Explosion in the cooling water loop of 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 ruptured and pieces were projected damaging the surrounding structure. 

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. Hydrogen was generated by radiolysis of water in the reactor core accumulated in the pipe, forming an explosive atmosphere. 

The IGNITION SOURCE was a pressure wave during the coolant injection test. 

As to the pipe failure: the geometry of the steam condensing line was modified in the year 1993-94, introducing horizontal sections. This has certainly caused the development of strain-stress concentration and the bending. 

The ROOT CAUSE was the lack of knowledge of the phenomenon and he obvious related hazards. When designing and operating a system containing water steam, it was assumed that no non-condensable gases (hydrogen, oxygen, nitrogen, etc.) could exist in its pipes. 
Moreover, with a pressure of 7 MPa and a temperature of 270 C, it was assumed that even in the case of formation of explosive mixture, the conditions were too low for its autoignition. The possibility of a situation like that created by a ‘water hammer’ effect had not been considered. Therefore, the ROOT CAUSE can be identified in a combination of:

1.  An inadequate risk assessment (radiolysis hydrogen is a known fact and must be managed by design). 
2. A shortcoming in design of the modification of the system, as well as in the execution of the modifications.

H2 in reactor coolant

The plant was built in 1977. Following safety inspections in 1993-94, a series of improvements had been executed. The steam condensing line 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.

[copied from: Residual heat removal system - Wikipedia]

"After shutdown of a nuclear reactor, the core continues to produce decay heat due to the nuclear decay of fission products created during reactor operation. In a boiling-water or pressurized-water reactor (BWR or PWR), the residual heat removal system (RHR system) is used to remove this decay heat, and also serves safety functions during an accident. The RHR system may also be called the decay heat removal system or the shutdown cooling system"

The blown-off portion of the pipe was found in five large fragments. 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 reactor was manually shut down immediately after the pipe rupture and there was no radioactive release into the environment.

The presence of hydrogen (and oxygen) in the coolant circuit of a Boiling Water Reactor is due to radiolysis of water occurring in the reactor core. Off-gas system are design to manage and reduce hydrogen concentration to safe values. This incident however, highlighted the still existing risk of hydrogen in the primary coolant loop of nuclear reactors. In the same year, just one moth after this case, a similar explosion occurred in Germany on a reactor of the  same brand (see HIAD ID 522).    

Although the consequences in both cases were negligible, the two incidents triggered concerns on possible scenarios with much heavier consequences. As summarised by US NRC (see references), design, it was necessary to identify the coolant loop zones most vulnerable to formation of H2-O2 flammable mixture, and to develop design improvements.

Since then, several studies were performed to identify the conditions under which oxygen and hydrogen generated by electrolysis come to an explosive event inside pipes. The analysis of the event performed by Naitoh et al. (2012 see References) concluded that in the section of the pipe which exploded in this event, a non-condensed gas atmosphere had formed consisting mainly of H2 and O2 because of factors such as the pipe geometry (a series of vertical and horizontal sections) and the temperature and heat experienced by the water vapour.  
According to A. Leishear (2020, https://doi.org/10.1115/1.4044807), fluid transients are then responsible to heat the explosive gas mixtures to autoignition. The same mechanism was responsible for pipe and pump damages to U.S. reactor systems since the 1950s, while at that time water hammer alone had been assumed as cause.

The corrective measures adopted by the Japanese plant are unknown. 

On the wake of the accident in Japan, the US nuclear authorities issued recommendations aiming at reviewing piping systems, to identify any potential vulnerabilities for the accumulation of radiolytic gases. 

Antaki et al (see References) proposed the following approach to assess the risks associated with the accumulation and explosion of hydrogen generated by radiolysis: 

1. To identify the systems susceptible to hydrogen accumulation, and their initial conditions (hydrogen-oxygen mixtures, their location and their initial pressures and temperatures). 
2. To determine whether the explosion is a deflagration, a detonation or a deflagration transitioning into a detonation
3. Based on the established conditions and the explosion regime, to develop the explosion pressure timelines and peak temperatures. This is the fluid dynamics aspect of the assessment. 
4. To calculate the structural capacity of systems, equipment and components subject to the calculated explosion loads.
   To perform the stress-strain analysis and fracture analysis. 
5. If the results of (5) shows a situation exceeding the structural integrity capacity of the structure, to investigate the failure mode to understand the consequences of such an explosion.