Availability of Present Fusion Devices - TELEGRID

AVAILABILITY OF PRESENT FUSION DEVICES

S. Ciattaglia, B.M. Angelinia, M. Coxb, O. Grüberc, D. van Houtted, K. Kuriharae, P. Petersenf, M. de Baarg, P.Sonatoh

EFDA CSU Garching, 85748 Garching, Germany, [email protected] aEURATOM/ENEA Association FUS FTU, Via Enrico Fermi 45, I-00044, Frascati, I

bEURATOM/UKAEA Fusion Association, Culham Science Centre, OX14 3DB, UK cMax-Planck-Institut fuer Plasmaphysik, EURATOM-Association, D-85748 Garching, D dAssociation EURATOM-CEA, DSM/DRFC/Cadar, F-13108 Saint Paul-lez-Durance, F

eJAERI, Naka Fusion Research Establishment, JT-60U fGeneral Atomics, MS 34-107C P.O. Box 85608, San Diego, CA 92186-5608, USA

gEFDA Close Support Unit, Culham Science Centre, Culham, OX14 3DB, UK hConsorzio RFX, Euratom-ENEA Association, Corso Stati Uniti 4, I35127 Padova, I

Abstract— Operation of tokamaks has now reached few hundred device-years. Availability is a top level parameter vital for the efficient management of complex plants, like fusion devices, for decision making and to judge the quality of design, manufacturing and operation. There is no standard way to analyze the operating experience: different techniques are adopted, various levels of detail are reached and also the kind of analysis and corrective actions are different. Nevertheless most of the results for the various tokamaks are expressed in similar terms like number of plasma pulses, operating time and delays, experimental sessions lost. An overall analysis of JET, JT-60, DIII-D, ASDEX-U, Tore Supra, FTU and RFX operating experiences has been performed to find out main results, lessons learned and possible suggestions to improve availability of the devices themselves and of those under design or construction, like ITER. The main results are here presented and discussed.

I. INTRODUCTION Reliability and availability represent important performance

parameters of a system, with respect to its ability to fulfill the required mission during a given operational period. The availability (A) is affected by anything preventing a 100 % loading factor.

Α = UΤ/UΤ+∆Τ (1)

where UT = Up Time and ∆T = Down Time

The two main contributing factors are reliability and maintainability. Availability of a plant depends on the availability of each system/component and on its design. It becomes more and more important, in general, with the increase of the ratio between construction cost and operation cost.

II. OPERATING EXPERIENCE The peculiarities of present tokamaks are the not

completely steady state operation (operation through pulses

from a seconds to hundreds of seconds), the limited operating experience (prototypes, new materials and technologies, complex plasma scenarios), the operation through experimental campaigns of few weeks (3-5 days a week, with a long or a double shift) spaced by shutdown periods.

Operating experience is collected with different tools. Main sources are Engineer in Charge, Session Leader, Control and Data Acquisition System (CODAS) and specific systems diaries-logs and dedicated trouble reports. If the tokamak pulse is not achieved within the minimum experimental time of the machine (10-30 minutes, depending on design constraints), the delay is associated to the system that caused the downtime. From these data, several technical indicators are produced to assess the behaviour of components and systems and the success of experiments: operating days per year, pulses per day, ratio between various kinds of pulses, effective operating time, delay time and relevant causes. In some devices the collection of operating experience goes further through the definition of detailed technical indicators - e.g. performance of Additional Heating (AH), Fueling and Diagnostics – and the collection and the statistical analysis of all malfunctions for each component of the main systems, to assess the relevant reliability [1]. From these technical indicators and the trouble reports it is possible to find out the causes and then the possible corrective actions to be taken at managerial level.

The consequences of troubles during the experimental campaigns can be classified in two types: experimental sessions lost-cancelled and delays during sessions.

The first one (roughly 5-25% of planned sessions) is due to significant problems on important systems, like leaks in Plasma Facing Components (PFCs) cooling circuits, vacuum leaks or major contamination in the vacuum vessel, severe problems on cryogenics and superconducting magnets. Other events are failures of essential components without redundancy, or, in few cases, lack of spare parts.

The delays during the experiments, mostly due to troubles of components, are events that can be recovered in a relative short time (from minutes to hours) and the experiment can be resumed. The relevant downtime is about 15-25% of the planned operating time (decreasing in longer and double shifts). Power Supply components are the main cause of delays

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Availability for DIII-D

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Figure 3. Availability of DIII-D

ASDEX-U [5] presents similar availability, around 80%, while RFX [9] availability is about 75%, and FTU [10] a bit lower (70%) also due to the presence of magnets at liquid nitrogen temperature.

