Fusione e Fissione

ENGLISH

Ogni tanto trovo qualcuno che confonde  queste due parole alquanto simili, forse perché entrambe hanno a che fare con grandi quantità di energia ed entrambe possono essere usate per fabbricare bombe.

E’ un errore scusabile?

Questo documento ha un obiettivodi mera divulgazione: sono state fatte molto approssimazioni e esemplificazioni per renderlo comprensibile, conservadone la base scientifica.

Spero di aver messo in pratica la massima del prof. Feynman: “Se non riusciamo a spiegarlo a una matricola, significa che non l’abbiamo veramente capito”

Esprimendoli il più semplicemente possibile, i due principi di funzionamento sono questi:

  • FISSIONE: la divisione di una atomo grande in due atomi più piccoli. rilasciando energia
  • FUSIONE: l’unione di due piccoli atomi per formarne uno più grande, rilasciando energia

Esaminiamoli uno alla volta.

Fissione

Nella fissione lanciamo ad alta velocità dei neutroni verso un nucleo pesante, ad esempio Uranio-235; i neutroni – con il loro carico di energia cinetica – atterrano su un nucleo semisolido di Uranio già piuttosto tremolante, transformandolo brevemente nell’ancor più instabile Uranio-236 che si mette ad oscillare assumendo una forma bilobata; quando i due lobi raggiungono una distanza superiore alla distanza efficace della forza Nucleare (di cui parleremo dopo) si dividono in (di solito) due nuclei più leggeri, sparando fuori altri neutroni e un po’ di energia sotto forma di raggi gamma (*); i neutroni liberi colpiscono altri nuclei di U-235 ed il processo si ripete, dando vita alla cosiddetta “reazione a catena”.

Per immaginare questa situazione, considerate un elastico tenuto teso da una molletta. Applicando una piccola quantità di energia alla molletta (il neutrone), l’elastico viene liberato e riacquista la sua forma naturale, rilasciando una grande  quantità di energia elastica.

Questa reazione a catena diverge rapidamente: in una bomba A (come quella di Hiroshima) in modo esplosivo, mentre in un reattore nucleare viene inserito un “moderatore”, cioè una sostanza che assorbe una parte dei neutroni liberi rallentando la reazione per renderla controllabile.

In linea di principio potremmo dividere qualsiasi nucleo, ma in pratica l’energia liberata è maggiore di quella applicata solo per quelli più pesanti e questa differenza cresce col peso, al punto che la maggior parte degli elementi trans-uranici decadono radioattivamente in modo naturale in frazioni di secondo. L’Uranio è il più pesante nucleo che esista stabilmente in Natura (**).

Fusione

Il processo di fusione è l’esatto opposto: mettiamo due elementi leggeri l’uno vicino all’altro fino a farli cadere nel pozzo energetico della forza Nucleare, per comprendere il quale è necessaria una breve digressione sulla struttura interna dell’atomo.

Sappiamo dalla fisica elementare che l’atomo è fatto da un nucleo di protoni e neutroni con una nuvola di elettroni che gli gira attorno. Per avere un’idea delle dimensioni, se il nucleo fosse largo 1 metro, gli elettroni sarebbero a 100 km di distanza. I protoni hannn carica positiva, i neutroni non ne hanno nessuna e gli elettroni hanno carica negativa; dal momento che cariche opposte si attraggono, come mai gli elettroni non cadono sul nucleo?

Prendiamo due magneti: lasciati liberi, il polo positivo dell’uno si attaccherà al polo negativo dell’altro. Ora colleghiamoli con uno spago e facciamone muovere uno: a causa dello spago, esso non può che muoversi intorno all’altro e se la sua velocità è sufficiente, la sua inerzia impedirà che cada su quello che resta fermo. L’equilibrio tra inerzia ed attrazione elttrostatica rende l’atomo quello che è.

Ma il nucleo, invece? Tutte le particelle che contiene hanno carica positiva o nulla, non dovrebbero respingersi?

In effetti lo farebbero, se non fosse per un’altra strana forza chiamata forza Nucleare; le forze che sperimentiamo fisicamente sono quella elettromagnetica (attrattiva o repulsiva a secondo della carica) o la gravità (sempre attrattiva) ed entrambe diminuiscono col quadrato della distanza. La forza Nucleare invece è trascurabile a distanze superiori al diametro di un nucleo, fortemente attrattiva a quella distanza per poi diventare repulsiva a distanze ancora minori! (***).

Come risultato, i protoni ed i neutroni vengono tenuti insieme nel nucleo, ma non possono comprimersi di più.

Dunque, quando spingiamo due piccoli nuclei sempre più vicini, è come se spingessimo due palline da golf oltre il ciglio di una buca: ad un certo punto supereranno il ciglio e precipiteranno l’una contro l’altra in fondo alla buca.

