Tuesday, March 15, 2011

Fukushima update

NEI is running continuous updates on the situation with the Fukushima reactors, however the goal here is to summarize everything that's been reported thus so far and what the general implications of these events are.

Units 1 and 3

Currently, the situation at Units 1 and 3 is stable - the reactors are under control and being cooled by seawater laced with boron. As indicated before, the addition of boron is a safety precaution to prevent any further fission in the fuel - boron "eats" any neutrons which may still be produced in the reactor through "spontaneous" fissions (from isotopes such as plutonium). The reactors are "off" and in "cold shutdown" mode now, so any fission chain reactions are no longer possible.

Both Units 1 and 3 are reported to have lost coolant for some time, with fuel rods becoming partially uncovered for some time at both reactors. Air is a relatively inefficient conductor of heat, thus when rods are uncovered, this leads to a buildup of heat (due to the decay heat in the rods) in the fuel and cladding material.
As a result, some damage may have occurred to the cladding or fuel itself.

The measured presence of cesium and iodine outside of the reactor may have indicated damage to the zirconium cladding, allowing some escape of fission gases. Iodine and cesium, as well as noble gases such as xenon and krypton, are generally present as fission product gases. One of the purposes of the clad material, beyond providing mechanical support for fuel pellets, is in containing these fission product gases.

A hydrogen explosion also occurred at both Units 1 and 3; this is believed to have occurred due to the buildup of hydrogen from steam being dissolved into oxygen and hydrogen when the cladding material grew very hot, in a process known as zirconium hydriding. In this process, Zirconium, normally resistant to oxidation, reacts with steam to produce Zirconium Oxide and hydrogen:

Zr + 2H2O -> ZrO2 + 2 H2

As hydrogen built up in the secondary containment (the reactor support building above the reactor), this eventually ignited and exploded. However, in both cases the "primary" containment (composed of the steel reactor vessel and a thick concrete wall) appears to be undamaged.

With the reactors now being safely cooled however, it would appear the worst is over from these units. Given that the fission reaction has stopped and decay heat decreases with time, as long as the reactors continue to be cooled, little further damage to the core for either of these units can be expected.

Unit 2

The unit of most serious concern right now is Fukushima Daiichi Unit 2. Late reports indicate that there was damage to a major emergency cooling component of the reactor known as the "suppression pool" - a doughnut-shaped object at the base of the core which circulates cold water, designed to condense steam from the reactor in an emergency as a means of reducing pressure inside of containment. (Recall that increasing pressure inside containment was why steam had to be vented from Units 1 and 3.)

Reports from TEPCO also indicate Unit 2 lost coolant for around 140 minutes following the damage, due to a rapid rise in pressure inside of the Unit 2 containment. This appears to have left the fuel rods uncovered for some time, possibly leading to damage to the clad or rods. This appears to have been the most prolonged period of the fuel remaining "uncovered" of the three reactors thus so far.

As a result, radiation levels at the plant boundary was reported to have briefly spiked to around 1000 millirem per hour - six hours later, the level has declined to around 60 millirem per hour. While this level is elevated, these levels do not pose any immediate threat to human health.

Is this a meltdown?

Given the discussion in the media, the question in the media has now inevitably turned to characterizing whether the events at Fukushima Daiichi constitute a "meltdown."

"Oh, meltdown. It's one of those annoying buzzwords.
We prefer to call it an unrequested fission surplus."
Nuclear Fissionary has a thorough rundown on the meaning of the term "meltdown". For our purposes here, it is helpful to discuss what we know has and has not happened:

  • Three of the reactors appear to have lost coolant for some period of time, possibly exposing the fuel rods.
  • The fission reaction is "off" in all of the reactors - this happened as soon as the earthquake hit and the control rods were dropped into place. 
  • Currently, all of the heat being produced in the rods is due to decay heat. This heat decreases rapidly and exponentially with time.
  • All three reactors (Units 1, 2, and 3) are being cooled by an injection of seawater containing boron. No fuel is currently exposed.

So far, it is unknown whether there has been any actual melting of the fuel pellets themselves. Bear in mind that fuel pellets are designed to withstand incredibly high temperatures - the melting point is nearly 3000 degrees Celsius. The melting point of Zircaloy (the zirconium alloy used in fuel cladding) is around 1850 degrees Celsius. In situation where water covering the fuel rods is lost, particularly after a shutdown, the fuel is designed to withstand short periods without cooling. It is possible (and appears likely) in this case that there may have been some damage to the clad (having a lower melting point), thus allowing for the release of some fission product gases while the fuel itself has maintained its integrity.

Thus so far, there is little evidence that the fuel itself has melted - likewise, for the entire core to melt would require that the entire length of the rod be exposed for a significant period of time. Thus so far, the worst case appears to have occurred has been at Unit 2, with a portion of the rods becoming uncovered when the suppression pool was damaged and operators were unable to inject coolant until pressure was relieved.

Likewise, it is useful to bear in mind as well that these are only the first lines of defense from a radioactive release. Were the fuel itself to melt, it would be contained by the reactor vessel itself (made of thick steel), followed by the concrete containment. This is what is meant by "defense in depth" - several additional barriers exist to protect the public against any kind of release of materials. In the case of fuel melting, what would likely happen based on prior incidents (such as Three Mile Island) is that any melted fuel would collect at the bottom of the reactor vessel itself, where it would quickly pool out and cool.

In the case of Three Mile Island, in which over half of the uranium fuel in the core melted (and over 90% of the zirconium cladding material failed), the fuel did not even penetrate the reactor vessel, much less escape primary containment. Rather, the melted fuel collected into a pool at the bottom of the vessel, quickly cooling into a lump of hot metal referred to as "corium."

Given the rapid response in the case of Fukushima, it seems unlikely that damage as substantial as Three Mile Island Unit-2 occurred in these cores. Likewise, it should be emphasized again that the radioactive release from an accident as severe as Three Mile Island was quite minimal - the average dose to the public around 10 miles from the plant was around 8 millirem (around the same as a chest X-ray), and no individual was exposed to a dose higher than 100 millirem (about one-third of a person's expected annual exposure due to natural sources).

As of right now, it is difficult to determine if any fuel itself has melted in the case of Fukushima, however it is useful to draw upon examples such as Three Mile Island as an example of why much of the media panic is unwarranted.

What's the danger?

Various media sources have been jumping onto the story of radioactivity releases from the plants without putting these levels into context, stoking fears of a widespread Chernobyl-style contamination when nothing could be farther from the truth. First, there is no danger to anyone living far away from Fukushima - none. Even the risk to folks living nearby is relatively small - the levels outside the plant boundary are below the levels at which any immediate health effects could be observed. Further, the radiation levels, while above the legal limits under normal operating circumstances, pose an extremely small risk themselves - the best advice offered by anyone thus so far in to those in the vicinity would be to roll up windows if inside and avoid going outside.

Those in the immediate vicinity have already been given potassium-iodine tablets in order to "inoculate" their systems against the uptake of any radioactive iodine present (in essence, by flooding the thyroid with natural, non-radioactive iodine such that any iodine contamination present will be rapidly excreted by the body).

On a more general note, one should not lose sight of the extreme devastation wrought by this earthquake and subsequent tsunami - over 6,000 people are already confirmed dead or missing, with tens of thousands now homeless or displaced by evacuation orders. Concerns about the relatively low risk from radiation releases would almost be comical were they not in stark contrast with the humanitarian crisis already present.

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