Occupational health and safety

The study has initiated a methodological approach in tackling occupational health and safety aspects. The hazard registry classifies relevant sources o f risks to permit identifying those, which can be addressed by standard approaches and those, for which project-specific assessments need to be performed, followed by a definition o f mitigation measures against residual risks. This preliminary activity has identified that two main hazards are present in underground areas: fire and oxygen deficiency (ODH ). The latter is a residual risk that emerges after applying “safety-by-design”

to cryogenic hazards, such as the avoidance o f cold surfaces and functional measures such as combined vacuum and superinsulation blankets. This report, focusing on the feasibility and concept elements o f a future particle accelerator research infras­

tructure does not permit the technical risk management files to be presented in a

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comprehensive manner. It therefore only focuses on the presentation o f the approach for the two main hazards (fire and oxygen deficiency). The results o f the studies create a longitudinal unidirectional flow. Smoke and ODH detectors initiate localised mitigation measures. A dedicated 350 mm diameter extraction duct traverses all com ­ partments and can extract smoke or helium. The extraction for each compartment powered and potential ignition sources. During other periods, personnel are present and they may inadvertently cause a fire, e.g. during welding. Table 8.2 shows the three fire scenarios studied.

Following the performance based design methodology, the current LHC tunnel equipped with the planned safety measures was challenged with three different fire scenarios representing credible incidents in the HE-LHC underground tunnel struc­

ture. For consistency, the fire scenarios considered were the same as those considered for FCC-hh [253].

Only the life safety goal for occupants was evaluated during this initial study (O 1-O 3 in Tab. 8.1) . For the sake o f consistency, the same performance criteria used for the FCC-hh study were set to ensure tenability limits for the evacuees. These criteria comprised incident heat flux, temperature, visibility and exposure to toxic materials accounted for by means o f fractional effective dose (FED).

both shafts, maintaining evacuation paths over-pressured with respect to the fire compartment and clear from smoke.

This strategy was evaluated within the fire compartment by means o f CFD simu­

lations o f the three accidental scenarios investigated. The simulation program is the Fire Dynamics Simulator (FDS) V 6 [254]. The pre-movement time and walking speed were defined according to standard recommendations [255] (British Standard PD 7974-6) and were further improved by adding some degree o f uncertainty. The results showed that the heat flux, temperature and FED values to which evacuees are exposed are well within the acceptable thresholds stated in the performance criteria.

However, for a single evacuation location visibility is locally reduced to below 10 m at 2 m height. The stratification is still maintained and visibility is unimpeded at 0.5 m height.

Based on the outcom e o f the simulations, it can be stated that the current design meets the life safety objective for occupants in case o f fire. However, there is a very limited safety margin and it is strongly recommended that additional safety measures aiming at increasing the robustness o f the design should be incorporated. The other safety objectives will be evaluated at a later stage.

Protection o f the environment calls for a careful selection o f fire-fighting agents (e.g. water, foam) which will be done at a later, detailed design stage. The system configuration will help keeping the release o f smoke, chemical and radioactive con­

taminants to quantities as low as reasonably possible.

8 .2 .2 O x y g e n d e f i c i e n c y h a z a r d most critical commissioning and maintenance phases. Based on the current access restrictions in the LHC for operational periods where large helium release flows can­

not be excluded, it is assumed that similar measures would be in place during the compartment walls in the tunnel every 548 m. The objective o f the study was to deter­

mine whether an accidental release o f 340 g /s produces a helium layer at the ceiling o f the tunnel that is large enough to prevent successful evacuation o f occupants.

The helium release is localised within one compartment. The compartment doors will remain open during a helium release. Gaseous helium rapidly warms up and rises to the compartment ceiling. In order to simplify the estimation, it is assumed that the longitudinal ventilation does not have an influence o f the stratification o f helium and that all helium is in the ceiling layer shortly after release. The maximum thickness

T a b l e 8 . 3 . I n p u t d a t a f o r t h e e v a lu a t io n o f a n a c c id e n t a l r e le a s e o f h e liu m in t h e t u n n e l.

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Q H e ( k g s 1 ) 0 .3 4 V H e (m 3 s - 1 )

@300K

2 .1 2

a (m ) 2

h (m ) 0 .8

A ( m 2) 1 .6 5

L (m ) 5 4 8

F i g . 8 . 1 . C r o s s - s e c t i o n o f t h e L H C T u n n e l. T h e g r e e n a r e a in d ic a t e s t h e h e liu m c e ilin g la y e r w i t h c r o s s - s e c t io n a r e a A a n d d e p t h h . a is t h e h e ig h t o f t h e c o m p a r t m e n t d o o r s a n d h t h e r e m a in in g h e ig h t u n t il t h e t u n n e l c e ilin g , 0 .8 m .

o f the helium layer at the ceiling is 80 cm, when its lower edge reaches the upper edge o f the compartment doors and it can spill over to the adjacent compartments.

Table 8.3 shows the input data used for the evaluation (see also Fig. 8.1) .

It can be seen from Figure 8.2 that it would take seven minutes to fill a com part­

ment with a helium layer o f 0.8 m, for a gas temperature o f 300K. For a temperature o f 273 K it takes almost eight minutes. Based on the data used for the FCC-hh mag­

net concept, the estimated volume to empty the helium inventory can be determined (c.f. Tab. 8.4) .

If the extraction duct was also used for the management o f the released helium, it would be operational at full capacity in the sector concerned for four minutes after the ODH alarm. The capacity o f the system is estimated at 2200 m3/h , with an extraction efficiency o f 80%. The remaining 20% o f the volumetric flow rate is air.

Fig. 8 .2 . Volume of gaseous helium in the compartments as a function of the gas tempera­

ture and time. The crossing of the dashed horizontal lines mark the time from the beginning of the release to when the helium penetrates the next adjacent compartments.

T able 8 .4 . Parameters used for FCC-hh dipole magnets.

VL H e(lm X) 33

L (m) Sectorisation (corresponding to 1 cell)

137

VG H e (kg) 670

Main conclusions

- Personnel access to the underground areas is forbidden in operational phases where the potential helium release exceeds 340 g /s. In LHC, this is the case for powering phase 2. A similar assessment o f possible helium release rates must be conducted for the HE-LHC magnets.

- Assuming the tunnel occupants are not in the “turbulent zone” o f the helium release [256], they would be minimally affected by the helium released, which is accumulated at the ceiling o f the tunnel. Safe evacuation from the tunnel is necessary. As an additional precautionary measure, personnel are equipped with self-rescue masks.

- The temperature o f the gas (between 273 and 300 K) does not have a m ajor effect on the time to fill the 0.8 m deep volume at the ceiling o f one compartment.

- If the mass flow o f the release is constant, it would take about 30 min to empty the helium inventory. B y that time, three compartments in a sector would have been filled with helium to a level o f 0.8 m from the ceiling.

- The addition o f the He extraction would only be considered as a com pensatory safety measure for flow rates above 6000 m 3/h .

- Further studies on the dynamics and influences o f the boundary conditions in such an environment are necessary. A simulation with a Computational Fluid Dynamics (CFD ) program is recommended.

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At this stage, the ODH analysis uses a simplified approach on a qualitative engi­

neering level, mainly due to the level o f technical detail at this stage o f the study. A quantitative approach will be used for the technical design report.