Enthalpy

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. 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.

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. 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.

The tsunami originated in Aceh was very destructive in India and Sri Lanka and easily observable without instruments in Africa.

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.

I seriously hope that both the Cooper pair and the BCS theories offer better explanations than the usual "pairs of fermions are bosons so they occupy the same state", which is an absolute nonsense. ONE pair of fermions can occupy a state and that's all. This is what happens in any molecule, where electrons are paired with opposite spins, and each orbital is full with a pair of electrons.

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. 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.

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.

Because of RoHS law, we must use a **** solder that doesn't solder anything, despite nobody will churn an electronic card... But mercury is used increasingly in power-saving lamps, and you'll inhale it if you break the lamp. Semiconductor industry... uses really nasty chemicals, HF not being the worst! Not even hydrazine is the worst there.

Chernobyl killed provably many dozens of people, made a province unusable for several centuries... If you include deaths due to low dosis, it's thousands more - that's why nuclear propagandists spread nonsense as a "minimal dose" for cancer. Compare that with a worst case scenario of wind or Solar power. About positive reactivity coefficient, you should have a look at the "Fouth generation reactors"... The ones cooled with sodium. As opposed to water cooling, loosing the sodium increases the reactivity. Some reactors have a confinement, some have none, in Russia (the VVER) as in the west (the research reactor in the middle of the city of Grenoble). Shall I remind you that the Fukushima reactors, whose confinement failed each and every time, were of Mark-1 type, designed in the US by GE? Did you notice you're on a science forum? Please keep your standard arguments for the kindergarten public.

The beat sensors clipped to patients' ear or finger observe the flesh optical transparency change as blood flows in surges. One more possibility, at least at the ear or the finger of a non-mobile person.

A handful of quarks, combined according to simple rules, explains hundreds of particles, once you tell these are composed of quarks. The interactions of composite particles follow "simple" conservation rules if described at the quark level. Desriptions at the quark level have predicted composite particles that were later observed, including excited states of these compositions. This is what one calls a successful model.

They do attract. Writing it as an energy rather than a force simply fits better the wave equation. Protons and electrons don't annihilate because they can't: this would not keep the baryonic number. (Less old formulations would put other numbers here). Protons annihilate with antiprotons, electrons with positrons, that's it. No repulsion, and in fact, spherical orbitals (1s, 2s, 3s...) have their maximum density of probability (per volume unit, not per radius unit) right at the nucleus. But non-spherical ones have a zero density there. Now, atoms have a diameter because electrons, as any particle, are waves. An electron crammed into a smaller volume means a shorter wave which has more kinetic energy. You can consider orbitals as an minimization of the electron's energy: electrostatic (=nucleus attraction) plus kinetic (=electron confinement). Feynman makes a short calculation this way in this (recommended) course and gets a reasonable atom diameter. This is the most obvious success of quantum mechanics: it explains why matter has a volume. They aren't. The fixed energy levels correspond only to stationary solutions, where the amplitude of the orbital does not evolve over time. So to say, these orbitals don't "vibrate" and accordingly don't radiate light - what a vibrating electron would do. This is one other early success of quantum mechanics: it explains why electrons in a atom don't radiate. Interestingly, these solutions which don't radiate are the ones where the electron's energy is constant: it's consistent. Now, if an electron bound to an atom is absorbing or emitting light for instance, its wave function is a combination of several stationary solutions. This combination is not stationary but does vibrate, at (or near) the frequency of the light. The proportion of the stationary solutions in the combination evolves over time, more or less quickly depending on the light's intensity. Here quantum mechanics is not so abstract. The quantization of orbital momentum is easier to understand. It corresponds to the number of 360° phase turns the wave makes in a 360° geometric turn. Since the wave function is identical after a geometric turn, the phase of the wave function makes an integer number of turns, and the orbitam momentum is integer. (Spin cannot be understood that way) I ignore it, and so do many people. For sure, neutrons and protons can transform in an other by emitting an electron (beta minus radioactivity), emitting a positron (beta plus radioactivity), or absorbing an electron (electron capture). So if a nucleus has too many protons or neutrons, radioactivity will correct this in order to put the baryons in a favourable number, which is nearly as many protons as neutrons for small nuclei. BUT. Nothing would oppose a di-neutron or even a di-proton from the little I've read, and these are not observed, though we have huge amounts of free neutrons in uranium reactors. Experimentators seek them in vane. Apart that the neutron itself is radioactive (quarter hour life) a di-neutron would put both nucleons in the ground state as efficiently as a deuterium nucleus. Could it be that only neutrons and protons attract an other, but not neutron and neutron, nor proton and proton? This is NOT standard theory; on the other hand, could it be that these interactions are not known with enough detail? No idea.

There is no physical limit to long waves, but it may take time to observe one period. On Earth, we produce >>kilometric waves (LW begin at 2km), though historic uses (Loran...) disappeared, leaving only submarines to use them. The longest wave propagating around Earth fits its half-wave between the ground and the ionosphere, about 50km, equivalent to 7kHz - the legal lower limit of "radio waves". These are monitored to count thunderstorms for instance. Longer waves, with approximately half a wave in one Earth diameter, correspond to a resonance of our planet. Thunderstorm monitoring as well. No more a propagation, it's a standing wave. Earthquake scientists measure E and B fields slower than that, but these are standing fields. Same with cave explorers who communicate through the terrain by injecting a current in it. You might try to measure or even produce even longer waves... But: - Antennas being so much shorter than the wavelength will be very inefficent - I'd say too bad for any observation; - Between any location and the antipode on Earth, your transmission will be a near field. -------------------------- Electromagnetic waves are energy. They're able to create a pair of charge carriers in the CCD retina of a telescope though the star that created this light has disappeared for billions of years. Just like any electric or magnetic field field contains energy, which can be stored in vacuum without any material insulator. In a propagating wave, this energy travels away from the antenna, atom or anything that created it.

