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i39

Energy and thermodynamics

Thermodynamics

The nuclear energy system is a technical means to unlock the potential energy embedded in uranium atoms and to convert it into electricity, useful energy, a process involving a number of energy conversions. Energy conversions are governed by the laws of thermodynamics. Thermodynamics is the science of energy conversions and lies at the base of all sciences. Consequently the generation of nuclear energy is subject to the laws of thermodynamics. Several basic notions are important in the assessment of nuclear power and the sustainability of the world economic system:

• energy

• First Law

• spontaneous changes

• entropy

• Second Law

Energy

Energy is a basic and conserved physical quantity, which cannot be deduced from other physical quantities, Consequently energy is a starting point of thermodynamics. Energy can be defined as the entity making changes possible.

First Law

The First Law is the well-known law of energy conservation: energy cannot by destroyed, nor created from nothing; only energy conversions are possible.

Spontaneous changes

Some changes happen spontaneously, other don't. A cup with hot tea cools down to the temperature of the surrounding air; a cup of cold tea never gets hotter by cooling down the surrounding air. A piece of charcoal (carbon) burns to hot carbon dioxide, but an amount of hot carbon dioxide never forms spontaneously a piece of pure carbon. Reheating the tea or reconversion of the carbon dioxide into carbon again is possible only by doing dedicated work.

Not all spontaneous processes will start spontaneously: sometimes a small amount of activation energy is needed to get the process going. For example one spark is needed to ignite any amount of oil, wether 1 gram or millions of tonnes: once started the spontaneous process continues until the last drop of oil has burned. The same holds true for fission of fissile uranium atoms: once started the chain eaction will go on as long as sufficient fissile material is available. An uncontrolled fission chain eaction results in a nuclear explosion. The function of a nuclear reactor is to keep the fission rate under control.

When a change occurs, the total energy in the universe remains constant, according to the First Law. Spontaneous changes are always accompanied by a reduction of the quality of the involved amount of energy: during the change the energy quality is degraded to a more dispersed form. Spontaneous processes are consequences of the natural tendency of the universe towards greater entropy, a greater dispersion of matter and energy.

The reversal of a spontaneous process, in many cases only theoretically possible, would require investment of useful energy and dedicated effort [more i40].

Examples of spontaneous processes are: the dispersion of CO2 from burning fuel into the atmosphere, the rusting of steel in the open air and the decay of dead organisms.

Entropy

Entropy may seem a somewhat elusive notion, but it is a key notion in thermodynamics.

Entropy is a measure of dispersal of matter, of energy and of oriented movement.

In practice only entropy changes of a system can be observed. For understanding some basics of nuclear power and sustainable energy a semiquantitative approach of entropy changes is satisfactory: we only need to know if the entropy of a system increases or decreases by a given action or phenomenon [more i40].

A rise of the entropy of a system means more dispersion of matter, energy and directional movement, or in other words: a loss of quality and usefulness of the observed system. For that reason entropy may be described in non-physical terms as a measure of ‘mess and uselessness’. A decrease of the entropy of a system means less randomness and consequently a gain of quality and usefulness of the system.

Second Law

The Second Law of thermodynamics is one of the most basic laws of nature. It says that any spontaneous process in a given system will go in the direction of more dispersal of matter, energy and directional movement: to more entropy of the system.

In a system without material exchange with its surroundings any spontaneous process will increase the entropy (disorder) of the system and decrease its quality and usefulness.

A most important consequence of the Second Law with regard to nuclear energy generation is:

The generation of an amount of useful energy from a mineral energy resource (fossil fuels, uranium) inextricably generates more disorder, more mess and more loss of quality (= more entropy) within the biosphere than can be compensated for by the produced amount of useful energy [more i41].

Potential energy

otential energy is energy embodied in some kinds of matter. Fossil fuels contain potential energy, in the form of chemical energy, which is liberated as heat when the fuel is burned.Uranium contains potential energy embodied in the nuclei of the atoms of the metal, which is liberated as heat and radiation when the nuclei are fissioned.

Useful energy

Useful energy is energy that can be used at will to attain an energy service: transport, lighting a room, cooking, running a computer, producing iron from iron ore, and so on. Useful energy is energy that flows in one direction, for example electricity flowing through a copper wire, heat that flows from a hot to a cold place, kinetic energy embodied in a spinning wheel. What is called the generation of energy is actually the conversion of potential energy into useful energy. A nuclear power station converts potential energy in uranium into heat and the heat partly into electricity.

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ijzer

Figure 39-1. Second Law

Rusting of a steel pole is a result of spontaneous processes, in accordance with the Second Law. When left unattended long enough, the steel pole will end up as a pile of dust. The entropy of the steel of the pole, the system in this case, has increased by the spontaneous process: the mess and uselessness of the original amount of steel have increased. The amount of iron in the observed system has not changed: the iron atoms of the original tube are still present in the pile of rust grains.