MODIFICATION OF THE LHC VENTILATION SYSTEM M. Nonis EN/CV INTRODUCTION The aim of the present document is to review the current LHC ventilation configuration and evaluate the need to modify part of the existing installations in order to increase its safety functions. The input of this review is coming from the result of the Task Force on Safety that has been set up in 2008 to analyse the incident on sector 3-4 that took place on September 19 th, 2008; the outcome of this study showed that new functionalities need to be studied in order to guarantee a proper performance of the ventilation system, for instance in case of controlled helium release. Recommendation number 14 of the Task Force report [1] says that the Safety Task Force considers that the ventilation system is relevant for safety of personnel and thus recommends to set up a study of the LHC ventilation system with respect to monitoring and reliability of the system. The present article refers about those aspects and is focused only on underground structures (tunnel and Experiments). The main topics that will be discussed in the following lines are, after recalling some of the principles of the possible referential to use for LHC underground areas, an overview of the present configuration for the 2009-2010 run and the evolution on the medium term, the first results on the impact of a controlled release of helium in the tunnel, the actions that might be taken in order to improve the current situation and finally a short reference to the consolidation plan that has been presented by EN/CV in 2009 for HVAC systems in LHC. REFERENTIAL As already mentioned in previous papers (see [2]), it is not possible to define a referential that is comprehensive of all the specificities of a complex such as the LHC; the most appropriate one seems to be the ISO 17873 Nuclear Facilities Criteria for the design and operation for ventilation systems nuclear installations other than nuclear reactors [3]. The most important guidelines mentioned in the standard and that will be taken into consideration in the following paragraphs are: Prefer static confinement with respect to the dynamic one, Keep duct networks separated, Ensure functionalities in degraded mode, Filter air with appropriate filter class before releasing to external environment, Foresee a monitoring system covering main parameters to be controlled during the running of installations, Avoid booster fans, Foresee an optimum air renewal ratio: between 1 and 2 volumes/hour, Test periodically (yearly) the ventilation system, Foresee redundant system, Avoid single point of failure, Allow manual operation of dumpers in case of fire. While some of these points are already integrated in the LHC ventilation system, the remaining part of them is the object of the present paper. PRESENT CONFIGURATION The compensatory measures taken in order to guarantee the maximum safe conditions during the run from 2009 to 2010 with respect to the consequences of a new accident in the accelerator, have an impact on the sectorisation of areas and therefore on the ventilation conditions in the underground. One conclusion of the Task Force on Safety is that the maximum credible incident (MCI) has been redefined with respect to previous definition (dating in 1999) and therefore new compensatory measures have to be taken to counteract the consequences of such an event. Without going into many details the most important parameters in case of MCI to take into account are the following [1]: 139
maximum helium flow during the leak: up to 40 kg/s (around 166 m3/s), helium loss in first minutes: up to 1 520 kg, total helium loss: up to 4 920 kg. Compared to the previous risk analysis, it has to be noted that the flow rate to take into account has doubled, whereas the helium loss during the fast release has become 2.5 times higher. It has therefore been decided to create helium escape paths from the underground to the surface in order to avoid major damages in case of a new MCI in the next run; this has been achieved by dismantling the ventilation doors from the tunnel and the UAs thus creating one single volume between these areas (previously considered two different ventilation zones); helium can then rise via the PMs shaft in the SD buildings that have been equipped with calibrated pressure panels that will open in case of an overpressure of around 10 mbar. Further calculations performed later in 2009 [4] indicate that, in the new conditions the overpressure shall reach around 55 mbar in the tunnel and 11 mbar in US. During the start up phase of LHC in Summer 2009 the air flow rates supplied and extracted from the tunnel have been slightly modified in order to reach a differential pressure between experimental caverns and tunnel as close as possible to 20 Pa. Instead of keeping a fixed flow rate of 36 000 m3/h both at supply and at extraction, the working parameters are presently 30 000 m3/h at the supply and 40 000 m3/h at the extraction (tunnel in non accessible conditions); later in 2010 it will be monitored whether this modification has an impact on the removal of the thermal charge from the tunnel. In addition to that, the fresh air blown in all UAs (equal to 22 000 m3/h), goes then in the tunnel and therefore the air speed in the sectors vary from 0.2 to 1.9 m/s (according to the part of the sector). Differential pressure between different ventilation areas is the following: pressurised shaft: above 20 Pa everywhere, as foreseen. ATLAS and CMS: above 20 Pa between caverns and between cavern and tunnel. 