Conventional Nuclear Power

Electricity from nuclear energy is generated in the same manner as in a coal – fired power plant except that the source of heat for firing the steam-powered tur­bine is from nuclear reactions. Most nuclear power plants in operation today are

Type of Reactor

Acronym

Typical Fuel

Coolant

Description

Lead fast reactor

LFR

U/Pu nitride mixture

Lead or Pb/Bismuth

TJTmlt = 600/800°C

Molten salt reactor

MSR

233UF4 or 235UF4

Molten salt (e. g., FLiNaK)

TJTout = 700/1000°C

Very-high-temperature reactor

VHTR

UOj

High-pressure helium gas

TJTout = 600/1000 °С; 50% efficiency. Electricity is generated via a gas turbine driven by hot helium. Designed for the co-production of hydrogen. Graphite moderated

Pebble bed modular reactor

PBMR

U° or UOX distributed in porous graphite

Helium

A version of the VHTR. The core uses tennis ball-sized spherical graphite “pebbles” containing ceramic-coated fuel particles

Gas fast reactor

GFR

MOX

He + supercritical C02

Operating temperature of 850°C; 48% efficiency

Supercritical water-cooled reactor

SCWR

UOj

Supercritical water

TJTout= 290/600°C. Can operate at very high pressures because supercritical water has good heat capacity and

TABLE 9.2 (continued)

Some Types of Nuclear Reactors

therefore can be used in smaller volumes

Sources: Adapted from International Energy Agency. 2007. Nuclear Power. ETE04. http://www. iea. org/publications/freepublications/publication/essentials4.pdf; Suppes, G. J. and T. Storvick. 2007. Sustainable Nuclear Power, Academic Press Sustainable World Series. London: Elsevier; Zinkle, SJ. and G. S. Was. 2013. Materials challenges in nuclear energy. Acta Mater. 61 (3):735-758.

Note: MOX, mixed oxide of uranium and plutonium.

thermal reactors in that the energies of the neutrons in the reactor are reduced by collisions with the moderator to thermal energy in the range of 0-1 eV. This process—deemed thermalization—is necessary to increase the likelihood of the neutron being captured by U-235 and resulting in fission. If the neutron energy is not reduced and, instead, fast neutrons (>1 MeV) are allowed to persist, the reactor is termed a fast reactor. Fast reactors are discussed in Section 9.4.3 in the context of Generation IV reactors.

Thermal reactors include both heavy water reactors and light water reactors (meaning they use D2O or H2O). There are two primary types of light water reactors: the pressurized water reactor (PWR) and the boiling water reactor (BWR). They are appropriately named; in the PWR water is under pressure so that it is not allowed to boil; in the BWR water is under lower pressure and it is allowed to boil, generating steam to turn the steam turbine. The light water-cooled and water-moderated PWR is the most commonly used reactor design in operation today.

The PWR essentially consists of three interconnected operations: (1) the reac­tor vessel, (2) the steam generator, and (3) the steam turbine/generator complex (Figure 9.3). The heat from the nuclear reaction heats the water surrounding the reactor core. This hot, pressurized water acts as the heating fluid for the separate production of steam in the steam generator. It is pumped through to the steam gen­erator, but the two supplies of water never intermix (see Figure 9.3)—two separate loops of water, one circulating from the reactor core and one circulating in the steam generator, are used. The produced steam goes to the turbine to generate electricity and the exhausted steam is condensed and pumped back to the steam generator.

The heart of the reactor vessel is the reactor core that contains the fuel rod assemblies. The type of fuel that is used depends on the reactor. Conventional pres­surized light water reactors require 3-5% enriched U-235 pellets, but other fuels,

including naturally occurring uranium oxide or a mixture of uranium and pluto­nium oxides (MOX), can be used in certain cases (see Table 9.2). The MOX fuel is derived from the reprocessing of spent nuclear fuel (Section 9.3.3.2) and gener­ally cannot be exchanged 1:1 for reactor-grade fuel. Light water reactors require the enriched uranium because of the effectiveness of light water as the moderator. The hydrogen nucleus is a very efficient moderator (too efficient, in fact) in that it can capture a slow neutron to become deuterium, thus removing neutrons from the chain reaction. (The use of heavy water, a less effective moderator, allows for the use of natural uranium. Graphite, also, is a less effective moderator than water and is used when the production of Pu-239 is desired for nuclear weapons production: it both captures fewer thermal neutrons and requires more collisions with the neu­trons to reach thermal energies. As a result, the chances are increased that U-238 will capture a neutron and convert into Pu-239. However, graphite obviously cannot be used as a coolant.)

The fuel pellets are encased in a zirconium alloy (zircaloy) tube, chosen both because it is strong and because it has low neutron absorptivity. An array of fuel rods is bundled into the fuel rod assembly with some spaces left empty for insertion of control rods containing a neutron absorber, for example, boron-10, that can control the rate of reaction by capture of a neutron (see Equation 9.8).

10 B + 1 n neutroncapture 7 j j + 4^e (9 8)

Cadmium-113 is also an effective control element. The operation of the control rods by their insertion or withdrawal from the core effectively controls the rate of reaction (see Figure 9.4). The entire assembly is placed in the reactor core in a pool of water that plays the dual role of both coolant and moderator. The entire reactor vessel is then contained within a protective dome to prevent release of radioactive materials.

Updated: September 25, 2015 — 2:51 am