A time travel with fission products

This brief yellow and blue study is dedicated to my friend Andrey Marushev from Ukraine, now living in Kyiv.

I met Andrey between 2002 and 2004 in France. We were both at the beginning of our carriers, working on nuclear accident computation codes, theory and validation.

The Core

In a common nuclear reactor, nuclear fuel is uranium dioxyde (UO2) containing a small amount of U-235 fissile isotope, the rest being non fissile U-238 isotope.

After some steps of in-reactor irradiation, the isotopic distribution has been deeply changed due to fission products. Here it is, along increasing nucleons numbers (A).

On the left, high concentrations correspond to the consequence of the neutron capture on fuel's oxygen. On the right, the same phenomena after neutron capture on U-238. (simplified explanation)

In the middle, one may recognise the classical "two hills" curve, which is relative to equilibrium fission product abundances, from U-235 fission .

This curve is easily understandable : Imagine a glass breaking into two fragments only. Which one do you think has the highest probability, two fragments of the exact same mass, or one quite heavier than the other?

Stable and radioactive Fission Products (FP)

Now, let's focus on this "two hills" curve and observe the balance between radioactive and stable isotopes.

Here are the distributions of stable and radioactive isotopes at the moment when the fission process is stopped (called hereafter "Reactor trip").

You may notice that the maximum density for each hill is:

• for stable FP: A=97 and A=138, the sum being 235
• for radioactive FP: A=100 and A=136, the sum being 236

This is a quite funny U-235 fission signature, isn't it?

Now let's look at the global balance between stable and radioactive isotopes.

....not so intuitive but yes, fission in a nuclear reactor produces more stable than radioactive isotopes during its operation !

1000 years of loneliness

Let's spend 1000 years now doing nothing, because we need time to think about our last statement.

• Stable and radioactive density distributions:

• Global balance:

I bet you were sure that radioactive part should be less, say it!

Isotopes do not want to die

Let's go now to 1000 billion years. Yes I am serious. Why 1000 billion years? Because why not. Because the latest computation code we developped can do it. And because it is also the mean observed time for my 13 year-old son to wake up every morning.

• Stable and radioactive density distributions:

• Global balance:

What the hell?!

Who are they?

From the beginning, you noticed them:

• isotope 100 of Molybdenum, a transition metal; half-life: 7,3 e18 years;
• isotope 136 of Xenon, a noble gas; half-life: 2,1 e21 years.

They seem to not need any life insurance!

Conclusions

I hate conclusions. So choose one of these:

• People saying that radioactivity disappears with time are not so right and not so wrong.
• Mo-100 and Xe-136 could be considered as stable isotopes regarding the age of our universe.
• The blue of peace always ends dominating the yellow of fire and war.

Grégory ORTEGA NICAISE, PhD

03/04/2022