Cassini: In-Depth Look

Recently, I published a post on the Cassini spacecraft that currently orbits Saturn. I received a comment from a reader stating that they wanted to know more about the process of actually using the alpha decay process of the plutonium for power.

I will apologize ahead of time but to have answered the question I had to use some technical/science terms.

A definition you will need:
Thermocouple: a kind of thermometer consisting of two wires of different metals that are joined at both ends; one junction is at the temperature to be measured and the other is held at a fixed lower temperature; the current generated in the circuit is proportional to the temperature difference.

This is a great question and I have to admit that I had to do some research in order to answer it because it is different from how nuclear power plants work (which makes total sense). Turns out that Cassini is powered by three...wait for it...radioisotope thermoelectric generators. Big word but here is a simple explanation of what it does: an RTG uses heat from radioactive decay in order to generate electricity by use of thermocouples. In Cassini's case the heat was from the energy coming from the radioactive decay of plutonium. The explanation of the process can get quite technical but I will simply put it that an RTG relies on temperature differences in order to create a voltage.

An RTG consists of the fuel, plutonium, which is kept in some sort of container. Thermocouples are hooked up to the container and the heat produced from the radioactive decay allows for electricity to be generated. You can see the RTGs at the bottom of the Cassini spacecraft on the image below.

  

RTG's can use fuel other than plutonium (i.e. strontium, polonium, curium, etc.) however, plutonium requires the least amount of lead shielding (2.5mm) in order to contain the radiation. And when you are designing a spacecraft, the weight of everything on board is extremely important. Plutonium also had an optimal half-life (87 years)  for the mission...meaning that it should have a continuous release rate of energy for an optimal amount of time (in Cassini's case it needed to power for at least 11 years).

I hope this helps!

NASA's Cassini...Thank You Plutonium

In an attempt to cure my terrible case of writer's block, I have decided to possibly go into a series concerning "other benefits of nuclear energy/radioactive materials". For my first attempt I would like to discuss something historical...and one of my favorite subjects.

I love anything and everything to do with space. Planets, stars, spacecrafts, shuttle missions, space stations, galaxies, and my favorite: planetary dust aggregation...you name it...I'm interested. So you could have imagined how shocked I was in nuclear energy class this week when my fellow students discovered for the first time that the successful Cassini mission (all the way to Saturn) was powered by plutonium 238.

Most spacecraft missions are powered by solar rays but because of Saturn's distance...Cassini was going to need something different. Therefore the idea was to use approximately 72 pounds of 238Pl. The heat given off by the alpha decay process of plutonium would fuel Cassini for the entire 11 year trip (awesome). Of course, nuclear energy has had enemies throughout all of history, and at one point the mission was almost canceled due to a high turnout of protesters. Many claimed that if the Cassini exploded as the Challenger did in 1986, it would spread high amounts of plutonium all over Florida. But NASA prevailed and went on with the project and launched Cassini in 1997. Loaded up with its 72 lbs of plutonium it was on its way...and to gain momentum throughout the flight all the way to Saturn, it performed several gravitational slingshot procedures when within distance of other planets (this actually means that it went the wrong way first-towards Venus- in order to gain momentum from Venus twice and then Earth before starting its real trip).

At this very moment Cassini orbits Saturn and delivers data to us. Here is a great link to one of NASA's pages that describes all of the Cassini events happening this year! Cassini has brought so much valuable information into our hands...more than we could ever have dreamed of. In 2004 alone, Cassini delivered images to us that allowed for the discovery of three new moons! And just think that this was only possible because scientists were able to understand the decay processes of radioactive elements and NASA was able to carry through with the project despite massive protests.

Video Blog: Risk versus Hazard

Dr. Cecil Presents: Basic Science of Nuclear Reactor Meltdown

Last Thursday, I attended a presentation by Dr. Ed Cecil, Colorado School of Mines Professor, which covered the basic science behind nuclear reactor meltdowns. I thought it would be interesting to attend a short presentation about a subject I already understand in order to see how Dr. Cecil would portray the complex information of nuclear physics and engineering to an audience of non-nuclear junkies and do it in less than an hour.

He started off with the simple idea of a nuclear power plant..."it works just like any other power plant setup" that is it contains a steam generator that spins a turbine for power. Everyone can get that...right?

