The first nuclear chain reaction occurred naturally about two billion years ago in Gabon, Africa, where a uranium deposit moderated by water spontaneously became critical. In 1942 as a result of the war efforts, a sustained chain reaction was achieved by E. Fermi in Chicago working on the Manhattan Project which eventually led to the atomic bomb.
Of the naturally occurring isotopes, only235U is fissionable with a neutron absorption cross section (s) of 582 barns with thermal neutrons at 0.025 eV. This cross section decreases as the energy of the neutrons increase (s/E½), and in the MeV range,sis about 2 barns.238U reacts with neutrons (s~ 01/4 for E <0.9 MeV) to form plutonium (Pu) by the reaction sequence:
238
92U nð ; gÞ 239 92U!b
½23:5 min 239
91Np!b
½2:33 days 239
90Pu (7.20)
Table 7.4 The approximate values of RBE for various types of radiation
Radiation RBE
b,g 1
a, p 10
n (Thermal) 5
n (Fast, E<40 MeV) 10
Heavy ions (cosmic ray) 20
Table 7.5 Units of radiation 1 Curie (Ci)¼3.71010disintegrations/sa 1 Becquerel (Bq)¼1 disintegration/s 1 Gray (Gy)¼100 Rads¼1 J/kg 1 Sievert (Sv)¼100 Rem
aThe activity of 1 g of pure radium
Table 7.6 Radiation damage
for the whole body exposure 0–25 Rem No clinical effects
25–50 Rem Decrease in white cells in blood 100–200 Rem Nausea, less white blood cells
500 Rem LD50
Table 7.7 The naturally occurring radioactive isotopes in a 70-kg human body
Source
Amount of isotope (g)
Number of atoms
Total activity (Bq)
Particle energy
MeV g
3
1H 8.41015 1.7109 3 0.018
14
6 C 1.9108 8.11014 3.1103 0.158 40
19K 8.3102 1.21021 4107 1.36 1.46
Plutonium is radioactive witht½ ¼2.4104years and is fissionable like235U. It is also possible to convert 232Th to fissionable 233U which has a half-life of 1.63105 years by the reaction sequence.
232
94Th nð ;gÞ !b 233
93Pa !b 233 92U
½23:6 min ½27 day
(7.21)
The process of fission in235U proceeds by the absorption of a thermal neutron and formation of the compound nucleus 236U in an excited state with about 6.5 MeV. During the fission process, the compound nucleus is distorted and splits into two fission fragments which by virtue of coulombic repulsion achieve a kinetic energy equivalent to about 80–90% of the fission energy (200 MeV).
The fragment nuclei decay by neutron and gemission leading to an average of 2.42 neutrons released for each neutron captured. The stabilization of the fragments and the radiation results in the thermal energy produced by the chain reaction.
The neutron flux can also be controlled by control rods made of cadmium or boron which are strong absorbers of neutrons and which are raised or lowered into the reactor to maintain the chain reaction at the appropriate level.
The enrichment of 235
92 Ufrom natural uranium was first performed by the differential diffusion of gaseous UF6through porous barriers. This very energy-intensive method was replaced by the simpler gas centrifuge. With the development of the tunable laser, it became possible to photochemically excite one isomeric uranium atom by virtue of its hyperfine splitting of the isotopic lines and thus enrich one isotope. Though the cost of the enrichment process has thus been greatly reduced,235U is still a very expensive fuel.
The yield of the fusion products as a function of the atomic mass is shown in Fig.7.4and indicates that the major elements formed are in the range A¼85–105 and 130–150. The major elements formed and their fission yields are listed in Table7.9.
Reactors can be classified in terms of the fuel, (235U, 233U, 239Pu) and its containment alloy, zircaloy, the moderator which is needed to slow down neutrons, and the heat exchanger used to generate steam. The most common type of classification is in terms of the coolant which can be either a liquid or gas.
The four categories are (1) the light water reactor (LWR) of which are 81% of all reactors, (2) gas- cooled reactors (GR) make up 11%, (3) 7% use heavy water (HWR), and (4) the remaining reactors use molten metals (such as sodium). (See Fig.7.5c.)