L’energia necessaria per la spintarella è minore dell’energia rilasciata nello scontro fino al nucleo di Ferro-26 dopodiché diventa maggiore; anche se l’abbiamo chiamata “spintarella” però si tratta di una quantità energia molto grande: in pratica i nuclei devono avere un’altissima velocità, cioè trovarsi alla temperatura di almeno 100M °K (sei volte la temperatura al centro del Sole)(****).

Confinare un plasma a 100 milioni di gradi non è uno scherzo e richiede energie enormi: fino a che queste energie non sono molto inferiori alle energie rilasciate, nessun reattore a fusione serve a nulla e la ricerca si concentra su tecniche di confinamento che adottano soluzioni molto diverse.

Conclusione

Fissione e fusione liberano l’energia di legame dei nuclei atomici: questa energia è massima per il nucleo di Ferro-26, dunque si ottiene energia dividendo i nuclei più pesanti del Fe-26 e mettendo insieme quelli più leggeri. Naturalmente il rilascio è massimo agli estremi:

binding energy

La Natura preferisce la fusione perché l’idrogeno è più abbondante di qualsiasi altro elemento e la sua densità energetica è un po’ maggiore, soddisfancendo meglio la fame insaziabile di entropia dell’Universo: da un punto di vista umano, la fissione è più facile da ottenere, il suo combustibile è più scarso (anche se comunque abbondantissimo) ed i suoi sottoprodotti più inquinanti.

D’altra parte la fusione è più difficile da controllare, ma usa un carburante disponibile in quantità quasi illimitata ed i suoi sotto-prodotti sono meno pericolosi.

Fissione e fusione hanno densità energetiche simili: 1 kg di combustibile rilascia rispettivamente 20,000 e 24,000 MWh di energia, confrontato con i miseri 14-15 kWh ottenuti bruciando 1 kg di qualsiasi combustibile fossile.

Possiamo concludere che, se anche i processi ed i materiali sono molto diversi, il principio fisico è molto simile.


NOTE

(*) in questa descrizione semplificata non teniamo conto di neutrini e anti-neutrini

(**) in effetti il Torio-232 andrebbe anche meglio, ma dato che i suoi sottoprodotti non hanno applicazioni militari, negli anni ’40 gli fu preferito l’Uranio

(***) la forza Nucleare NON È una delle quattro forze fondamentali esistenti in Natura, ma piuttosto un complicatissimo effetto collaterale della loro azione

(****) il Sole riesce a cavarsela con una temperatura minore perché si aiuta con l’enorme pressione gravitazionale dovuta alla sua massa


(More posts on Nuclear Power)

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More nuclear questions

I am right in the middle of a blogging hiatus, immersed as I am in the herculean task of getting a start-up off the ground. For reasons of personal survival, I hope to be able to report soon.

In this situation, my blogging is purely reactive, as in the case of the questions received by my friend Marco, the author of a cool divulgative page “La Fisica che non ti aspetti” (=Unexpected Physics).

For those willing to understand completely the answers, it may be a good idea to first read this other post which is not specific to the BWR (boiling water) reactor technology used at Fukushima Dai-ichi

Q1. News reports said the cores of one or more reactors melted and contaminated the underlying aquifer. Why wasn’t it contained by the concrete vessel encasing the reactor vessel?

All three reactors at Fukushima experienced core meltdown (while 4, 5 and 6 were on scheduled maintenance), and all three for the same reason: the 13 meter tsunami wave overcame the 10-meter seawall and struck the generators, cutting power; when the batteries ran out of energy, the coolant ceased to circulate and temperature rose.

The resulting corium breached the reactor vessel but not the Primary Containment (concrete) Vessel. It’s all still there, as is the Secondary Vessel encasing the Primary.

That is not to say that all the radioactivity was contained as neutrons have the bad habit of traveling around. Additionally, several controlled and uncontrolled events lead to the dispersion of mainly Ce-137 in ocean waters: given the very strong currents in the area, this is believed to have dispersed in a very large area, causing little or no damage.

Q2. What contaminated people if there was no radioactive cloud?

Fuel rods are encased in a Zirconium alloy which is inert at the normal functioning temperature of the core (300 °C); when overheated to 1200 °C however, they react with the surrounding water and create gaseous hydrogen; when the percentage of hydrogen in the air reaches the explosive saturation, chemical explosions occur, as it was the case in all three reactors, dispersing radioactive material.

Fusion vs. Fission

ITALIAN

Occasionally someone seems to be mixing up these rather similar terms, maybe because they both deal with enormous energies and they are both used to make bombs.

Is this justified?

