## 1 Introduction  
The elaboration of the breeder blanket is one of the most challenging tasks within the EUROfusion DEMO project. This activity aims to elaborate on the technological schemes and solutions that can ensure the technical feasibility of the introduced technologies. Operation of the DEMO facility with a closed tritium fuel cycle is the most basic and, at the same time, the most challenging requirement adopted in the EUROfusion project [1]. A sufficient tritium breeding capacity of the breeder blanket that ensures the functioning of the DEMO closed tritium cycle is a prerequisite for acceptance of the blanket design into the project. This implies a blanket engineering layout that can provide sufficient tritium generation to realize the closed cycle and to cover natural tritium losses due to its inherent decay. Accepted in the project is a tritium breeding ratio (TBR) that is used to assess the potential of each breeder blanket concept to generate tritium in a sufficient amount for the DEMO facility.  
A DEMO tokamak configuration, i.e., the type and number of auxiliary systems and equipment, is not yet fixed in the project. These so-called in-vessel components (IVCs) include different systems for plasma heating, diagnostic equipment and special limiter blocksthat control the plasma stability. With the evolution of the DEMO project, the number and configuration of these systems change. Due to this uncertainty, the exact space available in the tokamak for the breeder blankets has yet to be finalized. This results in uncertainty about the requirements that have to be applied to the blanket breeding capability in the project. A solution adopted in the DEMO project for support and continuation of the blanket development assumes a qualification of the tritium generation in the breeder blanket with a basic value, \(\text{TBR}_{\text{req}}\), indicating the TBR required to compensate all tritium losses in the fuel cycle and also to cover tritium production losses due to the arrangement of the auxiliary systems in the tokamak that consume the space available for the breeder blankets. The \(\text{TBR}_{\text{req}}\) is determined as \(\text{TBR}_{\text{req}}=\text{TBR}_{\text{target}}+\Delta\text{TBR}\), where \(\text{TBR}_{\text{target}}\) is the routine tritium production in the DEMO tokamak with all in-vessel components installed. Respectively, \(\Delta\text{TBR}\) should ideally account for all uncertainties associated with the insufficient information about the tokamak layout. As a common practice, the particular blanket design includes various simplifications that negatively affect the TBR, and therefore, the \(\Delta\text{TBR}\) accounts for these kinds of effects as well. \(\text{TBR}_{\text{target}}=1.05\) is accepted in the EUROfusion as a DEMO project requirement [1,2].  
The aim of this work is to present an assay of the \(\Delta\text{TBR}\) and nuclear power losses coming from the allocations of the currently accepted auxiliary system components. This study was conducted to account for the effects on the TBR and power production of the diverse possible cut-outs in the blankets required for the arrangement of the IVCs. The numerical simulations were carried out with fully heterogeneous geometry models of the HCPB [3], WCLL [4] and WLCB [5] blanket concepts to provide feedback for designing groups with the indication of the \(\text{TBR}_{\text{req}}\) margins and losses of the power production due to IVCs that should be demonstrated before any blanket concept can be accepted in the DEMO project.