Cryogenic oxygen deficiency hazard assessment at the National Synchrotron Radiation Research Center

The National Synchrotron Radiation Research Center (NSRRC) uses cryogenic fluids to create a low-temperature cooling environment for equipment and to conduct various experiments. However, exposure to these cryogenic fluids can cause frostbite, hypoxic suffocation, behavioral incapacitation, insanity, and even death in severe cases. To evaluate oxygen deficiency hazard (ODH) in the NSRRC, we adopted the Fermilab assessment methodology and conducted ODH assessments in the Cryogenic Compressor Room, Taiwan Light Source (TLS) Tunnel, and TLS15A hutch. The results of the evaluation of the Cryogenic Compressor Room and TLS Tunnel revealed that the ODH class is 0 both when the exhaust fan is operating normally and when the exhaust fan is damaged. The exhaust equipment in the TLS15A hutch is only for emergency use. Without the emergency exhaust fan, the ODH class in the area is 1. If the emergency exhaust fan is always on, the ODH class is 0. Therefore, we recommend that those in TLS15A should undergo safety education and receive hazard notifications. In addition, we strongly recommend installing oxygen detectors in the beamline hutch to ensure safety.


Introduction
The National Synchrotron Radiation Research Center (NSRRC) is the largest research facility in Taiwan, housing two accelerators, the Taiwan Light Source (TLS) and the Taiwan Photon Source (TPS).The TLS, with a beam energy of 1.5 GeV and a circumference of 120 m, is the first thirdgeneration synchrotron light source facility in Asia.The TPS, with a beam energy of 3 GeV and a circumference of 518.4 m, is one of the brightest synchrotron light sources worldwide.
Utilizing compressed and liquefied asphyxiant gases such as nitrogen and helium is a common practice at NSRRC.However, releasing these gases into the environment, particularly into low-oxygen atmospheres, can be hazardous.When exposed to such conditions, individuals may experience impaired cognitive function, loss of consciousness, or even fatality.To solve this problem, oxygen deficiency hazard (ODH) assessments must be conducted.The ODH assessment methodology was developed by Fermilab (Fermilab Environment, Safety, and Health Manual [FESHM] 4240) [1] and implemented at Jefferson Lab (Environment, Safety, and Health [ESH] Manual 6540 Appendix T4) [2] and SLAC (ESH Manual Ch. 36) [3].The NSRRC adopted the methodology to assess ODH.The results of these evaluations can serve as a basis to determine the ODH class of each area and to suggest protective and preventive measures, such as engineering controls and administrative management, to guarantee safety.

ODH assessment
Fermilab's ODH assessment methodology involves a dynamic model for ODH in specific areas during and after the accidental release of asphyxiant fluid (Figure 1).The methodology is based on parameters such as the ventilation rate, leak rate, and time, which are used to estimate the concentration of oxygen in the atmosphere.The NSRRC uses Fermilab's ODH assessment methodology to determine whether an area is safe for occupancy during and after the accidental release of asphyxiant fluid.

ODH assessment equations
The concentration of oxygen within a confined volume before and after the release of an inert gas can be estimated using the equations presented in Table 1 [4].These equations are based on the oxygen mass balance for the confined volume and solution, with the boundary condition of C = 0.21 at t = 0.The following assumptions apply to all cases [1]:  For cases A through C, assume complete and instantaneous mixing within the confined volume. Q, R, and V remain constant. Pressure in the confined volume remains constant and near atmospheric pressure because of the louvers or natural leakage. Gas entering from outside the confined volume is air with an oxygen concentration of 0.21 (21%).where C = oxygen concentration (%); Q = ventilation rate of fan(s) (cubic meters per second); R = leak rate (cubic meters per second); t = release time (seconds); V = confined volume (cubic meters).

ODH class
The ODH class is determined on the basis of the most severe risk condition, namely that entailing the possibility of death.For a specific area in which multiple events may lead to oxygen deficiency, each event is considered an independent occurrence with an anticipated incidence and severity of fatality.
The fatality rate associated with ODH is , where Ø = the ODH fatality rate (per hour); Pi = the expected failure rate of event i (per hour); and Fi = the fatality factor of event i.
For the sake of convenience, we used the Pi and Fi values provided in FESHM 4240.If possible, the value of Pi should be based on experience; otherwise, the data provided by other accelerator facilities or equipment manufacturers should be used.After determining the ODH fatality rate, the ODH class of an area can be identified (Table 2).

ODH control measures
The design and installation of equipment at NSRRC shall ensure that areas intended for human entry during normal operations are not higher than ODH Class 2. Humans should not be allowed in areas with ODH higher than class 2 unless ODH can be reduced to acceptable levels through engineering controls, administrative controls, and training.

Engineering controls
Before cryogenic equipment is installed, the safety design and layout of safety valves, pressure relief valves, pipelines, and ventilation systems must be evaluated to prevent environmental hypoxia and system overpressure.Engineering controls include the following:

Administrative controls
Some administrative controls for ODH areas are warning signs, training programs, and safety device testing.One critical administrative control for areas with ODH class 2 or higher is the buddy rule, multiple personnel in continuous communication.Table 3 presents the warning signs and managerial tasks necessary for each ODH class. the activities at the NSRRC involving oxygen deficiency,  the definition of ODH,  the effects of exposure to an oxygen-deficient atmosphere,  the ODH classification scheme,  required control measures,  personal protective equipment, and  emergency procedures and evacuation (Figure 3).

Evaluation results
We used Fermilab's ODH assessment methodology to calculate the ODH class for the Cryogenic Compressor Room, TLS Tunnel, and TLS15A hutch (Table 4).The analysis of the Cryogenic Compressor Room revealed that the ODH class is 0 when the exhaust fan is operating normally, and that it remains 0 when the exhaust fan is damaged.The analysis of the TLS Tunnel revealed that the ODH class is 0 when the exhaust fan is operating normally, and that it remains 0 when the exhaust fan is damaged.
In the TLS15A hutch, the exhaust equipment is used only for emergencies.For this reason, we analyzed situations both with and without the emergency exhaust fan operating.The evaluation revealed that without the fan, the ODH class is 1; with the fan constantly on, the ODH class is 0. Therefore, researchers using the TLS15A should undergo safety education and receive hazard notifications.We also strongly recommend installing oxygen detectors (Figure 4) in the beamline hutch to ensure personnel safety.Helium gas pipe 1×10 -5 1 1×10 -9 Exhaust system 1×10 -4

Conclusion
The analysis results of the TLS15A hutch indicate that all new beamline hutches that use liquid nitrogen in the TPS need to install with oxygen detectors.Other several ODH Class 1 areas, such as the corridor in Utility building II, the Cryogenic Pipeline Room in the TPS building, and the cryogenic system of the superconducting radio frequency (SRF) module in the TPS tunnel, have oxygen detectors installed and personnel working in these areas should undergo emergency response training to ensure their safety.

Figure 2 .
Figure 2. Oxygen detector and status light.

Figure 4 .
Figure 4. Oxygen sensor in the beamline hutch.

Table 3 .
ODH warning signs and management measures