Explanation of Hydrogen Explosions at Units 1 and 3

Explosions at units 1 and 3 occurred due to similar causes. When an incident occurs in a nuclear power plant such as a loss of coolant accident or when power is lost, usually the first response is to depressurize the reactor. This is done by opening pressure relief valves on the reactor vessel. The water/steam…

Explosions at units 1 and 3 occurred due to similar causes. When an incident occurs in a nuclear power plant such as a loss of coolant accident or when power is lost, usually the first response is to depressurize the reactor. This is done by opening pressure relief valves on the reactor vessel. The water/steam mixture will then flow down into the suppression pool, which for this design of a reactor is in the shape of a torus (technical term for the shape of a donut). By blowing the hot steam into the suppression pool some of the steam is condensed to liquid phase, which helps keep the pressure low in the containment.

The pressure in the reactor vessel is reduced  by  venting the water/steam mixture. It is much easier to pump water into the vessel when it is at a reduced pressure, thus making it easier to keep the fuel cooled. This procedure was well underway after the earthquake. Unfortunately, because of the enormous magnitude of the earthquake, an equally large tsunami was created. This tsunami disabled the onsite diesel generators as well as the electrical switchyard. Without power to run pumps and remove heat, the temperature of the water in the reactor vessel began to rise.

With the water temperature rising in the core, some of the water began to vaporize and eventually uncovered some of the fuel rods. The fuel rods have a layer of cladding material made of a zirconium alloy. If zirconium is hot enough and is in the presence of oxygen (The steam provides the oxygen) then it can undergo a reaction that produces hydrogen gas. Hydrogen at concentrations above 4% is highly flammable when mixed with oxygen; however, not when it is also in the presence of excessive steam.

As time went on, the pressure in the containment rose to a much higher level than usual. The containment represents the largest barrier to the release of radioactive elements to the environment and should not be allowed to fail at any cost. The planned response to an event like this is to vent some of the steam to the atmosphere, just to keep the pressure under control.

Exactly what happened next is not verified; however, the following is very likely the general explanation for the explosion. It was decided to vent the steam through some piping that led to a space above and outside containment, but inside the reactor building. At this point, the steam and hydrogen gas were mixed with the air in the top of the reactor building. This was still not an explosive mixture because large amounts of steam were mixed with the hydrogen and oxygen (from the air). However, the top of this building is significantly colder than inside the containment due to the weather outside. This situation would lead to some of the steam condensing to water, thereby concentrating the hydrogen and air mixture. This likely went on for an extended period of time, and at some point an ignition source (such as a spark from powered equipment) set off the explosion that was seen in units 1 and 3. The top of the reactor building was severely damaged; however, the containment structure showed no signs of damage.

Right after the explosions there were spikes in the radiation levels detected, because there were some radioactive materials in the steam. When the zirconium alloy cladding reacted to make hydrogen, it released some fission products. The vast majority of the  radioactive materials in the fuel will remain in the fuel. However, some of the fission products are noble gases (xenon, Xe and krypton, Kr) and will immediately leave the fuel rods when the cladding integrity is compromised. Fortunately, Xe and Kr are not a serious radiological hazard because they are chemically inert and will not react with humans or plants. Additionally, small quantities of iodine (I) and cesium (Cs) can be entrained with the steam. When the steam was vented to the reactor building, the Xe and Kr would have followed as well as some small amounts of I and Cs. Thus, when the roof of the reactor building was damaged , these radionuclides that were in the reactor building would have also been released. This is the reason a sudden spike was seen in radiation levels. These heightened radiation levels quickly decreased. This is because there was no damage to the containment which would increase the quantities of radionuclide released, and because the radionuclides released during the explosion quickly decayed away or dispersed.

Unit 2 explosion

Recent information indicates that unit 2 may have suffered a containment breach.  Pressure relief of unit 2 was complicated by a faulty pressure relief valve, which complicated the injection of sea water and the evacuation of the steam and hydrogen.  It is reported that the fuel rods were completely exposed twice.  More details to follow.

Unit 4 fire

A fire was reported at unit 4 which was in a shutdown state during the earthquake and tsunami for a planned outage.  Latest reports indicate that the fire was put out.  More details to come.