The purpose of this document is purely divulgative: many approximations and semplifications have been used to make it understandable while preserving the underlying scientific base.

Hopefully we complied with prof. Feynman’s principle: “If we can’t explain something to a first-year student, it means we don’t really understand it”

The physical principles, putting it in the simplest possible way, are:

  • FISSION: splitting a large atom into two smaller atoms, releasing energy
  • FUSION: bashing together two small atoms to form a heavier one, releasing energy

Let’s look at each separately.

Fission

In fission we throw some fast neutrons at a heavy atom nucleus like Uranium-235; the neutrons land, with their kinetic energy payload, on the semisolid, unstable puddle of energy which is the Uranium nucleus,  briefly transforming it into the even more unstable Uranium-236 and eventually making it oscillate into a dual lobed shape. When the lobes become farther from each other than the effective range of the Nuclear force (more later), U-236 splits in (usually two) lighter elements, shooting out more free neutrons and a few gamma rays (the energy) (*). The free neutrons hit more U-235 nuclei and the process repeats in a so-called “chain reaction”.

A good image is one of an elastic band held in tension by some pinching device like a clothespin. When a small amount of energy (the impinging neutron) is applied to the clothespin, the band is freed to snap back into its natural shape releasing a great deal of elastic energy.

This chain reaction escalates rapidly: in an A-bomb (like Hiroshima) the escalation is explosive, while in a nuclear reactor, a neutron-absorbing medium is inserted to soak up some of the free neutrons to make its progress (and therefore the release of energy) controllable.

While in principle we could split ANY nucleus except hydrogen (which contains a single proton), in practice the binding energy released is greater than the applied energy only for heavier nuclei, and the heavier they are, the larger is the difference, to the point that most trans-uranic elements are so unstable that they only exist for a fraction of a second before they naturally radioactively decay into lighter, more stable ones. Uranium is the heaviest nucleus which is stable enough to exist in Nature (**).

Fusion

The process of fusion is exactly the opposite: we bring two light elements closer and closer together until they fall into the Nuclear force well. But to understand it, we must briefly digress to describe the inner nature of an atom.

We know from elementary physics that the atom is made by a nucleus of protons and neutrons, around which rotates a cloud of electrons. To get a feel for the dimensions, consider that if the nucleus were 1 meter across, the electrons would be 100km away. Protons are positively charged, neutrons have no charge, electrons are negatively charged; since a positive charge attracts a negative one, how comes electrons don’t fall into the nucleus?

Visualize two magnets: if you let them alone the negative side of one will stick to the positive side of the other. Now imagine you connect them with a string and impress some kinetic energy to one of them; because of the string, the moving magnet will rotate around the other, and if the speed is high enough, its inertia will be enough to prevent it from falling on the one that’s standing still. The balance of these two forces (inertia and electrostatic attraction) makes the atom what it is.

But what about the nucleus? All the particles in the nucleus have positive or no charge, shouldn’t they repel each other?

They would, if it wasn’t for another weird force called the Nuclear force; the forces we experience at our size are the electromagnetic force (attractive or repulsive, depending on the charge) and gravity (always attractive) and they both decrease with the square of the distance. The Nuclear force instead is negligible at atomic distances, becomes much stronger and attractive at distances comparable to the size of the nucleus, to become repulsive at even smaller distances! (***).

The result of this weird behaviour is that protons and neutrons are held together at nucleus’ distance, but do not crush further.

So when we bring the two small nuclei closer and closer is like if we were pushing two golf balls over the edge of a well: a little nudge and they will crash into each other at the bottom of the well.

The energy needed to nudge the atoms together is smaller than the energy released from the crash up to the Iron-56 nuclei, after which it becomes larger; even if we called it “nudging” however, this is an extremely high energy, i.e. the nuclei must crash into each other at very high velocities which can only be achieved at the incredibly high temperatures of 100M °K, sixfold hotter than the Sun’s core(****).

Containing a 100M °K plasma is no joke and requires huge energies: until these energies are much lower than the energy output, no fusion device is of any practical utility: research is therefore focusing on the confinement of the plasma by using a variety of solutions.

Conclusion

Fission and fusion both release the binding energy of atom nuclei: this energy is maximum for the Iron-56 nucleus, so we release energy by splitting nuclei that are heavier than Fe-56 and by joining nuclei that are lighter. Of course the release of energy is higher at the extremes:

binding energy

Nature prefers fusion because hydrogen is much more abundant than anything else and its energy density is higher, satisfying the Universe’ insatiable appetite for entropy: from a human standpoint, fission is easy to achieve, its fuel is more scarce (but still very plentiful) and its by-products more polluting.

Fusion on the other hand is more difficult to control, but its fuel is almost in unlimited supply and by-products less dangerous.

