This paper describes a novel methodology for the analysis of transient nuclear criticality in layered aqueousemulsion-organic plutonium nitrate systems. The presented methodology includes point neutron kinetics
equations coupled with phenomenological one-dimensional nuclear thermal hydraulics models, which describe
the variation in mass, power, reactivity, temperature and voidage within the system. Equations to describe
the mean neutron generation time and changes in reactivity due to changes in the system’s temperature and
void fraction are formulated from fits of the results predicted by the criticality transient multiphysics model.
Using these developed models, the nuclear criticality incident that occurred at the Windscale Works in 1970 is
analysed and presented. In this criticality transient, two immiscible fissile liquids (an aqueous solution and an
organic solvent) were disturbed such that an emulsion layer formed between them, inducing a transient nuclear
criticality excursion and yielding around 1015 fissions. It was determined that parameters such as emulsion
globule size and vessel outlet pipe radius had negligible impact on the total fission yield of the corresponding
transient when compared to variations in organic solvent volume and emulsion band thickness. Several possible
configurations were identified which could result in a transient similar to that of the Windscale Works in 1970.
Notably, a system containing 39.75 L of organic solvent which underwent a transient with the formation of
an emulsion band that was a maximum of 5.4 cm thick and 54% aqueous solution by volume yielded 9 × 1014
fissions, and thus is expected to closely resemble the 1970 Windscale nuclear criticality incident. However, the
uncertainties associated with the original transient and the system parameters (e.g. thermophysical properties
of the three layers) are such that the configuration is likely to vary from these exact quantities.