140 LHCb: underpressure of the cavern with respect to the sector 78, very slight overpressure between UX and sector 81. Overpressure of the UX85 with respect to US85. ALICE: the cavern is in underpressure with respect to the tunnel on both sides. Finally the supervision system confirms that the stability of conditions is guaranteed only in close configuration; no possible pressure differential can be ensured in access mode. CONTROLLED HELIUM LEAK SCENARIO In order to avoid as much as possible major damages related to helium increase of pressure inside the accelerator equipment, safety valves have been installed in most of the sectors; these valves shall open and release 1 kg/s of helium in the tunnel for a maximum duration of 25 minutes. In order to evaluate the impact of such a discharge; two computational fluid dynamics simulations are presently running and preliminary indications on temperature, pressure and helium propagation in the tunnel are available. Since this scenario is also possible with presence of people in the tunnel (TA - accessible mode) other than during the run of the accelerator (TNA - non accessible mode), both cases are studied and first results (45 seconds of leak) show that the helium cloud follows the same direction of the air in case of TNA mode (45 000 m3/h), while the cloud propagates in both direction in case of TA mode (18 000 m3/h); the overpressure due to the helium is around 20 Pa at the end of the sector. MONITORING Several actions referring to the monitoring system are proposed. At present, not all the parameters of the air condition in the tunnel are monitored from remote stations; temperature probes are located in RRs and in front of REs but air speed is only obtained by calculation knowing the supplied flow rate. The cryogenic group TE/CRG shall install in the next shutdown some sensors to monitor the air speed and the pressure in case of a MCI; since the range of these sensors has to be of another order of magnitude from the ones needed for standard
conditions, additional probes will be installed to monitor the air speed and the differential pressure between different ventilation sectors (where already not existing): UX and US, UA and RA, UA and US. The signals from these sensors shall not interfere with the ventilation process but will generate an alarm in the control room that will allow taking decision on the procedures to adopt before going into underground areas. In all sectors and caverns the air is extracted via dedicated air handling units; the only exception is represented by UX45 and UX65 where the air is mechanically blown into the PX shaft and then goes via the shaft in the SX building. Although this does not represent a problem from a radioprotection point of view, it has been decided that the PX shafts shall be closed and that one extraction unit (that was running during LEP period) per shaft shall be re-commissioned; this will allow a radioprotection monitoring of the extracted air and its filtering. Most of the ventilation sectors are separated by static confinement and big efforts in tightening the areas have been done in the last year; some issues still need to be solved between UA and RA and between ALICE and LHCb caverns and tunnel. The drillings made between UAs and RAs for the passage of cables do not allow having the required air separation between those two areas with strong drawbacks in terms of air activation and risk in case of helium leak; the beam energy of the LHC is limited as long as this configuration will be kept. It is therefore foreseen to close all those cable passages and reinstall the ventilation doors between those areas; it is estimated that a few drillings per UA shall need to be ventilated in order to dissipate the cable thermal charge; this will be achieved blowing air from UA to RA by installing one dedicated fan per drilling and, possibly, fire dampers. Contrary to ISO recommendation, it would not be possible to install a redundant system. Since the tunnel is in overpressure with respect to the experimental caverns of ALICE and LHCb, static confinement has to be improved; in fact it is not possible to modify the pressure conditions since the fresh air for the tunnel is supplied in the even points. In case of LHCb cavern, while this can be achieved on RB84, it seems more complicated to be obtained in RB86. If this solution cannot be implemented and in order to avoid air going from 141 the tunnel to the experimental caverns, an extraction system should be then installed in RB84 and/or RB86 and the volume between the TAS and the existing chicanes made airtight; this will allow this areas to be in underpressure. Such an extraction system might be made redundant but will consist of a booster fan, solution that is not counselled by the ISO standard. Finally, the last issue related to confinement is the separation between the LHC tunnel and TI2 and TI8 tunnels to protect those tunnels from damages due to a MCI. If separation doors have to be foreseen, the need of bringing fresh air into TI2 and TI8 shall require the installation from UJ24 to UJ22 and from UJ86 to UJ88 of an DN1000 air duct, these dimensions are however not compatible with the existing available space in those areas; other solutions have therefore to be foreseen for those two tunnels. RELIABILITY All AHUs blowing or extracting air from underground areas are backed up by a redundant unit with the exceptions of the extraction from RF area in UX45 and the boosters extracting air from the tunnel in UJ76. The installation of an additional unit in UX45 should not represent a major problem, whereas the available space in UJ76 is reduced. In other cases (boosters in tunnel at Point 3, units in PX24, PM65 and PZ65 and the supply of the tunnel), in case of breakdown the redundant system can ensure a lower flow rate but this does not represent a operational nor a safety issue. FIRE SAFETY POLICY The existing fire policy with respect of the ventilation system is based on the following principles: Ventilation can ensure confinement of pressurized areas if doors kept closed, therefore contact alarms have to be foreseen where not existing. Actions to be taken are decided on site by Fire Brigade after assessment of the situation. The extraction system is designed for cold smoke extraction (without filtering) only. In case of fire detection, all the supply and extraction units are stopped and can be restarted only by fire brigade personnel.
