Fast Neutron Reactor and present-day weapons

[Enthalpy]

Several companies or administrations develop fast neutron reactors presently, one project being Astrid by France's CEA... Well, these reactors are perfect targets for present-day weapons.

 

Take for instance a 20t single-stage solid rocket that fits on a banal truck - simpler than a V2. It can propel a 2.5t passive steel head out of the atmosphere to 2,500km range where the head falls down at 5km/s. This pierces some 5m steel or 10m concrete - thicker than any present or future reactor can have. The recent Chinese anti-airplane-carrier-missile may be of this type; anyway, it punches a ship that is better armoured than a nuclear reactor.

 

Falling on a reactor, the head punches it from top to bottom, letting the accumulated radioactivity escape from the reactor as at Chernobyl or Fukushima. The core's deformation due to the impact may or may not provoque a significant "power excursion" (uncontrolled chain reaction) that contributes to spread the radioactive debris.

 

One refinement at fast neutron reactors is that hot sodium would be exposed to air and catch fire, dispersing the radioactive elements.

One other refinement is that fast neutron reactors' reactivity increases when they lose the coolant, leading to an uncontrolled chain reaction - as opposed to water-cooled reactors - and a pair of battletank's kinetic energy penetrator suffice to punch the containment vessels.

[John Cuthber]

If it's simpler than a V2 then nobody knows where it will land.

 

What sort of nuclear reactor is not a great target?

 

One thing that would disrupt the chain reaction would be to drop a few tonnes of steel into it.

You wouldn't be talking about "deformation" but vapourisation. The structures needed to maintain criticality would be lost.

 

And, odd as it may seem, this sort of thing has been thought about.

Nobody discusses the countermeasures.

[InigoMontoya]

The recent Chinese anti-airplane-carrier-missile may be of this type; anyway, it punches a ship that is better armoured than a nuclear reactor.

 

I highly doubt the missile in question pushes an inert payload. First off, carriers aren't particularly hard targets requiring such penetrating capabilities from a missile. Secondly, merely punching a hole through the hull of any modern warship won't sink it. That's doubly true for a large ship such as a carrier.

[Enthalpy]

If it's simpler than a V2 then nobody knows where it will land.

 

What sort of nuclear reactor is not a great target?

 

One thing that would disrupt the chain reaction would be to drop a few tonnes of steel into it.

You wouldn't be talking about "deformation" but vapourisation. The structures needed to maintain criticality would be lost.

 

And, odd as it may seem, this sort of thing has been thought about.

Nobody discusses the countermeasures.

 

You're right, propulsion is simpler, guidance isn't.

 

The passive head would go through the reactor, not stay in it. At the fissile material, the optional moderator, and other stuff, it creates a huge shock wave that increases the density and changes the distribution of varied materials, which may increase the reactivity. For instance a fast neutron reactor gets more reactive if it loses its sodium - by an amount that puts it in prompt supercriticality.

 

Countermeasures... Several tons of solid steel at 5km/s are difficult to deviate, and the launch can be from any banal truck or boat within 2,500km. Plus, as the rocket is much cheaper than the target, many can be launched at the same time. And as 2.5t aren't necessary, the enemy can MIRV the head or lauch many rockets per truck.

 

I highly doubt the missile in question pushes an inert payload. First off, carriers aren't particularly hard targets requiring such penetrating capabilities from a missile. Secondly, merely punching a hole through the hull of any modern warship won't sink it. That's doubly true for a large ship such as a carrier.

 

Common figures for the deck are 0.5m to 1m steel - harder than a nuclear power plant, but not needing 2.5t at 5km/s; consistently, the Chinese missile isn't as big.

 

The missile falling at 45° or steeper would punch a hole at the bottom as well. A wide one there, since the deck produces shrapnel from itself and the impactor. And in between, the shock wave and shrapnel destroys and ignites the planes, fuel depots, munitions... I expect a total damage.

 

But the design details of the Chinese missile aren't public, sure.

