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MYRRHA Accelerator eXperiment, research and development programme

Final Report Summary - MAX (MYRRHA Accelerator eXperiment, research and development programme)

Executive Summary:
MAX has been the follow-up of the EUROTRANS Work Package 1.3. It has been a 3.5-year collaborative project with a global budget of 5 M€ and an EC contribution of 2.93 M€. The success of MAX is expressed by the fact that all but one of its 53 initially defined milestones have been fully reached and that the corresponding performance indicators are green. This fact clearly points to the value of the collaboration and to the high level of motivation of each of the 11 partners.

Besides the production of the 18 scientific MAX Deliverables, the quality of the work produced in the frame of the MAX project is also underlined by the following list of achievements.
- More than 40 publications (in journals or international conference proceedings).
- More than 40 oral presentations in cross-projects collaboration workshops.
- More than 50 contributions to seminars and lectures (including the 2-day MAX accelerator training school, organized in Frankfurt on October 1st & 2nd, 2013).
- More than 60 internal MAX project meetings (incl. Governing Council, Project Coordination Committee, Project, WP and specific tasks meetings).
- Training of several students through internships, PhD or post-doc positions directly related to the MAX project.
- Development of a dedicated web site to ensure the public communication of the project and, through its secured private section, the exchange of internal working documents between the MAX partners.

With respect to the EUROTRANS outputs a very significant progress has been made on the path towards the accelerator for MYRRHA. From the very start, MAX has been organized around the actual needs of the MYRRHA linac and thereby it has been able to focus on the well defined requirements of this machine. This has led to a number of achievements that are all fundamental in view of the reliability goal.
- A fully reliability-oriented overall consolidated concept of the accelerator.
- A set of benchmarked modelling tools allowing for start-to-end beam simulations.
- An operational reliability model based on the SNS experience.
- An adequate and realistic injector design.
- A detailed engineering design of a few critical elements.

Specific experiments, matched to particular aspects of an ADS-accelerator, have also supported some of these achievements or provide valuable information for future and further developments.
- Cooling performance tests of the 4-rod RFQ model cavity in real CW RF operation.
- Investigation of the behaviour of a low-beta elliptical superconducting (SC) cavity in accelerator-like conditions (2K, high RF power).
- Assessment of a SC cavity fault-recovery scenario using a digital low level RF feedback system and featuring an ADEX (adaptative) tuner controller.
- RF test of a superconducting CH cavity at 4K and 2K in vertical cryostat.
- Performance of a 704 MHz solid state RF amplifier module and associated power combiner.

A particularly strong achievement of the results generated by the MAX programme and of the outcome of the International Design Review is the global level of confidence, in the concept on the one hand, and in the feasibility of its components on the other hand. This level of confidence is coherent with the fact that MAX has now brought us to the first major milestone on the road towards the realisation of the MYRRHA linac. The milestone may be labelled "ready for prototyping". It is the starting point of a new set of mandatory R&D activities where the emphasis should lie on experimental optimisation. It must be acknowledged that in this phase, the full exploitation of synergies with similar projects may be particularly awarding.
Project Context and Objectives:
The MYRRHA project, “Multi-purpose hYbrid Research Reactor for High-tech Applications”, is to be considered as a major milestone in the endeavour to develop a sustainable solution to the nuclear waste problem based on efficient transmutation of minor actinides. It aims at demonstrating the Accelerator Driven System (ADS) concept at a relevant power level, and also at providing an efficient fast spectrum neutron irradiation facility. Its liquid lead-bismuth cooled reactor would be a highly significant prototype of one of the Generation IV options, namely the Lead Fast Reactor. By simultaneously addressing outstanding R&D needs in the fields of future reactor technology and of fundamental nuclear science, MYRRHA is nominated on both the ESNII roadmap and the ESFRI list.

