105B and11348Cd.
Nuclear reprocessing
Eventually, the23592U fuel in a nuclear reactor becomes spent, and, rather than being disposed of, it is reprocessed. This both recovers uranium and separates23592U from the fission products. Short-lived radioactive products are initially allowed to decay while the spent fuel is retained in pond storage; after this period, the uranium is converted into the soluble salt [UO2][NO3]2 (see Box 6.3). In the series of reactions 2.18–2.21, the nitrate is converted into UF6.
½UO2½NO32 Hydrated salt
570 K"UO3þ NO þ NO2þ O2 ð2:18Þ
UO3þ H2970 K"UO2þ H2O ð2:19Þ
UO2þ 4HF "UF4þ 2H2O ð2:20Þ
UF4þ F2720 K"UF6 ð2:21Þ
At this stage, the UF6contains both23592U and23892U. Applica-tion of Graham’s law of effusion:
Rate of effusion/ 1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Molecular mass p
shows that23592UF6can be separated from23892UF6by subject-ing them to a centrifugal force; molecules of the two iso-topically labelled compounds move to the outer wall of
their container at different rates. The result is the isolation of23592U-enriched UF6. After this process, the hexafluoride is converted back to uranium-235 metal, thereby regenerating fuel for reuse in the nuclear reactor.