In the previous Figures, the sessions cancelled, because of major problems, have been not taken into account. There are not enough data available on that. From a few data relevant for JET, the sessions cancelled due to this reason are around 10%.

Figure 4 reports the availability of Tore Supra [6]. The contribution of in-Vacuum Vessel (VV) water leaks to the unavailability of the machine is significant; many sessions had to be cancelled, bringing the availability down to 50-55%.

Figure 4. : Availability of Tore Supra

Tore Supra presents two important systems in view of ITER: actively cooled Plasma Facing Components (PFCs) and Toroidal Superconductor Magnets (SC). The first component suffered several failures in terms of water leaks: two weeks are necessary, in average, to repair and to resume the operation. The second one presented only one major problem in 15 years of operation and several minor problems during experimental sessions: the magnets were de-energised due to spurious quench-detection signals (and it takes about 2 hr to resume the operation [7]).

in several machines (up to 50%), then CODAS (15-30%), AH and all the other systems. Human factors can also be an important cause.

Figure 1 shows the daily delay per system along the years at JET [2].

Figure 1. Daily delay per system along the years at JET

III. THE AVAILABILITY OF PRESENT FUSION DEVICES It is not straightforward to measure the output of present

tokamaks in operation: number of pulses, which pulses, operating time, etc. There is not a common definition of availability. The most adopted ones are the following: (scheduled time - downtime)/scheduled time and effective operating time/(effective operating time + troubleshooting time). Figures 2 shows JT-60 effective availability [3] defined from (1). After initial difficulties, the availability of the device is almost constant and around 80%. It resulted that many troubles occurred after hardware modifications, just before and after pulse and after plasma disruptions.

Figure 2. Availability of JT-60

Figure 3 presents the availability of DIII-D [4] defined by (1). The major contribution to the unavailability of the machine comes from Power Supply Components.

Many other technical indicators are produced to give a more complete picture about how the operation is going on and about the performances of various systems.

Most machines assess the successful pulses and total pulses per experimental day. Figure 5 shows this parameter for JET [2]. The ratio between these two indicators is about 90%. The successful pulses give an indication of the reliability of those systems “essential” for running the machines like Vacuum, Cooling Circuit, Power Supply, CODAS, Fueling and Cryogenic, but they say little on the success of the experiment itself: the final aim of the actual fusion devices is to produce successful scientific pulses (“good pulses”) for the benefit of the scientific programme.

Figure 5. Total (blue) and successful (pink) pulse per year at JET

Since mid 2002, JET has introduced the “good pulse” parameter [8] to define the success of the mission (however not straightforward measurable). The pulses are rated from unclassified (pulse failed or not for physics programme), zero (pulse of no scientific value), up to three stars (pulse of high scientific value). Figure 3 shows the good pulses in the last three years of operation: they depends also on the complexity of experimental programme and are about 50% of the total pulses.

Figure 6. Good pulses at JET per year

To get a good pulse, all systems relevant to the specific pulse must perform as requested, e.g. level of AH power delivered/coupled, number and quality of Pellets, Diagnostics. A proper preparation of the session is also important (e.g. a plasma disruption could require 4-5 pulses, i.e. about half session, to resume a good condition of vacuum and then the experiment).

Maximizing the device availability means to get a good pulse in the shortest experimental time interval that is defined only by machine design constraints.

IV. LESSONS LEARNED AND CORRECTIVE ACTIONS The systematic collection of operating experience together

with the relevant analysis is an essential tool to understand which feedback actions are necessary in order to improve availability.

From the analysis of operating experience of JET, JT-60, DIII-D, ASDEX-U, Tore Supra, FTU [9] and RFX [10], some common results can be pointed out.

The short-term corrective actions are those fixing the trouble as soon as possible, to allow the continuation of the experimental programme. In some cases the corrective actions can be implemented at the next maintenance or shutdown period (i.e. some vacuum leaks that can be fixed temporarily). In other cases (e.g. lack of requested specific AH power or unavailability of specific Diagnostic) it is necessary to swap to a "backup" programme.

To implement mid-long term corrective actions, a continuous analysis of troubles should be undertaken, to find out the real causes, such as:

• Staffing: Assess number of Personnel in support of key systems and skills. Identify areas of weakness (including risk of loss of individuals): provide extra staff and cross training

• Planning and implementation of modifications: systematically log and assess potential impact on operations

• Operational documentation and procedures: fixes/improvements must be documented

• Spares/Preventative Maintenance: review and optimize the maintenance plan and the spare parts policy according to the operating experience. Spares should be known to be working and ready to fit

• System improvements: improve diagnostic capabilities to identify repetitive troubles and for a timely maintenance

• Accurate preparation of the experimental session.

