Nuclear Energy

Welcome back to Ballarat Science in the Community,

To kick things off with our 1st “in depth” discussion I wanted talk to you about nuclear energy. What it is, how we use it, some of the pros and cons of the technology. As you can imagine this is an astoundingly big topic, as such we’re going to break it up into a series of several posts so that we don’t bombard you with too much information in one go. I’m going to lay some of the ground work in this first post then we’ll mix it up with later posts in this topic. I keep seeing reactionary posts/comments in social media talking about how “yucky” nuclear energy is but these posts never seem to go into too much detail regarding the science. Let’s do something about that.

To help me out, I’d like to introduce Jamie Grimwood who’s been collaborating with me on this blog series and will be helping to produce more content both here and in other BSCit projects through the coming year. Jamie is an Environmental Science student at Federation University, an environmentalist and horticulturalist and Nuclear/Atomic energy is something that we’ve both been excited about and we’re keen to share some of the things we’ve been reading up on.

So, what do you know about Nuclear Energy?

Introduction

To start, let’s have a little refresher on what makes our world tic….Physics.

We’re going to be talking about atomic decay and energy, so knowing what makes up the basic concept of an atomic structure as well as a bit about the conservation of energy and a little bit of relativity (E=mc^2) will be handy. If you’re going to build a reactor, sure, you should have a better grasp, we’re not going to go too deep into this because to get a basic idea of how and why we use nuclear energy we don’t need to.

So, Einstein’s theory of relativity (E=mc^2) becomes relevant to us as we talk about atomic decay and producing energy. I’m going to try and give you the crash course here and we can go into it in more detail, at a later date. 

Important things to note: 

  • Energy is the ability of a system to do work, it’s measured in joules. 
  • Energy can never be created or destroyed, it can only be converted.
  • Mass is equal to the volume of an object multiplied by its density. (How much stuff is in a given amount of space.) 
  • Mass is a constant feature of an object unlike weight which is relative to the action of gravity on said object. Mass is measured in kilograms
  • The speed of light (c= 299,792,458 m/s) is constant for all observers, it’s a universal speed limit and nothing goes faster. 

Basically, what the equation E=mc^2 tells us is that energy can be either stored in an object’s mass, referred to as mass energy or energy density. Or, it can be converted into a form of energy (typically that means a combination of light, heat and sound.)

Now, an atom is the fundamental chemical element required to form matter (“stuff”) there are 118 elements on the periodic table, most are naturally occurring, several are man-made (several were added last year.) It was thought that atoms were the fundamental building blocks of our universe. However, we now know that there are even smaller ‘subatomic’ particles called quarks but thats getting into quantum theory and a whole other blog.

atom

Figure 1. Atomic structure of a carbon atom, adapted from Universe Today.

All atoms consist of an atomic ‘nucleus’ which contains a given number of positively charged ‘protons’, a number of ‘neutrons’, which have no charge and a number ‘electrons’ which are negatively charged and balance out the protons charge. The overall number protons in an atom determines its atomic number and chemical properties

Changing the mass of the atom releases energy, you can do this by adding mass, as in fusion or removing mass, as in fission

Fusion and Fission

Fusion is the atomic process where two hydrogen atoms are forced to bind into a single Helium atom, releasing huge quantities of energy. It is this Fusion reaction that drives our sun releasing energy into to the cosmos. The mechanics of how the sun works are awesome and involved, so we might get back to this later.

Nuclear Fission, is the process of splitting atom and at present all commercial nuclear power plants on earth are nuclear fission reactors. They work by controlling the atomic decay of radioactive material. Controlled fission is a nuclear reactor. Uncontrolled, is a bomb.

A sustained fission reaction occurs when you have a quantity of radioactive “stuff” like Uranium (the U-235 isotope is the commonly used substance) and you shoot atomic particles known as neutrons at the nucleus of the U-235 isotope. This splits the “stuff” into two smaller isotopes and releases another three neutrons, which splits two more isotopes of the U-235. This controlled reaction releases energy in the form of heat which can be used to turn turbines and produce electricity.

fision

Figure 2. Fission reaction, adapted from Live Science.

The point to note is that, in a given amount of “stuff” there are billions of atoms, which can produce an immense amount of power. The energy produced by just 1kg of naturally occurring uranium containing about 0.7% U-235 (enriched/reactor grade uranium has about 5%) could power a 100W light bulb constantly, for nearly 200 years.

