Failure Knowledge Database / 100 Selected Cases

Silane Gas Explosion at Osaka University【October 2, 1991; School of Engineering Science, Osaka University】
Masayuki Nakao
Institution of Engineering Innovation and Engineering Research Institute Department,
Graduate School of Engineering, The University of Tokyo
An explosion occurred at the School of Engi neering Science, Osaka University , during a
student experim ent with a CVD (Che mical Vapor Deposition) system. The Silane container(Figure 1), which supplied gas to the CVD system, blew up. Two students were killed and fiveothers received m inor injuries. The acciden t was caused b y a reverse flow of Nitrous Oxide
due to a degraded O-rin g seal used in a check valve (Figure 2), and the m ixture of Silane andNitrous Oxide exploded in the container. The explosion ignited the city gas andorganochlorine solvent in the facility and started a fire.
To Eliminator
To CVDMIS gasSiH4 Sensor

Line1Line2Line3Line4Line4Line3Line2 Line1

Iron plate
Base

Purge line

Mixed gas
Burst

High-purity nitrogen Gasfor piping and gas-purge inCVD (47 liter)

Low-purity nitrogen Gas forswitching CVD gas intakevalve (47 Liter)

Dent
Entirely bulged

Figure 1: Piping outline around the containers for CVD system (estimation) [3]1

Failure Knowledge Database / 100 Selected Cases

Figure 2: Check valve [3]1. Event
During student experiments at the School of E ngineering Science, Osak a University with aplasma CVD system , a Silane container that supplied gas to the CVD system suddenlyexploded. Two students were killed and five others received m inor injuries by the blast andflying debris from the explosion.
2. Course
Around 4pm on October 2, 1991, at the School of E ngineering Science, Osaka Uni versity, astudent closed a valve (DV1 in Figure 1) of a plasm a CVD system . The closure triggered asudden explosion in the Silane container that supplied gas to the CV D system . The Silane
container bursted, and the blast forced everythi ng including the test equipm ent and people tothe walls. The explosion and flyi ng debris from it killed tw o students and injured five others.
The explosion also ignited the city gas and organochlorine solvents to start a fire.
3. Cause (Refer to the piping layout of gas supply facilities in Figure 3)
What exploded in the Silane cont ainer was m ixed gas of Silane (SiH 4) a nd Ni trous Oxi de(N2O). The O-ring within check valve (CV3) in Figure 2 had been degraded by Nitrous Oxide,
and the check valve (i.e., non-retu rn valve) had lost its function. As a result, a reverse flow ofNitrous Oxide through the check valve (CV3) flew into the Silane container via the purge lines.
The purge lines rem ove noxious fum es and com bustible gas inside the pipes by introducing
inert gas such as Nitrogen after each system operation. When the student closed the valve(DV1) the m ixed gas in pur ge lines com pressed, ge nerated heat and ign ited. The flam e thentraveled back to the con tainer th rough the pip ing and explo ded the con tainer. The pressure
inside the container at the tim e of e xplosion was estim ated at 2,000 to 3,000kgf/cm 2. T hesafety valves for preventing heat meltdown of the container had no time to activate.

2Failure Knowledge Database / 100 Selected Cases

Decompression ValveCheck Valve
Diaphragm Valve
Ball Valve
FilterBall valve forchannel switching

Since valves of N2 source container switch were closed,
these valves are considered to be all closed.

Common Purgeline

Plasma CVD
SystemValve switch (open/close) for N2(Low-purity
oxygen47 litercontainer)(High-purity
oxygen47 litercontainer)

Thinner channel

Cylinder Cabinet

N2 source switch for gas
inlet valve of CVD system
Nitrous Oxide Cylinder

Silane Cylinder
Bold line is the piping directly involved withthe explosion.

Check Valve with assumed
O-ring with degradation

Figure 3: Piping layout of gas supply facility [2]4. Immediate Action
Two m onths after the incident, High-pressure Gas Control Law was revised to mandate
reporting the use of spe cific high-pressure gas su ch as Silane, regardless of the a mount. The
revision was posted to jurisdic tional prefecture a nd city governm ents. Each university
voluntarily started inspecting applicable gas lines.
5.