[swansont]

The passive head would go through the reactor, not stay in it. At the fissile material, the optional moderator, and other stuff, it creates a huge shock wave that increases the density and changes the distribution of varied materials, which may increase the reactivity. For instance a fast neutron reactor gets more reactive if it loses its sodium - by an amount that puts it in prompt supercriticality.

 

How is the core going to lose sodium without the core geometry being changed by the impact?

[Enthalpy]

How is the core going to lose sodium without the core geometry being changed by the impact?

 

Geometry does change! What makes fast neutron reactors worse is that their working shape, composition, assembly... is NOT the one that produces the maximum reactivity. At least, water-cooled reactors lose reactivity when their water goes away.

 

For instance a hole in the vessel by a battletank's penetrator lets the sodium leak away, and the remaining fuel has more reactivity - enough to be supercritical without the retarded neutrons.

 

Or the shock wave of a penetrator passing through can compress all core materials into a hollow cylindre at some time, where the reactivity near the periphery is greater than at the original uncompressed full cylindre.

[InigoMontoya]

Common figures for the deck are 0.5m to 1m steel - harder than a nuclear power plant, but not needing 2.5t at 5km/s; consistently, the Chinese missile isn't as big.

A 1.5' x 1000' x 150' slab of steel weighs in at about 55,000 tons. You're telling me that on the *conservative* end of the spectrum that the deck accounts for over 50% of the mass of the ship? How does such a ship stay right-side up?

 

Don't get me wrong, decks are armored, but not like that. Not against heavy/hard penetrators.

 

The missile falling at 45° or steeper would punch a hole at the bottom as well. A wide one there, since the deck produces shrapnel from itself and the impactor. And in between, the shock wave and shrapnel destroys and ignites the planes, fuel depots, munitions... I expect a total damage.

Agree to disagree.

 

Agree that it would punch a hole in the bottom as well (thus my prior comments in this thread). Disagree on total damage.

[swansont]

Geometry does change! What makes fast neutron reactors worse is that their working shape, composition, assembly... is NOT the one that produces the maximum reactivity. At least, water-cooled reactors lose reactivity when their water goes away.

 

For instance a hole in the vessel by a battletank's penetrator lets the sodium leak away, and the remaining fuel has more reactivity - enough to be supercritical without the retarded neutrons.

 

Or the shock wave of a penetrator passing through can compress all core materials into a hollow cylindre at some time, where the reactivity near the periphery is greater than at the original uncompressed full cylindre.

 

But you're positing that the projectile has enough energy to penetrate the containment, but will not damage the core at all, preserving its ability to be supercritical. How does that happen? Your scenario is cracking an egg with a hammer and not making a mess.

[Enthalpy]

A 1.5' x 1000' x 150' slab of steel weighs in at about 55,000 tons.

You're right, this armoured deck thickness is impossible.

So a carrier-punching weapon would not necessarily suffice against a nuclear reactor.

But an adequately designed weapon would, and its size is easy.

 

Agree that it would punch a hole in the bottom as well (thus my prior comments in this thread). Disagree on total damage.

The size of the hole at the bottom depends essentially on the distance from the first impact, that is from the height of the ship, as soon as the impactor disintegrates at the deck.

 

An impactor just big enough for the deck, which would spread out for sure, would dissipate its impact too widely at the hull to punch it, while an oversized impactor would stay concentrated and punch a "small" survivable hole in the hull.

 

But then, the impactor can be designed to split during the impact at the deck, just as some forbidden bullets do.

[John Cuthber]

Can someone check my maths here?

A kilo of stuff travelling at 5000 m/sec has 12.5 MJ of kinetic energy.

On impact, most of that energy will be converted to heat.

 

Iron has a heat capacity of 25J/mol/k

12.5MJ of heat will raise the temperature of a kilo of iron (about 17 moles) by about 30,000 degrees (I'm ignoring the phase changes here for simplicity).

This vapourised metal will spread out a bit.

Consider the ideal gas laws as a ballpark estimate of the volume of 17 moles of gas at roughly 100 times room temperature which gives about 40 cubic metres.

