Veja também nosso suplemento literário
Comparison of greenhouse effect gases (GHG) emissions from nuclear electrical generation with other sources in Brazil
Note: Study carried out by e&e through Ecen Consultoria Ltda about greenhouse effect gases (GHG) in the nuclear fuel cycle in Brazil, with the support of Eletronuclear (ETN) and Indústrias Nucleares do Brasil (INB). The complete results are in the reports sent to Eletronuclear.
Direct emissions from nuclear electricity generation are practically zero. Emissions only from generators and auxiliary boilers should be considered.
This type of calculation has rightly been contested regarding both nuclear energy and some alternative sources since this type of calculation does not consider emissions along all the electricity production chain. In some cases it has been questioned if emissions really are avoided in some cases of fossil fuels substitution for alternative ones.
Direct and indirect GHG emissions along all electricity production chain should be compared with other energy cycles using coherent procedures. Analysis should comprehend from mining to electricity generation and also include deactivation of equipment and waste disposal.
It should be remembered that the doubts regarding emissions balance were raised when fossil sources were substituted for alternative ones in the 1970s and 1980s due to the oil prices shock. Alcohol from sugar cane and nuclear energy in Brazil - by the time it was assumed that low content ore would be extracted from Poços de Caldas and uranium would be enriched using the jet-nozzle process (highly electricity intensive) - were analyzed regarding energy balance along the whole cycle. This analysis is the base for emissions calculations made here using similar methodologies.
As an example of emissions along the nuclear cycle, one can mention the use of diesel in machines and vehicles in the extraction and transport of uranium ore (mining step), as well as electricity from fossil plants that produce CO2 and other greenhouse effect gases emissions. There are also emissions associated with the energy used for fabricating machines and mine installations that should be calculated. 
Naturally, it is necessary to establish boundary conditions or limits for the calculation, since there is the risk of double accounting in the analysis of different sectors or even of only one. Another concern is that equivalent criteria that define the limits should be adopted for all cycles. This is not always possible using the available data. In this case, homogenizing the limits (not considering some specific phase of the cycle) should be made or clearly indicating the different approaches.
Another necessary care regards the boundary definition of equivalent criteria for all cycles. This is not always possible with the available data. In this case homogenization of limits (such as not considering some specific phase of the cycle) should be made or different approaches should be clearly indicated.
Evaluations in the present study permit to quantify greenhouse effect gases emissions along the nuclear cycle as compared with those obtained for fossil fuels and some alternative energy sources. Care has been taken to maintain coherence regarding the cycles.
Emissions evaluations were carried out in a process of successive approximations where it was calculated:
a) Direct emissions from electricity generation;
b) Direct emissions from the cycle and finally;
c) Direct and indirect emissions in generation and in the whole cycle.
Direct emissions are those resulting from the use of fuel; CO2 emissions as well as those of CH4, NMVOCs, CO, N2O are calculated and they are converted into CO2 equivalent by the GWP (Global Warming Power) criterion recommended by IPCC. These emissions are directly calculated using data from the National Energy Balance and the Useful Energy Balance and they coincide (except for small contributions from auxiliary equipment) with those calculated in the National Emissions Inventory presented in the Brazilian Declaration to the United Nations Framework Convention on Climate Change.
Direct emissions of greenhouse effect gases from electricity generation are shown in Figure 1:
Figure 1: Scheme of direct emissions calculation; width of arrows is not proportional to emissions. In the case of nuclear and hydro sources emissions due only to auxiliary activities are accounted for but in the biomass case CH4 and N2O emissions are accounted for. Emissions due to external electricity are not accounted for in this process (direct emissions).
Direct + indirect emissions along the fuel cycle and generation are shown in Figure 2. Direct emissions from the cycle are calculated in a way analogous to the generation power plants shown in Figure 1 and emissions from input and equipment are integrated along the life time of the plant as well as those associated with external electricity (includes those from input and equipment).
Figure 2: Direct Emissions from Generation and from the Fuel Cycle (white arrows) and indirect ones (dotted arrows), and the latter include emissions associated with external electricity production and those contained in inputs and equipment used.
Emissions calculations associated with external electric energy is carried out considering the fuel profile used in generation. In the Brazilian case, emissions from the present park with high hydroelectric share are very low. Since it is considered the convenience of future energy use, it would be important to know the energy profile of sources used in generation in the actual use. This involves considerable uncertainties.
