Availability requirements for diagnostics in nuclear fusion: the ITER Collective Thomson Scattering case-study

• We created a Reliability Block Diagram of the ITER Collective Thomson Scattering Diagnostic. • We showed the availability dispersion in cases where maintenance is not readily available. • We propose new availability requirements for systems without access to maintenance. ITER will explore nuclear fusion under unprecedented operational conditions in terms of neutron exposure, magnetic fields and fusion power. Diagnostics play a vital role in the operation of ITER, providing plasma control, machine protection, and allowing for physics experiments, whilst withstanding these extreme conditions. The ITER Reliability, Availability, Maintainability, and Inspectability (RAMI) program was developed to address the impact of these operational conditions on its systems from the early design phases. However, the specifics of each diagnostic operation are not considered when determining availability requirements. In this work, we present simulations of the Reliability Block Diagram (RBD) of the ITER Collective Thomson Scattering (CTS) diagnostic to obtain its mean inherent availability distribution, using the methodology to estimate the availability distribution parameters to compare the effects of different operation and maintenance conditions. Finally, we assess the applicability of the mean inherent availability as a system design requirement and show that it is only a suitable metric for systems that can undergo maintenance at any time. Additionally, we show that diagnostics need alternative requirements that consider the spread of the mean availability distribution. For that purpose, we propose two types of requirements for the design of diagnostics: the first uses the mean availability and its standard deviation, while the second sets probability limits for desirable and undesirable availability values.