Radiation

What’s commonly referred to as “Radiation” is actually a few different things. There is ‘pure’ energy in the form of gamma rays, and particle radiation made up of very small particles that carry lots of energy.

Radiation from a given source can be ionising like Alpha and Beta particles with  an electromagnetic charge or Gamma and Neutron wave radiation which has no charge. Or it can be non-ionising like radio waves or the spectrum of visible light. In the context of this discussion we’re typically talking about the Ionising radiation produced by the atomic decay of a radioactive substance, as the isotope breaks down to a simpler substance it produces Ionising radiation which has enough energy and charge to strip the electrons from atoms and molecules as it passes through a given medium.    

radiation-penetrationFigure 3. Radiation penetration, adapted from Mirion Technologies, Types of Radiation

There are two factors to consider when we talk about radiation they are, the form of nuclear radiation and the dosage or exposure to said radiation.  Types of radiation include, Alpha (α) particles, Beta (β) particles, Gamma (γ) ray and Neutron radiation. Each having a different range and ability to penetrate a given medium (air, concrete, aluminium, you) the nature of these radioactive particles and waves differs significantly.

Exposure to radiation does not make you radioactive, except for Neutron radiation (but becoming radioactive when you’re at ground zero of an atomic bomb is kind of a moot point.)

As the frequency and wavelength of the radiations increases so does it’s energy and it’s ionising power.

ionizingvsnon-ionizing

Figure 4. As the wavelength increases so does the ionising potential. Adapted from Mirion Technologies, Introduction to Radiation Safety.

Alpha particles are highly ionising they have a low penetration power and are easily stopped by a simple sheet of paper however Alpha particle radiation can be harmful if inhaled or ingested. Beta radioactive particles are much more energetic than alpha particle, having a moderate penetration and ionising power but are still able to be stopped by a thin sheet of wood of Aluminium.

Neither Gamma or X-ray radiation have associated particles like alpha or beta, they are pure energy, they are also very energetic and able to penetrate through most substances. This gamma radiation is why nuclear reactors require substantial shielding and control over exposure/dosage to protect people from harm.

Neutron radiation occurs at location of nuclear fission (i.e. controlled/uncontrolled reaction,) although usually in both instances the neutron is readily absorbed by hydrogen atoms like in water or concrete.  Still…Being present at either event would be ill advised.

We measure radiation relative to its dosage, the units Grey (Gy) and Sievert (Sv) are measures of the absorbed radiation over time Sv per sec or Gy per day, whereas the Becquerel (Bq) is a measure of the radioactive decay or nuclear disintegration of a particular radioactive material. With 1 Bq = 1 disintegration/second. People who work regularly with radioactive materials “Radioisotopes” commonly wear carry specialised equipment like a Geiger counter or special tags that record their exposure and alert the wearer when it’s becoming too dangerous.

Table 1. Radioactivity of everyday and other materials.

1 banana 15Bq
1 kg of brazil nuts 400Bq
1 kg of coffee 1000Bg
1 kg of granite 1000Bq
1 kg of coal ash 2000Bq
The air in a 100 sq metre Australian home  (radon) 3000Bq
1 adult human (65 Bq/kg) 4500Bq
1 household smoke detector (with americium) 30000Bq
1 kg uranium ore (Australian, 0.3%) 500000Bq
1 luminous Exit sign (1970s) 1000000Bq
1 kg low level radioactive waste 1000000Bq
1 kg 50-year old vitrified high-level nuclear  waste 10000000Bq
1 kg uranium 25000000Bq
Radioisotope for medical diagnosis 70000000Bq
Radioisotope source for medical therapy 100000000Bq

Dosage AND exposure are what’s important, that’s why radiologists sit behind a wall. While the dosage you receive is quite large (100 x 10^6 Becquerels) the time you are exposed to said radiation is only very small and then you leave the room, they work all day.

 

What’s in a Reactor?

As mentioned earlier, a reactor is a way of having nuclear reactions happen in a controlled, sustained way. Put simply, a nuclear reactor is a machine that is able harnesses the strong nuclear forces.