Countermeasure(1) To prevent the O-ring d egradation by Nitrous Oxide, the ch eck valve (CV 3) and theball valve (BV 3) were r elocated. The or iginal location of th e check valve clo se to agas cylinder kept the O-ring exposed to N
itrous Oxide. The pressure of Nitrous
Oxide (typically 50 atmospheres) is highe r than the p ressure of Nitrogen (usually afew atm ospheres). A new ball valve with m etal contact was placed next to the gascylinder.
(2) Separate purge lines were placed for each ga s type. Shared pipes have advantage ineasy operation and lower system cost, however, could generate dangerous gas
mixture like in this case.
3Failure Knowledge Database / 100 Selected Cases

6. Summary
Due to the degradation of an O-ring in a check valve, Nitrous Oxide flew into Silane throughpurge lines and caused the explosion. When designing systems with potential hazards like thiscase, it is n ecessary to m ake sure that all th e s afety features such as check valves function
properly at all tim e. It is also im portant to periodically inspect such safeguards components.
Furthermore, it is essen tial to desig n inherent s afety into such system s like separ ate pur gelines. At th e tim e of the accid ent the industr y rum ored the university ’s loose m anagement
being the root cause. Shared purge lines, however, were also used in some industry systems.
7.

Knowledge(1) Silane gas is dangerou s. It can explode when its den sity exceeds 1%, and isflammable when mixed with Nitrous Oxide. The mixture usually does not explode atroom temperature, but if it is ignited the power of explosion is large.
(2) Degradation of material may lead to an acci dent. Systems should use m aterial that is
hard to degrade in the planned environm ent, or otherwise, they should be designedwith inherent safety to prevent damage even when the material degrades.
(3) Efforts to improve working property or reduce cost can reduce the margin of safety.
(4) If fatal accidents are possible, designing inhe rent safety into system s is ab solutelyessential. The original layout in Figure 1 is dangerous , for example, if an earthquake
happens during an experim ent, piping can ge t s lacked to mix gases, an d any spark
can readily cause an explosion like this case.
(5) Gas-leak sensors are no t versatile. It appe ars that they thou ght the sy stem was safeenough with the gas-leak sensor (indicated in the center of Figure 1).

8. Background
Technopolis Initiativ e (Law for Acceler
ating Regional Develop ment Based upon
High-Technology Industrial Complexes: Con
cept to develop industrial tow
ns, whichorganically bound academic research functions and living functions, in order to introduce anddevelop high-tech industry to local regions) driv en by the Ministry of International T rade andIndustry (now the Ministry of Economy , Trade and Industry) went into ef fect in March 1983.
The initiative triggered rapid introduction of foreign technology with foci on sem iconductor
technology and m echatronics. The sem iconductor technology involves various chem ical
materials including Silane. Many accidents were caused by the spontaneous combustibility of
Silane; a fire of a sem iconductor f actory in Kiyotake-cho, Miyazak i, caused by Silane gas
leakage in October 1983, Silane gas explosion at a sem iconductor prototyping facility in
Kodaira, Tokyo, in Decem ber 1989, a fire caused by Silan e at a sem iconductor f actory ofMiyazaki Oki Electric in Takasaki, Gunma, in March 1990, a fire caused by Silane leak age ata Silane manufacturing plant in Aomi, Niigata, in June 1990.
4Failure Knowledge Database / 100 Selected Cases＜References＞[1] Contam ination issues caused by chem ical material, Hokkaido University inform ation
initiative Center. http://www.hucc.hokudai.ac.jp/[2] Machinery Creativity , Y otaro Hatam ura, Koz o Ono, Masayuki Nakao (Editors),Maruzen Co., Ltd.
[3] The Practice of Machine Design Book 3, Lear ning from Failure, Yotaro Hatamura ed.,Practice of Machine Design Research Group, The Nikken Kogyo Shimbun, Ltd.
[4] High-pressure Gas V ol.29 No.251 (1992), Os aka University Silane Gas Explosion
Accident Investigative Committee Interim Report[On the Side]
Article 23 of the Constitution guarantees the fr eedom of learning as “Academic freedom shallbe guaranteed.” Some understand this constitution not on ly guarantees the acad emic freedom
but also guarantees autonom y in universities. However , people should f ully understand thatuniversity laboratories ar e not extraterrito rial and should take ev ery possible m easure toensure their safety.

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