The cost of such corrective actions must be compared with the expected availability improvement, in order to decide which one to undertake.

The operating experience gives also useful feedbacks to the machine under construction or under design in order to improve their future availability.

In particular ITER is a "first of a kind" experimental complex nuclear machine.

Many of its systems are derived from the present tokamaks with mid-large extrapolation. Some others refer to systems of Nuclear Power Plants (NPPs) and of big experimental laboratories. Because of the presence of more and complex

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systems, the availability of ITER could be lower than the availability of the present tokamaks (i.e. Tore Supra).

Furthermore the nuclear environment will reduce further the availability of ITER because of licensing prescriptions, hostile environment (e.g. dose limits, access restrictions, ALARA criterion). Therefore, significant effort has to be put in all phases of ITER in order to reach and maintain a level of availability adequate to accomplish its mission.

During the design phase, an availability target should be defined for the machine, and subsequently for each main system. Design criteria, like redundancy/diversification, safety margins, diagnostics on components/systems, layout optimization, components reliability and Quality Assurance (QA) criteria, should be adopted in a harmonized way.

During Manufacturing, Assembly, and Commissioning phases, QA and strict quality control should be followed, in particular for prototypes, cooling circuits [11], in-vessel material/components and cryogenics. Furthermore, a component database should be organized to support maintenance type and planning, test procedures, spare parts, operating limits and conditions, operating instructions, assembly procedures (and trials for the most complex ones), training of Personnel.

Figure 7 shows an overview of the availability improvement approach for a fusion device like ITER.

Figure 7. Availabilty optimization flow diagram

Availability should be reviewed at significant stages of design and construction.

The upper limit in availability improvement is driven by design choices, reliability of components and budget constraints.

V. CONCLUSIONS The analysis of the operating experience and availability of

JET, JT-60, DIII-D, ASDEX-U, Tore Supra, FTU and RFX

gives also possible suggestions for improving the availability of devices under design or construction.

The average daily delay during the experiments is quite similar in the various devices (around 2 hr): the main contributions are due to Power Supply, CODAS, and AH.

The availability (effective operating time versus scheduled operating time) is not so different in the various tokamaks (about 75%). However, in tokamaks with magnets working at cryogenic temperatures and with water actively cooled plasma facing components, as Tore Supra, the additional delays, especially those related to the water leaks, lead in general to a lower availability (about 55%).

The main corrective actions influencing availability in present fusion devices are staffing, design and system diagnosis improvement, operational procedures, maintenance and spares holding, QA and quality control during modifications, preparation and management of experimental sessions.

The availability of the present fusion machines has been kept constant and in some cases improved all over the years in spite of the aging of the systems and components and of more demanding and complex plasma scenarios.

By extrapolating these results to ITER, taking also into account the presence of more systems, the complexity and the nuclear (hostile) environment, we could infer that the availability will be lower, and therefore not adequate to its mission. It is necessary to improve the availability of critical systems through design choices, R&D, QA.

An availability target should be defined since the design phase, and kept under careful control during the entire life of the project.

REFERENCES [1] T. Pinna et Al, "Collection of data at JET", SOFT 2004 [2] M.Edwards, JET Stats, personal communication (PC) Oct 2004 [3] M.Kikuchi, Special Committee on ITER Project of AEC, chap. 3 July

2000 and PC Sept 2005 [4] P.Petersen, Seminar at JET: “DIII-D HW upgrades, operation experience

and TRS”, 2002 and PC, July 2005 [5] G.Pautasso, C.Fuchs, ASDEX-U, PC Sept 2005 [6] Didier van Houtte, “Availability analysis of five years of operation of

SC tokamak Tore Supra@ -SOFT-1996, and PC Sept 2005 [7] J.L. Duchateau et Al, "Pioneering SC magnets in large tokamaks, Tore

Supra operating experience", SOFT 2004, Venice [8] P.Lomas, 2002, Att. I to JET WG report on improvement of the exp.

session/ Top Tips for success [9] B.M. Angelini: FTU Stats, PC, Sept 2005 [10] P.G.Sonato: RFX Stats, PC Sept 2005 [11] J.J. Cordier"Preliminary results and lessons learned from upgrading Tore

Supra actively cooled PFCs", SOFT 2004, Venice