The MICE Hydrogen System Safety Review Introduction Tom Bradshaw, Yury Ivanyushenkov, Elwyn Baynham, Tony Jones, Mike Courthold and Matthew Hills Rutherford Appleton Laboratory

Overview of talks 1.Introduction (TB) Design philosophy Basic operation including hydride bed Basic design calculations Motivation and approach – R&D cycle Safety review process 2.Process and Instrumentation Diagrams (MH) 3.Control System (MJDC) Overview of control system Prototype flow diagrams & control sequences Pumps and Instrumentation Implementation and hardware 4.Hall Infrastructure and R&D Cryostat (AJ) 5.HAZOP and failure analysis (YI)

Introduction The muon ionisation cooling experiment is an pre-cursor to a neutrino factory. Its objective is to demonstrate muon cooling – i.e. produce a collimated beam of muons. This is achieved by cooling and slowing down the muons in an absorber (Hydrogen, Helium or plastic) before accelerating them in an rf field. The absorber for use with liquid hydrogen or helium consists of a chamber with thin aluminium windows. This requires a hydrogen delivery system which is the subject of this review. The design of the absorber and its implementation is the subject of a separate safety review.

Safety Review Process We are here

There are two phases to the implementation of the hydrogen delivery system: R&D Phase where a single system is developed and tested on a test cryostat which represents an absorber. Implementation phase where the delivery system is matched to a real MICE absorber The objective of the R&D phase is to demonstrate a safe hydrogen delivery system for the absorber. This will consist of a first model MICE hydrogen delivery system together with a test cryostat that does not have thin windows but does contain the same instrumentation. Objectives of R&D

Cross section of Absorber a)Windows are mounted off RT interface – see thermal model later b)Space for change in pipe dimension close to magnet c)Large “bucket” at base to contain any rupture This is not the subject of this review Windows are rated up to: Design pressure1.6bar Test pressure2 Burst pressure6.4

Safety Review Process KEK Absorber design

Past Experiences

Hydrogen incidents listed in “Control of Liquid Hydrogen Hazards at Experimental Facilities” by A A Weintraub.

Hydrogen Accidents - Industrial Number of Percentage of Total Category Incidents Accidents Undetected leaks Hydrogen-oxygen off-gassing explosions Piping and pressure vessel ruptures Inadequate inert gas purging 12 8 Vent and exhaust system incidents 10 7 Hydrogen-chlorine incidents 10 7 Others Total Source: Safety Standard for Hydrogen and Hydrogen Systems, NASA NSS , p.A-109

Design Philosophy We have three absorbers and have three independent hydrogen systems, this is to: Avoid consequential failures – a failure or fault in one is easier to deal with than a fault on a large system This will ease the staging for MICE as only one absorber is required early on Smaller systems are easier to work on

Design Philosophy Other considerations:  Minimise venting – many accidents are caused during this process  MICE has to be flexible – there will be many filling cycles of the absorbers and we want to minimise the amount of hydrogen that we have – hence the use of a hydride bed  Control system automates the filling, emptying and purging system (many accidents from ineffective purging)  Must be safe in the event of a power loss or system shut-down (looking at default valve positions)  No surfaces below the BPt of Oxygen – this is to prevent cryopumping of oxygen on any surface that may come into contact with hydrogen in the event of a failure  Safety volumes to contain gas, relief valves to prevent back flow in case of catastrophic release

RAL Codes Hydrogen zones definition according to RAL Safety Code No.1: Zone 0: An area or enclosed space within which any flammable or explosive substance, whether gas, vapour, or volatile liquid, is continuously present in concentrations within the lower and upper limits of flammability. Zone 1: An area within which any flammable or explosive substance, whether gas, vapour, or volatile liquid is processed, handled or stored and where during normal operations an explosive or ignitable concentration is likely to occur in sufficient quantity to produce a hazard. Zone 2: An area within which any flammable or explosive substance whether gas, vapour or volatile liquid, although processed or stored, is so well under conditions of control that the production (or release) of an explosive or ignitable concentration in sufficient quantity to constitute a hazard is only likely under abnormal conditions. Intention is to have no Zone 0 or 1 regions in the design

System Overview Gas Delivery System Hydride bed for gas storage Control Valves, pumps, alarms and indicators Buffer volume Control System Controllers Interface to the rest of MICE Test Cryostat Cryocooler Instrumentation Hydrogen volume

Window rupture Must be safe in the event of a window rupture: Introduction of a buffer vessel limits pressure excursions Pipework sized to accommodate gas release

Assumes mixing of gas - cold from absorber + buffer volume Temp in buffer calc on basis of constant Cv - this is optimistic for Tgas ~50K but pretty good for Tgas >100K For large outflow through relief valve the algorithm is not correct because the valve essentially shuts Buffer volume gives a huge safety margin over just the pipe system with vol ~ 0.1m^3 for 50m of 50mm dia pipe The buffer vessel will keep the gas warmer due to its thermal mass - this is not included - it will increase the pressure rise Typically with 1m^3 Tgas ~100K pressure rise rate is 0.1 bar/sec valve opening time of sec would be OK Expected boil-off rate Latent heat446000J/kg Power into liquid10179W Hydrogen boiled off (kg/s) kg/s Start mass of liquid1.544kg Liquid density Start pressure (bar)0.5 or 1 Rgas4157 dt0.2 Buffer vol1m^3 density 300K0.08kg/m^3 relief valve pressure1.60E+05Pa outlet mass flow1.20E-02kg/s Effectiveness of Buffer Volume

What is temperature of Outer Window ? Thermal balance between radiation [to inner window at 18K] and conduction to 300K Particular concern is that the centre of the outer window will fall below condensation point of Oxygen

Pipe sizes Diameter (cm)30 Area (cm2) x Specific load (W/cm2)3.6 Load (W) Safety factor x2 (W) Latent heat (J/g)446 Hydrogen boiled off (g/s) Vel sound m/s Estimates of pipe sizes required in the case of a catastrophic vessel rupture Pipe transitions are inside vacuum space for venting CERN measured worst case x2

Hydrogen Storage Trade-off Options for hydrogen storage: A) in a low pressure tank Pros: truly passive system Cons: - size (about 30 m3), 3 tanks are required; - dispersed system with long pipes => difficult to collect hydrogen in case of leak; - not feasible for neutrino factory (where to put them ?). B) in a metal hydride bed Pros: - very compact system ( easier to collect hydrogen in case of leak; - hydrogen is stored as a solid compound; - more feasible for neutrino factory. Cons: not a passive system => requires active heater/cooler.

Cryocooler Maintaining a positive pressure Cryocooler operation will keep temperature in range 14-20K Helium gas will be introduced to keep pressure in system positive The use of helium to maintain a positive pressure needs thought as it will be added after the hydrogen has condensed

Summary The hydrogen system is being developed through an R&D process Many aspects of the safety have been considered through calculation, design and review We have a well defined safety review process The design is well advanced and will be detailed in the following talks

MICE Hydrogen System END