Explosion at a Nuclear Power Plant
The event occurred in the steam condensing line of the residual heat removal system the nuclear reactor. Radiolysis generated hydrogen and oxygen which then accumulated in high concentration in the pipe. During the periodical manual start-up test of the High Pressure Coolant Injection (HPCI) system, a pressure spike due to pressure fluctuations in the pipe led to ignition followed by detonation. The pipe burst open and pieces were projected damaging the surrounding structure. Noble metals (Pt and Rh) used to prevent stress corrosion of the lower part of the reactor containment vessel, might have been involved in the detonation, but this is not certain (see discussion in NIT No. 90, among References).A 3-dimensional simulation of the event was simulated by finite element analysis (see Naitoh et al. in references). It showed that the detonation pressures in the straight pipes were about 120MPa and the pressure wave propagation speed was approximately 3,000 m/s. The peak pressures at the elbows were 2.02.5 times higher than those in the straight part of the line, due to reflection and overlapping of the pressure waves. These values at the elbow above the surface of the accumulated water exceeded the critical strain, explaining why the pipe ruptured there. The results were reasonably matching the actual pipe deformation observed after the accident.
Event Date
November 7, 2001
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
The immediate cause was the formation of explosive atmosphere in the steam pipe due to radiolysis. The references do not report the root causes. nevertheless it is mentioned (NIT no. 88 and no. 90), that the affected steam line had been replaced years before with a new one with completely different geometry, apparently to better control steam flow. This may hint at a inadequate risk assessment.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
steam condenser pipe, pressure vessel, reactor coolant,
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
A manual start-up test was performed just before the accidents.The plant was built in 1977. Following safety inspections in 1993-94, a series of improvements had been executed, including the steam condensing line, which was originally vertical and straight. It was replaced with L-shaped pipes connected to horizontally-placed pipes,to create a detour for steam coming from the reactor.
Currency
Property Loss (onsite)
500948
Lessons Learned
Lessons Learned
The references do not report the root causes: in particularly, the excessive presence of hydrogen remains unexplained. Radiolysis is only one of the possible explanations. Also the source of ignition was not definitively ascertained. Some calculation showed that a much higher would have been necessary to ignite the hydrogen at the operative temperature of the steam. As to the pipe failure: the fact that the geometry of the steam condensing line as modified in the year 1993-94, introducing horizontal sections has certainly caused the presence of strain-stress concentration and the bending. The only measure reported by the sources available, was the instruction, by the Nuclear Authority, to drain water from similar steam condensation system in all the other boiling water reactors of he same type. The NIT reports (see references) are very critical on the measures adopted.
Event Nature
Emergency Action
Unknown
Release Type
Release Substance
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
Masanori NAITOH et al., "Analysis on Pipe Rupture of Steam Condensation Line at Hamaoka-1, (II)
Hydrogen Combustion and Pipe Deformation",
Journal of NUCLEAR SCIENCE and TECHNOLOGY, 40 (2003) p. 1041â1051
Chihiro Kamisawa,
"Hamaoka Nuclear Power Reactor 1 -Pipe Rupture and Water Leak from Pressure Vessel" , Nuke Info Tokyo, No. 88 Marc April 2008, pages 1-5
https://cnic.jp/english/?p=374
(accessed October 2020)
"Insufficient Investigation: Hydrogen Explosion and Water Leak at Hamaoka 1 Nuke Info Tokyo No. 90",
CNIC (Japanese Citizens' Nuclear Information Center) Report, by By Chihiro Kamisawa,
https://cnic.jp/english/?p=392
(accessed October 2020)