While the units managing air for the experimental caverns are on the secure electrical network (however no pre heating is possible), this is not the case for the units related to the LHC tunnel. A first study has been done to understand the work required to modify such situation and the related cost sums up to 10 MCHF (excluding civil engineering costs), since in very few LHC Points there s enough secure power available. At the same time it has to be assessed the need of including the preheating coils in the secure network for all units backed up by diesels, taking into consideration that freezing problem can appear during cold season. CONSOLIDATION It has to be reminded that the present ventilation system dates mainly from LEP period and important part of it is still the original one. Therefore some consolidation work has to be foreseen in the coming years to allow a proper run and operation of the installations without impacting into the accelerator program. The most important aspect concern safety features, absolute filtering, supply air plenum, thyristors replacement, instrumentation replacement and vibration analysis as reported in the following table: Absolute filters** Priority Before Risk score After Amount* [MCHF] H 6 3 0.8 Safety features H 9 2 1 Supply air plenum M 6 3 0.6 Thyristors replacement H 6 3 0.8 Instrumentation replacement M 9 3 0.5 Vibration analysis AHU M 9 3 0.8 * CE, EL etc. costs not included ** To be confirmed by SC/RP Planning 2012 2010-2012 2012-2015 2013 2014 2013-2017 142 A more detailed explanation of the consolidation program for EN/CV consolidation program can be found in the technical note: Mid-term consolidation plan for the cooling and ventilation facilities, 2010-2017 August 2009 [5]. CONCLUSIONS The existing ventilation system satisfies almost the totality of requests that were made during its construction; some new issues have then been raised at a later stage, mainly related to the consequences of a helium leak in the underground areas, requesting additional functionalities or the reinforcement of existing installations. In addition, the compliance with a new referential, such as ISO 17873, is quite costly and technically difficult to be achieved; this standard can therefore only be used as a possible reference document but not taken as a new set of rules or guidelines to fully comply with. Other aspects, such as the fire policy to follow or the confinement of TI2 and TI8, need a risk assessment of the situation before taking a decision about approving new work that might cost several millions. The remaining open issues, linked to existing redundancies, pressure differentials, monitoring ventilation conditions are already foreseen or can be rapidly launched if a positive decision is taken. The cost related to these actions shall be estimated in the coming months but it is considered that most of the action will sum up to a few millions maximum. Work shall fit into future shutdowns, according to other interventions foreseen in the tunnels and caverns. It has anyhow to be remarked that the actions taken do not always represent an optimized solution according to ISO 17873 standard (use of boosters, redundancies, accessibility problems, dynamic versus static confinement, etc.), since we have to adapt to existing premises and installations. ACKNOWLEDGMENTS The work resumed in the previous paragraphs has been possible only to the help of several people and colleagues that gave a very important help allowing the study to be done within the very tight schedule. The author is therefore grateful to all of them and, in particular, to J. Inigo Golfin, G. Roy, D. Forkel-Wirth, S. Roesler, O. Beltramello, E. Thomas, C. Schaefer, M. Tavlet, S. Weisz,
R. Trant, B. De Lille and to the EN/CV people for the work they have and are doing. REFERENCES [1] Safety of Personnel in LHC underground areas following the accident of 19 th September 2008, Task Force report, CERN ATS-2009-002, [2] The LHC Ventilation system, safety considerations; J. Inigo Golfin, Proceedings of Chamonix 2009 Workshop on LHC performance, February 2009 [3] ISO 17873 Nuclear Facilities Criteria for the design and operation for ventilation systems nuclear installations other than nuclear reactors. [4] LHC Machine Maximum credible incident Pressure build up inside LHC tunnel Summary; P. Azevedo, B. De Lille, DG- SCG/TR/2009-19, October 2009 [5] Mid-term consolidation plan for the cooling and ventilation facilities, 2009-2016; M. Batz, Y. Body, S. Deleval, J. Inigo-Golfin, C. Martel, M. Nonis, G. Peón, B. Pirollet - August 2009 143