One way to circumvent this difficulty is to use the net value of the emissions/electric energy coefficient. This is done by not considering emissions associated with electricity use and subtracting from the generated energy the auto-consumption and the electricity used along the process. For this purpose it is necessary to maintain, as it is usual in life cycle analysis, separate emissions due to electricity from the other ones. The “net” emissions process is illustrated in Figure 3.
Figure 3: Scheme for calculating emissions by net supplied electricity. In the emissions/electric energy coefficient, emissions due to electricity generation are not accounted for (in the numerator) and it is considered the net supplied electricity (Generation - Auto-Consumption - External Electricity in the whole process).
The objective of this work is to compare, for the different cycles, GHG emissions (in t CO2 equivalent) by electric energy unit (MWh). Two types of emission/electricity coefficients were calculated: in the first one (usual), emissions from the generation of the electricity consumed in the fuel cycle are added to the numerator and the denominator is the generated energy; in the second one (net emission) the auto-consumption and the external electric energy used along the cycle are subtracted from the generated energy.
It should be remembered that direct emissions from the fuel cycle are, rigorously speaking, indirect ones in electricity generation. It was possible to calculate in the present study the cycle’s direct emission. It was used here coefficients from the National Inventory of GHG emissions and data from the National Energy Balance – BEN. Additional information, available for some years, from the BEN data base system and information from BEN’s coordination itself regarding auto-consumption in the generation units were used. Calculation is more complete for some types of fuel than for others since BEN has different approaches for the different energy cycles. Coal and sugarcane bagasse require special attention.
Emissions along the cycle can be important; for example, for petroleum product, since about 9% of the carbon contained in oil is emitted during the extraction and refining steps.
Indirect emissions are those related to equipment fabrication and inputs outside the installation or the cycle. The usual procedure is to separate emissions from the use of thermal and electric energy. This separation permits to distinguish, along the process, emissions due to electricity generation, used in inputs and equipment, from those directly due to fossil fuels. Some double accounting can be avoided using emission coefficients per net electric energy and this was one of the procedures adopted in the present study.
Nuclear cycle calculation is based on data directly supplied by INB and Eletronuclear. As far as we know, calculation of the nuclear cycle in Brazil using actual data has not been carried out before. The processes and fuel consumption and other inputs of the nuclear cycle refer to the year 2007. The fuel cycle steps carried out abroad (conversion and enrichment) used international coefficients. In the case of steps made in Brazil national coefficients from the National Inventory were used whenever possible as well as real data supplied by Eletronuclear and INB.
The steps considered for emissions in the cycle were basically those of an open cycle (Figure 4): Mining, Treatment, Conversion, Enrichment, Pellets Fabrication, Components Fabrication, Fuel Element Assembly, Reactor Construction, Generation and Reactor Decommissioning. GHG emissions here do not include the waste conditioning and final disposal because there are no data available (including those for the other fuel cycles to be compared with) and because the PWR-type reactor waste contains energy that can be used. In case of reprocessing, the contained plutonium could be used and it would also produce new emissions.
The use of plutonium entails environmental and economic risks and concerns regarding non proliferation. By adopting the medium term storage of wastes, Brazil maintains open the possibility of future use of the energy contained in plutonium.
Figure 4: Emissions in an open cycle considered in the present study except for waste conditioning and final disposal.
Emissions calculation results for the nuclear cycle are summarized in Table 1.
Table 1 – Electricity Consumed in each Step of the Fuel Cycle and Emissions due to Total Energy and only to Thermal Energy (values for production of 10.000 GW.year)
Note: does not include operation (0,77 gCO2/kWh)
Table 1 shows emissions in each step in gCO2eq/kWhel (equivalent to tCO2eq/GWhel), reaching the value of 27.64  gCO2eq/kWhel considering emissions in operation. It is verified that 1.65% of the generated energy is consumed along the cycle. Furthermore, 7% (information from MME) of the generated energy is consumed by the power plant itself (auto-consumption), that is, the net energy supplied to the network is 91.4%  of the generated energy. This share and the emissions value (excluding those due to electricity consumption in the cycle) permit to calculate emissions per supplied energy, namely, 27.8  gCO2eq/kWhel. This share (that is very close to the previous one) does not depend on the profile of the generation park. It corresponds approximately to the emissions per electric energy supplied if all electricity in the cycle would have the origin of the used source.
