Tonio Pinna

The DONES (Demo Oriented NEutron Source) objective is to irradiate in a simulated fusion irradiation environment a sufficiently large number of specimens with required neutron fluxes for an adequate period of time. To achieve the required results, a target of 70% of operational availability was established for DONES facility design, which means that, the facility is expected to be available for irradiation (i.e. with the beam on at full power) for 255.5 days per year. Such an average operational availability requirement combined with the foreseen scheduled annual maintenance scheme (20 + 3 days) implies an inherent availability requirement of 74.7% (i.e. ̴75 %) for the DONES facility. Allocating this target to individual systems within the whole plant, the following system targets for inherent availability were defined: Accelerator Facility 87 %, Lithium Target Facility 94 %, Test Facility 96 %, Conventional Facilities 98 % and Central Control System & Common Instrumentation (CCS & CI) 98 %. Key elements to ensure the availability objective set for a complex and innovative plant such as DONES are to accompany the engineering development of DONES with reliability and availability analysis, reliability testing of components and systems and the application of "reliability growth", the structured process of identifying the root causes of reliability problems and predicting and monitoring the increase in system reliability through successive phases. Since reliability and availability (RA) are closely linked to the maintenance and inspection activities carried out during plant operations, the integrated approach for reliability and availability optimization needs to be based on all four issues: Reliability, Availability, Maintainability and Inspectability (RAMI). Accordingly, RAMI analyses cover all stages of DONES progress and specific studies have been performed for the various systems. This paper presents the RAMI studies conducted to date.

As part of the pre-conceptual design activities for the European DEMOnstration plant, a carefully selected set of safety analyses have been performed to assess plant integrated performance and the capability to achieve expected targets while keeping it in a safe operation domain. The DEMO divertor is the in-vessel component in charge of exhausting the major part of the plasma ions’ thermal power in a region far from the plasma core to control plasma pollution. The divertor system accomplishes this goal by means of assemblies of cassette and target plasma facing units modules, respectively cooled with two independentheat-transfer systems. A deterministic assessment of a divertor in-vessel Loss-of-Coolant Accident is here considered. Both Design Basis Accident case simulating the rupture of an in-vessel pipe for the divertor cassette cooling loop, and a Design Extension Conditions accident case considering the additional rupture of an independent divertor target cooling loop are assessed. The plant response to such accidents is investigated, a comparison of the transient evolution in the two cases is provided, and design robustness with respect to safety objectives is discussed.

Integration of safety aspects in IFMIF-DONES design is a main objective of EUROfusion and European Commission projects. IFMIF-DONES will be a radioactive facility of the first category, and stringent safety objectives must be achieved and demonstrated. A very low acceptable risk for the worker, the public and the environment is the main principle in the design phase. The progress of safety activities is performed iteratively as detailed engineering develops, taking into account the uniqueness of the facility: a high-power deuterons accelerator (125 mA, 40 MeV), a target of flowing liquid lithium, traps for activation products, a dedicated-design module for irradiated samples, a massive shielding cooled room with confinement function, and a number of conventional systems with safety functions. Several phases are developed: (i) identification of sources and materials at risk, radioactive and nonradioactive, subject to potential mobilization, (ii) failure mode analysis and effects of systems, starting at the functional level, and support with probabilistic analysis, (iii) identification of scenarios leading to unacceptable risk if unmitigated, (iv) proposal of layers of defense by means of safety-credited components and design features, (v) deterministic analysis of scenarios in support of requirements, and (vi) definition and demonstration of safety requirements charged to components.

The Sorgentina-RF project will use fusion neutrons to produce ⁹⁹Mo, a precursor of ^99mTc, by irradiating natural molybdenum. ⁹⁹Mo is produced by means of the inelastic reaction ¹⁰⁰Mo(n, 2n)⁹⁹Mo on ¹⁰⁰Mo, which is an isotope of natural Mo. From a functional point of view, the project consists of two parts: an irradiation neutron source at 14 MeV and a radiochemistry facility dedicated to the extraction of ⁹⁹Mo from the solid sample irradiated by the neutron source. Given the degree of complexity of such a facility, the risk management strategy is based on an integrated approach that combines the engineering method of safety with that of radiation protection. Therefore, design issues were studied and systems were planned according to both radiation protection and safety criteria already in the preliminary phase, allowing a general strengthening of the safety of the plant. This work discusses the preventive analysis and the related activities to identify the ways in which potential exposures to radiation may occur. In particular, the preliminary safety analysis is presented for the innovative rotating target, developed for the project, and, accordingly, some specific technical solutions are given to refine the initial design of the facility.

The IFMIF-DONES (International Fusion Material Irradiation Facility-DEMO Oriented NEutron Source) facility is being designed with the general objective of providing irradiation of representative samples of power fusion machine materials under prototypical conditions. A linear accelerator will deliver deuterons at high intensity to circulating lithium in a loop, which will produce neutrons capable of obtaining the required damage conditions. As a result of this process, radionuclides will be produced as a by-product, which is characterized by several degrees of mobility. Shielding and radiation protection measures will be required in the facility. IFMIF-DONES will be classified as a first class radioactive facility according to national regulations, with Spain being the European candidate to site the facility. Several aspects of the main safety instructions affecting the facility’s design are explained and discussed in this paper.

Two of the key elements of the engineering development of the DEMO reactor are the definitions of reliability and availability requirements (or targets). The availability target for a hypothesized Fuel Cycle has been analysed as a test case. The analysis has been done on the basis of the experience gained in operating existing tokamak fusion reactors and developing the ITER design. Plant Breakdown Structure (PBS) and Functional Breakdown Structure (FBS) related to the DEMO Fuel Cycle and correlations between PBS and FBS have been identified. At first, a set of availability targets has been allocated to the various systems on the basis of their operating, protection and safety functions. 75% and 85% of availability has been allocated to the operating functions of fuelling system and tritium plant respectively. 99% of availability has been allocated to the overall systems in executing their safety functions. The chances of the systems to achieve the allocated targets have then been investigated through a Failure Mode and Effect Analysis and Reliability Block Diagram analysis. The following results have been obtained: i) the target of 75% for the operations of the fuelling system looks reasonable, while the target of 85% for the operations of the whole tritium plant should be reduced to 80%, even though all the tritium plant systems can individually reach quite high availability targets, over 90% - 95%; ii) all the DEMO Fuel Cycle systems can reach the target of 99% in accomplishing their safety functions.