Tuesday, 28 September 2021

Part 3 - Nuclear Power and the Reduction of Carbon Dioxide Emissions

In the previous article I concluded that intermittent renewable energy sources, like wind and solar PV, will not be sufficiently reliable to fully replace fossil fuels. Furthermore, even with an installed overcapacity of such sources, energy storage options are insufficiently developed to power electricity supply grids during periods of low renewable energy production and, if nuclear generation is ruled out, fossil fuel powered backup generating stations will be required.

Ross Koningstein and David Fork are engineers at Google who worked together between 2007 and 2011 on the renewable energy initiative somewhat geekily known as RE<C". From a position of strongly supporting renewables as the answer to the climate crisis, and following modelling studies, they came to the view that renewables alone would be insufficient to address the requirement for enough carbon free energy to halt the increase in atmospheric carbon dioxide.
“As we reflected on the project, we came to the conclusion that even if Google and others had led the way toward a wholesale adoption of renewable energy, that switch would not have resulted in significant reductions of carbon dioxide emissions," wrote Koningstein and Fork.”

So what would it really take to limit or even reverse climate change if today's renewable technologies by themselves aren't enough?

Koningstein's and Fork's research led them to think that a new technology is required, which will disrupt the existing status quo, but beyond discussing smarter, distributed, small scale power generation, they didn’t specify which technology, or deal with the need to act now and not wait in the hope that something new will emerge.

Environmentalists for nuclear and renewables
Taking into account the variability of wind and solar, and the current status of energy storage technologies, I've come to the conclusion that, alongside renewables, the only energy production technology that is available to be deployed on a global scale over the next twenty to thirty years, which will not contribute to climate change, is nuclear power.

In this video James Hansen explains why he has reached the same conclusion.

Other environmentalists also expand on their reasons for changing their view of nuclear power in this video.

So, together with a growing community of engineers, scientists and environmental campaigners, I am proposing nuclear power alongside renewables as the future for reducing carbon dioxide emissions and slowing the increase of the atmospheric concentration of greenhouse gases.

The example of France
As a result of the 1973 Oil Crisis, and its lack of natural fossil fuel resources, France decided, on the grounds of energy security, to rapidly expand its fleet of nuclear reactors and most of the plants currently in operation were built in the 70’s and 80’s. There are at present 56 active power reactors and, over the 30 years between 1990 and 2019, nuclear power generation has saved around 49 million tonnes of carbon dioxide from being discharged to the atmosphere. France exports electricity to nearly all its neighbours and earns €3 billion per year from this.

I quote this example to illustrate that a major change of technology can be implemented in a relatively short time, with highly effective results.

What does a nuclear power station contain?
Since, however, not many people have studied nuclear technologies; I will take the liberty of briefly explaining how reactors work and the degree of diversity of designs that have been built.

The primary components of any nuclear power generation system are the following;

· A critical mass of fissile material;
· A containment structure;
· A power supply and control system;
· A cooling/heat transfer loop;
· A power generation system (a means of turning the heat into electrical power).

A critical mass of fissile material is created when there is enough of the  radioactive components and they are sufficiently close to each other that the neutrons generated by the fission of one atom set off a chain reaction by stimulating other atoms to fission. This process generates heat and the neutron flux must be carefully controlled so that the rate of fission does not outpace the ability of the cooling system to remove heat from the reactor core. This is traditionally done by inserting and adjusting control rods of neutron absorbing materials like boron or hafnium. Clearly these reactions need to be contained and not allowed to disperse into the environment. The type of containment depends on the conditions of temperature and pressure inside the reactor but usually comprises a metallic reactor vessel inside a reinforced concrete containment structure.

There are many types of nuclear reactors.

Reactor regime                Thermal or Fast neutron spectrum

Fissile material                 in solid or liquid form

Cooling/heat transfer        by water, molten sodium, molten lead, molten fluoride                                         or chloride salts, carbon dioxide or helium gas.

Thermal spectrum reactors with a moderator, such as graphite or water, are the most common. As of April 2020, there are 440 power reactors in the world, operating in the thermal spectrum, with a combined electrical capacity of 390 GW. Additionally, there are 55 reactors under construction and 109 reactors planned, with a combined capacity of 63 GW and 118 GW respectively,

The fast spectrum is currently employed in four active reactors worldwide.

How does a nuclear reactor work?
The article “NUCLEAR 101: How Does a Nuclear Reactor Work?” explains in a simplified way how the two most common types, “The Pressurized Light Water Reactor PWR” and the “Boiling Water Reactor BWR”, function.

Pressurized water reactors

Solid fuel, second generation pressurized light water reactors (PWRs), are the most common type of nuclear reactor currently in service and make up two thirds of the US fleet but, whilst they have made a major contribution to carbon free power over the decades, I am convinced that they are not the best option for the future, other than as a short term stop-gap solution. 

PWRs were originally designed to be used in submarines for which, being surrounded by emergency coolant, they are ideally suited and they represent a design that dates from the late 1960's. There has been little improvement since then.

Explosion Risks
In order to generate high temperature steam at around 300 deg C, PWRs run at a pressure of over 300 bars (atmospheric pressure is equal to one bar) and therefore carry an inherent risk of explosion. To guard against a reactor vessel failure they need thick heavily reinforced concrete containment vessels. They burn enriched uranium fuel which generates waste with very long lived radioactive transuranics and actinides. Also they can only burn 4 or 5% of this fuel before the fuel rods deteriorate and must be replaced. Typically one third of the fuel rods are replaced every two years and kept in storage ponds until they are reprocessed to recover the remaining reusable fuel.