The object of the “MYRRHA Accelerator eXperiment” (MAX) collaborative project, as described in the Annex B of its proposal, in its first Deliverable 1.1 or in its public web-site, is to pursue the R&D on ADS type driver accelerators and deliver an updated consolidated reference layout of the MYRRHA proton linac (linear accelerator). This accelerator has to satisfy a list of criteria that have been defined by the reactor design team in accordance with the accelerator design team. They primarily concern the particle type, energy, beam current and beam delivery time structure. They constrain the operational tolerances on the parameters. Finally they fix a beam reliability goal, expressed as a tolerance to beam trip frequencies according to beam trip durations. These tolerances may be translated to values of Mean Time Between Failures (MTBF). The MYRRHA accelerator main beam requirements are presented here below.
- Particle: protons
- Beam energy: 600 MeV
- Peak beam current: 0.1 to 4 mA
- Beam duty factor: 10-4 to 1
- Time structure: (microstructure) CW ; (nominal macrostructure) 200 µs beam holes at 1 to 250 Hz
- Beam power stability: ± 2% (100 ms integration time)
- Footprint on target: circular (ø 85 mm)
- Footprint stability: ± 10% (1 s integration time)
- Beam trip tolerance: (t > 3 s) 10 per 100 day period ; (0.1 s < t < 3 s) 100 per day ; (t < 0.1 s) infinite
- Corresponding MTBF: (t > 3 s) 250 hours

It must be stressed that the beam trip tolerance criterium is particularly stringent in view of present day operational data from existing accelerators with comparable performance in terms of beam power. This fact was already fully acknowledged during the FP5 project PDS-XADS, where it led to the fundamental choice of proposing a SuperConducting (SC) linac as the driver machine in the ADS context. Thus the Spallation National Source (SNS) linac is particularly relevant: a comparison of the MYRRHA request with SNS experimental beam trip data is enlightening and perfectly underlines that a strong focus is still needed to improve the reliability of present accelerators and reach the ADS goal. Therefore, even if today the SC linac choice is entirely corroborated for the ADS purpose, it is more obvious than ever that its implementation needs extensive R&D efforts to be undertaken and proper prototyping to be pursued. This statement is doubtlessly reinforced by considering the borderline position of MYRRHA in today’s accelerator landscape.

In general terms, the goals of the MAX project may be itemized as follows.
- Obtain a consolidated concept of the entire MYRRHA linac (MLA).
- Build selected engineering design files.
- Perform selected experiments that have been defined earlier within the preliminary concept.
- Gather and interpret return on experience data.
Project Results:
The withheld MAX topics and obtained results may be assigned to 5 main categories. These categories are presented here below. For each of them it refers to the relevant MAX deliverables containing the detailed descriptions/data/calculations/design/analyses.

1. Global design and coherence
1.1. Definition of the MYRRHA linac consolidated concept (D1.2 D1.4)
1.2. Advanced beam optics simulations for fault tolerance (D1.4 D1.2)
1.3. MYRRHA linac reliability modelling (D4.4)
1.4. Cryogenic working temperature assessment (D4.1)
1.5. Buildings and infrastructures (D1.3)

2. Injector layout
2.1. LEBT design (D1.2)
2.2. Choice of frequency & injector concept (D2.1)
2.3. Simulation codes benchmarking and optimal optical solution (D1.4 D1.2)

3. Construction readiness design files
3.1. Spoke cavity and cryomodule engineering design (D3.3)
3.2. Room-temperature CH cavity engineering design (D2.3)
3.3. 4-rod RFQ engineering design (D2.4)

4. Experiments
4.1. Operation & reliability assessment of an elliptical cryomodule (D3.1 D3.4)
4.2. Assessment of the ADEX cavity tuner control loop with LLRF (D3.2 D3.1)
4.3. Fabrication & cryogenic test of a superconducting CH cavity (D2.2)
4.4. RFQ thermal mock-up evaluation and test (D2.4)
4.5. 704 MHz SS RF amplifier module development and test (D4.5)

5. Return on experience
5.1. Reliability analysis of the SNS linac (D4.2)
5.2. Accelerator operation in the GUINEVERE experience (D4.3)

A detailed summarizing overview of these different MAX activities and results is also available in Deliverable 1.5 (MAX final report), here attached (PDF), section "MAX topics & main achievements".
Potential Impact:
By combining these R&D results generated by MAX and the comments formulated by the members of the International Design Review, it is now possible to emit certain recommendations with respect to a potential pursuing of MYRRHA linac related R&D activities.