Now, consider 2.5 tonnes of metal at that speed.

The energy is greater in proportion to the mass, so the temperature change will be about the same.

The volume will be greater because there's simply more stuff.

 

Something like 100,000 cubic metres of ultra hot gas (ok it cools as it expands and mixes with the air.)

 

If I'm right ( or even close to right) this isn't a subtle " punch a nice clean hole in things" weapon.

It would, by simply "distributing the fissile material over a wide area", rather rapidly shut down any criticallity in a reactor.

It would obviously make a huge mess but the one thing you wouldn't need to worry about was an uncontrolled critical mass.

Edited July 9, 2012 by John Cuthber

[Enthalpy]

...preserving its ability to be supercritical.... not making a mess.

I didn't tell for how long, my fault. The scenario I imagine happens during the very few milliseconds after the reactor is punched through, when its constituents (fuel, cooler, cladding, control rods...) have been repelled from the impactor rocket's trajectory and move outwards in a compression wave.

 

I've tried to figure out if the coolant would separate from the fuel during this phase, and estimate presently they should not much. But both water and sodium are seriously compressible as the 5km/s impactor pushes them to the sides at, say, 1km/s: this would induce over 3GPa in pure water, which nearly equals its bulk compression modulus. Coolant and the contained fuel would, at the shock wave, be squeezed to a higher mean density hence increase their reactivity - from a previous situation of 1.000 reactivity, and where >0.7% increase results in prompt supercriticality that doesn't need the delayed neutrons. Hence the few milliseconds are a serious worry.

 

A clear assessment would require more means than I have, but if you think at peripheral fuel, the neutrons it emits tangentially are more probably absorbed as the compression wave arrives there than during normal operation, while the neutrons flying towards the centre still meet as much fuel. Hence I imagine a reactivity surge even with this outwards compression, not only inwards as in a plutonium bomb. And anyway, the impact can be much off-axis, in which case the compression is inwards even if not symmetrical.

 

In the case of a battletank penetrator, it is accurate and might target the fast neutron reactor so as to leak quickly the sodium but keep the fuel debris within the vessel, producing for sure a huge criticality accident where the fuel vaporizes. At a target tank, these penetrators kill the crew with shrapnel but don't explode the hull; the turret flies away when the target's ammunition detonates.

 

It doesn't even need to leak the sodium: shaking the fuel to collapse at the vessel's bottom is enough, since there, it's separated from the absorbing sodium. Worse: at fast neutron reactors, neutrons escaping the core are essential to the total reactivity, so fuel concentrated at the bottom is hugely more reactive.

 

Remember Fukushima? Fuel pellets have sunk to the vessel's bottom at several reactors, but these get less reactive when they lack water (within some limits...). The same accident or intentional damage with fast neutron fuel would have triggered a fully blown (if I dare to write) criticality accident, with all the fuel vaporized in the atmosphere.

 

A kilo of stuff travelling at 5000 m/sec has 12.5 MJ of kinetic energy.

On impact, most of that energy will be converted to heat. [snip snip snip]

We wrote our posts at the same time...

 

Rockets do achieve kinetic energies per kilogram similar to the heat contents of a flame, yes. That's why they can access space. Such an energy density exists alo at meteoroids for instance, which can also get hot. Rockets achieve this by being made essentially of propellants.

 

Converted to heat only where some process achieves it, and not necessarily at the beginning of the target. Here an (oversized) 2.5t penetrator would lose little of its energy at the reactor, but rather in the very deep soil. In fact, battletank penetrators do punch clean holes through less armoured targets - losing less energy in the intermediate room, though it's enough to damage weaker hardware and personnel there.

 

For sure, the penetrator going through a core will disperse it because the core is liquid+solid, but as I wrote at the same time as you did, I believe a huge criticality accident can happen while the compression wave propagates through the core in very few milliseconds.

[John Cuthber]

Not my field, but all the reactor designs I have seen put the scram rods on the top.

The first thing that the bomb would do is push them into the reactor.

