Desktop version

Home arrow Engineering

  • Increase font
  • Decrease font


<<   CONTENTS   >>

Section 3: Wind/Solar/Hydro/Nuclear

: Nuclear Energy, the Largest Source of CO2 Free Energy: Issues and Solutions

Anthony J Baratta

Introduction

Nuclear energy is the only low-carbon source of energy capable of producing 1,000s of MWs1 day in and day out, regardless of weather or time of day. Yet many developed countries, including Germany and the U.S., are moving away from the use of nuclear energy in favor of renewables supplemented by “clean” natural gas. In the U.S., the main reason for the shift is money, secondarily influenced by public safety concerns. In Germany, it is public concern over safety in the aftermath of the Fukushima Daiichi accident.

This chapter examines these factors and explores alternatives. The fact that nuclear is currently the largest producer of low-carbon energy is discussed. The process of converting the energy in the nucleus of uranium to a usefl.il form is explained. The risks of the process are analyzed and the three principle nuclear accidents, Three Mile Island, Chernobyl, and Fukushima Daiichi, are examined and their health consequences considered. Finally, the question of cost and competitiveness is considered for current nuclear plants; the new generation of reactors might overcome these concerns.

The Nuclear Process

Fission and fusion are the two types of nuclear reactions capable of producing large amounts of energy. Fission is currently the only practical process capable of commercialization. Discovered in the 1930s by Otto Hahn and Fritz Strausmann, the process of fission involves the breakup of the nuclei of heavy elements such as uranium. In the process of the breakup, energy is released in the form of radiation. This radiation is the energy associated with the movement of the fragments of the original nucleus away from the fission site.

By comparison, fusion involves combining the nuclei of light elements such as hydrogen to form heavier elements. The result is energy in the form of radiation and energy in the form of the motion of the resulting heavier nucleus. The original theory of fusion was proposed by Arthur Eddington in the 1920s to explain energy generation in stars such as our sun. Fusion in the laboratory was achieved in the 1930s by Mark Oliphant.

The energy released in both fission and fusion eventually appears as heat. Current nuclear reactors use the fission of uranium to produce energy. The heat generated in the fission process is used to produce steam which drives a turbine generator, resulting in an electric current. Aside from the source of the energy, the process of generating electricity in a nuclear power plant is identical to that of many fossil fuel power plants. While no practical fusion reactor has been built, it is expected that the final result of electrical generation will rely on a similar process.

A practical system must be capable of running at high power levels for extended periods of time. In the case of fission, the process involves a chain reaction where neutrons are emitted when the uranium fissions are used to induce fissions in other uranium nuclei. As long as sufficient fuel is available, the process can continue. In fact, most commercial nuclear reactors have enough fuel to generate 1,000 MW of electricity for more than two years. The fuel is contained in the reactor core and the amount of fuel is extremely small compared to the amount of fossil fuel needed to power a conventional power plant. The reason for the difference is that a single fission generates 100 million times as much energy as the combustion of one fossil fuel atom. Nuclear processes are much more energy intensive than the chemical process of combustion due to the extremely strong nuclear forces at work.

To date, no comparable fusion reactors are able to operate for extended periods of time. However, there have been significant advances, with the latest systems achieving fusion and maintaining conditions for fusion for times measured in minutes. While this may seem small compared to fission reactors which operate for years, it does represent significant progress in the field. The U.S. has generated more energy from fusion than was consumed to cause the fusion (Herrmann, 2014). The Chinese created the conditions necessary for fusion and maintained fusion for over 100 seconds (Chinese Academy of Science, 2017), no mean achievement, since only a few years ago the times were measured in fractions of a second.

The principle safety concerns with fission reactors are the highly radioactive fragments of the uranium nucleus that are formed when the uranium fissions. These “fission products” are sufficiently radioactive that a person exposed directly to the fuel would receive a lethal dose of radiation in a matter of seconds. In normal operation, the “fission products” are contained safely in the fuel which is confined to the reactor, located inside a rugged containment structure. Under accident conditions, these barriers can and have been breached, resulting in the release of radioactive material to the environment. It is the potential release of the fission fragments to the environment during a severe accident that prompts public concern about the safety of nuclear energy.

A fusion reactor will also produce radioactive material. The difference between the two concepts is that the radioactive material from a fusion reactor will decay much more rapidly than the fission products formed in current fission reactors. It is expected that the radioactive material created in a fusion reactor will have half-lives measured in days to years compared to hundreds or thousands of years and in some cases tens of thousands of years for those from a fission reactor (Baratta, 2017). Since the radioactive waste produced in a fusion reactor is short-lived, the consequences of an accident and problems with storage would be significantly reduced.

Carbon Emissions from Various Energy Sources

No matter the source of energy, there are some Green House Gases (GHG) emitted. Even the generation of electricity from solar and wind have emissions of GHGs associated with them. It simply is not enough to consider just the actual production of energy from an energy source. All necessary phases of the energy production cycle, including the manufacture, deployment and maintenance of the generating system as well as the actual energy production, must be included. The upstream processes, such as the manufactur e of the solar cells or the mining of uranium, must also be included in order to determine an energy source’s GHG contribution.