Boiling water reactors

Boiling Water reactors run at lower pressures of 70 -150 bars but are otherwise similar to PWRs in their fuel types and safety issues.

Emergency reactor shutdowns
Another major concern with many of the second generation reactor designs currently in service is that, when a reactor shuts down unexpectedly, and there is no power available from the grid, they need backup power supplies in order to run pumps and therefore maintain the cooling needed to remove the heat from the radioactive decay of fission products. This was what failed at Fukushima Daiichi, where the backup generators were in a location which was only designed to resist a 3 metre tsunami and were consequently put out of service.

More recent generation III+ PWR designs currently awaiting approval or under construction do, however, incorporate passive cooling, meaning that once shut down, they don't need power to dissipate decay heat. Other safety features are also included.

Liquid Metal Fast Breeder Reactors

Some types of reactor operating in the fast neutron spectrum, like the liquid metal fast breeder LMFBR, which uses molten sodium as the coolant medium, have other disadvantages. Sodium metal in liquid form is a good coolant, which operates at higher temperatures and lower pressures than water, and is a very good heat transfer material, but it reacts with air, and violently reacts with water. So the possibility of a leak between the liquid sodium and the steam generating sides of the heat exchangers is a serious safety concern. These types of reactors, depending on their size and design, can also have an overall positive temperature coefficient of reactivity, meaning that as the temperature rises, nuclear reactions increase and create more heat.

Such matters can, however, be overcome by good design. The Experimental Breeder Reactor at Argonne (EBR II) included passive safety features and was successfully tested in 1985 under conditions simulating power failure.

Terra Power
, a company sponsored by Bill Gates, is proposing a reactor using the fast spectrum, with cooling by molten sodium.
The project features a 345 MW sodium-cooled fast reactor with a molten salt-based energy storage system. The storage technology can boost the system’s output to 500 MW of power for more than five and a half hours when needed.

Fast spectrum nuclear reactors can also be designed to breed fissile fuels, such as Plutonium 239 and Uranium 233, from naturally occurring fertile materials like Uranium 238 and Thorium 232. They can also be designed to burn waste from operational reactors using the Uranium/Plutonium fuel cycle.

Service temperatures
Reactors currently in service can run at temperatures ranging from 300 deg C for PWRs to 650 deg C for the Advanced Gas Cooled Reactor. The higher the temperature, the more efficiently the power generation system can function.

Still higher operating temperatures of up to 1000 deg C are proposed for the Gen IV High Temperature Gas Cooled Reactor. This opens up the possible application of nuclear power to generate process heat for industry and thus replace fossil fuel usage.

Much research is being directed to determining the long term resistance of materials to the neutron flux in advanced reactor designs, their limitations, and their suitability under operating conditions of high temperature and pressure.

Nuclear safety
Most people object to nuclear power due to safety concerns and the longevity of nuclear waste. Let’s discuss safety first; nuclear waste will be dealt with in part 4 of this series of articles.

In spite of the age of most of the nuclear plants currently in service, on a global scale, there have been very few incidents involving nuclear reactors which have had major consequences, and two of those were at plants with fundamental design flaws.

At Chernobyl, where 64 people died of acute radiation sickness after emergency work, the reactors were never provided with containment vessels and they had other design flaws which would never have been permitted in Western countries.

At Fukushima Daiichi the backup power plant was flooded by the very large tsunami that we tend to forget resulted in over 18,000 people dead or missing. The several dozens of direct casualties at Fukushima are attributed to the effects of the evacuation of the surrounding area on the sick and elderly, but some workers have received radiation doses which exceed lifetime safety limits.

At Three Mile Island operators did not react appropriately when a pilot valve failed and, by sticking open, allowed coolant to escape. This fault had occurred on 11 previous occasions!

Estimates of future deaths related to radiation exposure from these accidental releases of radioactive material vary widely depending on their source, the methods used and the assumptions made. For example for Chernobyl in 2006 the WHO estimate of radiation related premature deaths was 4,000 whereas the Greenpeace estimate was 200,000! In 2008 another WHO report urged caution in the development and use of projections (paragraph 110).
The UN Scientific Committee on the Effects of Atomic Radiation makes the following statement in their report revised on 6th April 2021 concerning Chernobyl.

Quote<< Apart from the dramatic increase in thyroid cancer incidence among those exposed at a young age, and some indication of an increased leukaemia and cataract incidence among the workers, there is no clearly demonstrated increase in the incidence of solid cancers or leukaemia due to radiation in the exposed populations. Neither is there any proof of other non-malignant disorders that are related to ionizing radiation. However, there were widespread psychological reactions to the accident, which were due to fear of the radiation, not to the actual radiation doses. >> Unquote

Estimates of premature deaths are controversial and the methods for calculating them are disputed by respected scientists. A full discussion would occupy too much space in this context, as would an examination of the number of fatalities in other industries associated with energy production, like coal mining. As the table below shows, all energy production carries risks and is responsible for fatalities.

Rates for each energy source in deaths per billion kWh produced. Source: Updated (corrected) data from: World Health Organization; CDC; Seth Godin; John Konrad.

In fact you are exposed to less radiation standing next to a nuclear reactor than flying in a plane at 30,000 feet but the specific point I wish to make is that even safer nuclear power options are available than those which are currently in operation. 

In the next article, I deal with innovations in nuclear power generation and discuss safer options than the familiar pressurized water reactors that have been in use since the 1960-70’s.


Post a Comment