Nuclear fission is the basic process that generates energy within power systems around the world. Fission is a complex process, splitting a nucleus typically with more than 200 nucleons into any of a large number of possible fragments, alongside emitted neutrons and other particles. Understanding the probabilities of having emitted particles and fission fragments from fission events is essential to the modelling and simulation of all systems with fission.
The study of fission physics has a long and rich history, but no theoretical models exist that can predict the liklihood of generating any fission products. Detailed measurements are required and paired with semi-empirical models to create evaluated nuclear data files that can be used in application studies. The uncertainties on these fission product yields must also be quantified and, due to the basic nature of how fission works (two fragments in binary fission, conservation of mass and charge, etc.) these data must be highly correlated. The quantification of these correlations and use of these matrices for uncertainty quantification is one of the ongoing major challenges for nuclear data.
The reaction physics of the fission products are also crucial for advanced modelling of nuclear reactors, since the inventory of these products will change not only due to production from fission - but also from the reactions on the products. Since these are nearly all radioactive, exotic isotopes, few measurements have been performed and these data are generally less well known.
The NEA has organised several subgroups of the Working Party on International Nuclear Data Evaluation Co-operation (WPEC) focusing on the physics of fission products, applications of fission product yield and decay data, and new measurements and evaluation techniques for fission product yields, including a focus on covariance information. These have directly resulted in the creation of first-of-a-kind libraries of evaluated nuclear data for fission products and new fission yield evaluations with supplementary covariance information.
In response to these activities, including the subgroup 25 on the assessment of fission product decay data for decay heat calculations and subgroup 37 on improved fission product yield evaluation methodologies, the IAEA has launched follow-up Co-ordinated Research Projects (CRPs) and national programmes have launched new fission yield and observable efforts, including new measurements and evaluation efforts.