Fukushima - The issues, in 'layman terms'
Trying to disseminate information from the news about what’s going on at the Fukushima plant in Japan in the aftermath of their recent earthquake and subsequent tsunami is a tricky business. BBC is all about pictures of explosions and worried looking people in paper masks. Sometimes it’s interesting to have a little more detail about what trouble they may be having in managing a nuclear fission plant in a critical state. Well as it turns out, I know a nuclear engineer who was kind enough to give me a summary.
He has asked me not to name him. Suffice to say, he has been working in the field both maintaining and building nuclear plants for many years and in several countries. The man knows his trade. The following is his summary:
What I know about Fukishima follows and has been taken from the International Atomic Energy Agency - IAEA ( http://www.iaea.org/press/ ), the Canadian Nuclear Association - CNA ( http://twitter.com/talknuclear ), Canadian Nuclear Safety Commission - CNSC ( http://cnsc.gc.ca/eng/mediacentre/updates/march-11-2011-japan-earthquake.cfm ) websites.
There were two totally separate, but connected, events. The first was the earthquake which at 8.9 on the Richter scale was greater than the 8.2 (standard required) against which the plant was constructed. This meant that the actual forces were seven times greater (the Richter scale is log, not a linear scale). That being said it appears that the plant withstood the event and the after shocks and is demonstration that it has been robustly constructed.
The the three operating Fukishima plants, along with all other nuclear and fossil plants in the affected areas, were automatically shutdown (as per design). The earthquake caused electricity pylons to collapse and as a result no external power (from unaffected areas in Japan) was available to run the motors of the pumps required to pump cooling water into the reactor units. So, as designed, the standby diesel generators started automatically and provided the necessary power. All was in order and running per design.
About an hour following the earthquake a tsunami occurred. This huge wall of water caused these diesel generators to fail. It is unclear to me as to precisely what happened; the available data suggest that the diesel fuel was either cut off or contaminated by water. In either case the result would have been the same - loss of power generation. So now the next back-up power supply was employed and that was from the batteries which were used until they ran down. At this point all forced cooling would have been lost.
Since that time I am led to believe that large “transportable” diesel generators and fuel have been delivered to the site and power restored to the pumps (how much power and how reliable the supply I do not know) to cool the fuel. It would appear that the explosions and fires may have impacted on the “recovery” process.
The essential thing is to ensure that the fuel is cooled.
Following reactor shutdown the heat from fission is immediately cut off, however the decay heat remains. Somewhere between three and six percent % of full power, assuming 3000 MW thermal, (probably about 90 to 180 MW thermal - the equivalent of 90,000 to 180,000 electric kettles) is the amount of heat being provided by decay heat from fission products. This heat input drops over time as the fission products decay.
If the cooling is lost then the fuel cladding (a zirconium alloy) will heat up. As the cladding heats significantly above its normal about 350 degrees C temperature (when reactor is at power) a reaction between water and zirconium is possible. In this case hydrogen can be generated as the zirconium picks up oxygen (water is made of hydrogen and oxygen). [You may recall the issues with HMS Sheffield during the Falklands war which were compounded by high temperature water metal (aluminium) reactions leading to hydrogen production.] This hydrogen is in addition to that is added on purpose to control corrosion when under power operation.
Hydrogen at concentrations between 4 to 75 percent in air forms an explosive mixture. An explosion will occur should a spark and/or sufficient energy be available in this concentration range. The explosions that have occurred in the Fukishima units were the consequence of hydrogen air mixtures existing in vent pipework/ducting. The consequence has been to damage the exterior of the building but not damage the area in which the reactor is housed. The hydrogen had to be released due to pressure build-up primarily due to steam.
Back to fuel in the reactor. The zirconium water reaction at about 1100 degrees C (water is now steam) becomes exothermic (gives out heat) and the alloy will get hotter, hence keeping the fuel cool is essential. The zirconium alloy melts at about 1850 C. Once the alloy has melted then the fuel is now exposed. The fuel starts out as uranium dioxide. Uranium dioxide has excellent properties to capture fission products. As the hot fuel oxidizes on exposure to water/steam the uranium oxide (structure) changes. The ability of the fuel to hold fission products is reduced. At this point the volatile fission products can be released. The filters in the venting systems are designed to retain these volatile fission products. If the venting system has been damaged then the extent of that damage will affect the fission product retention capability.
Fuel in the cooling pond/bay. Fuel from Unit 4 is in the cooling pond. Even though this fuel was removed some months ago heat from decay of fission products must still be removed. Should cooling be lost the bay water temperature will increase and similar reactions to those described above with the zirconium are possible. However, the immediate danger is reduced as after about 40 days the majority of the remaining radioiodine will have decayed away, but other longer lived fission products will remain. HOWEVER, I have been reminded that this is not CANDU (natural uranium isotopic 0.7%) but enriched (U-235) fuel and in one unit mixed oxide (U & Pu) fuel. In order to prevent criticality in the spent fuel bay soluble poison is used, usually a boron compound in BWRs. The bad news is that if a bay drains there is the possibility of a criticality event. The good news is that fuel that has been used (“burned”) has reduced fissile (U-235/Pu-239/Pu-241) content. That being said, the fact that the fuel was enriched initially means that the fissile content will certainly be greater than for CANDU fuel.
I have recently heard that the fuel pool, or cooling pond, of one unit has drained and that the fires may have been related to hydrogen generation from the zirconium steam interaction described above.
At the start of this event I was fairly upbeat about the issue with respect to Japan and Fukishima area. As time has passed and we have learned more, I have become less optimistic that the issue will be solved easily and quickly. The good news as far as Countries to the west of Japan is that the prevailing winds are to the east - towards the Americas.
With respect to the Uk and Canadian reactors I believe safety is more than adequately covered. In all honesty I believe that the CANDU reactors are the safest in the world.
He has also passed on to me the following presentation. Alas, it’s in Microsoft Silverlight and so I can’t see the thing. Hopefully it will be of interest to you though.
Presentation Details: Title: Public seminar on Fukushima event Date: Thursday, March 17, 2011 Time: 7:00 PM EDT Duration: 3:00:00