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Operation of the fuel cycle of a thermonuclear fusion reactor naturally leads to accumulation of surplus protium but in some cases it can also lead to accumulation of surplus deuterium. Both surplus protium and deuterium have to be separated, detritiated and discharged to the environment normally passing a final detritiation stage based on either the LPCE (liquid phase catalytic exchange) or water distillation process. A concept of a multicolumn cryogenic distillation system capable of discharging time varying surplus of deuterium is presented in the article together with its dynamic model. The model is based on a UV flash formulation and equation of state (EOS) thermodynamic model for hydrogen isotopologue mixtures. The UV (internal energy -volume) formulation of thermodynamic state although fundamental for the constant volume system has not been used widely in transient simulations, in particular, for distillation dynamics modelling other approaches are much more common. At the same time in helium cryogenics the UV formulation has reached a wide usage in large scale dynamic simulations. It is known from the literature that a UV formulation of the distillation problem is very challenging for a numerically stable implementation. We present our findings on the sources of numerical instabilities and approaches to cope with it.

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