3-Week Update on Japan’s Nuclear Crisis

, co-director and senior scientist | April 2, 2011, 10:40 am EDT
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Two nuclear plants northeast of Tokyo were initially the main focus of concern after the March 11 earthquake and tsunami: Fukushima Dai-Ichi (or Fukushima I) with six reactors and Fukushim Daini (or Fukushima II) with four reactors.

Currently, all the reactors at Fukushima Daini have reached cold shutdown, meaning the water in the reactors is below boiling temperature and should remain that way as long as nothing disrupts the cooling. There have been no reported problems with the spent-fuel pools at this site.

Serious problems remain at Fukushima Dai-Ichi with both the reactors and spent fuel pools, which contain large amounts of radioactive fuel. All the operating reactors were shut down in the period after the earthquake but before power was knocked out by the tsunami.

However, fuel in an operating reactor becomes highly radioactive and that radioactivity continues to generate heat even after the reactor has been turned off and the fuel has been removed from the reactor, so the radioactive fuel rods must be continually cooled for years. This is done by circulating cooling water around the fuel that is in the storage pools and in the reactor.

If the cooling stops, the fuel begins to heat up. If this continues long enough the cladding may get hot enough to react with water in the air to release hydrogen, which can explode if it builds up. If the cladding continues to heat up and react with water, it can expand and rupture, releasing radioactive gases. If the fuel heats up enough, the fuel pellets can begin to melt, which releases larger amounts of radioactive gases.

While the reactor has several layers of containment to keep gases in the core from escaping to the atmosphere—a steel reactor vessel and a steel and concrete containment structure—damage to the reactor vessel and reactor containment can allow some of this to escape, as can intentional venting of gas from the containment to reduce pressure. Gases released from spent fuel in the pools are contained by the reactor building, but can be released if the building is damaged.

The earthquake and tsunami caused a loss of AC power to the facilities so that motor-driven cooling of the reactors and spent fuel pools stopped. Cooling was provided for a few more hours to the reactors by steam-driven pumps, but those stopped when the batteries needed to operate the systems ran out of power. Workers have struggled to resume cooling to minimize damage to the fuel and release of radioactivity.

Current Status at Fukushima Dai-Ichi

The Fukushima Dai-Ichi facility has 6 reactors, all built in the 1970s. Three—Units 1, 2, and 3—were operating at the time of the earthquake, while Units 4, 5, and 6 were shut down for maintenance. All the fuel had been moved from the Unit 4 reactor vessel into the spent fuel pool, so there is no concern about the Unit 4 reactor vessel. The Unit 5 and 6 reactor vessels still contain more than 75% of the fuel they use when operating, so they require cooling.

Units 5 and 6 are located a short distance away from the other reactors and do not appear to have been as badly damaged. The cooling systems have reportedly been restored to both these reactors and their associated spent fuel pools, so they are not considered a threat at this time.

Image from New York Times

The Fukushima Dia-Ichi facility also has a common storage pool for spent fuel, which contains fuel that has been out of a reactor for at least 19 months. Since the radioactivity of the fuel rods falls with time, this fuel is not generating as much heat as fuel more recently removed from the reactors, which is stored in the spent fuel pools located in the reactor buildings at each of the 6 reactors. Workers have reportedly been adding cool water to the common pool as needed, and this pool is apparently not currently seen as a threat.

So the concern at the Fukushima Dai-Ichi site is focused on the fuel in the reactor cores at Units 1, 2, and 3, and the spent fuel in cooling pools at Units 1, 2, 3, 4.

Reports say that electric power has now been reconnected to all four reactor buildings. While lights have been turned on in the control rooms of these reactors, few other systems appear to be working, including instrumentation that would allow workers to know what is happening in the reactor cores and spent fuel pools.

Atmospheric radiation releases

A significant amount of radiation has been released to the atmosphere from this site since the beginning of the crisis. Two of the main health hazards from the radioactive gases that have been released are from iodine-131 (I-131) and cesium-137 (Cs-137). One analysis estimated that roughly 20% of the I-131 and up to 50% of the Cs-137 released in the Chernobyl accident was released from Fukushima to the atmosphere within the first few days of the accident.

Very high radiation levels are being detected at some points many kilometers away from Fukushima, outside of the evacuation zone, although there is no clear picture at this point because the locations of the readings are not publicly available and there has not been a systematic survey.

Japan initially ordered residents to evacuate out to 3 km (1.9 miles) around the Fukushima site, with residents out to 10 km (6.2 km) told to stay indoors. By late on March 12, Japan expanded the evacuation zone to 20 km (12 miles) with sheltering to 30 km (19 miles). On March 25, Japanese officials said they were encouraging residents to evacuate out to 30 km.

In contrast, on March 17 the U.S. embassy told US citizens to evacuate out to a radius of 80 km (50 miles) from the site.

As the radiation is carried by winds across the ocean, it spreads out and becomes diluted. While trace amounts have been detected in the US, these amounts have been much lower than the natural background levels of radiation that people are constantly exposed to, and are not a serious health hazard.

