Saturday, March 12, 2011

Nuclear Safety and the Fukushima Dai-ichi Explosion

The explosion at the Japanese Fukushima Dai-ichi nuclear plant, will undoubtedly and other events that are still unfolding will no doubt lead to further debates about nuclear safety. We know that the explosion was probably a hydrogen explosion inside the containment building rather than a nuclear explosion inside the reactor core. The core vessel appears to still be intact. What we know is that the cooling systems of two reactors in the Fukushima Dai-ichi nuclear plant failed and that heat began to build up un their cores, along with pressure inside the air tight containment structure. In the event of an emergency reactor shutdown, electricity from back-up electrical sources operates the emergency cooling system, but the earthquake appears to have damaged Fukushima Dai-ichi back-up electrical sources. The pressure inside the containment structure probably was the result of the venting of radioactive gases as well as other volatile fission products and hydrogen from the reactor core. The fission products would have been released following the rupturing of uranium fuel pellets inside the reactor core. Hydrogen can be produced by a high temperature chemical process involving the disassociation of hydrogen and oxygen in coolant water and steam, that would in close proximity to the overheated nuclear fuel pellets inside the core.

The air tight containment structure is designed to prevent the escape of volatile fission products and radioactive gasses in the event of a nuclear accident. The airtight barrier also contains any hydrogen gas released from the nuclear core, as well as steam. It appears that air leaked into the containment structure, and an explosive oxygen-hydrogen reaction was triggered destroying the outer containment building. (Update: I would now back off from the assessment that a hydrogen explosion occurred.) The inner steel containment vessel remains in tact, at least for now.

While the explosion of the containment building was impressive, it pales to insignificance compared to the disaster that has fallen on Japan due to the magnitude 8.9 earthquake, and subsequent tsunami. So far there have been no deaths or injuries associated with the reactor accident, and no one is known to have received a high level radiation exposure. (Update: We now have reports of up to 4 injuries to plant workers.) The Radioactive gasses are quickly dissipated as the mix with air in he atmosphere, and pose little threat to people. Volatile fission products tend to precipitate out of air as they cool, and much of them will fall within a few hundred yards of their source reactor. Reportedly radioactive materials released by the Fukushima Dai-ichi blast are currently blowing out to sea, but there is no reason to assume that this will continue to be te case.

As I have indicated the Fukushima Dai-ichi explosion and other possible further events at the nuclear plant are relatively minor events compared to the enormous property damage and loss of life in Japan. Much of the Japanese energy structure has been subject to catastrophic failure, Natural gas pipelines have ruptured, and escaping gas has caught on fire, triggering fires in homes and other buildings. Oil and oil product tanks have ruptured and been set on fire. Many power lines are down, and the damage to hydroelectric dams has yet to be assessed. No one has any idea yet of the extent of the loss of life and property damage, but the dead are likely to number in the tens of thousands, and property damage could run into the trillion dollar range.

Could this accident been avoided? The answer is "probably yes," with the best recent reactor designs, the AP-1000 and the ESBWR, and certainly yes with advanced Molten Salt Reactor (MSR) technology. With the Integral Fast Reactor (IFR) the safety issues in the even of a large scale earthquake are less clear.

The AP-1000 and the ESBWR rely on passive emergency cooling features, that simply rely on the laws of nature rather than outside power sources. Had the Fukushima Dai-ichi reactors been AP-1000, or ESBWRs, they would have survived the Earthquake event intact with no emergency cooling failure.

In the case of molten salt technology, for example the Liquid Fluoride Thorium Reactor, as core temperature rises above operational range, a solid salt plug in a reactor core drainage system will automatically melt and the core fluid will automatically drain into passively cooled tanks. Thus the entire MSR/LFTR emergency shutdown and coolant system is dependent on the unfailing laws of nature.

The Fukushima Dai-ichi accident, as it now stands has set a new standard for thinking about nuclear safety. It is clear that some new nuclear plant designs could have successfully withstood the 8.9 magnitude earthquake, other reactors, similar to the Fukushima Dai-ichi nuclear plants would be less likely to do so. Safety standards should undoubtedly be reevaluated, and the consequences of the fission product escape from the Fukushima Dai-ichi reactor carefully and rationally assessed.

UPDATE: News sources are now reporting that up to 4 Fukushima Dai-ichi nuclear plant workers have been injured.

UPDATE 2: The Nature of the explosion is now open to question, as far as I am concerned.

Update 3: CBS News reports:
Eleven reactors shut down automatically when the earthquake hit. And with most of those, the cooling has been reasonably straightforward, because there's been a power supply to the plant, either from the grid, or from the backup generators. But with the first two units of the Fukushima Dai-ichi plant, the generators cut in and ran for about an hour and then stopped. And we understand the reason they were stopped is because they were overwhelmed by the tsunami. And that precipitated the crisis, really, and then the challenge of keeping the reactor cool, because they then have to default to their battery power. And as far as I understand it, that may not have been sufficient to do everything that was fully required."
We still hack, as far as I am concerned a satisfactory account of how the explosion occurred, although CBS news continues to describe it as a hydrogen explosion.


