Conversations in Science

New Radio Show and Heavy Water

You never know what opportunities crop up when you put yourself out there. My recent appearances on various shows with KLRNRadio have been no different.

I now have my own show on KLRNRadio: Conversations in Science. I'm still trying to figure out exactly how that happened. Rick Robinson and Jessie Sanders have been trying to convince me for some time, but I was resistant. Producing my own radio show? That was the last thing I wanted to be doing. But apparently, I have the knack of explaining science in a way that everyone can understand. Maybe that's why they kept calling me for help on the science stuff.

On a recent episode of Jessie's POV, Jessie Sanders called out for help, not understanding the science behind the issues of Iran and its stockpiles of heavy water. She even shouted out, "Judy, if you're listening... HELP!" Of course, I wasn't listening at the time (I was off playing "Mom"), but I got a phone call later that day...

"Judy, when do you have the time to record an episode about this and explain what heavy water actually is?"

Everything from that point was a blur; I think my head is still spinning. What was supposed to be the odd appearance on other shows has now become my own show. Jessie Sanders is producing the show for me, so all I have to do is the research, which I was doing anyway for my blog and personal writing. The whole situation went by so fast that I haven't even told my parents yet. (BTW, mom, I now have in internet radio show about science.)

Conversations in Science will air the first Monday of every month at 4pm EST (equates to the first Tuesday of every month at 9am for those in New Zealand, but this NZ time will change come summer — daylight savings).

(Someone please remind me why I agreed to this... Oh yeah, that's right. I remember now. My husband said it was a good idea. Gijs, I blame you if things go horribly pear-shaped.)

The first episode has now aired and the topic: heavy water and why its so important for nuclear power generation. It's a topic that was asked for by Jessie Sanders, all because of what is going on in Iran.

Even though I'm not a nuclear physicist, I have enough of a scientific background to understand the technical details and hopefully explain them so listeners of KLRNRadio can understand.

What is heavy water? Why is nuclear power plant waste dangerous?
(First Aired on KLRNRadio, Tuesday, September 7, 2016)

For my readers, here's the dealio:

Heavy water is just water, but the hydrogen atom used in the molecule is actually the deuterium isotope.

Okay... Techno speak is on overdrive here, so let me bring this back a few steps.

Every atom on the periodic table will exist in different isotope forms. If you remember back to high school chemistry, each atom is made up of protons, neutrons and electrons. The number of protons an atom has in the nucleus will determine what type of element the atom is. Different isotopes will have the same number of protons but different number of neutrons. (Electrons don't come into it.)

The three most common isotopes of hydrogen are protium (often simply called hydrogen), deuterium, and tritium. (Source:

The three most common isotopes of hydrogen are protium (often simply called hydrogen), deuterium, and tritium. (Source:

Hydrogen has seven different isotopes. H1 has one proton and is also called protium (but most people call this particular isotope hydrogen). H2, also known at deuterium, often given the symbol D, has one proton and one neutron. Both protium and deuterium are stable isotopes and are NOT radioactive. The heavier isotopes, on the other hand...

H3, tritium, given the symbol T, has one proton and two neutrons, and is radioactive. It's one of the main reactant in fusion reactions between deuterium and tritium, the reaction that takes the least amount of energy to initiate.

The heavier isotopes are highly radioactive, with half-lives in the order of seconds. There I go again... The term half-life refers to the amount of time for half of the number of atoms within a sample take to decay.

Right... Back to the term heavy water. The molecule of water is made up of two hydrogen atoms and one oxygen. Here's the thing, it doesn't matter what isotope of hydrogen is used, a molecule with two hydrogens and one oxygen is still water, however, water made from deuterium is a heavy water molecule.

Normal water naturally contains heavy water molecules. In fact, approximately 0.0156% of all water molecules in natural water are of the heavy variant. When scientists are referring to heavy water, they are referring to water where majority of the molecules present are of the heavy variety. (CANDU reactors use 99.75%-enriched heavy water.)

Now that the technical description of heavy water is out of the way, time to turn to why heavy water is so important. But before we get too carried away, I should explain about uranium.

