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# PT17.S2.Q21 - Nuclear Reactors

Core Member
edited December 2020 19 karma

Hi all,

I need some help with this question. i watched and read the explanations, and while i now understand how to eliminate the other choices, i still don't quite get why answer C is the necessary assumption. Here's how i broke down the passage --

Context: Some nuclear reactors are in geologically quiet region, which only has minor faults and lives far away from plates.
Premise: no minor fault in this region produces earthquake more than once in any 100000 year period. That means there is at least a 100000 year waiting period between earthquakes.
Conclusion: in this region, potential nuclear sites near a fault that produced an earthquake in living memory are least likely to be struck.

So i imagine that X is a nuclear site in a quiet region near a minor fault that produced an earthquake in living memory, say 100 years. From the premise, X will not be struck by another earthquake for at least 99900 years. That's pretty unlikely. But i need to compare it to other sites, so this is where the necessary assumptions come.

C: in this region, every potential nuclear site is near 1 or more minor fault.

The negation test means there is a nuclear site Y in this region that's not near any minor fault. But the passage says nothing about the frequency of earthquakes in areas without minor faults. For instance, if Y's area has earthquakes every year, even though there are no faults, X would be less likely to be struck than Y. But that means the conclusion still holds, failing the negation test.

Am i wrongly using the negation test for this question? Hopefully someone can explain my error here. Thanks a lot.

• Member
614 karma

Hello,

C is correct because the stimulus says that geologically quiet regions only contain minor faults. Since there are only minor faults in the region, it's reasonable to assume that building away from these faults would result in a decreased chance of experiencing an earthquake, since the stimulus saying that only minor faults exist in the region allows us to reasonably assume that the parts of the region that don't have minor faults would have an even lower chance of having an earthquake. If this is the case, then it wouldn't make sense for areas with minors faults that have recently had earthquakes to be the safest place to build a nuclear reactor unless there was no option other than to build a nuclear reactor on a minor fault.

• Alum Member Sage
edited December 2020 8198 karma

It's not that there couldn't be an earthquake off of a minor fault. It's that the criteria used for being least likely to be struck by an earthquake is that it is on a fault that has been hit already by an earthquake. It limits the argument to the set of just reactors in the zone that are on minor faults.

Sure there could be an earthquake anywhere including off a fault. But there could also not be... ever. So if some reactors are off any faults, they could potentially be even less likely to be struck by an earthquake than those on faults that already had earthquakes. Are they actually? We don't know and it doesn't matter. Just the fact that they could be makes C necessary.