Anonymous said...

No-one injured related to this accident yet?

charlesH said...

Doesn't LFTR have the option of continuous removal of fission products (thus they don't build up in the reactor)? Therefore heat removal under shutdown is a relatively minor issue with LFTR vs LWRs?

Charles Barton said...

Charles, the removal of some or all fission products is an option for all MSRs, and would be a significant enhancement to its passive safety features.

Jason Ribeiro said...

Charles, I think you write prematurely here, it doesn't seem that layer 5 containment was breached.

Charles Barton said...

Jason the video of the explosion seems to show the roof being blown off the containment building and smoke gushing from inside it. That is clear evidence of a containment breach. What the Japanese say is that the steel containment vessel is still intact.

crf said...

They are poor communicators.

The press are, understandably, intensly interested in what is going on.

But, because the Japanese are providing few facts to the public, the press are turning to outside experts to interpret what is going on, given the limited information available.

Apart from whatever nuclear technical design issues that might be learned in this earthquake, the massive failure of clear communication with the public ought to publicly scrutinized.

Jason Ribeiro said...

Charles, according to many reports, this was not a level 5 containment breach. That's a good thing. It was, until further notice, just the outer container surrounding level 5.

Frank Kandrnal said...

About three weeks ago I made a comment on Brave New Climate in favor for natural passive safety features of Molten Salt Reactor. I was scolded for my comment by Barry Brook who basically said the comment was too silly. How much more nuclear safety is needed for already too safe system? Well, just like Alvin Weinberg believed, I too believe that no amount of engineering can outperform mother nature. Simply, MSR has so many natural safety features no other reactor can match.
The hydrogen explosion in nuclear plant in Japan is another thorn in the side for nuclear industry. The anti nuclear crowd will be showing the pictures of exploding power plant for a long time to come with distorted spin propaganda against nuclear power.
IFR proponents have big uphill battle ahead of them. Any reactor working with fast neutrons, filled with large quantity of extremely reactive liquid sodium and filled with solid fuel/blanket in over critical mass quantity does not meet natural safety criteria and will never be as safe as MSR.
In addition, the IFR plutonium fuel cycle is hard sell for general public who cannot differentiate between Pu isotopes, hence for uneducated masses any Plutonium is suitable for bomb making, because Green Peace or some other Yahoo told them.
Electrorefining process or any other process dealing with plutonium in an attempt to make it proliferation proof will not penetrate the brain dead skulls of anti nuclear activists.

Charles Barton said...

Frank, you re right about LFTR safety, and right about the problems that the IFR crowd face. I have also advised them of all this, and they are not prepared to listen. I cannot help them.

horos22 said...


Whilst I appreciate the thought exercise with regards to LFTR, there is one place that we have to be careful about - and that is the solubility of the molten salt.

As far as I can gather, the candidate salts for LFTR *are* water soluble, so a lot of care is going to have to be taken in preventing the coolant from direct contact with water, and perhaps a separate level of containment that filters any water that comes in contact with the plant.

Anyways, I'm not sure what the thoughts of the LFTR community were on this so I thought I'd throw it out for discussion.


Charles Barton said...

horos22, anyone who is familiar with the history of MSR technology is aware of the corrosion problems that even a small amount of water in a MSR can lead too, and how easily even exposure to humid air can produce corrosion in the reactor vessel.

BilloTheWisp said...

Charles, Just getting back to the problems at Fukushima Dai-ichi, which is now busily being paraded as the world's third worst nuclear disaster. A disaster - with as yet, no fatalities.

I agree this is a serious diversion from the major problems caused by this earthquake. The sheer amount of coverage this has received compared to the unfolding real disaster surrounding it beggars belief.

When push comes to shove, a 40 year old reactor with an obsolete design, built on top of a major fault line was badly damaged by an exceptionally strong earthquake.

A matter of concern maybe. Embarrassing maybe.
But a disaster? Are they serious?

Atlanta Roofing said...

The explosion is a very very bad sign. They are claiming that they don't know if the the reactor core has been damaged. That unfortunat¬ely isn't a true statement they know perfectly well it has been breached. They have announced elevated levels of Cesium in the air. They are talking about "radiation leakage". That is misleading¬,Cesium 135 and 137 are isotopes that are byproducts of the fission reactor using
Uranium 235 a a primary fuel. Cesium isotopes can only come from the reactor core which means it has been breached. In a best case they are deliberate¬ly venting the reactor vessel to try and keep the amping control rods from collapsing¬.

Anonymous said...

Okay, BillOtheWisp, you're right. It's not a disaster but merely a "matter of concern." We look forward to hearing your reassuring reports to that effect from inside the Dai'ichi Fukushima facility.


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