Nuclear Fuel Cycle

When a heavy nucleus such as $^{235}$U undergoes fission, it splits into two (occasionally three) smaller nuclei called fission products. Approximately 200 different isotopes of some 40 different elements are produced.

The fission products are initially neutron-rich and unstable. They decay through several stages (typically 3 or 4) by beta and gamma emission, with half-lives ranging from milliseconds to thousands of years.

The Bimodal Yield Curve

The yield of fission products is not uniform across all mass numbers. Instead, it follows a characteristic bimodal (double-humped) distribution:

• Light peak: Mass numbers approximately 85—105 (e.g. Sr-90, Zr-95, Mo-99)
• Heavy peak: Mass numbers approximately 130—150 (e.g. I-131, Cs-137, Ba-140, Ce-144)
• Symmetric fission (mass ~ 117): Very rare --- yield is about 10,000 times lower than the peaks
• Peak yields: Approximately 6—7% per fission for the most probable products

Why bimodal? Symmetric fission (splitting exactly in half) is energetically unfavourable for thermal neutron fission of $^{235}$U. The nucleus preferentially splits into one lighter and one heavier fragment.

There are minor differences in yield depending on:

• Which fissile isotope is undergoing fission ($^{235}$U vs $^{239}$Pu vs $^{241}$Pu)
• The energy of the neutron causing fission (thermal vs fast)

Key Fission Product Groups

For health physics and accident analysis, only a limited number of fission product groups are of primary concern. These are isotopes that have high yield AND are volatile or gaseous AND are radiologically significant: