  Mn-Ni-Si precipitates (MNSPs) are known to be responsible for
irradiation-induced hardening and embrittlement in structural alloys used in
nuclear reactors. Studies have shown that precipitation of the MNSPs in 9-Cr
ferritic-martensitic (F-M) alloys, such as T91, is strongly associated with
heterogeneous nucleation on dislocations, coupled with radiation-induced solute
segregation to these sinks. Therefore it is important to develop advanced
predictive models for Mn-Ni-Si precipitation in F-M alloys under irradiation
based on an understanding of the underlying mechanisms. Here we use a cluster
dynamics model, which includes multiple effects of dislocations, to study the
evolution of MNSPs in a commercial F-M alloy T91. The model predictions are
calibrated by data from proton irradiation experiments at 400 {\deg}C.
Radiation induced solute segregation at dislocations is evaluated by a
continuum model that is integrated into the cluster dynamics simulations,
including the effects of dislocations as heterogeneous nucleation sites. The
result shows that MNSPs in T91 are primarily irradiation-induced and, in
particular, both heterogeneous nucleation and radiation-induced segregation at
dislocations are necessary to rationalize the experimental observations.