Emissions in other cycles were calculated using the available national and international data. Emissions considered were those from natural gas, fuel oil and steam coal in public service plants (auto-producers were excluded). It was also considered emissions associated with sugarcane bagasse generation as well as wind energy and photovoltaic. Emissions from hydraulic generation are outside the scope of the study since the objective was to compare sources that are alternative or complementary to hydro source.
Calculations concerning the other cycles focused on the utilization conditions and information available in Brazil. The fuel cycle studied were Oil and Natural Gas (extraction and production were treated together), Mineral Coal and Sugar Cane Bagasse. Regarding wind and solar energy there is not the so called fuel cycle.
Fuel cycle direct emissions were calculated from BEN’s “Energy Sector” data and international ones which were adapted to the Brazilian reality whenever possible. Emission coefficients used were preferably those used in the Brazilian Inventory coordinated by MCT.
A complementary analysis of the cycle data using international parameters adapted to the national conditions was also made. The results practically coincide regarding direct emissions but concerning cycle emissions the values of emission coefficients per generated energy are much higher for natural gas and oil products and slightly lower using information of the Energy Sector from BEN. In the present summary the option was to use for natural gas, fuel oil and diesel national data combined with those regarding construction to calculate “upstream” emissions. Data for decommissioning (downstream) are based on international coefficients.
The results of CO2 equivalent per kWhel emissions are shown in Table 2 for the examined sources.
Table 2 – Direct and indirect emissions in electricity generation- gCO2/kWhel
a CO2 emissions from biomass fuel are not accounted for in the GHG inventory
In Table 2 are presented the upstream emissions (before generation), during generation and after generation (deactivation of installations). The fugitive emissions, so called because they are GHG gases (mainly methane) that escape in the different cycle steps, are also presented. It should be mentioned the low capacity factor of the natural gas plants that in Brazil were considered as 20%, taken from the historical use and EPE’s projections for normal years. The 80% capacity factor would correspond to the current use abroad.
Figure 5 shows emissions due to alternative sources (including nuclear) that are generally an order of magnitude below those of thermal ones. Emissions per kWh generated by nuclear energy are higher only compared to wind energy.
In Figure 6 indirect emissions (excluding emissions during generation) in the nuclear cycle are compared with other cycles.
Greenhouse Effect Gases Emissions in gCO2eq/kWhel
Figure 5: GHG Emissions per generated electric energy in the Brazilian park
Indirect Greenhouse Effect Gases Emissions in gCO2eq/kWhel
Figure 6: Indirect emissions (total – direct ones in generation), showing that nuclear emissions are lower than those of thermal and alternative ones except for that of wind energy.
It can be observed in Figure 6 that indirect emission from nuclear generation is lower than all other forms of thermal generation. That is, when it is calculated the emissions that are avoided considering only direct emissions, the result is underestimated in absolute value. Including emissions associated with the life cycle, the value of emissions avoided by nuclear energy increases.
Another polemic question regarding emissions calculations in Brazil relates to the use of electricity that are underestimated because emissions from hydro electricity are low considering the present electricity generation profile. The option of adopting net emission coefficients eliminates this type of objection. Table 3 shows the values used in order to obtain these coefficients.
Table 3: GHG Emissions per Electric Energy Supplied
As it can be seen in Table 3 and Figure 7, the values of the “gross” and “net” emission coefficients are not significantly different. Anyhow, using “net” emissions for comparison purposes, nuclear energy would be advantageous. The maximum difference using each one of the coefficients would be 5%.
Coefficients per Generated Energy
Figure 7: GHG emission coefficients in electricity generation, and comparison of “gross” values (total emissions / generated energy) with the “net” ones (without emissions due to electricity consumed in the cycle) and considering the supplied electric energy subtracting the auto-consumption and the electricity used in the cycle.
The calculation of avoided emissions in the use of energy is a useful parameter regarding the choice of a national policy that can contribute to the reduction of GHG emissions. In the present study this evaluation was made for different fuels comparing direct emissions. It was seen in the previous item that the calculation produces underestimated results for avoided emissions due to the use of nuclear energy. Nevertheless, emissions avoided by nuclear energy in 2006 reached 3% relative to the total, as shown in Figure 8. This means that fuel emissions in 2006 in Brazil would be 3% higher if fossil fuels (considering the same relative share in that year) would be used instead of nuclear energy for electricity generation.
