Hazard Analysis
Prepared:
Checked:
Approved:
Jiawei Mei
H. Nann
M. Snow
(1) Leak in the hydrogen fill/vent line.
Hazard: hydrogen gas leaks from the tubing and demountable joints leading into and
out of the fill/vent line.
Mitigation: The hydrogen gas will be conducted in vacuum through the fill/vent system
using stainless steel (SS) tubing; all components are made with certified materials and
welded together by certified welders. The possibility of a leak is reduced by the use of
welded joints. Helium leak testing will be required in the operating procedure to be
performed before introduction of hydrogen gas into the fill/vent line. Pressure gauges
and a residual gas analyzer (RGA) on the main vacuum chamber are present which
would indicate any such large leak occurring inside the vacuum chamber.
(2) Overpressure in the hydrogen fill/vent line.
Hazard: The hydrogen gas pressure in the fill/vent line rises and threatens to burst the
line.
Mitigation: The hydrogen line possesses a relief valve RV104 and a rupture disk RD101
in parallel. The diameter of the line is sized to accommodate pressure rises experienced
in accident scenarios as well as normal venting. The thickness of all line components is
sufficient to prevent deformation of the line outside the elastic regime. The bellows are
protected from bellows squirm by external mechanical guards. The demountable Conflat
joint at the end of the line has been successfully tested at internal pressures larger than
those to be experienced.
(3) Leak in the vacuum line.
Hazard: air leaks into the main vacuum system, thereby breaching triple containment
and introducing the oxygen in the air closer to the hydrogen. Also frozen air in the
vacuum system can expand upon warmup and generate high transient pressures.
Mitigation: The vacuum system uses welded components and standardized commercial
flanges wherever possible. The possibility of a leak is reduced by the use of welded
joints. Helium leak testing will be required in the operating procedure to be performed
before introduction of hydrogen gas into the fill/vent line. Pressure gauges and a RGA
on the main vacuum chamber are present which would indicate any leak. Helium gas is
introduced outside all the non-welded joints inside the experimental cave to catch any
such leaks immediately on the RGA.
(4) Overpressure in the vacuum line.
Hazard: The gas pressure in the vacuum line rises and threatens to burst the vacuum
system and/or implode the hydrogen line.
Mitigation: The vacuum line possesses two rupture disks RD201 and RD202 in parallel.
The diameter of the vacuum line between target and vent exit line is sized to
accommodate pressure rises experienced in accident scenarios. The thickness of all
line components is sufficient to prevent deformation of the line outside the elastic
regime. The bellows are protected from bellows squirm by external mechanical guards.
The demountable Conflat joint at the end of the line has been successfully tested at
internal pressures larger than those to be experienced.
(5) Damage of the fill/vent line from thermal contraction.
Hazard: During target venting the hydrogen gas is cold and the hydrogen line will
thermally contract. This can produce enough stress to damage or rupture the line.
Mitigation: The hydrogen line possesses bellows in both the horizontal and vertical
sections which suffice to relieve the stress while remaining within the operating
envelope of the bellows well over their expected lifetime for expansions and
contractions.
(6) Loss of Power/Backpressure in the Vent Isolation Box.
Hazard: In the case of a failure of the two target refrigerators (e.g. power failure) with no
vacuum failure the LH2 target vessel will slowly warm up causing the liquid hydrogen to
boil off. This will cause a slow pressure increase in the 1.5-inch relief line up the
pressure relief valve RV104, situated in the vent isolation box, till RV104 opens at its set
pressure of 20 psid. Then hydrogen gas will flow into the vent isolation box causing a
pressure increase in the box. If this pressure is too large the rupture disks could burst.
Mitigation: The choice of the set pressures of the pressure relief valves combined with
the gas flow rate in this scenario cannot generate a pressure high enough to burst the
disks. See Appendix for estimates.
(7) Damage of the fill/vent line from earthquake.
Hazard: A seismic event damages or rupture the line.
Mitigation: The hydrogen line has several alignment spiders which limit the motion of the
line transverse to the axis of the main vacuum line it is inside of. It is mechanically fixed
at the bend location to the cold head of the cryorefrigerator (more than one fixed place).
The internal components of the relief system that the hydrogen line connects to are
tack-welded and mechanically limited where needed to limit relative motions of massive
objects inside. The same is true of the hydrogen target vessel: its motion is constrained
by G-10 spacers and mechanical connections to the cryostat refrigerators.
(8) Blockage of the fill line.
Hazard: a blocked fill line upsets the steady-state nature of normal hydrogen target
filling and thereby requires operator action, leading to an increased possibility of error.
Mitigation: Blockage of the fill line is made unlikely by the liquid nitrogen cold trap
upstream of the fill/vent line. Should the fill/vent line become blocked despite this
precaution, the relief valve RV102 and RV103 will protect against overpressures. The
operating procedure will include a clear set of actions to be taken in the event of a
blockage in the line involving the action of pumps in the gas handling system. The
effectiveness of the proposed operating procedure will be tested with neon gas to
confirm that the procedure is safe and effective.
(9) Rapid rise in gas pressure
Hazard: the pressure of the cold hydrogen gas undergoes a rapid increase, thereby
causing pressure changes in other areas of the gas handling system and confusing
operators, leading to possible operator error.
Mitigation: Such a pressure rise could be caused in various ways: for example (a) a
failure of the refrigerator during operation, (b) a plug in the fill line, (c) a leak in the main
vacuum chamber, thereby changing the temperature and therefore pressure of the cold
gas in the fill/vent line, (d) failure of the pump which evacuates the vacuum chamber.
The hydrogen fill/vent line possesses a relief valve (RV102 and RV103 in the drawing)
which is connected to the main vent system. The potential for a plug in the fill/vent line
is reduced by the upstream cold trap CT and the large diameter of the fill/vent line. The
pump that evacuates the vacuum chamber will be valved off from the chamber during
operation.
(10)
Deceptive RGA readings
Hazard: the RGA signal is contaminated by false background signals, confusing the
operators and leading to operator error.
Mitigation: The vacuum system is designed as closely as possible to high vacuum
standards without major sources of outgassing or virtual leaks. The pressure gauges do
not make ions which can pass into the RGA mass spectrometer to produce false
signals.
(11)
Prevention/mitigation of the possibility of liquification of hydrogen in
the hydrogen gas cooler.
Hazard: the hydrogen gas flowing into the gas cooler is liquefied inside the section,
thereby creating liquid hydrogen in an unintended location.
Mitigation: The unmodified cryocooler has a base temperature that is low enough to
liquefy the hydrogen. We propose to prevent this by introducing a passive, mechanical
thermal short between the first and second cold stages which keeps the temperature of
the coldest stage above 30 K. Test measurements (see appendix below) have
established that this temperature, which is several degrees above the temperatures at
which hydrogen can liquefy at gas pressures present in the hydrogen fill line, is stable
over long periods of operation. It is possible that over long periods of time the thermal
contact between the two cold stages which defines their base temperatures might
slowly change (through, say, slow relaxation of the force exerted by the bolts on the
thermal link). The bolts will be secured by lock washers to reduce the probability of such
relaxation, and thermal conducting grease (Apiezon) will be used in the area of thermal
contact to help make the area of thermal contact defining the base temperature of the
cold stages more insensitive to small changes in the force of the bolts. Diode
thermometers on the coldest portion of the OPC will be used to monitor the temperature
to ensure that any such possible change in the thermal contact between the stages can
be observed.