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Causes of Unit 4 Explosion Revisited
One of the more puzzling incidents during the initial phase of the Fukushima Daiichi nuclear accident was the unexpected explosion of unit 4. Unit 4 was offline at the time for maintenance. Workers had offloaded all of the fuel from the core into the spent fuel pool and were in the process of replacing the reactor shroud and other core internals. TEPCO assumed unit 4 was of less concern until it exploded in the early hours of March 15th.
Neither the TEPCO web cam nor the TBS live camera caught the explosion of unit 4 as both were offline at the time. This left no visual record of the blast to analyze. Initially most assumed the blast to have been from hydrogen generation out of the spent fuel pool. Early reporting cited concerns the pool had boiled down to the point that the fuel had caught fire. There were even efforts discussed to dump sand into the pool to put out the assumed fire. Over time the idea of the fuel actually having caught fire has been considered less likely. TEPCO also presented a theory of hydrogen generation at unit 4 having come from unit 3. This idea was adopted by many causing little additional review of the events at unit 4. The causes behind the explosion at unit 4 still seemed to be not fully determined. This caused us to take a second look.
The structures involved in any of the theories on unit 4 include the unit 4 building, spent fuel pool and the standby gas treatment system (SGTS). The SGTS system includes a network of building ductwork similar to HVAC ducting used for building heating and cooling along with a series of HEPA filters and valves. Unit 3 is also implicated in some of the theories where the emergency venting system and the SGTS system there play a role. The structures between the units also play a role in some of the theories. This includes the shared vent stack tower, emergency vent ducts and the SGTS pipes that run outside the buildings and join at the vent stack.
Delivery From Unit 3
A theory developed by TEPCO concluded that the hydrogen assumed to have damaged the unit 4 building came from unit 3. The assumption is that hydrogen gas flowed backwards through the SGTS piping into unit 4. The assumed route is from the connections of the emergency vents and SGTS pipes at the vent stack, causing gasses released by unit 3’s venting operations to flow backwards through unit 4’s SGTS pipe. The SGTS pipes from the vent stack to unit 4 take a 2 story vertical rise as the pipes enter the building The pipes all join into a central pipe at the base of the vent stack creating a situation where gasses could travel between reactor units.
TEPCO cited the high levels of contamination in the HEPA filters at unit 4 inside the SGTS system. The filters during normal operation would show any contamination highest at the filter closest to the internal building ductwork with the filter last in line being lower in contamination. Instead what TEPCO found upon physical inspection was the filter closest to the outside SGTS pipe to be the highest in contamination with the other filters in the row having succeedingly lower contamination. TEPCO’s inspection also found the valves that open and close the air flow of the SGTS system to be open. This open pipe configuration would allow gasses access to flow backwards through the SGTS system to the building ductwork. The SGTS system does not possess a blackflow valve that would prevent gasses from flowing backwards through the system. The SGTS system also “fails open” when it loses AC power. When all AC power was lost at the plant soon after the Tsunami the SGTS system would lose power and those valves would go to the open position.
The heaviest damage at unit 4 is from the locations where the duct work for the SGTS runs. This includes the west side of the building that has large building failures area. The open valves and backward contaminated filter situation leads TEPCO to assume this to be the route of hydrogen gasses into unit 4. Unit 3 was vented repeatedly from 9:17 JST March 13th right up until the explosion. The emergency venting was pushing gasses into the pipe system on route to the vent stack and the SGTS for both units were open but had no pressure purposely generated through those systems. The higher pressure in Unit 3’s system could have caused gasses to be pushed backward through unit 4’s SGTS piping but Unit 4’s building pressure is unknown. This makes it harder to judge the ease with which gasses would have flowed into unit 4. The SGTS piping is also quite small in diameter raising questions if it is capable of delivering enough gasses to create an explosive hydrogen concentration on its own.
Hydrogen or Carbon Monoxide
Hydrogen is the most assumed gas to have been generated and to possibly travel between the two units. Carbon Monoxide is also a possibility as it is generated as part of the meltdown process and has explosive potential. Hydrogen is 14 times lighter than air, Carbon Monoxide is about equal in weight to air. The explosive concentration of hydrogen is 4–74%. The explosive concentration for Carbon Monoxide is around 17.5%. Carbon Monoxide would have been created by the meltdown at unit 3 and the corium to concrete reaction where the melted fuel burns and consumes the concrete floor of the reactor containment. Unit 4 would have not been likely to create Carbon Monoxide gasses through the spent fuel pool.
The events at Fukushima Daiichi play a considerable role in what happened and the forensic investigation of unit 4’s explosion. Power was lost after the tsunami, knocking out the remaining AC power to the plant that was being provided by diesel generators. This caused the spent fuel pool at unit 4 to lose cooling. This is critical as the entire core had been offloaded to the pool for the maintenance work being conducted. The core from unit 4 was offloaded into one area rather than being offloaded into a checkerboard configuration as is normally suggested. A checkerboard configuration spaces the fresh fuel between older spent fuel in the pool to lower heat and reactivity conditions.
As unit 4’s spent fuel pool lost cooling the offloaded fuel would have begun to heat up. Unit 3’s meltdown progress resulted in an explosion at 11:01 JST on March 14th. The explosion severed both the SGTS pipe and emergency venting pipe leaving unit 3. This would have terminated any ability of gasses to flow backwards into unit 4. 18 hours passed between unit 3 exploding and unit 4 exploding. The long time frame between unit 3 exploding and unit 4 exploding raises questions about the backflow of gasses being the sole cause of unit 4’s explosion. The time frame of 18 hours without addition of hydrogen as the sole cause of the explosion at unit 4 is improbable.
Hydrogen Production From Spent Fuel
Oak Ridge National Lab ran MELCOR models to look at the potential of unit 4 to generate hydrogen via the spent fuel pool. They found that in their hypothetical situation enough hydrogen could be generated in 3.64 days to reach the explosive concentrations needed. With water 4.02 meters above the fuel assemblies the rapid 3.64 days is needed to reach explosive levels. If the pool water stayed at the full pool level it would take 14.65 days to reach explosive levels. According to these models the fuel does not need to be uncovered, catch fire or melt to produce large volumes of hydrogen. The hydrogen production curve under most scenarios takes a steep and abrupt increase. This estimated time frame falls within the time after unit 3 exploded and before unit 4 exploded.
TEPCO’s findings of evidence that the backflow of hydrogen took place are notable but appear to not be sufficient to be the sole cause of the explosion at unit 4. The diameter size of the SGTS vent pipe is quite small, limiting the total amount of gas that could backflow. The long time frame of 18 hours between unit 3’s explosion and unit 4’s explosion challenges the theory that unit 3 was the sole cause of the explosion at unit 4. If 3 had provided enough quantity of hydrogen to cause unit 4 to explode, there is no viable explanation for the 18 hour delay in explosions.
The MELCOR models put the drastic increase in hydrogen levels within that window between the two explosions giving a cause that occurs in the 18 hour window. It is our assumption that the hydrogen generated by the spent fuel pool in unit 4 could be the sole cause or the major contributing cause of the explosion at unit 4. TEPCO did find evidence of some backflow through the SGTS but the theory lacks in a number of ways as the sole cause yet it could have at least contributed to the hydrogen in the building. It is more likely that unit 4’s spent fuel pool generated the bulk of the hydrogen through radiolysis in the spent fuel pool. The offloading of the core into the pool in a concentrated fashion likely contributed to this occurrence.
One aspect of this event that has yet to be resolved is the ignition source or event that caused the actual explosion. We are continuing to research this portion of the events at unit 4 in depth.
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