There's no doubt that the Fukushima catastrophe has decimated public support for nuclear power.
NuScale RPV and containment vessel
Suppose you are a proponent of large-scale nuclear expansion, and you are arguing your case with someone who has become opposed to it after the Fukushima disaster. They cite Fukushima as an example of -- if not danger, the unreliability, and potential for economic devestation, of nuclear power plants. Setting aside the issue of operating Gen-2 reactors, what's your defense of new construction -- for building Gen-3.5, Gen-4 NPPs? Granted, human engineering is never perfect; but how would you make a convincing case that nuclear power can become more reliable than the engineering of 50 years ago?
I think the first place to start is with decay heat removal. Fukushima's failure to do this under extreme conditions directly caused much of the disaster: the core drying up and uncovering fuel, causing it to overheat; the high temperatures allowing zirconium oxidation, leading to hydrogen explosions breaching containment; the overheated fuel melting, releasing fission products; the melted fuel (probably) boring through reactor pressure vessels, resulting (again) in radiological release.
So then: how fundamentally can you "eliminate" the design limitations involving decay heat removal? How far can you go towards making heat removal passive and intrinsically safe?
I'm not an engineer, but I'd like to provoke debate by suggesting one solution. It's based on very basic physics, and there's already a Gen-3.5 design that exemplifies it.
The concept is to put small reactors in very large, atmospheric-pressure water pools. Large enough that they can reject decay heat indefinitely by boiling away into the atmosphere. At least one small/modular reactor (SMR), NuScale, seems designed to do this:
The NuScale design has twelve 150 MWt/45 MWe reactors, each in their own steel containments, submerged in a common 4 million gallon (15,000 m3) pool. With about 20 megawatt-days of decay heat per reactor over the first 30 days (see above), or 240 MWt-days combined, the pool is more than enough to absorb all of it through evaporative cooling. (It would take at least 396 MWt-days to evaporate all of it, just by multiplying by the heat of vaporization).
This is described by NuScale as a "30-day supply" of cooling water. By their reasoning, this is all that is needed: air cooling is sufficient afterwards, when each core is down to just <400 kW of decay heat.
So, this design's emergency heat rejection seemingly does not depend on
- External water supply (Fukushima: lost seawater pumps)
- External AC power (Fukushima: lost power lines, diesel generators)
- Battery power (Fukushima: batteries did not last long enough)
- Safety-critical piping (Fukushima: possibly damaged by earthquakes)
On that last point: NuScale argues their design eliminates "large break" loss-of-coolant accidents, since the reactors are completely submerged in a pool. The core can always be flooded simply by opening a valve in the containment.
Should this type of SMR be the nuclear industry's response to Fukushima? What do you think?NuScale plant layout: