Hacker News Reader

Comment by 542458 | Hacker News Reader

Re: Indian Point - containment buildings are pretty armoured by design. The Indian point containment vessel walls are several feet of high-grade steel reinforced concrete. You might be thinking of the skinny, low quality rebar used in common construction at this point - this isn’t that, the rebar here is 2.25 inches thick, made of good-quality steel, and spaced every fifteen inches or so. The containment building is also smaller than a 747, so you can’t transfer the entire plane’s energy to it like you could with a larger target like the towers.

The Indian Point engineers assert that theirs reactors and all other structures contains radioactive material could survive a 9/11 style impact.

http://webarchive.loc.gov/all/20130108221220/http://www.nei....

View on HN

When the towers were first built, one of the design questions was whether or not they could survive being hit by a 747 — on accident, presumably. So, that issue was taken into consideration.

At the time of the attack, more modern aircraft were used, which had much thinner skins and were much more lightly constructed. Which means that they got much more shredded by the building construction as they passed through, and which also helped take off a lot more of the fire insulation from the trusses in the building.

The 747s simulated were also near-empty and flying slow, as the assumption was an accidental collision from an airplane lost in fog trying to land. They never simulated one nearly full of fuel running full-tilt.

Interesting. Do you have a reference by chance? I had no idea such simulations were run but in curious to learn more about it.

https://youtu.be/4eE8d94qGPo?t=222

And they sere simulations.

Reality bites

The two towers were designed to sustain an accidental hit by a 707 on a descent trajectory.

They were instead purposely hit by much larger 747s and at a much higher cruising speed. This meant that the kinetic energy transfer was approximately 8x larger than expected. The 747s at the time of impact also held significantly more fuel (at least 100% more) than those utilised in the 707 attack model.

they were not hit by 747s

Correct. Sorry, I had originally typed 767 and then in an edit swapped as everyone else said 747 and I thought perhaps I was having some sort of Mandela Effect going on. The size and speed differential holds true - a cruising speed extended-range 767 is much faster and larger than the equivalent, landing-approach 707 the building was “rated” for.

The twin tower planes were: 767-200 767-223ER

>When the towers were first built, one of the design questions was whether or not they could survive being hit by a 747

no, it was a 707 https://www.youtube.com/watch?v=4eE8d94qGPo&t=232s

You'd be more likely to damage enough of the cooling and control systems to cause an issue but it most likely wouldn't lead to a meltdown since Indian Point was a PWR reactor and those use the liquid water as a moderator so if it starts to overheat it naturally lowers it's own reactivity.

Of course it's a little more complicated than that, because decay heat is the concern in a loss of cooling scenario. Having said that, I can't speak to the Indian Point design at all.

Yeah it will but at least that's also a negative feedback too as the fuel heats up the neutrons appear to be moving faster on the average.

My dad told me that when he studied construction engineering in the late 70s at TU Munich one of their assignments was to calculate if the Isar nuclear reactors are built strong enough to hold off a Mach 2.2 impact of a Phantom II. If I recall correctly the correct answer was that they were quite a bit stronger than necessary for that.

However, I always wondered if a large civilian plane like a 747 with a speed close to Mach 1 creates more or less impact. The 747 is ~12 times heavier, but it's also less than half the speed and spreads the impact across a much larger area.

This video shows an F-4 Phantom being slammed into a concrete wall using a rocket sled: https://www.youtube.com/watch?v=U4wDqSnBJ-k

At about 2 minutes in they show the aftermath. The plane is just gone, while the wall is a bit discolored...

Unconvinced.

That concrete block was not ridged. It moved when the plane hit it.

So the strongest aspect of concrete, compression, was tested. But the weakest, flexion, was untested.

What fun though.

To test it as you suggest, you'd have to build the full containment building, which are built as domes. That dome structure should lend itself to exploiting concrete's compressive strength and minimize tensile forces. They are also specifically reinforced to increase their tensile strength, since they are primarily designed to contain explosions from within. I have little doubt that one of those domes could shrug off a Phantom crashing into them.

More generally, in order to be successful a traditional armor piercing projectile needs two traits in particular: it needs to be harder than the target, and the projectile needs to be robust enough to not disintegrate on impact. Airplanes are neither of these; they're built out of soft aluminum and are built light to maximize payload, not to be robust. Consequently I would not expect an airplane striking a reinforced concrete building to ever behave as an armor penetrating projectile would.

The part that I'm a bit skeptical of is the suggestion that shrugging off a Phantom is equivalent to shrugging off a 747, which is more than an order of magnitude heavier. Throwing an airplane at a concrete building is like throwing a wet ball of modeling clay at a glass window. Likely to splat, like that phantom did, but if your ball of clay is big enough it might go through. A large enough plane might even knock the building off its foundation, rather than penetrate the concrete.

> The containment building is also smaller than a 747, so you can’t transfer the entire plane’s energy to it like you could with a larger target like the towers.

I know very little about physics and found this to be an unexpected point I’d like to understand more. Could you explain?

The plane isn’t perfectly rigid, so it would only be able to impart as much energy to the structure as contained in the part of the plane that gets sheared off while colliding with the containment building.

As a layman there are probably few theories behind this statement: 1. The plane is not an energy monolith, any part of the plane larger than the building (eg wings) would have its own inertia which continues to rip off and around the building rather than drive all energy/mass attached to plane to the point of impact. 2. A smaller building may act like a plow if circular or hit on a corner, diverting energy as in 1. 3. If you simplify the side of a building to a supported plane, a larger plane may deform to failure more than a small plane due to the distance between supports ( other walls of building)

My read is that the sensitive area of the reactor is a lot smaller than the plane, so only part of the energy of the plane would hit it, the rest would go into the ground around it or whatever else the plane hits.

I’m no expert at physics but the towers were more “collapsible” so they would absorb more of the momentum from the planes.

I can imagine the reactor shell itself surviving intact, but as with Fukushima, loss of the secondary cooling system (and its power supply) could well have resulted in a runaway reactor situation followed by a hydrogen explosion blowing the cap off, again as with Fukushima.

It might not have impacted NYC though, I just checked and the exclusion zones would likely have been well outside NYC city limits, but still, an epic disaster would have been the likely outcome.

Nuclear reactor security has been increased since then, but they're a major target today.

Reply on news.ycombinator.com