Avoided Carbon Emissions in 2006 Relative to the Effective Total
Figure 8: Carbon emissions Avoided by Nuclear Energy in 2006
Figure 9 shows the evolution of emissions avoided by nuclear energy relative to other two important sources that reduced greenhouse effect gases emissions. It is noticeable that the percent of avoided emissions relative to the effective ones (in the energy area) have been decreasing since the beginning of the 1990s. Because the nuclear production remained static (the graphic does not show the period when Angra I was not operating) and the fossil fuel consumption has been increasing, the relative volume of the emissions avoided by nuclear energy has been decreasing. The emissions avoided by the two other sources have not been following the growth pace of emissions in the country.
Figure 9: Evolution of avoided emissions
Anyhow, the presence of nuclear energy contributes to maintain a high level of avoided emissions in Brazil.
For the future, a projection based on the 2030 Plan was made. Two situations were simulated based on the basic reference scenario, extrapolating utilization rates of the different sources; in one of them the generating park had the planned configuration (including that of nuclear) and the other one where there would be no nuclear generation and the energy would be distributed in the same planned proportion between natural gas and coal. The result shows that between 2005 and 2030, 437 million tons of CO2 equivalent, corresponding to a reduction of 19% in emissions that would take place if nuclear generation would not be used.
The main objective of the study was to quantify emissions associated with the nuclear cycle in Brazil and this was carried out using real data supplied by INB and Eletronuclear. The particularity of trying to apply in a uniform way the emissions calculation for the different cycles has consolidated the effectiveness of nuclear energy use in Brazil as a means of reducing GHG emissions. Comparison with other thermal sources has shown a large advantage even when generation emissions are excluded. Nuclear generation emissions are even lower than those from sugar cane bagasse and photovoltaic but it is higher than wind energy which, as known, has important daily and seasonal limitations.
Nuclear energy will also be important to maintain in the future an electricity generation profile with low emissions of greenhouse effect gases.
 Even in this extreme case, the balance energy was considerably positive (MSc Thesis of Davi, E. M. from UFMG in 1982, advised by Carlos Feu Alvim), as well as CO2 emissions balance regarding nuclear energy in all references used for the present study. Concerning sugarcane alcohol, there is consensus that the balance is positive; regarding alcohol from corn, there are doubts.
 This analysis could be extended almost indefinitely by, for example, calculating the energy and emissions contained in the industrial machines that produced equipment. Since the objective is to compare emissions from alternative sources, this analysis is cut off using homogeneous criteria for the different sources.
 When it is calculated, for example, emissions due to the generation of the electricity used in the fabrication of a vehicle used in mining, part of it is already computed in the cycle under analysis itself.
 Information that complement the National Energy Balance Complete Tables (49 sectors per 47 energy sources) present for some years a subdivision of fuel consumption in different activities comprised in the so called “energy sector”. It was possible to extrapolate this information for other years.
 Allocation criteria in the Energy Balance are not uniform: for oil and natural gas information includes extraction and refining whereas for steam coal and uranium consumption it is allocated to the “Mining and Pelletizing” sector; in the same way, emissions from biomass production are allocated to agriculture.
 Double accounting in emissions calculations can only be avoided using an input X product matrix. The Brazilian matrix is still poor regarding energy specification and therefore its use in emissions calculations is limited. Anyhow, in the “net emissions” methodology, also used here, double accounting of electric energy is avoided in the cycle under study.
 On one hand, the criterion used was to reflect as close as possible emissions as they occur in the Brazilian production cycle. However, when no national coefficients were available, international coefficients and data regarding input and national equipment were used. There were cases where the input data used were the international ones while the coefficients used were those from the National Inventory, so that they were more representative in the hypothesis of the complete Brazilian cycle. However, in the case of electricity per gross generated energy, emissions will be lower when all the fuel cycle steps will be carried out in Brazil (the conservative procedure); in case one considers the net electric energy, there will be significant difference between calculations considering international or Brazilian data.
 2.87+0.77 = 27.64
 100%-1.65%-7% = 91.45%
 Discrepancies found are due to the fact that most of the natural gas produced in Brazil is associated with oil and its exploration and production costs are shared with those of oil; in the case of oil products, it is natural that these values are higher than those abroad because oil is extracted in offshore platforms.
 In the case of electricity, besides the mentioned factors, as the comparison is made relative to the average composition of thermal generation, the introduction of natural gas has reduced the value of emissions per electric energy unit that was supposedly substituted.
Graphic Edition/Edição Gráfica:
Thursday, 12 January 2012.