As mentioned previously, in order for significant proportions of energy to be released from uranium it is necessary to initiate a chain reaction. In other words, every fission event must trigger at least one further fission. The fission of U235 creates 2 or 3 neutrons on aver- age [67]. There are several possible outcomes for these neutrons: a) absorption and fission of U235, b) absorption and transmutation of U238 or other reactor materials (e.g. Fe58) and c) leakage from the reactor. Clearly a) is favourable if one wishes to initiate a chain reac- tion. There are several ways in which conventional reactor designs ensure a bias towards fission of U235. Moderators are used to slow down (thermalise) the very high energy (fast) neutrons to velocities that are more likely to cause fission of U235 (or Pu239). The enrich- ment of uranium increases the ratio of U235to U238and, therefore, increases the chance of neutron-U235collisions. Typically, reactor materials that are transparent to neutrons, such as Zircalloy (for fuel cladding), are selected to minimise parasitic neutron losses. Neutron reflectors are also used at the reactor boundaries to minimise neutron leakage to the envi- ronment. A generalised schematic of the two most prominent UK designs is presented in Figure 1.6a.
The following discussion will deal with the selection of appropriate reactor materials in terms of their thermal and neutronic properties according to the operating conditions of a particular design. A balance is made in all cases to select materials with high radiation damage tolerance, high thermal conductivity, neutron moderating abilities and neutron transparency but materials that can also operate at high temperatures to promote thermal efficiency. Maintaining these properties throughout the fuel lifetime is a critical factor in maximising the energy extracted from the fuel. Even in modern reactors the burnup is limited to 6 % of heavy metal atoms (i.e. 60 GWd/t) [68].
During 60 years of nuclear operations around the world, a wide range of nuclear reactors have been developed. Commonly, early non-commercial reactor programs, such as Wind- scale [69], were developed in the aftermath of world war two as countries hurried to breed
Charge tubes Control rods Radiation shielding Pressure vessel Graphite moderator Fuel rods
Hot gas ducts Steam Heat exchange
Water circulator
Water Cool gas duct Gas circulator Charge tubes Control rods Graphite moderator Fuel rods Contrete pressure vessel Heat exchange Water circulator Water Gas circulator Steam
a)
b)
Figure 1.6: a) Schematic illustration of a MAGNOX reactor and b) of an AGR reactor based on the description of [3]. The MAGNOX uses a two loop coolant system, similar to the PWR design but with a CO2coolant rather than water, whilst an internal steam generator is used in the AGR design.
Pu for their own nuclear weapons programs. Metal uranium fuel was frequently used for ease of Pu extraction after irradiation. The work presented here is not concerned with the production of nuclear weapon materials, however, the early UK commercial MAGNOX re- actors used metal uranium even as the generation of electricity became an end in itself. The natural uranium metal was bound in a cladding made of magnesium-aluminium alloy from which the MAGNOX reactor gets its name. This cladding material was selected, in addition to a CO2gas coolant, due to their low neutron neutron absorption properties. Thermalisa-
tion of fast neutrons was achieved by using a graphite moderator. The high uranium density of the metal in conjunction with the neutron-transparent reactor materials negated the need
for uranium enrichment. Very similar reactor designs have been built in France and North Korea [70]. Nonetheless the low damage tolerance and poor chemical stability of natural metal uranium was not sufficient for the second generation high burnup power reactors, so the world moved towards enriched UO2during the 1970s.
The second generation of reactors can generally be split into light water reactors (LWR) and gas-cooled reactors. In virtually all generation II designs UO2fuel was adopted due to
its chemical stability, high melting point and enhanced radiation damage tolerance, thus, enabling improved thermal efficiency and higher burnups to be achieved [3]. The ability of UO2 to accommodate high levels of non-stoichiometry [71] is particularly useful as the
chemical composition changes during irradiation. The high temperature operation of such reactors means that LWRs can be split into boiling water reactors (BWR) and pressurised water reactors (PWR). The PWR prevents the boiling of the coolant water under normal operation by maintaining a pressure of around 152 atmospheres in the primary loop and uses a heat exchange to generate steam in a lower pressure secondary loop [3]. The BWR design has a lower primary loop pressure and utilises steam generation directly from the re- actor core to power the steam turbines. Although efficiency is enhanced in BWRs, activation products and FP release to the turbines creates a radiological hazard during maintenance. Unlike most other LWR designs, the Reaktor Bolshoy Moshchnosti Kanalnyy (RBMK) [3] design uses graphite in addition to water as a moderator [72]. Alternatively, gas-cooled reactors have also been used, particularly in the UK. The UK advanced gas cooled reactor design (AGR) used a CO2 coolant in conjunction with a separate graphite moderator. The
design is a natural evolution from the MAGNOX reactor but the use of enriched UO2 fuel
in conjunction with CO2coolant enables high temperature operations for which neither the
zircalloy cladding of LWRs nor the MAGNOX cladding are suitable, so stainless steel is used instead. The preferential oxidation of LWR zirconium based clad limits hyper-stoichiometry in the UO2matrix [73] but the extent to which this is true for AGRs is not clear, as stainless
steel has a highly passive Cr2O3layer.
conductivity is relatively poor so the fuel pellet radius is kept very thin to prevent melting. An additional feature of AGRs is a central bore hole in the pellet designed to avoid centre line temperatures exceeding the melting point. For nearly all the generation II designs the UO2fuel must be enriched to compensate for its low uranium density, as well as, neutron absorption by either the cladding in AGRs or by the coolant in LWRs. However, one notable exception is the CANDU reactor [3] which uses a pressurised water system but, instead of conventional water, it uses deuterium water (or D2O). Deuterium can be described as neu-
tron saturated and, therefore, has a low neuron capture cross section making it relatively transparent, however, it still acts as a moderator [3]. Furthermore, the relatively low operat- ing temperature enables transparent Zr cladding to be used. Consequently, natural UO2is
sufficient to provide the reactivity required to achieve criticality despite the relatively low uranium density of UO2.
Generation III reactors designs are very similar to generation II reactors but with a focus on increased power output and enhanced passive safety. Fundamentally, the reactor cores are much the same as for previous LWR designs but each plant will operate more primary reactor loops in tandem to maximise the power output of a particular site. This is, in part, due to difficulties in creating a new licensed site for reactor operation. Instead the current UK renaissance will aim to build on existing nuclear sites. A strong focus on passive safety mechanisms for loss of coolant accidents should help mitigate the risks associated with the most prominent disasters of the past, ensuring lessons have been learnt. In particular, the European pressurised reactor (EPR) features an improved core catcher [74] designed to limit the consequences of meltdown, whilst AP1000s feature a passive backup coolant system that uses condensation [75].