Japan’s Fukushima Daiichi reactor complex has been headline news since the earthquake and tsunami led to a partial melt of the enriched uranium fuel in at least one of its reactors.
How we humans have used energy generation technology is one of this blog’s core topics, its reason for existing. In this series of posts, we will use the Fukushima Daiichi horror and other terrible incidents to discuss whether nuclear fission reactors should be considered a reasonable choice for meeting world energy needs.
The Fukushima installation has 6 power generating reactors, R1, R2, and R3 were generating power and 4, 5 and 6 were off-line and considered safe. R1, R2 and R3 all had major overheating of the reactor core when (a) power failed, then (b) the emergency backups took a minute to come to full power. That response time delay was sufficient cause run-away temperatures in the fuel rods and caused major damage. Part of the damage was to the water cooling system for the spent fuel rods.
The full extent of the damage is still being assessed, 3+ weeks after the event and it is not yet clear whether or not the spent-fuel rods in the 6 storage ponds have experienced an overheating incident, too.
|LOCA||Loss Of Coolant Accident||BWR||Boiling Water Reactor|
|PWR||Pressurized Water Reactor||FBR||Fast Breeder Reactor|
Reactor Fuel: As with all reactors worldwide, uranium fuel is packaged in long tubes (fuel rods) of special zirconium alloys. When the fission activity drops below a level for efficient power generation, the rods are removed to a nearby (on site) storage volume similar to a swimming pool. Here, they kept underwater for years, until the self-heat from the residual fission can no longer melt the casings. Although not a “core melt” because it is in the storage pond, overheating of spent rods can generate severe issues in the world ecosystem.
There are four well-known historical events where the reactor core lost coolant, causing the fuel to over heat and melt the rod casings. In at least one overheating incident, the molten fuel dripped down onto the concrete floor, causing the concrete to evaporate. If the entire fissile core were to melt, it is hard to envision what could dissipate the molten bolus of material. The fanciful picture is of a mass that melts “straight down to China” when viewed from U.S. locations. No ‘China Syndrome’ as yet; the human species has been lucky.
1966, Oct 05 – Fermi Station
Near Detroit MI. Unit 1, an experimental 94 MWe liquid sodium cooled FBR, suffered LOCA and some fuel rods in the core melted. During the past 45 years the events have been variously described as straight forward containment event of a fuel melt situation, or an event that nearly destroyed Detroit.
1979, Mar 28 – Three Mile Island.
Near Middletown PA. Unit 2, a 430 MWe PWR, experienced operator error causing a LOCA event leading to a partial core melt. The problem was contained within the safety enclosure. The 3-Mile incident is frequently cited as the reason that no reactors have been built in the U.S. since this time.
1986 Apr 26 – Chernobyl
Near Pripyat Ukraine. Reactor R4, a 1 GWe graphite moderated BWR unit, suffered a LOCA event during a coolant safety test and the graphite moderators caught fire. This caused massive contamination in Ukraine and Belarus. A concrete ‘sarcophagus’ was built over the damaged reactor to block further radioactive contamination.
2011 Mar 12 – Fukushima Daiichi
Six BWRs generating 4.7 GWe power are located near Fukushima, Japan. This station experienced a LOCA after a 9.0 magnitude earthquake and related tsunami. Although units 1, 2, 3 and 4 are clearly in trouble, this is an ongoing incident and, more than 3 weeks later, the the full extent of the issue has not been determined.
This list is not comprehensive but does illustrate that the major concerns with a reactor are with its cooling strategy. If the cooling process is stopped for even an instant, a major incident could develop. LOCA-type events, such as the 4 just described, have been considered as an important failure mode since the start of the reactor age.
Right now, as oil production peaks in the world, there is strong interest in power generation free of CO2 emission, one not relying on hydrocarbon combustion.
There is renewed attention in nuclear power as an energy source. Here is the KKG pressurized water plant in Northeast Switzerland, showing the reactor’s confinement dome, its huge heat exchanger stack that removes heat from the circulating water, and (to the left of the dome) the reactor’s waste gas stack that allows release of radioactive biproduct gases into the atmosphere. As with most reactors, it is by a river for extra cooling water, when needed. (Some plants, such as the one at Fukushima Japan are near an ocean and do not need the huge cooling tower structure.) This is a top down overview of a reactor power station. The questions is: Should power production “go nuclear?”
I see two major obstacles to responsible deployment of fission reactors.
First is the danger posed by loss of coolant leading to core melt down.
Second is the safe storage of spent fuel. The spent fuel issue has been the object of wishful thinking up to now. No viable solution exists at this time.
These two issues are the subject to the next two posts.
Update: 12 Apr 2011: Fukushima Daiichi has had its incident analysis upgraded from the initial IAEA INES accident level 5 (Accident with wide consequence but limited radioactive release) to INES Level 7 (Major Accident, with large release of radioactive material and potential major environmental impact). Fukushima Daiichi joins Chernobyl as the only Level 7 events recorded.
Note the Three Mile accident was a Level 5 and the Fermi-1 accident has not been classified. IAEA = International Atomic Energy Agency; INES = International Nuclear and radiological Event Scale.
Charles J. Armentrout, Ann Arbor
2011 Apr 08, Minor addition: 2011 Jun 03
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