Table 7.8 Yearly radiation
exposure Natural radiation
Cosmic rays sea level at 2,500 m 40 mRem
(Banff AB) 90 mRem
Terrestrial exposure due to radium and other isotopes in ground and buildings
40 mRem Internal radiation mainly due to potassium 40 18 mRem Cosmic radiation during 10,000-km flight at 10-km altitude 4 mRem Man-made radiation
1 Chest X-ray 40 mRem
1 Dental X-ray 20 mRem
Fallout 3 mRem
Misc.—TV, monitors, etc. 2 mRem
7.6 Nuclear Reactors 115
Successful operation of current light water reactors and implementation of advanced nuclear energy systems is strongly dependent on the performance of fuels and materials. A typical light water reactor (LWR) contains numerous types of materials that all must perform successfully. A majority of the LWRs in the USA are extending their operating licenses from a 40-year period to a 60- year period, with initial discussions about 80-year lifetimes now underway. Many proposed advanced systems (also known as Generation IV systems) anticipate operation at temperature and radiation exposures that are beyond current nuclear industry experience, as well as most previous experience with developmental systems. Table 7.10 summarizes the expected environments during normal operation for the six Generation IV systems. For comparison, the operating conditions for a pressurized water reactor (PWR is a type of light water reactor) are also listed. The Generation IV systems are expected to operate at high temperatures and in some cases with coolants that present more challenging corrosion problems than current LWRs. Generation IV systems are expected to operate for at least 60 years.
Fig. 7.4 The distribution of fission products from the reaction of235
92Uwith slow neutrons
The LWR is further classified into the pressurized water reactor (PWR) which operates at about 150 atm and 318C with a thermal efficiency of about 34%. The other type of reactor is the boiling water reactor (BWR) which operates at 70 atm pressure and 278C with a thermal efficiency of 33%.
These reactors require fuel with enriched235U to about 3% to have a sufficient neutron flux for the chain reaction. The fuel, as UO2, is in the form of pellets enclosed in a zirconium alloy, Zircaloy-2.
The gas-cooled reactors were developed in Great Britain using CO2as the gas coolant and graphite as the moderator with natural uranium metal as the fuel. The thermal efficiency is about 25%. With uranium enriched to 2.2% as the oxide (UO2) fuel, the thermal efficiency increased to 41% and is called the advanced gas reactor (AGR). Helium is also used in a high-temperature version.
The heavy water reactor was developed in Canada and is known as the CANDU reactor. The D2O is used as both coolant and moderator. The relative moderating efficiency of various materials is given in Table7.11. Because of the superior moderating property of D2O, it is possible to use natural uranium as the fuel in the form of UO2pellets in zircaloy tubes. This makes the CANDU one of the best designed reactors in the world. The coolant cycle and the moderator are separate flow circuits shown in Fig.7.6. The fuel elements in the pressure tubes and the D2O flow is shown in Fig.7.7where the coolant is at about 293C and 100 atm pressure. The moderator is at lower temperature. The efficiency is rated at 29%.
The average composition of deuteriumð2
1DÞin water (H2O) is about 150 ppm which varies from place to place, as shown in Fig.7.8for Canada.
Heavy water is extracted from water usually in two steps: the first being a dual temperature deuterium transfer process and the second through vacuum distillation.
The dual temperature process is based on the atomic exchange of hydrogen and deuterium between hydrogen sulfide gas (H2S) and fresh water with a deuterium concentration of approximately 148 parts per million.
When hot water is in contact with hot H2S gas, the deuterium atom migrates from the water into the gas; when both are cold, it migrates in the reverse direction.
HODþH2S!H2OþHSD K¼0:99 25ð CÞ (7.22) By repeating this process in successive stages, the deuterium oxide is increased to a concentration of 15%. The ten extraction towers of the plant contain sieve trays. Water flows downward across each tray in succession. H2S gas is forced upward, bubbling through the sieve holes for contact with the water. This is shown in Fig.7.9.