 

 

 

Explosions at Units 1 and 3

Explosions at units 1 and 3 occurred due to similar causes. When an incident occurs in a nuclear power plant such as a loss of coolant accident or when power is lost, usually the first response is to depressurize the reactor. This is done by opening pressure relief valves on the reactor vessel. The water/steam mixture will then flow down into the suppression pool, which for this design of a reactor is in the shape of a torus (technical term for the shape of a donut). By blowing the hot steam into the suppression pool some of the steam is condensed to liquid phase, which helps keep the pressure low in the containment.

The pressure in the reactor vessel is reduced  by  venting the water/steam mixture. It is much easier to pump water into the vessel when it is at a reduced pressure, thus making it easier to keep the fuel cooled. This procedure was well underway after the earthquake. Unfortunately, because of the enormous magnitude of the earthquake, an equally large tsunami was created. This tsunami disabled the onsite diesel generators as well as the electrical switchyard. Without power to run pumps and remove heat, the temperature of the water in the reactor vessel began to rise.

With the water temperature rising in the core, some of the water began to vaporize and eventually uncovered some of the fuel rods. The fuel rods have a layer of cladding material made of a zirconium alloy. If zirconium is hot enough and is in the presence of oxygen (The steam provides the oxygen) then it can undergo a reaction that produces hydrogen gas. Hydrogen at concentrations above 4% is highly flammable when mixed with oxygen; however, not when it is also in the presence of excessive steam.

As time went on, the pressure in the containment rose to a much higher level than usual. The containment represents the largest barrier to the release of radioactive elements to the environment and should not be allowed to fail at any cost. The planned response to an event like this is to vent some of the steam to the atmosphere, just to keep the pressure under control.

Exactly what happened next is not verified; however, the following is very likely the general explanation for the explosion. It was decided to vent the steam through some piping that led to a space above and outside containment, but inside the reactor building. At this point, the steam and hydrogen gas were mixed with the air in the top of the reactor building. This was still not an explosive mixture because large amounts of steam were mixed with the hydrogen and oxygen (from the air). However, the top of this building is significantly colder than inside the containment due to the weather outside. This situation would lead to some of the steam condensing to water, thereby concentrating the hydrogen and air mixture. This likely went on for an extended period of time, and at some point an ignition source (such as a spark from powered equipment) set off the explosion that was seen in units 1 and 3. The top of the reactor building was severely damaged; however, the containment structure showed no signs of damage.

Right after the explosions there were spikes in the radiation levels detected, because there were some radioactive materials in the steam. When the zirconium alloy cladding reacted to make hydrogen, it released some fission products. The vast majority of the  radioactive materials in the fuel will remain in the fuel. However, some of the fission products are noble gases (xenon, Xe and krypton, Kr) and will immediately leave the fuel rods when the cladding integrity is compromised. Fortunately, Xe and Kr are not a serious radiological hazard because they are chemically inert and will not react with humans or plants. Additionally, small quantities of iodine (I) and cesium (Cs) can be entrained with the steam. When the steam was vented to the reactor building, the Xe and Kr would have followed as well as some small amounts of I and Cs. Thus, when the roof of the reactor building was damaged , these radionuclides that were in the reactor building would have also been released. This is the reason a sudden spike was seen in radiation levels. These heightened radiation levels quickly decreased. This is because there was no damage to the containment which would increase the quantities of radionuclide released, and because the radionuclides released during the explosion quickly decayed away or dispersed.

Unit 2 explosion

Recent information indicates that unit 2 may have suffered a containment breach.  Pressure relief of unit 2 was complicated by a faulty pressure relief valve, which complicated the injection of sea water and the evacuation of the steam and hydrogen.  It is reported that the fuel rods were completely exposed twice.  More details to follow.

Unit 4 fire

A fire was reported at unit 4 which was in a shutdown state during the earthquake and tsunami for a planned outage.  Latest reports indicate that the fire was put out.  More details to come.

 

 

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Damage to Fukushima Daiichi 2 [World Nuclear News]

Loud noises were heard at Fukushima Daiichi 2 at 6.10am this morning. A major component beneath the reactor may be damaged. Confirmation of loud sounds this morning came from the Nuclear and Industrial Safety Agency (NISA). It noted that “the suppression chamber may be damaged.” It is not clear that the sounds were explosions. Also…

Loud noises were heard at Fukushima Daiichi 2 at 6.10am this morning. A major component beneath the reactor may be damaged.

Confirmation of loud sounds this morning came from the Nuclear and Industrial Safety Agency (NISA). It noted that “the suppression chamber may be damaged.” It is not clear that the sounds were explosions.

