New Radio Show and Heavy Water

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.)

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.

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.)