 

If the reactor were just sub- critical before impact then it might go critical briefly when hit.

So?

Reactors are built to go critical for years not just milliseconds.

[swansont]

Reactors are built to go critical for years not just milliseconds.

For prompt supercriticality, milliseconds is enough to be a problem. The characteristic time it take for the prompt neutrons from one fission to cause another is a few tens of microseconds, so a few milliseconds can cause a rather large power spike. One thing that isn't clear to me is that the core damage from such an event is any big deal, given that in this scenario the core has been hit with a large projectile.

 

Remember Fukushima? Fuel pellets have sunk to the vessel's bottom at several reactors, but these get less reactive when they lack water (within some limits...). The same accident or intentional damage with fast neutron fuel would have triggered a fully blown (if I dare to write) criticality accident, with all the fuel vaporized in the atmosphere.

 

I would expect that fuel pellets at the bottom are hugely less reactive because of the change in geometry of the system.

[John Cuthber]

A brief criticality incident will produce a lot of energy but the core of a power station is big and heavy It can absorb a lot of energy by simply getting warmer. Since it's in the process of hit by 2500kg of hypersonic metal it hardly matters if it is molten or even boiling when that happens.

The weapon only carries about the energy of a few kilos of TNT.(EDIT I meant a few kilos of TNT per kilo: about 10 tonnes of TNT in total)

 

As far as I can tell we are engaging in pure speculation about whether the effect of any criticality would be trivial compared to the mess made by spreading the core of a power station about or whether it would be comparable with a nuclear bomb's criticality. I rather doubt that anyone here has the background to calculate that actual energy released by any prompt criticality in the millisecond or so before the reaction blows itself apart.

 

Form a practical point of view I think they would both make a city sized area uninhabitable, though in one case it would also be demolished with the attendant immediate loss of life.

The best you could hope for would look like Chernobyl and the worst would look like Hiroshima.

 

The comments made about compressibility seem to me to be roughly as true for any sort of nuclear reactor.

If you could drop 2 tons of rock from space and get it to hit a nuclear plant then you would make life very difficult (or brief) for the people living nearby. Whether it was a new reactor or an old one wouldn't matter much.

 

BTW,re.

"Rockets do achieve kinetic energies per kilogram similar to the heat contents of a flame, yes. "

if you know any flames that run to 30,000 degrees, could you let the rest of us know about them?

Edited July 11, 2012 by John Cuthber

[swansont]

Given that the premise is that this is something that might happen if you hit it with a "battletank weapon" or missile, that presupposes a fairly substantial war, and all the damage that goes with it. I don't see this as a serious disincentive to construction. You could say the same of dams and probably other infrastructure.

[Enthalpy]

The first thing that the bomb would do is push [the scram rods] into the reactor.

 

A 2.5t impactor is small, for instance a cone of D=0.5 and L=5m. The reactor's parts not touched directly by the impactor would be pushed to the sides, at a serious speed, for instance 1000m/s.

 

Found data for liquid sodium at +145°C : 917kg/m3 and 2500m/s sound velocity, so a shock wave of 1000m/s would induce at least 2.3GPa and the bulk modulus at 1b is 5.7GPa - just to give a sense of the density increase. Water near its critical point is highly compressible as well.

 

The Mark-1 has its control rods at the bottom, a criticized design.

 

...One thing that isn't clear to me is that the core damage from [prompt supercriticality] is any big deal, given that in this scenario the core has been hit with a large projectile.

I feel the power spike is an added worry. It wouldn't produce much more radioactivity than normal operation has acumulated, but since only the core's vaporization ends the supercriticality, it would spread elements that other accidents keep solid hence disperse little, for instance plutonium.

 

I would expect that fuel pellets at the bottom are hugely less reactive because of the change in geometry of the system.

At Fukushima yes, because the thermal neutron reactors (almost) "need" their moderating water to sustain a chain reaction.

At a fast-neutron reactor, the coolant is a hindrance to the reaction. Losing it, and much worse, concentrating the fuel at the bottom of the vessel increases the reactivity a lot. Neutron losses at the core's surface limits much the reactivity of a fast-neutron reactor.

