Life cycle energy and greenhouse gas emissions of nuclear energy: A review

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Abstract

The increased urgency of dealing with mitigation of the looming climate change has sparked renewed interest in the nuclear energy option. There exists a substantial stream of research on the amount of embodied energy and greenhouse gas emissions associated with nuclear generated electricity. While conventional fossil fuelled power plants cause emissions almost exclusively from the plant site, the majority of greenhouse gas emissions in the nuclear fuel cycle are caused in processing stages upstream and downstream from the plant. This paper distils the findings from a comprehensive literature review of energy and greenhouse gas emissions in the nuclear fuel cycle and determines some of the causes for the widely varying results.

The most popular reactor types, LWR and HWR, need between 0.1 and 0.3 kWhth, and on average about 0.2 kWhth for every kWh of electricity generated. These energy intensities translate into greenhouse gas intensities for LWR and HWR of between 10 and 130 g CO2-e/kWhel, with an average of 65 g CO2-e/kWhel.

While these greenhouse gases are expectedly lower than those of fossil technologies (typically 600–1200 g CO2-e/kWhel), they are higher than reported figures for wind turbines and hydroelectricity (around 15–25 g CO2-e/kWhel) and in the order of, or slightly lower than, solar photovoltaic or solar thermal power (around 90 g CO2-e/kWhel).

Introduction

Despite its heat and electricity generating stages not causing any greenhouse gas emissions, nuclear energy is not a zero emissions energy source. Its extensive system of upstream supply stages requires energy inputs throughout, and given that in practice, a substantial part of these energy inputs are provided by fossil fuelled sources, nuclear energy indirectly involves the emission of greenhouse gases.

With climate change being increasingly viewed as one of the most pressing global problems, nuclear power has found its way back onto policy roundtables and into the media [1]. But, just how much CO2 nuclear plants will be able to avoid depends, amongst other aspects, on the indirect emissions associated with the nuclear fuel cycle. This topic has been the subject of controversial debates,1 and as a result, as part of his Uranium Mining, Processing and Nuclear Energy Review (UMPNER), the Australian Prime Minister called for an independent assessment of this question, the results of which were revealed to the public in December 2006.

This paper distils the findings from this, probably, most comprehensive review to date by summarising the energy and greenhouse gas life cycle analyses of the nuclear fuel cycle and by determining some of the causes for the widely varying results of previous studies. The following sections take the reader on a journey through the nuclear fuel cycle, with the goal of stating overall energy and greenhouse gas intensities, that is, the ratio of the primary energy consumed, or greenhouse gases emitted during all stages of the nuclear fuel cycle, per unit of output of electrical energy over the lifetime of the power plant.2 A few definitions are necessary upfront:

The load factor or capacity factor λ of an energy supply system is defined as the equivalent percentage of time over one year during which the system supplies electricity at 100% load, that is, supplies electricity at its nominal power rating P. For example, a 1000 MW power plant running constantly at 800 MW power output has a load factor of 80%. Equally, a 1000 MW power plant running for 292 days a year at 1000 MW has a load factor of 80%.

The energy intensity η of an energy supply system of power rating P and load factor λ, is defined as the ratio of the total (gross) energy requirement E for construction, operation, and decommissioning and the electricity output of the plant over its lifetime T:η=EP×8760hy-1×λ×T.In calculating E, it is (a) convention to a exclude the energy from human labour, energy in the ground (minerals), energy in the sun and hydrostatic potential and (b) not to discount future against present energy requirements [13], [14]. This review follows these conventions.

Similarly, the greenhouse gas intensity γ of an energy supply system of power rating P and load factor λ, is defined as the ratio of the total greenhouse gas emissions G for construction, operation and decommissioning and the electricity output of the plant over its lifetime T:γ=GP×8760hy-1×λ×T.It is obvious that an increase in the assumed lifetime and load factor of an energy supply system causes a decrease of its energy and greenhouse gas intensities because the lifetime electrical output increases. This influence can be eliminated by normalising the modelled energy and greenhouse gas intensities to a constant load factor of L and a constant lifetime of Y years according toηnorm=ηλLTY=EP×8760hy-1×L×Y,γnorm=γλLTY=GP×8760hy-1×L×Y.The inverse of the energy intensity is often called the energy ratio R. Calling Eout = P × 8760 h y−1 × λ × T the lifetime electricity output of a system, the energy ratio isR=EoutE.This ratio describes the amount of electricity delivered per unit of fossil energy expended on it throughout the economy [13, Eq. 6.7]. In computing the total energy requirement E, all its constituents must be of the same energy quality (the “valuation problem”, see Refs. [14], [15], [16], especially Ref. [17, p. 5–9] for the case of nuclear energy).

Energy intensity η and energy ratio R are related to the energy payback time. This is the time t that it takes the energy supply system to generate an amount of electricity tEoutT that, had it been generated conventionally, for example fossil fuelled, would have had a primary energy embodiment 1RfossiltEoutT equal to the system’s energy requirement E.tpayback=η1×T×Rfossil=RfossilRT.The energy payback time can be normalised just as the energy intensity. Note that the definition of an energy payback time implicitly assumes an initial energy sink associated with the construction of the energy supply system, followed by a continuous net energy source. This definition is less useful for technologies that are characterised with large energy sinks during stages towards the end of their lifetime [14]. Nuclear facilities, for example, require lengthy periods for dismantling and clean up.

Section snippets

Uranium mining

One tonne of rock and soil contains on average 1–5 g of uranium, and 3–20 g of thorium. Concentrations in sediments can reach magnitudes of about 1 kg of uranium per tonne. One tonne of sea water contains about 3 mg of uranium. Amongst the two uranium isotopes, only U92235 is fissile. Since the half life of U92235 is about 1 billion years, which is smaller than that of U92238 at 4.5 billion years, the concentration of U92235 in natural uranium has decreased steadily. While, at the time of the

Meta-analysis

Section 2 has clearly demonstrated the large range of estimates of energy in the nuclear fuel cycle. Clearly, there exist considerable variability, which could, on one hand, be caused by real differences in energy and greenhouse gas characteristics of different technology choices and countries and, on the other hand, be the result of methodological aberrations, such as systematic errors or deliberate scope settings. As a first approach to analysing this variability, I apply multiple regression

Conclusions

The increased urgency of dealing with mitigation of the looming climate change has sparked renewed interest in the nuclear energy option. In addition to the traditional areas of debate, such as reactor and processing plant safety and secure long-term storage of radioactive waste, a substantial stream of research has dedicated resources to establishing the amount of greenhouse gas emissions associated with nuclear generated electricity in comparison with fossil fuelled and renewable sources.

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