Uranium is the primary fuel used for nuclear power generation. Natural uranium contains two main isotopes: U238 and U235. U238 is what is termed as fissionable, requiring a highly energetic neutron to get the reaction started. U235, on the other hand, is fissile, requiring low energy neutrons for fission. To put it in other words, it takes a lot less effort to get a nuclear reaction started with U235 than it does compared to U238. (BTW, fission refers to a reaction where the nucleus of an atom is divided into smaller elements. Fusion is the reaction where two atoms are fused together to form a larger one.)

Natural uranium ore contains only 0.711% U235. As such, many nuclear reactors will use enriched U235. Weapons-grade uranium contains 85% or greater U235. However, the process to refine uranium is expensive. It is desirable to use natural uranium in nuclear reactors where possible. This is where heavy water comes in.

Within power generation, heavy water is used as a neutron moderator. (Here I go again, bringing out the big terms, but I can't help it. To properly explain the situation, it's unavoidable.) To increase the chances of successful reaction, the neutrons injected into a reactor need to be slowed down. Heavy water is used for this purpose. Once the reaction has been started, the fission reaction of U235 produces neutrons with sufficient energy to react with U238. From there, it's a chain reaction and it no longer matters that you have so much U238 on hand.

As a side note, reactors that use heavy water moderators do not need to use graphite moderators, making them safer. Chernobyl used a graphite moderator. It was after the incident at Chernobyl that reactors of that design were phased out.

Now here's the deal as to why everyone is up in arms about Iran having so much heavy water on supply and the amount of waste from their nuclear reactors that they're being allowed to keep.

During the normal operation of a heavy-water reactor, the U238 in the natural uranium fuel is converted into Pu239, the isotope of plutonium used in nuclear weapons. As a result, if the fuel in the reactors is changed out often enough, then significant amounts of weapons-grade plutonium can be extracted from the reactor waste.

Fission of U-235 or Pu-239 by a neutron results in two unequal pieces (fission products), about 3 neutrons and a small amount of mass turned into a large amount of energy. Atomic bombs are all U-235 or Pu-239 so everything splits uncontrollably, releasing an enormous amount of energy in a microsecond. In power reactor fuel, it’s only 3%-5% U-235 or Pu-239, so very little splits, just enough to produce heat and keep the reaction going in a controlled way. Instead, the mostly U-238 can capture a neutron and transform into Pu-239, which itself fissions. Power reactors use this Pu to produce as much energy as the U, resulting in very little Pu remaining for use in weapons plus having some of the wrong type of Pu (Pu-240) that acts as a poison in weapons. But weapons reactors optimize Pu production without burning much of it, and the fuel is removed after only months to be dissolved up in order to separate the Pu. The two types of reactors are easily distinguished. (Source:

Fission of U-235 results in two unequal pieces (fission products), while throwing out 3 neutrons and a small amount of mass turned into a large amount of energy. In power reactor fuel, it’s only 3%-5% U-235, so very little splits, just enough to produce heat and keep the reaction going in a controlled way. Instead, the mostly U-238 can capture a neutron and transform into Pu-239. (Source:

In addition, the heavy water used will contained trace amounts of tritium, resulting from the water absorbing some of the neutrons. (Remember, tritium is a radioactive isotope of hydrogen.) While there are more efficient ways of producing tritium, if sufficient quantities of tritium could be extracted, then it could be used in the construction in a boosted fission weapon.

In the 1970s, India proved that a nuclear bomb could be manufactured using the plutonium extracted from the spent fuel of a heavy-water reactor. If you combine this knowledge with the possibility of what could happen if sufficient tritium could be extracted from spent heavy water, it's not surprising that the world is up in arm about Iran's current situation.

Am I concerned about all this? Well, let's just say that I'm glad I live in New Zealand, the one country so far removed from all those using nuclear power or nuclear weapons. (And for those of you who don't know, New Zealand doesn't have either.)

(Conversations in Science no longer airs, but you can still find links on this blog to the various episodes.)

P.S. I'd love to meet you on Twitter or Facebook.

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© Copyright, Judy L Mohr 2016

Posted in ConvoScience Podcasts (Archive), Science and tagged , , .

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