IFMIF accelerator facility RAMI analyses in the engineering design phase

Abstract

The planned International Fusion Materials Irradiation Facility (IFMIF) has the mission to test and qualify materials for future fusion reactors. IFMIF will employ the deuteron-lithium stripping reaction to irradiate the test samples with a high-energy neutron flux. IFMIF will consist mainly of two linear deuteron accelerators, a liquid lithium loop and a test cell. Accelerated deuterons will collide with the lithium producing a high-energy neutron flux that will irradiate the material samples in the test cell.
 A timely and relevant fusion neutron source is essential in the path towards DEMO and future fusion power plants. For this reason, IFMIF is required to have high availability to obtain a fusion materials database to find suitable materials for DEMO design within the anticipated timeline. RAMI (Reliability Availability Maintainability Inspectability) analyses are being performed in the very early stages of design to meet such requirements. 
 The IFMIF accelerator facility is composed of two independent linear accelerators, each of which produces a 40 MeV, 125 mA deuteron beam in a continuous wave mode at 175 MHz. These beam characteristics pose several unprecedented challenges: the highest beam intensity, the highest space charge, the highest beam power and the longest RFQ (Radio Frequency Quadrupole). As a result of these challenges, many design characteristics are counter to high-availability performance: the design is reluctant to accept failures, machine protection systems are likely to stop the beam undesirably, cryogenic components require long periods for maintenance, and activation of components complicates maintenance activities. These design difficulties, together with the high availability requirements and the demanding scheduled operational periods, make RAMI analysis an essential tool in the engineering design phase. 
 These studies were performed in collaboration with system designers, enabling the creation of RAMI models that reflect current accelerator design. This feedback has been of the utmost importance to propose plausible design modifications in order to improve the availability performance of the machine. Parallel activity on the design and construction of the Linear IFMIF Prototype Accelerator (LIPAc) provided the detailed design information needed to conduct these studies properly. 
 An iterative process was followed to match IFMIF design and availability studies. These iterations made it possible to include recommendations and design change proposals coming from the RAMI analyses into the accelerator reference design. Iterations consist of gathering information from the design, creating or updating the RAMI models, obtaining and analyzing results, and proposing ways to improve the design.
 Three different approaches were carried out in the iterative process. First, a comparison with other similar facilities was performed. Second, an individual fault tree analysis was developed for each system of the accelerator. Finally, a Monte Carlo simulation was performed for the whole accelerator facility considering synergies between systems. These approaches make it possible to go from detailed hardware availability analyses to global accelerator performance, to identify weak design points, and to propose design alternatives as well as foresee IFMIF performance, maintenance and operation characteristics.
 The IFMIF accelerator facility design was analyzed from the RAMI point of view, estimating its future availability and guiding the design towards a high reliability and availability performance. In order to achieve the high-availability requirements several design changes have already been included in the accelerator reference design whereas other important design modifications have been proposed and will be further analyzed in future design phases.