Hence I maintain: the same situation, with fuel fallen down in the vessel, would be much worse with a FNR - in fact, it couldn't even reach that point.

 

A brief criticality incident will produce a lot of energy but the core [...] can absorb a lot of energy by simply getting warmer. Since it's in the process of being hit by 2500kg of hypersonic metal it hardly matters if it is molten or even boiling when that happens.

 

The mechanical impact of 30GJ (a bit less energy than the 20t of propellants) melts a part of the core and the debris with a high melting point stay in the vicinity. As opposed, prompt criticality (typical time constant 100ns) induces a huge temperature (a nuclear one, not chemical) before various kinds of pressure disperse the reacting components. This means that all elements, including refractory ones like plutonium, are widely dispersed. Without the prompt criticality, things like iodine and caesium fly away.

 

The comments made about compressibility seem to me to be roughly as true for any sort of nuclear reactor. [...] Whether it was a new reactor or an old one wouldn't matter much.

Fast neutrons reactors lose many neutrons at their surface, thermal neutrons reactors few. So density is more important to FNR.

And in the case of a battletank impactor, loss of coolant stops the water-cooled reactor but brings the sodium-cooled one into prompt criticality.

 

30,000 degrees: hey, I wrote "similar". This figure isn't accurate neither.

 

You could say the same [the damage that goes with a substantial war] of dams and probably other infrastructure.

 

Take the extreme case of France with >50 reactors: bursting them would make the country about unusable. In constrast, dam bursts would make permanent total damage to very few cities. Paris for instance would be partly evacuated for some days after a dam break, then cleaned and inhabitated back.

 

I really dislike the deterrent-type threat by weapons not very difficult nor expensive to build, whose launchers can be banal and located quite far.

Edited July 11, 2012 by Enthalpy

[John Cuthber]

"This means that all elements, including refractory ones like plutonium,"

Plutonium is flammable so it tends to get dispersed if it was the metal before the impact.

Also, everything is volatile at the sort of temperatures produced by this impact, whether things go critical or not.

 

In effect it's a damnably effective dirty bomb.

[Enthalpy]

Obviously, French politicians are unable to understand the unreasonable risk, or unwilling to resist some obscure appeal.

 

Ségolène Royal, minister for environment and energy, declared yesterday that France needs to develop Astrid, a fast-neutron reactor. (Remember the industrial disaster of the EPR, still not working, whose penalties for late delivery already exceed the sale price. Remedy: develop Astrid in addition.)

 

A reactor in operation is "critical": its neutron flux is at equilibrium thanks to (1) natural retroaction that reduces the reactivity if the temperature increases (2) man-made hence slow retroaction, only fast enough to intervene on the "retarded" neutrons, which make <1% of the total with plutonium. If some external action increases the reactivity by 1%, means of human control are lost, and the natural retroaction has a limited efficiency too.

 

Worry: a fast neutron reactor lets a big proportion of neutrons escape the core; opacity to this flux is paramount to the core's reactivity, and it depends on the core's density. This is how a plutonium bomb is started, by implosion. This is also how a big kinetic impactor would detonate a fast neutron reactor: by compressing it. At 3km/s, the impactor would compress the plutonium/sodium suspension by 30% easily, and even separate the sodium and plutonium, so that the reactivity jumps by far more than 1%

 

How big the following nuclear explosion is remains unclear. The compression is damned quick, faster than in a bomb. 100 to 1000 time more plutonium is available than in a bomb. But the shape of a reactor core isn't optimized to detonate, nor is the impact at the worst possible location and angle, so I expect the nuclear explosion to take place but use only a part of the available plutonium.

 

Then, you can expect an enemy to add worsening materials in the projectile: a "booster". This doesn't even demand nuclear technology. Such a scenario looks really bad.

 

Angela has a PhD for nuclear physics, and Germany closed its fast-neutron reactors when I drew public attention on this risk. But French politicians have a degree for politics.