High radiation fields are an occupational hazard during operation, routine maintenance, and decommissioning of nuclear power plants. Over the last several decades, progress has been made in lowering worker radiation dose, predominantly due to the reduction of radioactivity in out-of-core reactor systems. However, this does not preclude further reduction of fields around system components as the CANDU design continues to evolve. The effort to minimize dose and the release of radioactive materials to the lowest possible amount is known as the ALARA (As Low as Reasonably Achievable) safety principle. An accurate model for the transport of soluble and particulate species in the PHTS is essential for predicting radiation fields around components and in keeping with the ALARA criterion.

To formulate a comprehensive mechanistic model of activity transport in CANDU-6 reactors, the concepts of previous models that treat such corrosion-product behaviour empirically are much improved by incorporating the mechanisms of crud formation and behaviour. Solid particulate (crud) deposition and release mechanisms on isothermal and non-isothermal surfaces, including the effects of in-core boiling, are modelled and removal via the purification circuit is considered. The radionuclides ⁵⁵Co, ⁵°Co, ⁵1Cr, ⁵9Fe, ⁵⁵Fe, ⁵°Mn, and ⁵3Ni arise from crud residing under neutron flux and are released to the coolant as in-core crud dissolves;​ their activation of surfaces around the circuit is determined by considering the underlying material corrosion and the oxide solubility properties - whether under isothermal conditions or under temperature gradients. Soluble (Fe, Ni, Co, and Cr) and particulate species transport is modelled alongside primary side activity levels for times up to 30 effective full power years (EFPY). The predictions of surface activity, particularly in the steam generators, are in agreement with plant measurements.