There are several kinds of nuclear reactor but they all take advantage of this naturally occurring reaction to produce electrical energy without producing CO2 emissions. As you can imagine nuclear engineering is quite the complex subject, currently there are more than 440 commercial nuclear reactors and upward of 250 research reactors around the world (including ANSTO’s Open Pool Australian Lightwater (OPAL) reactor.)  You should also check out the phenomenon of naturally occurring nuclear fission like that found at the Oklo uranium deposit in Gabon, Africa.

The commercial reactors are made of several types of nuclear reactor these include, Light water reactors (LWR), Pressurised heavy water reactors (PHWR), the Advanced gas-cooled reactors (AGR) and Fast neutron reactors (FNR). Your generic nuclear reactor works as a heat transfer system, taking the heat energy produced by the atomic decay of the fuel and transferring it to the steam turbines to produce electrical energy. In this sense, it’s no different to any other heat exchanger, the fuel is used to heat water, turning it into steam, the steam is then used to turn the turbine and produce electricity.

gross_and_net_power

Figure 5. Basic construction of a nuclear reactor, adapted from world-nuclear.

The components.

The fuel source in a nuclear reactor is typically comprised of hollow rods packed with uranium oxide (UO2) pallets are configured to make reactor core. Moderators and Control rods are used to inhibit and control the rate nuclear decay and neutron emission, this varies the heat produced allowing for power generation to be regulated. This heat is transferred from the reactor core to a Coolant fluid, all these are housed in a Pressure vessel and connected to a Steam generator with a Containment structure built around this to protect the system from interference and to contain any radioactive output from either regular operation or malfunction. Failsafe systems for major incidents, like core catchers are also used in newer systems to can collect melted reactor and other radioactive material.

 

Natural Nuclear Fission.

 

Yes, that’s right, it’s a thing. The theoretic model for natural nuclear fission was first published in the 1950’s and verified through observation and experiment at the Oklo uranium deposit in the former French colony of Gabon, West Africa, in the early 70’s.

oklo

Figure 6. Natural Nuclear Reactor in Oklo, Gabon. Adapted from Flatrock.

It happened like this. While doing routine number crunching analysing the uranium content of ore being processed it was discovered there was a discrepancy between the amount of Uranium 234 which should have been present in the ore (0.720%) and the amount that was present in the Gabon samples (0.717%) That’s a difference of 0.03 g per 1 kg of ore. This difference was enough to alert the CEA (French Atomic Energy Commission) after a full analysis and investigation it was discovered that the site of the Oklo mine (and several other sites in the area,) had been undergoing spontaneous and naturally occurring nuclear fission reaction for millions of years.

Research into the Oklo phenomenon has since theorised that, over the course millions of years the uranium deposit got to a state of critical energy (we mentioned neutrons setting of a chain reaction earlier.) This was only possible because groundwater at the deposit ran through the site acted as a coolant allowing the fission reaction to take place. However, the temperature at the reactor would eventually get so great as to boil off this coolant ceasing the reaction. Every few thousand years’ groundwater would flow back through the uranium deposit and the fission reaction would start again.

With the resulting waste being safely locked into surrounding minerals, rather than migrating out of the fission zone with the groundwater, the big takeaway from this is. Given the right structure and material nuclear waste products can be safely stored underground. But, that’s for another day.

 

Conclusion

The aim of this article was to lay out some of the ground work so that you have a bit more of an understanding of the physics and science behind nuclear energy. We’ll go into more detail about nuclear waste, emissions, and other facets of nuclear science as well as things like Einstein’s theories of relativity and quarks in later blogs. I hope this has been helpful.

Until next time.

Ask your questions, support your opinions with evidence, discuss,

EDIFY.



 

Bibliography

Basic Physics intro

From atomic structure to atomic decay, these links can help get you on track to understanding our universe.

 

Nuclear Science

Everything from understanding the basics of radiation, to how reactors work and how many there are in the world right now. I used these 3 resources extensively for this blog (almost every topic) and will be using them again in future ones.

Radiation

More on radiation and safety measures to deal with it

Nuclear reactors

An interesting piece from MIT review about some new nuclear tech coming out of China and America. As well as a link to ANTSO’s Open Pool Australian Lightwater (OPAL) reactor.

 

Oklo Naturally occurring fission reaction.

Have a look at this extraordinary phenomenon which looks at the fact that nuclear fission is a natural event (albeit rare in nature.) and how waste has been safely stored underground trapped safely in mineral deposits


 

 

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