In JET experiments using extrinsic impurity seeding, it was observed that the plasma core contamination and subsequent radiative collapse could be avoided if the deuterium gas inlet level was above a certain level for high triangularity type I ELMy H-mode plasmas. Originally these experiments were modelled using the 1.5D core transport code JETTO/SANCO and the 2D SOL transport code EDGE2D/NIMBUS. In this paper we will use the COCONUT code that couples both transport codes by exchanging the boundary conditions between these two codes in every time step.