The facilities that involve the presence of neutron fields get the materials activated. That means a radioactive inventory that emits decay photons everywhere in the facility. Depending on the location in the facility, the decay photons field might be challenging to the survivability of the electronics, or tot he human intervention. It has a strong impact on the economics. The understanding of the decay photon field to minimize it and mitigate its effect is key to ensure the viability of those facilities.

However, the accurate determination of the decay photon field and its effects is one of the most complex activities in the field of the nuclear analysis nowdays. It involves sophiscated algorithms, the coupling of computational codes and the use of High Performance Computing (HPC) infraestructures. The correct analysis of the phenomenon requires an advanced understanding of nuclear engineering and physics.

In the last two decades, two modern computational methods to determine the decay photon field have been developed as an international joint effort: the mesh-based Rigorours-two Steps (R2S) and the Direct-one Step (D1S). They represent a step change as they allow full 3D resolution.

TECFIR / UNED developed in 2015 the tool called D1S-UNED. It is an implementation of the D1S method in the code MCNP5, accompained by a script to create D1S-specific nuclear data. It presents the main advantage of integrating the neutron and decay photon transport in the same calculation, what saves millions of activation calculations and allows a continuous treatment of the spatial resolution. D1S-UNED is one of the most advanced and powerful implementation of the D1S method, enabling unique features. In a typical calculation for ITER, it represents a factor 50x acceleration in calendar and analyst time with respect to R2S-UNED.

D1S-UNED is recommended for facilities for which the decay photon field features are already characterized and the neutron fields are moderate. This is typically the case of ITER. D1S-UNED is so powerful that ITER and TECFIR / UNED have reached an agreement to make D1S-UNED available to the entire ITER neutronics community.

D1S-UNED has been used in analysis and design activities of ITER: