Lesson 4 ◆ 4.2 Fission Product and Actinide Distribution

In addition to fission products, the fuel accumulates actinides (elements with atomic number 89 and above) through successive neutron capture and radioactive decay. The most important chain is:

238U→n,γ239U→β−(23 min)239Np→β−(2.4 d)239Pu→n,γ240Pu→n,γ241Pu→β−(14.3 y)241Am^{238}\text{U} \xrightarrow{n,\gamma} {}^{239}\text{U} \xrightarrow{\beta^{-}(23\,\text{min})} {}^{239}\text{Np} \xrightarrow{\beta^{-}(2.4\,\text{d})} {}^{239}\text{Pu} \xrightarrow{n,\gamma} {}^{240}\text{Pu} \xrightarrow{n,\gamma} {}^{241}\text{Pu} \xrightarrow{\beta^{-}(14.3\,\text{y})} {}^{241}\text{Am}

Key points about the actinide chain:

  • 239^{239}Pu and 241^{241}Pu are fissile and contribute up to 30% of reactor power towards end of core life.
  • 240^{240}Pu is fertile (can capture a neutron to become fissile 241^{241}Pu).
  • 241^{241}Am is a long-lived alpha emitter (T1/2T_{1/2} = 432 years) and a strong neutron absorber.
  • The actinides have much longer half-lives than most fission products and dominate the radioactivity of irradiated fuel after about 400—500 years.

Motivating fact for reprocessing: If the minor actinides (americium, curium, neptunium) could be separated from spent fuel waste and transmuted in fast reactors or accelerator-driven systems, the time required for the remaining waste to decay to the radioactivity level of natural uranium ore would be reduced from approximately 10,000 years to only 400 years. This dramatic reduction in the “toxic lifetime” of nuclear waste is one of the strongest scientific arguments advanced in favour of advanced reprocessing and partitioning & transmutation (P&T) research.