Table 7.9 Most important products of thermal-neutron fission of uranium-235
Isotope Half-life Fission yield % Isotope Half-life Fission yield %
137Cs 33 years 6 95Zr 65 days 6
90Sr 19.9 years 5 95Nb 38.7 days 6
140Ba 13.4 days 5.7 99Tc 2.1105years 6.2
91Y 61 days 5.9 129Te 35.5 days 0.2
140La 1.65 days 5.7 131I 8.1 days 3
141Ce 33 days 5 129I 1.7107years –
144Ce 282 days 3.6 103Ru 39.8 days 3.7
143Pr 13.5 days 5.3 106Ru 290 days 0.5
147Nd 11.9 days 2.6 105Rh 1.54 days 0.5
147Pm 2.26 years 2.6 133Xe 5.3 days 6
155Eu 1.7 years 0.03
7.6 Nuclear Reactors 117
Fig.7.5Aswiththeboilerinacoal,oil,orgasburningoilplant,anuclearpowerreactorproducessteamtodriveaturbinewhichturnsanelectricgenerator.Insteadofburning fossilfuel,areactorfissionsnuclearfueltoproduceheattomakesteam.(a)ThePWRshownhereisatypeofreactorfuelledbyslightlyenricheduraniumintheformofuranium oxidepelletsheldinzirconiumalloytubesinthecore.Waterispumpedthroughthecoretotransferheattothesteamgenerator.Thecoolantwateriskeptunderpressureinthe coretopreventboilingandtransfersheattothewaterinthesteamgeneratortomakethesteam.(b)TheBWRshownhereisatypeofreactorfuelledbyslightlyenricheduranium intheformofuraniumoxidepelletsheldinzirconiumalloytubesinthecore.Waterispumpedthroughthecore,boilsandproducessteamthatispipedtotheturbine.(c)The HTGRshownhereisatypeofreactorfuelledbyuraniumcarbideparticlesdistributedingraphitecore.Heliumgasisusedasacoolanttotransfertheheatfromthecoretothe generator.(d)InanLMFBR,moltensodiumintheprimaryloopispumpedthroughthereactorcorecontainingthefuel.Thissodiumcollectstheheatandtransfersittoa secondarysodiumloopintheheatexchangerfromwhichitiscarriedtothesteamgenerator.Inadditiontoproducingelectricity,thisreactoralsoproducesmorefissionable materialthanitconsumes,whichiswhyitiscalleda“breederreactor.”Inthereactor,uranium-238istransmutedtofissionableplutonium-239whichisextractedperiodically andfabricatedintonewfuel
Table 7.10 Approximate operating environments for Gen IV systems Reactor type
Coolant inlet temperature (C)
Coolant outlet temperature (C)
Maximum dose (dpaa)
Pressure
(Mpa) Coolant Supercritical water-cooled
reactor (SCWR)
270 500 15–67 25 Water
Very high temperature gas- cooled reactor (VHTR)
600 1,000 1–10 7 Helium
Sodium-cooled fast reactor (SFR)
370 550 200 0.1 Sodium
Lead-cooled fast reactor (LFR) 600 800 200 0.1 Lead
Gas-cooled fast reactor (GFR) 450 850 200 7 Helium/SC
carbon dioxide
Molten salt reactor (MSR) 700 1,000 200 0.1 Molten salt
Pressurized water reactor (PWR)
290 320 100 16 Water
adpa is displacement per atom and refers to a unit that radiation material scientists used to normalize radiation damage across different reactor types. For one dpa, on average, each atom has been knocked out of its lattice site once
Table 7.11 The relative moderating character of various materials
Substance Moderating ratio Average slowing down length from “fast” to thermal (cm)
Light water 72 5.3
Organic liquids 60–90
Beryllium 159
Graphite 160 19.1
Heavy water (99.8 %) 2,300
Pure D2O 12,000 11.2
Fig. 7.6 The simplified flow diagram for the CANDU reactor
7.6 Nuclear Reactors 119
The 15% enriched heavy water is delivered to a finishing unit where, through vacuum distillation, the concentration is upgraded to 99.8% (reactor grade) deuterium oxide. The distillation unit consists of two towers containing special packing.
Deuterium is also enriched during the electrolysis of water since 1
1His more readily liberated as H2than 2
1Das HD. Hence, the first samples of heavy water were obtained from Norway where water electrolysis is the method of producing H2and O2since electricity is inexpensive there—even today where it is the major method of heating homes.
It is possible to increase the fuel efficiency by selecting the zirconium isotope94
40Zrwhich has the lowest neutron absorption cross section (0.08 barns; see Table7.2) for the fabrication of the zircaloy.
Several attempts have been made since the high cost of an enrichment process is a major capital investment which could be of continuous benefit since the 94
40Zrcan be recycled from spent fuel and reused with little reprocessing.