Also known as the torus, this large doughnut-shaped structure sits in the centre of the reactor building at a lower level than the reactor. It contains a very large body of water to which steam can be directed in emergency situations. The steam then condenses and reduces pressure in the reactor system.

The pressure in the pool was seen to decrease from three atmospheres to one atmosphere after the noise, suggesting possible damage. Radiation levels on the edge of the plant compound briefly spiked at 8217 microsieverts per hour but later fell to about a third that.

A close watch is being kept on the radiation levels to ascertain the status of containment. As a precaution Tokyo Electric Power Company has evacuated all non-essential personnel from the unit. The company’s engineers continue to pump seawater into the reactor pressure vessel, in an effort to cool it.

Prime minister Naoto Kan has requested that everyone withdraw from the ten kilometer evacuation zone around the nuclear power plant and that people that stay within remain indoors. He said his advice related to the overall picture of safety developments at Fukushima Daiichi, rather than those at any individual reactor unit.

http://www.world-nuclear-news.org/RS_Possible_damage_at_Fukushima_Daiichi_2_1503111.html

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Status Update – 3/14/11 at 1:00 pm EST (Source – The Federation of Electric Power Companies of Japan (FEPC))

As of 1:00PM (EST), March 14, 2011 * Radiation Levels o At 9:37AM (JST) on March 14, a radiation level of 3130 micro sievert was recorded at the Fukushima Daiichi Nuclear Power Station. o At 10:35AM on March 14, a radiation level of 326 micro sievert was recorded at the Fukushima Daiichi Nuclear Power Station.…

As of 1:00PM (EST), March 14, 2011

* Radiation Levels
o At 9:37AM (JST) on March 14, a radiation level of 3130 micro
sievert was recorded at the Fukushima Daiichi Nuclear Power
Station.
o At 10:35AM on March 14, a radiation level of 326 micro
sievert was recorded at the Fukushima Daiichi Nuclear Power
Station.
o Most recently, at 2:30PM on March 14, a radiation level of
231 micro sievert was recorded at Fukushima Daiichi Nuclear
Power Station.
* Fukushima Daiichi Unit 1 reactor
o As of 12:00AM on March 15, the injection of seawater
continues into the primary containment vessel.
* Fukushima Daiichi Unit 2 reactor
o At 12:00PM on March 14, in response to lower water levels,
TEPCO began preparations for injecting seawater into the
reactor core.
o At 5:16PM on March 14, the water level in the reactor core
covered the top of the fuel rods.
o At 6:20PM on March 14, TEPCO began to inject seawater into
the reactor core.
o For a short time around 6:22PM on March 14, the water level
inside the reactor core fell below the lower measuring range
of the gauge.  As a result, TEPCO believes that the fuel
rods in the reactor core might have been fully exposed.
o At 7:54PM on March 14, engineers confirmed that the gauge
recorded the injection of seawater into the reactor core.
o At 8:37PM on March 14, in order to alleviate the buildup of
pressure, slightly radioactive vapor, that posed no health
threat, was passed through a filtration system and emitted
outside via a ventilation stack from Fukushima Daiichi Unit
2 reactor vessel.
* Fukushima Daiichi Unit 3 reactor
o At 11:01AM on March 14, an explosion occurred at Fukushima
Daiichi Unit 3 reactor damaging the roof of the secondary
containment building. Caused by the interaction of hydrogen
and oxygen vapor, in a fashion to Unit 1 reactor, the
explosion *did not damage the primary containment vessel* or
the reactor core.
o As of 12:38AM (JST) on March 15, the injection of seawater
has been suspended.
* Fukushima Daini Unit 1 reactor
o As of 1:24AM on March 14, TEPCO commenced the cooling
process after the pumping system was restored.
o At 10:15AM on March 14, TEPCO confirmed that the average
water temperature held constant below 212 degrees Fahrenheit.
* Fukushima Daini Unit 2 reactor
o At 7:13AM on March 14, TEPCO commenced the cooling process.
o As of 3:52PM on March 14, the cooling function was restored
and the core temperature was stabilized below 212 degrees
Fahrenheit.
* Fukushima Daini Unit 3 reactor
o As of 12:15PM on March 13, reactor has been cooled down and
stabilized.
* Fukushima Daini Unit 4 reactor__
o At 3:42PM on March 14, cooling of the reactor commenced,
with TEPCO engineers working to achieve cold shutdown.

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