A. Regarding the global linac design

The present conceptual design is convincing and satisfies (is able to satisfy) all the requirements. This statement is obviously largely based on simulation results. This calls for 2 important observations.
1. The link with reality has to be ensured through experiments.
2. Further optimisation has to be pursued in a persistent way, by taking into account new input data or by applying updated boundary conditions and by improving the existing models.

In this way it should be foreseen in the following phase of the activities to review and further optimize:
o the global linac layout,
o the redundancy configuration,
o the detailed design of the injector.

Certain global accelerator items had been omitted in the MAX project definition. In particular, diagnostics and controls were missing. Given the fact that both diagnostics (in general, not being restricted to beam diagnostics alone) and controls (the entire 3-tier control system) have an essential role to play in the extreme reliability scenario, it is now mandatory to include these items in an upcoming R&D program. The time has also come for an overall updated cost evaluation and cost optimisation – the latter being strongly connected to the above observation #2.

B. Regarding specific future R&D activities

Besides considering the global aspects which are mentioned in the preceding section, it is adequate to highlight some topics that have to be addressed in priority. There are two principal lines: prototyping and modelling.

o Correct (i.e. conform to reality) initial beam data are of fundamental importance in all beam dynamics simulation tools. Making use of the platform consisting of the ECR ion source and the LEBT that is presently under construction, an experimental campaign for fully characterizing the emerging beam (beam entering into the RFQ) should be launched.

o The injector has to be investigated in detail, in a step-by-step approach allowing for testing with beam. The logical first step is thus the construction of the 4-rod RFQ that has been designed in MAX (keeping in mind that the output longitudinal beam emittance needs to be minimized) and coupling it to the source-LEBT assembly presently under construction. This obviously implies the procurement of all the auxiliary equipment. The solid state RF power amplifier and the digital LLRF will themselves be prototype items.

o Since the superconducting linac is highly modular, thorough prototyping is very efficient. The MYRRHA-type spoke cryomodule has been fully designed in MAX and should now be constructed and tested. More generally, concerning SC cavities cryomodules, significant synergies is potentially existing with the ESS project and, to a lesser extent, with the CERN SPL project. A later series production could be discussed with potential manufacturers.

o The effort around the reliability modelling tool put in place by MAX should be pursued with vigour so as to obtain confidence and predictability. This tool is indispensable for the optimisation task.

o A virtual accelerator, performing on-line machine modelling with real-time machine data, will be a powerful and essential operational tool in the high reliability context. It is challenging, though, both from the simulation and from the control system point of view. The injector tests should serve such a development.

C. Regarding the connections with other projects or R&D programmes

The Superconducting Linacs for High Power Proton beams (SLHiPP) series of workshops, managed by CERN and ESS with a participation from SCK•CEN (MYRRHA), is a very useful forum for the exchange of engineering design information and of test results. MAX has been actively present in these meetings and its continuation should be supported.

However, in order to exploit the synergies that obviously exist between the different high power hadron accelerator projects, a network of bilateral collaboration agreements is required. From the MYRRHA side such MoU's are already available with CNRS/IN2P3, SPIRAL-2 at GANIL and CERN; the agreement with ESS is missing. Especially in view of cryomodule prototyping, the relevant collaborations may reveal their usefulness and should be activated. Note that a MYRRHA-type elliptical cryomodule design is up-to-now inexistent and that a close collaboration with (for instance) the CERN/SPL or the ESS cryomodule development activities would be highly valuable.

Within the European Framework Program FP7, synergies between MAX and EUCARD-2 have been clearly identified during a common workshop on ADS accelerators. Practical possibilities of future collaboration between EUCARD-2 and a potential follow-up of MAX lie in the joint organisation of topical workshops on subjects of common interest.

More generally, the R&D topics oriented on accelerator availability and reliability, that have been specifically explored within MAX, should be actively pursued in the following years. These developments will indeed bring substantial impact for all emerging and future accelerator projects featuring high power proton beams.

Finally, on a wider societal aspect, MAX has participated, through the R&D on Accelerator-Driven Systems, in addressing the issue of high level long lived radioactive waste transmutation.

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