Safety Concept and Tests Fakultaet Maschinenwesen, Institut fuer Energietechnik, Bitzer-Stiftungsprofessur für Kaelte-, Kryo- und Kompressorentechnik Jülich, August 20 th, 2015 Hans Quack

A short look back The overall project has not made the task for the moderator team easier: The optimisation of the moderator has led to a shape, which is difficult to manufacture. Operation close to the safety valve setting is expected. The 350 m distance between the helium refrigerator and the CMS has made the helium system difficult to control. 80% of the helium is in the two transfer lines. The 45 m long hydrogen transfer lines make the protection system of the hydrogen loop more difficult. 60 % of the hydrogen is in the transfer lines. The request for >99 % availability requires operation with two pumps. o-H2 content should be less than 0,5 % but there is no information about the o-H2 production in the moderator. Temperature range of 17 – 20 K is required. SNS operates between 18.5 and 21.5 K. Schedule and allowed costs are fixed, technical requirements are still being optimised. „In-kind“ procedures require start of work without allocated funds. Fortunately we are engineers, who like challenges.

Response to challenges We have tried to make the hydrogen loop simpler and thus more reliable: Replaced the helium backed bellows accumulator by a mostly passive expansion vessel. Moved the compensation heater from the hydrogen loop to the helium loop. Placed the two pumps in series instead of in parallel. Main operation will be at lower speed i.e. longer life time of the unlubricated ceramic bearings. Uninterrupted switchover to emergency running with one pump. Minimize control actions: Intend to operate with variable pressure level e.g. 15 bar with beam and 13 bar without beam. Use initial years with lower energy beam to operate the hydrogen loop at subcritical pressure to train the control system far away from safety valve settings. Hope to convince ESS to remove all valves out of the main loop.

Envisaged hydrogen loop No valve in the main hydrogen loop Three bypass valves V4 together with the heater controls the loop pressure. V5 controls the flow through the o-p converter V6 controls the flow rate through the moderators The feed stream is first cooled in HX2 and then converted in the o-p converter before joining the suction stream. The controlled release valve releases gas so that the safety valve has never to open. The expansion vessel mitigates all pressure variations. In early operation the set value for the pressure level will be reduced from 15.5 bar to a lower pressure.

Helium refrigerator Necessary decisions for the helium refrigerator: The large distance between the refrigerator and the CMS forced us to move the He/H2 exchanger to the high pressure side of the helium cycle. The small temperature span of the hydrogen forced an unusual rewarming exchanger in front of the expansion turbine. Increased refrigeration capacity and large turn-down requirements forced a switch to two parallel compressors and two parallel expansion turbines. The requirement for efficient operation in the whole 100 – 15 % capacity range is without example. Continuous technical contacts with potential vendors makes sure that they agree with these unusual requirements. We expect high quality and competitive proposals from vendors.

Safety systems Both the helium and the hydrogen systems have process spaces ad insulation vacuum volumes. All have to be protected against overpressure. The requirements and technical solutions in the helium system are relatively standard. The hydrogen loop is rather special and has to be investigated from scratch.

Hydrogen process loop protection First level Actively operated release valve(s) Programmable pressure and/or certain process signals Should cover all operational causes, hopefully only one unit at top of cold buffer Second level (PED) Spring loaded safety valves e.g. 17 bar (absolute) Number and positions to be determined by loss of vaccum calculation Third level Bursting discse.g. 18 bar (absolute) If possible avoid totally Serious causes: Blocked passages Loss of insulation vacuum Operational causes: Overfill Controls instability Trip of the refrigerator Trip of both pumps If there are valves or discs away from the cryostat, a thermally insulated line has to be foreseen. What about warm-up of cold hydrogen before it enters the stack?

Protection of Insulation Vacuum Spaces Small air leakAdsorption on cold lines, will not be noticed immidiately Evacuate during each warmup Small H2 leakWith dynamic pumping: No problem Static pumping: H2 will be adsorbed Evacuate during each warmup Simultaneous small leak of air and hydrogen Freezing of air, adsorption of hydrogen Explosive mixture during warmup: Simultaneous flooding with nitrogen during warmup? Cryostat vacuum is dynamic All others are static vacua ? Vacuum spaces Cryostat H2 transfer line Distribution box 8 jumpers 4 moderator insulations

Protection of Insulation Vacuum Spaces Large air leakQuick warming of hydrogen in this in this section will lead to pressure rise Release valves and probably safety valves of process will blow. Safety valves of vacuum room will probably not blow. Large H2 leakQuick increase of pressure in vacuum space. Warming of hydrogen in this section will lead to pressure rise. Release valves of process wiil blow. Safety valves of vacuum space will blow. (Distribution has to be calculated) Simultaneous large leak of air and hydrogen Explosive mixture in vacuum room, pressure increase in hydrogen process. Explosive mixture: Simultaneous flooding with nitrogen or helium during warmup? Cryostat vacuum is dynamic All others are static vacua ? Vacuum spaces Cryostat H2 transfer line Distribution box 8 jumpers 4 moderator insulations

Tests Compressors, turbines, H2 pumps Tests at manufacturers site Pressure and leak, maximum speed, efficiency with model fluid H2 pumps with LH2 Special setup in third party trest facility Performance field, efficiency, reliability CMS and moderators with LN2 JülichPressure and leak, thermal expansion, fluid flow pressure drop, heat leak Refrigerator aloneESSPerformance and acceptance test Refrigerator with transfer lines ESSTest of transfer line, dynamics of refrigerator and transfer line Total system with He in H2 loop ESSPressure and leak, thermal expansion, pressure drop, heat leak, active gas release system All equipent should be tested as early as possible

Tests All tests have to be prepared carefully. Are all test requirements part of the component specifications Which extra equipment is needed? Are all controls and instruments on site? Hazard and risk analysis Are all costs in the cost-book? Is there sufficien time?