Explosion and fire at a bubble chamber

This event occurred at the accelerator bubble chamber facility. During final stages of chmaber cooldown and its loading with LH2 (95% fill, about 95 gallons), an explosion and fire occurred causing extensive damage to facility and related equipment. The initial fire with hydrogen was believed to be short and the major damages caused by secondary fires. The initial explosion and fire caused overpressures of about 1.5 psi (10 kPa). The explosion pressure was relieved by the lifting of the facility roof.

The bubble chamber body and vacuum box were equipped with beam port windows made of beryllium. Most probable cause was the failure of first the inner and then the outer beryllium ports which sealed the LH2 from the chamber insulating vacuum and the vacuum from atmosphere. Failure of the outer window was due to impact of inner window fragments and the sudden temperature shock caused from LH2. The material fractured in the brittle mode due to stresses induced by differential thermal contraction forces as well as by the internal pressure. Surface imperfections due to machine tool fabrication operations produced stress intensifiers high enough to cause ultimate failure.

windows ,flanges

At the time of the explosion the accelerator was in operation for two experiments, and a technical team was in the hall monitoring the filling of the chamber with liquid hydrogen for the first time. The Bubble Chamber was in the final stages of a 10 day continuous program of cooling-down and filling with liquid hydrogen for the first time, and was about 95% filled

"A bubble chamber is normally made by filling a large cylinder with a liquid heated to just below its boiling point (usually liquid hydrogen). As particles enter the chamber, a piston suddenly decreases chamber pressure, and the liquid enters into a superheated, metastable phase. Charged particles create an ionization track, around which the liquid vaporizes, forming microscopic bubbles. Bubble density around a track is proportional to a particle's energy loss."
[https://en.wikipedia.org/wiki/Bubble_chamber]

The chamber of this event was "...used helium as the working fluid in the expansion engine located adjacent to the bubble chamber. Low temperature helium gas was circulated through condensers in the upper portion of the Bubble Chamber, liquefying the hydrogen gas. The hydrogen from a battery of compressed gas tanks outside the building went through pressure reducing valves, a purifier and flow control valves to the chamber."
[http://washuu.net/cea-bang.htm#:~:text=The%20burning%20roof%20required%20the,valves%20to%20the%20Bubble%20Chamber]

Major damage occurred to the roof, which was formed of light, precast reinforced concrete panels topped by a layer of insulation and a tar and gravel weather topping. The explosion lifted and dropped the concrete panels on the girders where the panels broke and shattered, dropping large quantities of rubble and tar on the floor and contributing significantly to damage of equipment. The burning roof required the use of much water which flooded the floor to a 14 inch depth and also added to equipment damage.

The explosion and fire occurred at 3:32. .
By 4:00 several intense local fires were burning on the floor of the hall, and insulation from the severely damaged roof was burning.
The fire was fought by water beams: it was contained by 5:00, but water was continued on the burning roof until 10:00 am or later.

The various sources highlighting this case are cautious in drawing a root cause analysis. As in all incidents characterised by escalations and extensive fire(s), the identification of the root cause is hindered by the great damage to structures and equipment.
It is very probable that the chamber failed by releasing hydrogen through the beam windows, however there were additional flammable materials and fuels in the chamber, contributing from the start other fire, including probably a gaseous hydrogen supply lone.

The bubble chamber of this experiment was one of the first developed for the detection of charged elementary particles from experiments with accelerators. The accelerator laboratory had started only few years before, the first prototype of the bubble accelerator dated back to 1960. The chamber was starting operation on the day of the incident. It is not known which validation and qualification tests were performed before, nevertheless the chamber must be considered as a pioneering experimental equipment.

A scientific article written in 1962 was recognising the difficulty of a sound safety-design: “The principal source of danger lies in malfunction or failure of equipment, releasing gas into the atmosphere with the formation of an explosive hydrogen-air mixture. Important safety considerations are: proper design of equipment and housing facilities, hydrogen control before, during and after bubble-chamber operation, and elimination of ignition sources. Design and operation are complicated by the lack of universal standards for handling liquid hydrogen and the use of conflicting procedures for controlling hydrogen hazards at various facilities. ”
Weintralub, A. A.; Weinstein, M. S., Bubble Chambers, Health Physics 8(1):p 11-15