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In VVER-1000/1200 type reactors much higher and geometrically more complex assemblies are applied compared to the VVER-440 design. Additionally, assembly shrouds are not used in the improved reactor types. This implies that the existing sub-channel wise models and hot channel calculation methodologies used in the safety assessment of VVER-440 reactors have to be improved. For this purpose a full core thermal hydraulic model was developed. In the first part of the paper the new model and its connections with the reactor physics of KARATE-1200 code system are demonstrated with results regarding the sub-channel outlet temperature and DNBR. In the second part of the paper the hot channel calculation methodologies to be used in the transient safety analyses of VVER-1200 type reactors are discussed. The appropriate frame parameter concept concerning the limitation of the heating up of the coolant and the single closed sub-channel approach vs. the multi-channel approach are studied, as well.

Hot channel calculation methodologies in case of VVER-1000/1200 reactors