​Introduction
The sprawling development of cryogenic liquid hydrogen (LH2) mobility solutions for land [R1], sea [R2] and air [R3] applications as a way to reduce the carbon emissions and its global warming effect as long as the renewed interest towards the development of cryogenic helium (He) cooled systems (i.e. linear accelerator for physics research [R4], superconducting magnets for fusion reactors [R5]) pose a significant technical and scientific challenge particularly in the domain of cryogenic safety.
When working with cryogenic liquids at low temperature (i.e. 4 K for LHe and 20 K for LH2 at the normal boiling point) in vacuum insulated architectures, the consequences of accidents involving the complete system has to be taken into account due to the hazardous and potentially catastrophic results that a bursting component may entail to the worker safety. The failure of the insulating vacuum accident (LOVA) is one of the major scenario when working with cryogenic fluids. In such a situation, the ambient air rushes into the vacuum space, used to insulate the cryogen-filled component from the external thermal losses, and freezes or condenses on the cold surfaces. The resulting large heat flux is suddenly transferred to the cryogenic fluid and leads to an extremely complex and rich phenomenology of temporally and spatially transient thermal-hydraulic phenomena, which have to be mastered to assure the safety of the user. This heat flux causes a large pressure increase inside the cryogenic circuits.
To avoid any damage, the system has to be protected by means of pressure relief devices, which have to guarantee a sufficient discharge flow rate of the cryogenic fluid to ensure that the maximum pressure remains below the safety limit [R6]. The cryogen is then discharged outside through a potentially complex system of piping and additional check-valves (depending on the cryogenic system, its operational use and constraints) which may affect the transient behaviour of the cryogenic fluid. Depending on the system configuration, its temperature, the cryogen tank initial conditions and the heat transfer as long as the pressure losses the emergency discharge may lead to a two-phase flow with sonic blockage possibly occurring downstream the safety relief device. Under such circumstances, it is clear that the safety of complex cryogenic systems shall be evaluated considering the whole system and its broader operating conditions and not only by focusing solely on the safety device.
Within this framework, the DSBT (low temperature system department) at CEA/IRIG in Grenoble has developed a significant experience on the topic by carrying out experimental [R9] and numerical [R7] research activities but further investigations are required to improve the knowledge concerning the sizing of the safety devices and the assessment of the thermal-hydraulic behaviour of complex cryogenic system.

Objective of the activity
The goal of the activity is to contribute to the development of a quantitative methodology to assess the behaviour of complex liquid helium cryogenic systems when a LOVA occurs with the use of the two-phase unsteady 6-equations CATHARE 3 code [R8]. The activity will focus firstly on the effect of relevant parameters (i.e. geometrical configurations, boundary and initial conditions, safety devices characteristics etc.) on the temporal and spatial dynamics of the thermal-hydraulic phenomena occurring at cryogenic temperatures. Secondly, a methodology will be developed to take into account the statistical variability of the parameters and to assess margins with respect to the safety criteria. The activity will be divided in several phases:
• Phase 1: Bibliographic review and numerical modelling update. The phase will focus initially on the collection, analysis and synthesis of the existing literature on the topic by the Candidate as long as a hands-on training on the use of CATHARE 3 for cryogenic systems.
• Phase 2: Modelling of complex cryogenic systems. During this phase, a CATHARE thermal-hydraulic model of a liquid helium cryogenic systems will be built and run within a step-by step approach of increased complexity. The thermal-hydraulic behaviour driving parameters will be explored following a sensitivity exploration matrix
• Phase 3: Development of the methodology. Once the models anchored on satisfactory results, the Candidate will contribute to the development of a quantitative methodology to assess the effects of the statistical scattering of the driving parameters on the results and the evaluation of the safety margins.
• Phase 4: Synthesis. The entire activity will be completed by the detailed synthesis of the work performed, the results and the outcomes as long as the perspectives.

Candidate requirements
The Candidate will explore the less known domain of cryogenic engineering where physics, material properties, thermodynamic and transport properties will change significantly at low temperatures with respect to standard (ambient temperature) conditions. The candidate required competences are:
• Numerical modelling and thermal-hydraulic simulations with CATHARE 3 (or 2) and its post-processing tools (Guithare),
• Scientific programming skills with Matlab™ and/or Octave and Excel™,
• Fluids mechanics and thermodynamics,
• Curiosity and autonomy. Knowledge of cryogenic processes will be an added value.

A BAC+5 (Master or Engineering school) internship if foreseen with a duration of 6 months with a starting date between January and March 2024.

Laboratory, supervisors & contacts
The internship will be carried out at the CEA site of Grenoble within the Low Temperature System Department (DSBT) at the Laboratory of Refrigeration and Thermal-Hydraulics (LRTH) under the supervision of Davide Duri

Bibliography
[R1] Technology Pitch: Subcooled Liquid Hydrogen (sLH₂). Available here. 
[R2] Pratt J.W., Klebanoff L.E., “Feasibility of the SF-BREEZE: a Zero-Emission, Hydrogen Fuel Cell, High-Speed Passenger Ferry”. Report SAND2016-9719, Sandia National Laboratory, Livermore CA (2016). 89. NORLED, World’s first
[R3] Airbus reveals new zero-emission concept aircraft. Available here. 
[R4] Link to Mhyrra/Minerva project.
[R5] Link to RenaissanceFusion project : 
[R6]. Sécurité en cryogénie. Ercolani E. et al., Tech Ing Froid Ind, vol. base documentaire : TIP204WEB, no. ref. article : be9814; 2015. doi: 10.51257/a-v1-be9814.
[R7] Adaptation of the nuclear safety code CATHARE3 to supercritical helium flow, S. Shoala et al., Cryogenics, 113 (2021), p. 103135, 10.1016/j.cryogenics.2020.103135
[R8] CATHARE modelling of the HELIOS loop for the study of heat transfer to supercritical helium flowing in pipes following loss of insulating vacuum. S. Shoala et al., Cryogenics Volume 126, September 2022, 103520, https://doi.org/10.1016/j.cryogenics.2022.103520
[R9] Etude expérimentale et numérique des entrées de chaleur accidentelles dans les circuits cryogéniques des grands instruments PhD thesis S. Shoala. To be defended in Nov. 2023 Abstract.