He explained to the audience that from the binding energies one can determine that the average fission process gives off 200 MeV's of energy and that by using that number we can calculate that we only need 10 kg of uranium to supply a 1 GW reactor for 6 months (AWESOME). Cecil accidentally stumbled onto the "iron death of the universe" but he quickly recovered himself. 

Well that's all great and dandy, but most importantly Cecil discussed the problems with nuclear power and how to interpret "radiation". Two topics I feel that the public today knows only enough to be dangerous about. On the first point, a downfall of nuclear power is the decay products left over from the process. Below is one of Cecil's slides listing the radioactive by products.

Q. So what are the radioactive by products of nuclear fission?

A. Lots and lots:

•Isotope                 Half-life         Fraction per fission

•137Cs, 137Xe,…  30 yr, 8 min,…    6.2%

•90Sr, 90Rb,….  28 years, 4 min,..     5.8%

•144Ce,…..            284 days,…         5.5%

•95Zr,…                   64 days,             6.5% 

•etc.  etc. etc

•131I,…                8 days,…              2.9%

•                                               Total = 200%

 

It's because of those elements with half lives of 30ish years that we have to come up with ways of safely storing the waste. (i.e. the shorter half life elements will eventually decay away...but it takes 30 years for some of them)

Another great topic that Cecil discussed was how to interpret radiation. People have become afraid of the word radiation...and I don't blame them. But you have to realize that radiation must be quantitatively described in order to know if it is really dangerous or not. In our world today we have background radiation of up to 0.1 rem/ yr. Bananas alone give off about .0001 rem/yr...should we all be running away from bananas? No. For example, the max dose for industry (nuclear submarine workers or nuclear plant workers) is approximately 10 rem/yr. Therefore, before the public goes off and reads articles about how high the radiation levels are somewhere...they need to educate themselves on what exactly is a "high" number. It was just the other day in my nuclear energy class when we read an article about Fukushima. The article had a quote in there about how "tiny amounts of radiation were leaking..." so what exactly does "tiny" mean. Who knows.

One last thing that I would like to bring up that Dr. Cecil discussed was just a small historical fact. There are a lot of people out there that bring up Chernobyl whenever nuclear power is discussed. Well, yes everyone knows that Chernobyl was a sad day, but there are quite a few of us that don't understand that there was something different about Chernobyl than with every other nuclear reactor in the world. Chernobyl was designed to operate at a criticality above one. Everybody but Russia seemed to realize that this was a bad idea...Russia was warned repeatedly but still went ahead with plans and built the reactor. That design failed just like everyone knew it would.

Bravo Dr. Cecil, what a daunting task to try to explain most of how a nuclear reactor works and about radiation in less than an hour. The talk was a bit jargony but we were all engineers in there so... I hope those 40 some people all went home armed with accurate knowledge about nuclear energy. 

Social Media Sites Dig Nuclear Energy (the graph says so)

Sorry but I just had to make a little blurb about something I found...it reminds me a lot about what we were doing in class with the BP presentations. For example, comparing how media reports certain controversial topics and how geography also affects what is reported. So, I just stumbled upon a site called Social Radar that monitors social media sites to understand how hot topics are being discussed. For those who have no idea what it is (like I didn't) Social Radar's site says: "Social Radar is a social analytics application that collects billions of articles and messages from millions of sources such as blogs, social networks, news sources, microblogs, forums, and more to provide instantaneous insights and measurement into online chatter."  It can even count positive vs negative talks on a subject and where in the world the talking is coming from. Cool, huh?

I pulled up the nuclear energy topic on Social Radar...turns out that they had a graph specifically made to describe the social media conversations from before the Fukushima incident to after. Social radar even determined if the conversation had positive or negative (describing nuclear energy) context. The graph (below) actually has some really interesting data.   

Graph from Social Radar

Before the accident, most of the social media conversations about nuclear energy were "positive". However, once the Fukushima incident happened negative comments took over, which we all could have predicted. The surprising part is that now it seems that people are once again talking positive. I knew nuclear energy would overcome this...but that was fast.

