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					                     ASSESSMENT OF JACKETING REQUIREMENTS
                     FOR MICE HYDROGEN R&D TEST CRYOSTAT

1. ORIGINAL THOUGHTS
A double-fault-scenario safe system was considered as a design philosophy for the MICE
hydrogen R&D system. In case of the test cryostat possible faults which could occur
simultaneously are:

      Cryostat is leaking hence there is an ingress of air into the cryostat. This air is then
       frozen on the 20K-surfaces. There is no detectable pressure rise in the cryostat vacuum.

      Parts of hydrogen circuit inside the cryostat are leaking. Hence there is a leak of
       hydrogen into the cryostat vacuum.

 As a result of these two faults an explosive mixture of hydrogen with air might be formed
during the system warm up. This consideration has originally lead to conclusion that the
cryostat needs a tertiary container filled with Ar or N2 to prevent of ingress air into the
cryostat volume.

2. CLOSER LOOK INTO THE PROBLEM
2.1 It is a triple-fault-scenario case.
The first comment is that it is not a double-fault but is a triple-fault scenario case since the
ignition source is still needed to ignite the explosive mixture. The electrical/electronic
system components might be a potential spark source unless this system is designed to
exclude this from happening.

Requirement: electronic/electrical system in the cryostat vacuum must be designed to exclude
sparking.

2.2 Probability of forming hydrogen-air mixture
2.2.1 Normal operation- hydrogen vessel is filled with LH2
In normal operation the temperature of hydrogen vessel and hydrogen condensing pot is
about 20 K, so the ait is frozen on their surfaces. The explosive mixture can not be formed.

Probability of forming hydrogen-air mixture: negligible.

2.2.2 Operation of filling up hydrogen vessel with LH2
In this situation the temperature of hydrogen circuit is about 20 K, and explosive hydrogen –
air mixture can not be formed as air is frozen on cold surfaces.

Probability of forming hydrogen-air mixture: negligible.



2.2.3 Operation of empting the hydrogen vessel
In this operation hydrogen boils off at 20.2 K at 1 bar pressure or at a slightly higher
temperature if the pressure is above 1 bar. But air is still frozen on the cold surfaces, and the
explosive mixture can not be formed.
Probability of forming hydrogen-air mixture: negligible.

2.2.4 At what situation the explosive mixture can be formed?
The explosive mixture can only be formed when both hydrogen and air are in gaseous state,
i.e. the temperature inside the cryostat is above 79 K. As it is shown above, during normal
operations, the temperature of the coldest parts inside the cryostat is about 20 K, and the air is
in solid state.

The temperature inside the cryostat is above 79K only when the system is being warmed up,
but in this situation there will not be any hydrogen left inside the hydrogen chamber as it has
been boiled off at 20 K.

Then the only hydrogen that could be potentially present inside the vacuum vessel during the
warm up, is the hydrogen that might leaked out into the cryostat when the system was cold.

How much hydrogen can leak into the cryostat ? If the cryostat volume is been continuously
pump off then all hydrogen leaked will be removed. Otherwise the hydrogen will slowly start
accumulate in the cryostat volume giving rise to the pressure which is detectable (estimation
of pressure rise to be made). Then the cryostat volume must be evacuated and hydrogen
removed.

Probability of forming hydrogen-air mixture: low.

Requirement: continuously monitor the pressure in the cryostat volume.

Question: is it safe to pump off hydrogen from cryostat volume?

3. CONCLUSION
1. Probability of forming hydrogen-air explosive mixture inside the cryostat volume is low
provided that:

      Pressure inside the cryostat volume is continuously monitored

      AND if pressure rise is detected then the volume is immediately evacuated

      OR the cryostat volume is being continuously pumped off (question: is it safe?)

2. The jacketing of the test cryostat is not required (subject of fulfilling of the above
requirements).

4. COMMENT ON ZONING INSIDE THE CRYOSTAT
Following the consultation with Marc Simon and David Haynes (both from ISIS ), it is
concluded that the test cryostat volume is NOT Zone 2 because the explosive hydrogen-air
mixture can not be formed during the normal operation.

References:
    In ATEX the definition of Zone 2 is as follows:
Zone 2 – a place in which an explosive atmosphere consisting of a mixture with air of
dangerous substances in the form of gas, vapour or mist is not likely to occur in normal
operation but, if it does occur, will persist for a short period only.
(http://www.hse.gov.uk/electricity/atex/definitions.htm )
      In RAL Safety Code 1 the definition of Zone 2 is:

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
(http://www-internal.clrc.ac.uk/staff/health_safety/SafetyCodes/RAL/ralsc1.html )




Prepared by Y. Ivanyushenkov, 21Aug2007

				
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