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).
According to a simulation of the event by finite element analysis (see Naitoh et al. in references), 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 2.0–2.5 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.

The INITIATING cause was the formation of explosive atmosphere in the steam pipe due to radiolysis.
The most likely explanation for this is
1. Formation of hydrogen by radiolysis of reactor water in a core.
2. Hydrogen accumulated in the pipe.
3. Hydrogen detonation, triggered by a pressure wave.

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.
This may hint at a inadequate risk assessment or shortcoming in the design.

steam condenster pipe, pressure vessel, reactor coolant,

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.

The coolant pipe was burst open, The blown-off portion of the pipe was found in four large fragments and another fragment was found on a later day. It is possible that these pieces shot off like missiles and damaged adjacent equipment. The surrounding area was badly affected: metal support structures were torn off, and the grating bent over. The walls in the vicinity of the area had some dents which might have been caused by violent motion of the pipe.

The operator estimated that the steam leakage was approximately two tones while the total radiation leakage was approximately 800 million Becquerel (see NIT report No. 88 among references).

The issued by the local or governmental authorities in relation to this specific accident are unknown.
Also the countermeasures adopted to prevent similar accidents are not available .In fact, the analysis of the causes present some uncertainty.:
(1) The excessive presence of hydrogen remains partially unexplained. Radiolysis is only one of the possible explanations.
(2) 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.

On the wake of the accident in Japan, the US nuclear authorities issued the following general recommendations:
(0) strongly urging all BWR licensees to perform detailed evaluations and implement mitigating actions as appropriate.
(1) review piping systems to identify any potential vulnerabilities for the accumulation of radiolytic gases,
(2) assess the detonation potential of vulnerable piping, (3) consider design or system operation modification(s), a (4) consider the potential for accumulation and detonation of radiolytic gases