The radiation released to the atmosphere at Fukushima came from two main sources. First, when cooling stopped in the reactor cores, the fuel began to heat up and the pressure in the reactor vessels increased. To reduce the pressure, workers vented to the atmosphere some of the radioactive gas that had built up in the vessel and primary containment. There are also reports that the primary containment of Unit 2 and possibly Unit 3 may be damaged; if that is true, that would also allow radioactive gases to escape.

Second, loss of water in the spent fuel pools led to fuel assemblies being exposed to air, which caused damage to the fuel that then released radioactive gases. While the pools are contained in the reactor buildings, hydrogen explosions in the buildings of Units 1, 3, and 4 created holes in the walls that allowed these gases to escape. And vents were opened in the walls of the Unit 2 reactor building to prevent a buildup of hydrogen that could cause an explosion.

Fortunately, monitoring indicates that deposition of Cs-137 is currently no longer occurring around the site. This is because efforts to cool the reactors and spent fuel pools have been successful enough to eliminate the need for additional venting and to stop further releases from the spent fuel pools. However, as discussed below, additional venting may soon be needed.

It is also important to note that the amount of Cs-137 and other radioactive material that remain in the fuel in both the core and spent fuel pool is much larger than the amount that has already been released. Some of this remaining radioactive material could be released if new problems occur, so this remains a very serious concern.

Other releases of radiation

The other source of radioactive contamination around the plant is from contaminated water. To attempt to cool the reactors and spent fuel pools, many thousands of tons of water were dropped by helicopter or sprayed by hoses at the plants. Some of the runoff water from these efforts has apparently become contaminated and run into the ocean, since radiation has been detected in the coastal waters.

More recently, there is a concern about very highly contaminated water in trenches outside the buildings, especially at Unit 2, which appears to be coming from water that has collected in the lower parts of the reactor turbine buildings. Japanese officials apparently believe this is water that was pumped in to cool the fuel in the reactor that has somehow leaked out into the turbine buildings.

On Monday March 28, press reports said the radiation level of this water from the Unit 2 reactor was 1,000 milli-Sv/hr. This is high enough that an hour-long exposure would give someone a radiation dose sufficient to cause acute radiation syndrome. At an April 2 press conference Japanese officials said that this highly contaminated water is leaking into the ocean.

Less highly radioactive water has also been found in tunnels under the turbine buildings at Units 1 and 3.

This issue is creating new problems for workers at the plant. The volume of radioactive water is so large that they are running out of places to store it. To cut down on the volume of water they need to remove and store, they are trying to reduce the amount of water they pump into the reactors to cool the fuel in the cores. But without that cooling, the fuel in the cores has been heating up. This leads to a buildup of pressure in the reactor that may require additional venting of radioactive gas to the atmosphere. If the heating becomes great enough, it can also lead to additional fuel damage and further release of radioactive gases from the fuel.

There is speculation about the amount of fuel in the reactor cores that may have melted, and given the lack of cooling it may be substantial. But because of the lack of monitoring in the reactor vessels no one really knows the condition of the fuel. The state of the fuel at Three Mile Island was not known for several years after the accident there. Similarly, because of lack of water in some of the spent fuel pools early during the Japanese crisis, people assume that some of the fuel in the pools may have melted, but the status is not known.

I-131 and Cs-137

Because I-131 has a half-life of only 8 days, it reaches a stable concentration when the reactor is operating but decreases relatively quickly once the reactor stops. So very little of this material remains in the fuel in the spent fuel pools; for example, fuel that has been out of the reactor for two months would have less than 1% of the I-131 it had when it was removed from the reactor. This means that the I-131 found outside the plant likely came from venting the reactor cores.

If at this point workers can control the temperature and pressure in the cores to eliminate the need for additional venting, this would essentially cap the amount of I-131 released. Moreover, even if future venting is required, the longer venting can be delayed the more the level of I-131 in the core will decrease. Already, the amount in the fuel in the core is only about one-fifth of the amount present when the earthquake hit.

Recent reports say that seawater collected near the Fukushima I facility showed I-131 levels several thousand times safety standards. This high level seems to indicate that the iodine must have come from fuel in the core.

Cs-137, however, has a half-life of 30 years so it decays much more slowly and its release remains a serious concern. This is the main contaminant that has caused long-term evacuation of areas around Chernobyl.

The fuel in the core of Unit 3 has been a particular concern because it contains mixed-oxide (MOX) fuel, which is made from both uranium and plutonium oxide rather than just uranium. While releases from fuel with larger amounts of plutonium raises additional health concerns, the MOX fuel in Unit 3 only makes up about 6% of the core (32 out of 548 total fuel assemblies), so the increased risk due to the presence of MOX fuel is probably negligible. Public opposition to MOX in Japan slowed down the program and is the chief reason why there is so little MOX in the core and why the risk from the additional plutonium is limited.

A 7-week update on Fukushima is available here.

Posted in: Japan, Nuclear Power Safety Tags: , ,

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