Monday, March 28, 2011

The Renewal of the Nuclear Debate

Following a brief period in the late 1950's when the Atom was the new wonder of modern technology, reality set in. Taming the atom was not as simple and inexpensive as predicted by visionaries of the day. The debate over the safety of nuclear power has had its ebbs and flows. As with most political debates, facts are of little use.

Design flaws and potential design flaws of the forty year old veteran General Electric Mark I nuclear design are having a Deja Vu moment. I have had my own criticisms of the design and the emergency response procedures. I, of course, feel my criticisms are founded in reality while of course everyone else feels the same. Of what I feel are the more informed people to comment on the design, I really have only one point where I disagree. That disagreement is not particularly a major issue.

Corium catch basins is one flaw the GE Mark I design is criticized for by experts. Corium, the molten combination of fuel and other reactor components, should be contained in the event of a total melt down. The GE Mark I is perceived to be inadequate to maintain the potential corium that could develop in a large power reactor. I say "perceived" because it is not proven. Critics of the design use the typical words, could, maybe, possibly etc. without any real numbers to back up their points.

I am not particularly a fan of GE, mega scale reactors or boiling water reactors. I can do math fairly well and I also can compare real examples from the past to possibilities in the future. Like TMI for example.

From Wikipedia, "Following corium relocation to the lower plenum, the potential exists for corium to breach the primary pressure boundary (in light water reactors, this is the reactor pressure vessel). What happens when the corium reaches the bottom of the reactor pressure vessel in a Western light water reactor is the subject of actual experience and considerable speculation, and depends on temperatures, the age of the fuel, the amount of activity the fuel has been exposed to, as well as the physical composition of the RPV, the dimensions of the RPV, the pressure of the primary coolant system (whether or not pressurized) and numerous other considerations. It is not likely for the corium to remain critical in the bottom of the RPV unless - first - the corium is quenched by a large excess of coolant water and turned back into solid phase, allowing the interposition of a water moderator and the formation of a critical geometry - second - after the quench of the corium, there remains sufficient unborated water in the lower plenum to moderate the reaction and support criticality - third - the corium remains unadulterated with a neutron-absorptive alloy or substance from the melt of the control rods, such as boron carbide or cadmium." RPV is reactor pressure vessel. As you can see there are a lot of ifs. It cannot be completely ruled out. The statistical probability is very low however.

This situation occurred at TMI but ran into a few ifs. "It was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed,[9][37] with five feet of the core gone, and around 20 tons of uranium flowing to the bottom head of the pressure vessel, forming a mass of corium.[38] The reactor vessel, the second level of containment after the cladding, maintained integrity and contained the damaged fuel with nearly all of the radioactive isotopes in the core.[39]"

Note: there is one minor error in the Wikipedia quote. The RPV is actually the third level of containment, Cladding first, fuel composition second, Reactor pressure vessel third and containment building fourth. Really there is a fifth level of containment, the over burden of the containment building. I'll let people think about that final one.

So there is actual real world data to determine the ability of the RPV to contain corium with a 50% core melt. There is actual real world data available to determine the probability of the total complete melt and relocation of all corium to the base of the RPV. Many of the arguments now are based on suppositions posed prior to TMI. It is probably impossible to have the arguments limited to "real" possibilities with "real" probabilities. I can dream though.

My other main concern with the GE Mark I design is emergency response planning. First the pressure vessel should have pressure relieved as soon as a real loss of coolant accident (LOCA) was determined. The lower pressure increases the chance of adequate water flow from back up cooling systems to prevent fuel damage. TMI showed that vented steam posed a much lower public health risk than advertised. With that knowledge the Japanese operators should have been able to make a better decision at the onset. Sorry, that is just the facts. Trying to do too much does too much damage. At Fukuhima the limited availability of cooling water caused more damage than no cooling water. Quenching the hot reactor, then having it heat up again, then quenching the again hot reactor causes damage to both the core and containment components. The mysterious cause of the radioactive core water leak will be found to be piping connections weakened by repeated heating and quenching. No, I am not clairvoyant, it is basic metallurgy. The piping connections are the weakest point.

So once again the debate will lead to less nuclear power of any kind or worse, more pressure on operators to not vent for irrational fear reasons that will lead to more damage and potential fall out.

It will be about five years until all the results are in, check back then.

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