You should put some figures on it. As far from our Sun as Earth is, the incident pressure is 4.5µPa, plus some reflected pressure depending on the design. Polyester and polyimide weigh around 1300kg/m3, and film 25µm thin is a shelf product. Then, putting a spacecraft on a Solar polar orbit for instance takes much shorter with a sail than going first to a Jupiter flyby to achieve the huge delta-V needed, and your craft is on a more interesting orbit. Same if going to Mercury. That's why a Japanese craft tests solar sails presently. By the way, if you desire a thinner film (25µm is still easy to handle) I describe at Saposjoint.net a process for the controlled thinning of a standard film. This should improve Solar sails. -------------------------------- A spacecraft emitting light (by laser or any means) is an old idea. It's impractical for many reasons, essentially because it needs a huge power to produce any thrust. chemical energy would be better used in a chemical rocket; nuclear energy (radioactivity or fission) would be better used to expell the reaction fragments or to heat hydrogen; Solar energy is better used in a Solar sail of the same area. Some people consider a laser somewhere (Earth orbit, Moon surface...) whose light pushes the sail of a spacecraft. I suppose we don't achieve light more concentrated than Solar light at any significant distance.

Nickel itself isn't the worst. Inconel contains 20...24% of Chromium to make it corrosion-resistent through a CrO2 layer.

Replying to my: "The centre of gravity has no relationship with the engine's thrust in stability, because the thrust is axial, not vertical." You believe what you want... Give a thought at how big the moment of two coaxial forces is, and whether it depends on how far apart the coaxial forces are. By the way, the British admirals had refused rear screws for boats because this, they said, would have made boats unstable - despite the invention had been demonstrated in front of them. Next, they lost the US independence war.

Or a complete neutral cell, yes: better than a pair or ions, for being more general. You could compute an equivalent of a cell in terms of dipole and tetrapole moments, which would tell you from which distance all the cells can be neglected, since these multipolar moments give a finite sum when integrating to infinite distance. Taking higher-order multipolear mometns would also reduce the number of cells needed to achieve a given accuracy.

Mamma mia! The centre of gravity has no relationship with the engine's thrust in stability, because the thrust is axial, not vertical.

I don't expect cracks to appear deep in a rail during its service. The big stress is at the contact with the wheel, at the surface. If a rail has a void from its manufacture, this has to be detected at the production site, where ultrasound is an excellent technique indeed. Here I concentrated on wear mechanism for rails in service, and I ilke the sensors to work on a train at commercial speed. I considered sound for this purpose, but it looks difficult. The train itself creates huge noise in the rail, especially at rail joints, and the rails produce strong echoes. This would rather need to send a pedestrian team when the traffic is stopped, meaning a survey once in many years, as compared with permanent monitoring made possible if regular trains are equipped.

This other proposal illuminates the rail at an angle that lets the reflected light escape if the rail is sound. A crack scatters light to abnormal directions, which a detector (close to the source in this sketch) monitors: A laser diode for DVD burner is a candidate for the source, as it allows fast modulation and narrow wavelength filtering, both useful against stray light. The target position on the rail is preferably where the vehicle's weight induces bending and shear in the rail, widening possible cracks. Here as well, I'd put several such sensors to avoid unjustified detections. And again, software can use successive measurements of the received background signal and compute an interpolation as a reference to compare the received signal with. Rust could scatter light as well, so I expect this method to work only with rails used regularly. Light shall target the part of the rail cleaned by the wheels. Marc Schaefer, aka Enthalpy

I wonder how many people will design an instruction set in their career. Not to mention: a 16 bit machine with irregular instruction set. I'm just coming from the thread with 150ns pipeline machine and wonder: has this subforum fallen in a time warp? A worm hole? How do these people of the past have an internet access to discuss with us? This was technology when I was a student, and I have grey hair now.

Knowing that your teacher took 30 years to propagate such values from his studies to yours, how many students will go through his pipeline of inept data before he eventually retires? Presently, the time unit is PICOseconds. And engineering IS all about actual values. By the way, a register doesn't have "a delay". It has at least a setup time and a propagation time. And nothing tells you the actual clock frequency of this archaeotechnological machine - only a maximum frequency, if only you had sensible register data. ----- Just for fun: the Cray-1 (1976!) had a pipeline for its floating-point adder where one register was missing. This was the critical path for the clock frequency of the whole machine, and the designers found that the propagation time was precise, predictable and stable enough (and equal for rising and falling edges, thanks to ECL) that the intermediate data was stored in the propagation time of the intermediate gates. A bit like a machine gun fires new rounds before the previous ones hit the target. By sorting out the gates before assembly, it did work and gained >10% clock frequency. ¡Ole!

You should resist this too easy trend to make everything in simulation. Simulation is NOT reality and does NOT teach the real world. Schools use simulations to save money, but that's bad teaching. Some suppliers in case your school is a desert: eBay, Conrad, RS...

General computer science is the exact opposite of mathematics: mathematics makes proofs of assertions that need few hypothesis and work always; software observes that some program may work sometimes for unknown reasons under huge contingencies. Sadly for you, cryptography is THE part of software that needs some maths. Not very complicated: only finite fields and reasonable algebra.

I get the same figures as yours (except 10+13 molecules, not 10-13).