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Life cycle analysis of the nuclear energy system

Nuclear process chain

A nuclear power plant is not a stand-alone system, it is just the most visible component and the midpoint of a sequence of industrial processes which are indispensable to keep the nuclear power plant operating. This sequence of industrial activities is called the nuclear process chain. Like most industrial processes the nuclear chain comprises three sections: the front end processes, the production process itself and the back end processes. This can be compared with a common sequence of household activities: preparing a meal, enjoying the meal and washing the dishes and cleaning up.

The front end of the nuclear chain comprise the processes to produce nuclear fuel from uranium ore and are mature industrial processes. The midsection encompasses the construction of the nuclear power plant plus operating, maintaining and refurbishing it. The back end comprises the processes needed to handle the radioactive waste, including dismantling of the radioactive parts of the power plant after final shutdown, and to isolate it permanently from the human environment. The most important back end processes are still existing only on paper.

Cradle to grave

Comparison of the diverse implications of different energy systems (nuclear, fossil, renewables) is possible only on basis of life cycle assessments of the full process chain of each energy system, from cadle to grave. Implementation of a given energy system has various aspects, for example concerning issues of economy, environment, safety, politics, society and availability of natural resources. These aspects are rarely observable at the same time. The cradle-to-grave period of an energy system is the period from the start of a given project through restoration of the site to greenfield conditions. The cradle-to-grave periods of the various energy systems, each type with an assumed operational lifetime of 40 years, vary from some 50 years for fossil-fuelled and renewable power stations to 100-150 years for nuclear power stations.

Energy balance of the nuclear system

Any industrial process consumes energy and materials. The sole purpose of the nuclear energy system, consisting of the reactor plus the industrial processes needed to generate electricity from uranium ore and handle the radioactive waste, is to deliver useful energy to the consumer. A conditio sine qua non for the nuclear energy system is that its indispensable assembly of industrial processes consumes less useful energy than is generated by the reactor. In other words: the net energy balance of the nuclear system should be positive. This condition is analogous to that of a viable economic activity, which should have a positive financial balance. The energy investments of the nuclear system from cradle to grave turn out to comprise five main components of similar magnitude:

• construction of the power plant,

• front end, from ore to fuel,

• reactor operation, maintenance and refurbishments (OMR),

• back end, definitive removal of all radioactive waste from the human environment,

• reactor decommissioning and dismantling.

Analysis of the complete nuclear system proves that the choice of enrichment technique, diffusion or ultracentrifuge, has a negligible impact on the energy balance of nuclear power.

The energy balance of the nuclear system turns out to be not a constant factor, but depending on a number of variables. The operational lifetime of the reactor, measured in full-load years, determines the gross energy production of the nuclear system. At low ore grades the thermodynamic quality of uranium resources from which the nuclear system derives its uranium becomes a dominant variable of the energy investments for the front end and the back end of the nuclear process chain [more i38]. The figures of the construction energy investments exhibit a considerable spread. Large uncertainties exist with respect to the last phase of the nuclear chain: decommissioning and dismantling of the reactor. Preliminary estimates point to a multiple of the construction energy investments.

Full-load years and energy payback time

A full-load year is equivalent with maximum amount of electricity delivered to the grid, if a nuclear power plant would operate at 100% of its nominal capacity during a full year without interruptions. A nuclear power plant which operated with an average load factor of 70% during a period of 30 years has an operational lifetime of 21 full-load years. The number of full-load years is a vital quantity for the calculations of the energy balance, the energy payback time and the energy return on energy investment. The energy payback time is the number of full-load years a certain reactor has to operate to generate as much useful energy as has been invested to construct and operate the system. The energy payback time of the currently operating nuclear energy systems, measured over the full cradle-to-grave period, is about 9 full-load years at the current world average uranium ore grade. The average operating lifetime in 2011 of the world operating nuclear fleet was about 21 full-load years.

Energy return on energy investment EROEI

The energy return on energy investment (EROEI) is here defined as the ratio of the energy delivered to grid over the energy investments, both measured over the full cradle-to-grave (c2g) period. The energy return on energy investments of the world averaged nuclear energy systems are EROEI = 2-3 under the current conditions, but will decline over time when leaner uranium ores are to be exploited [more i38].

Methodology of energy analysis

To assess the physical aspects of the nuclear energy system a comprehensive energy analysis of the system is required. The analysis starts with a life cycle assessment (LCA), to describe all processes comprising the nuclear system, the nuclear process chain. Each process of the chain is analyzed separately: the inputs of materials and energy are quantified per unit product. The direct inputs of useful energy (electricity and fossil fuels) are analyzed, but also the indirect energy investments, which have consumed for the production of the materials needed for the observed process. The production of one kilogram of steel from iron ore, for example, consumes a certain amount of useful energy; this amount is called the embodied energy of steel.

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chainxsimp8

Figure 12-1. Nuclear process chain

Broad outline of the nuclear process chain, also called the nuclear energy system. The three main parts are the front end processes, the powerplant itself and the back end processes. For more details see text.

c2gperiod

Figure 12-2. Cradle-to-grave (c2g) period

Cradle-to-grave (c2g) period of nuclear electricity generation compared to fossil-fuelled electricity. Assumed operational lifetime of both systems is 40 years. Construction period fossil: 5 years, nuclear: 10 years. Back end (waste handling, decommissioning + dismantling) fossil: 5 years, nuclear: unknown, at least 60 years, possibly more than a century. The full timeframe of a nuclear project may run to more than a century.

Edebt16

Figure 12-3. Dynamic energy balance of the nuclear energy system.

During the construction phase useful energy is invested into the nuclear energy system. During the operation phase useful energy is produced (electricity), but a part of the produced energy is required to run the front end processes: the production of nuclear fuel from uranium ore. The energy requirements of the back end processes (waste management, decommissioning and dismantling) are to be invested after closedown of the nuclesar power station. The energy requirements of the back end are called the energy debt, for these are still to be invested during the century following closedown. This diagram is at scale.

EROEItime070v2

Figure 12-4. The energy return on energy investment(EROEI) of nuclear power.

The energy return on energy investment (EROEI) is here defined as the ratio of the energy delivered to grid over the energy investments, both measured over the full cradle-to-grave (c2g) period (see also Figure 12-3). At EROEI = 1 the nuclear energy system consumes as much useful energy as it produces. Below a value EROEI = 1 the nuclear energy system becomes an energy sink, instead of an energy producer. This graph applies to a nuclear power plant of the current state of technology under favourable conditions. It turns out that the year of the plunge into the energy sink does virtually not depend on the assumed parameters of the nuclear reactor, but mainly on the quality of the available uranium resources. This is another way to present the energy cliff [more i38].

Eanalysis5

Figure 12-5. Outline of the methodology of energy analysis.

This outline pictures the inputs and outputs of materials and energy of a generic industrial process. The input of services, processed materials and capital goods represent an indirect (embodied) energy input. Human labour and raw materials are not attributed embodied energy.