Don't Eat the Yellowcake: Uranium Mining

There are a lot of us out there that don't exactly understand the processes behind "mining" uranium. It's actually a lot like the mining processes for other materials. People seem to have this notion that uranium is a very rare commodity when actually the earth's crust has about a 2.8 parts per million concentration of uranium, about the same as tin. My expertise is actually in the metallurgy field which is greatly connected with mining, so I want to take the time to clear up some misconceptions (hopefully) and explain just how uranium is mined.

There are three main types of uranium mines out there: In Situ, Open pit /underground, and heap leaching.

In Situ leaching (or solution mining) of uranium can be used if an orebody lies in some type of porous material and in ground water. Bore holes are drilled through the ore body and subsequently a leaching solution is poured down the bore holes. Once the leaching solution comes into contact with the ore body it dissolves the wanted material and thus when it is pumped back up to the surface the solution now contains uranium. The uranium is then recovered as a precipitate. Common leaching solutions used for uranium are sulfuric acid or sodium bicarbonate (but others can be used). One benefit of in situ leaching is that it has little ground disturbance (vs. open pit/underground mining) and there is no need for crushing.

Open pit and underground mining of uranium is pretty self explanatory (it's probably the picture that comes to mind when we all think of "mining"). If the uranium deposits are located somewhat close to the surface then open pit mining is preferential. A large pit is made and piles of ground material (uranium plus the rock material) are removed and sent to a processing/separation facility. All of the material will be crushed and then leached with an acid in order to dissolve the uranium oxides. Once uranium oxides are dried out (85% uranium by mass at this point) the resulting product is what we all know as "yellowcake". This is then packed into steel drums and sent to where they are needed. Underground mining is generally the same expect for the uranium deposits sit deeper within the crust and so tunnels must be dug into the ground in order to mine the uranium. These two options are by far the most popular, they account for 57% of the uranium mines.

Finally, heap leaching is very similar to in situ leaching. Except now very low grade ore is piled up and acid is poured on it. The new solution with dissolved uranium is then collected from underneath the pile and processed in the same way as with in situ.

So where is this being done? Turns out that there are uranium mines in over twenty countries. However, the top six of these countries produce about 85% of the mined uranium. They are: Kazakhstan, Canada, Australia, Namibia, Russia, and Niger in that order (but Australia has by far the largest reserves). In 2009 approximately 60,000 tonnes of uranium were mined which met 76% of the world's demands at the time. One good thing is that it is actually relatively simple to find uranium sources...considering the element is radioactive. A map of a region's radioactivity can be made and thus uranium can be found.

Well, is it safe? In fact...it is. Because the uranium is in such a low concentration there are no major health concerns for workers in the mine. There are however, a few uranium mines that deal with high concentrations. In these rare cases, special measures are taken to ensure that uranium dust does not get into the air and there is limited contact with the actual ore.

These methods have been used for decades and I don't see them changing anytime soon. It works and so it will continue. However, the search for better leachants and solutions is always on in order to recover more of the uranium.

Not Everyone Follows Crowds-South Africa Pushes Forward Despite Japan's Situation

I came across an article today about South Africa moving on to the last step for approval of an energy plan that will increase its nuclear capacity. This is shocking news to a lot of people. Recently, it had made it into the news that Greenpeace (a global lobby group) had urged SA to abandon the plans for adding more nuclear to their energy budget...saying that they should focus on "renewable energy sources". Greenpeace even went as far as calling SA's plan "absurd" due to the current events in Japan. One quote from the group: "We urge the government to rethink its coal and nuclear plans. Instead of dirty and dangerous power generation, it should be working towards a true energy revolution by investing in energy efficiency and renewable energies."

SA's plan involves adding 9600 MW of power from nuclear energy by 2030. And it has got everyone thinking...what is SA thinking? After the Japan incident, Switzerland and Italy postponed meetings about nuclear energy. Germany temporarily shut down 17 nuclear reactors. And, even China put a hold on all applications for new reactors. Yet here is SA deciding to move forward.

I think it's great. It was bound to happen...everyone knows that Fukushima is not going to shut down all of nuclear energy. I don't even think it'll slow it down. Governments know that the way things are going now with coal and such, it's bad...we just can't go on like that forever. If it wasn't South Africa it would have been another country...but hats off to SA for not listening to lobbyists or jumping on the media's bandwagon of nuclear fear.

“A little more persistence, a little more effort, and what seemed hopeless failure may turn to glorious success.” Elbert Hubbard