Naturally occurring uranium is composed mostly of two isotopes, U235 and U23S. For most reactors, the amount of U235 in natural uranium is insufficient and must be increased through the enrichment process. The process is energy intensive, requiring 1,000s of MWs of electricity. However, where the electrical energy comes from determines the amount of GHG emitted in the upstream fuel manufacturing phase. If the electrical energy to drive the enrichment process comes from a nuclear power plant then the GHG emission is less than when the electrical energy comes from a fossil fueled electric power plant.

Similarly, the amount of GHG emitted in the production of solar cells depends on the type of cell produced. The energy required to produce crystalline solar cells is larger than for amorphous solar cells, hence, more GHG would be emitted.

Estimates of GHG emissions are also subject to where the energy is eventually generated. In the case of solar, higher efficiencies are obtained nearer the equator, resulting in fewer cells needed and, therefore, less emissions. Table 1, adapted from the International Atomic Energy Agency, shows the amount of carbon emitted in grams CO, equivalent per kW-hr (IAEA, 2016).

Table 1. GHG emissions for various energy sources.

Source

g CO,-eq/kV-h

Coal

932-1132

Natural Gas

449-662

Fossil Fuel with Carbon Capture

82-232

Geothennal

62[1]

Biomass

64

Solar Photovoltaic

49

Thermal Solar

27

Wind

16

Nuclear

15

Hydropower

7

Нолу Nuclear Energy Can Assist in the Reduction of GHG Emissions

From the table, the use of any of the low emission technologies would be beneficial in reducing GHG emissions more than the continued use of fossil fuels. Unfortunately, solar and wind are intermittent sources due to weather conditions, including a lack of sunshine. Typical capacity factor for wind and solar has an average of 30% worldwide (the U.S. ranges from 28% to 55% with other countries being lower) (Lazard, 2018). Capacity factor is a measure of what is generated compared to what could be generated under ideal conditions. For solar, the capacity factor for the world is in the mid 20% range (for the U.S. it ranges from 26% to 28%) (Lazard, 2018). For comparison, a typical fossil fuel or nuclear plant has a capacity factor of 94% or more. To complement an intermittent source, an additional source is necessary, one that can be operated at will and at any power level to make up the difference when the intermittent source is not available or unable to run at frill rated capacity.

Hydro power is one possibility, but is very limited by the availability of sites that are acceptable. Biomass can be used, but the supply of material is limiting and costly. Nuclear can provide a low GHG source of energy to augment intermittent renewables. Nuclear does not depend on weather conditions and fuel is not of concern since sufficient uranium is available. Unfortunately, there are a number of barriers that are limiting the use of nuclear.

The first is economic. As pointed out in a previous work, the cost of natural gas (Princiotta, 2011) and the associated cost of the construction of natural gas generating stations have placed new nuclear power plants at a significant economic disadvantage. For the most recent year that data is available, the cost of electricity from a current frilly depreciated nuclear power plant is $33.50 per MW-hr (NEI, 2018). While this cost is below the projected cost of new combined cycle gas turbine power plants, it is above the cost of subsidized wind. For new nuclear, the cost is dramatically higher, in the range of $100 per MW-hr to nearly S200 per MW-hr (Lazard, 2018).

As a result, if natural gas prices remain low (compared to historical prices), there will be an increased use of natural gas across all energy sectors and an increase in liquefied natural gas exports. The electric power sector will therefore experience a continued shift to electricity generated by these fuels, driven in part by these historically low gas prices. The increase in natural gas electricity generation combined with a larger share of intermittent renewables will likely result in additional retirements of less economic coal and nuclear plants in the future (USEIA, 2019). For each MW-hr of electricity generated from a combined cycle gas-fired plant, there are 0.51 Tons of CO, equivalent emitted. Thus, for each MW- hr generated by a combined cycle plant that could have been generated by a nuclear plant there is 0.51 Tons of CO, equivalent that could have been avoided without additional carbon abatement technologies. Assuming there isn’t an increase in the capacity factor of renewables (highly unlikely at this time), there will continue to be a need for some form of dispatchable electric source, such as gas or nuclear. The replacement of nuclear by gas will lead to unnecessary GHG emissions for this sector.

For maximum efficiency and lowest cost, large thermal power plants, such as nuclear, are run at or near their maximum output with capacity factors in the 90% to 95% range. When combined with intermittent sources, the nuclear plant must operate at lower power levels. Since the largest single contributor to the cost of power from a nuclear plant is the capital construction cost, the total cost needed to bring a plant to commercial operation, which is a fixed cost, the cost of the power increases significantly when the plant is run at a lower power level. There are also some technical considerations that limit the ability of the current generation of nuclear power plants to be run at very low levels without modifications. Both French and German nuclear power plants routinely adjust their output to match the needs of the electrical grid. In the U.S., this is not the practice and there are regulatory restrictions that limit the “load following” practices followed by France and Germany. Nonetheless, the designs of most of the current generation and the new generation of nuclear plants under construction are capable of load following, possibly with some modifications (Lokhov, 2019).

  • [1] For renewables, the variation in GHG emission is small, so only the median values are presented.
 
<<   CONTENTS   >>

Related topics