Page 224 - Managing Global Warming

184 Managing Global Warming

maximum temperature of the coolant and significantly limits the efficiency of the power
conversion cycle.
The specific heat of He is higher than that of CO 2 and liquid metals. The thermal conduc-
l
tivity of He is 10 times greater than that of CO 2 . This characteristic facilitates heat transfer
and reduces the size of heat exchangers. He is more inert than CO 2 , does not absorb neutrons,
and cannot become radioactive on its own.

4.5 Concise overview of conventional and alternative
nuclear fuels

This Section is mainly based on Chapter 18 from [1] and Chapter by Peiman et al. [39].

4.5.1 Introduction

Nuclear fission is a reaction in which the nucleus of a heavy nuclide splits into smaller
nuclides; a few new neutrons are created; gamma rays are emitted and a significant
amount of energy is released. Nuclear fission has been used as a basis for production
of heat in all the current nuclear reactors. Even though reactors can be categorized
based on their cooling medium, pressure boundary, type of nuclear fuel, or neutron
spectrum, they all have one common feature, which is the production of heat via a
fission chain reaction in the nuclear fuel.
An important part of every reactor design involves the selection of a nuclear fuel
and the design of fuel assemblies (bundles). As general requirements, a nuclear fuel
should have a high melting point, acceptable thermal conductivity (higher is better),
sufficient mechanical stability, good dimensional and irradiation stability as well as
chemical compatibility with the cladding (sheath) and the coolant. Another important
parameter that influences the design and selection of a nuclear fuel is the dominant
neutron spectrum of a reactor. In this context, nuclear reactors can be categorized as
fast-neutron spectrum, epithermal-neutron spectrum, and thermal-neutron spectrum.
This classification is based on the energy group of neutrons that maintain the fission
chain reaction. In a fast-neutron-spectrum reactor, the chain reaction is sustained
mainly by fission of fast (e.g., high-energy) neutrons, while in an epithermal or thermal
reactor, fission of epithermal (intermediate-energy) or thermal (low-energy) neutrons,
respectively, maintain the chain reaction.
The neutron spectrum has an impact on the reactor design, selection of materials
for the reactor core, the type of nuclear fuel, and the associated fuel cycle. Unlike
fast-neutron-spectrum reactors, thermal-neutron-spectrum reactors utilize a moder-
ator such as water, heavy water, or graphite in order to reduce the energy of high-
energy neutrons. These two types of reactors use different coolants. Thermal-
neutron-spectrum reactors use water or CO 2 , which are composed of light elements,
especially those having high scattering cross sections. Liquid-metal coolants, such
as sodium or lead, are used in some fast-neutron-spectrum reactors. The neutron
spectrum is dependent on the isotopic concentration of fissile and fertile nuclides
in the fuel. As shown in Table 4.17, fast-neutron-spectrum reactors require a higher