Fission and fusion have similar energy densities: 1 kg of fuel releases 20,000 and 24,000 MWh of energy, respectively, compared to the measly 14-15 kWh obtained by burning 1 kg of any fossil fuel.

Therefore, although the processes and the materials are completely different, the physical principle is closely related.


NOTES

(*) we will ignore neutrinos and anti-neutrinos in this simplified description

(**) actually Thorium-232 is an even better candidate, but since its cycle cannot be used for military purposes, in the 40’s Uranium was chosen instead

(***) the Nuclear force is NOT one of the four fundamental forces existing in Nature, but rather a very complex side effect of their interactions

(****) the Sun can get away with a lower temperature because it is aided by the enormous gravitational pressure due to its mass


(More posts on Nuclear Power)

Fast vs. Slow Nuclear Power Plants

A friend asked me what is the difference between a “Slow” and a “Fast” Nuclear Power Plant, a curiosity probably stirred by the recent announcement of the Beloyarsk 4 NPP going online.

First of all, 789 MWe Beloyarsk 4 is not such a big deal, capacity-wise. It is part of the Beloyarsk complex, designed to deliver around 2.3 GW of power when fully operational, which is big but not in the league of the true giants like the 8 GW Kashiwazaki plant in Japan or the 6.2 GW Bruce plant in Canada. Russia itself has several 3.8 GW plants like the one powering St.Petersburg (still called the Leningrad Plant).

Beloyarsk 4 is however the world’s largest FAST reactor, about 1.5 times larger than the second-largest, its sibling Beloyarsk 3.

Fast reactors use an “inefficient” moderator like liquid sodium: neutrons are not slowed up as much and when they hit U238 nuclei (the non-fissile vast majority of natural uranium), transform them into Pu239 which then decays into U239 which is even more fissile than traditionally used U235. In short, fast reactors produce some of the fuel they consume and, in certain conditions, all of it and even more. This last kind is called “breeder” and it was popular in the late Seventies as it also bred Plutonium, used in you-know-what.

Excluding this last species, for which nobody has much use nowadays (and instead is faced with the decommissioning headache) fast reactors are more fuel-efficient than traditional light-water reactors: Beloyarsk 4 should achieve 80 GW/ton/day (compared with about 50-60 for a light water reactor) and an upcoming follow-up unit is expected to achieve 120 GW/t/d.

Of course, the engineering of a liquid sodium moderator is a little more complex (for one thing, it is prone to exploding), but the design has been around for decades and is therefore rather stable and well-understood.

The question for me is why, given Russia’s richness in uranium (4th largest reserves in the world after Australia, Kazakhstan and Canada) they are investing in more fuel-efficient plants; for more discussion of NPPs see also these other posts.

9 things you probably don’t know about Nuclear Power

As some of the readers here will know, academically speaking I am a Nuclear Engineer, although I never practiced it, since I went into computers and later communications straight after University.

This meant that – like anybody who possesses a “weird” body of knowledge compared to the people around him/her – I have been time and again asked to explain this or this other thing about these arcane pieces of marvelous engineering.

Notice that I carefully avoid the discussion on the politics around Nuclear Power, because I find that they are usually between people who are equally uninformed and biased.

So – in light of the proposal by my good friend Stuart Bruce we should talk about this subject at some juncture – I thought of giving him some heads up with my list of

Things most people do not know about Nuclear Power

  1. Nuclear fuel is VERY cheap; so cheap, in fact, that the cost of nuclear-generated energy is essentially the cost of amortization and maintenance of the plant itself.
  2. Nuclear fuel consumption is very low when compared to traditional power plants; depending on the design, a 1,000 MW plant consumes less than a hundred tons of fuel per year – by comparison, generating the same amount of energy consumes 4 million tons of coal, 2.6 billion liters of oil or 1.9 billion cubic meters of gas.
  3. A consequence of the above is that a traditional plant requires a constant flow of fuel to operate; turn off the tap and the plant stops in a few hours. Not so for a Nuclear Plant: stopping the supply does nothing because the core already contains all its fuel.
  4. Another consequence of the above is that the Nuclear Power plant is essentially NEVER turned off: it can be put in a state where energy output is zero, but this state is the equivalent of a car engine in neutral: the car does not move, but the engine is running: touch it and you’ll burn yourself.
  5. Yet another consequence of the above is that a NPP MUST have electricity. At all times, even when it’s in idle; that is why next to every NPP you find big diesels. The reason is that (#4) the core cannot be turned off but only idled, and to do so, the core must be crammed with moderator, a substance (water, molten salt, liquid sodium, graphite…) which will absorb neutrons, thereby slowing / stopping the chain reaction. No power = no moderator = no stopping of core. In reality reactors are designed with a “dead man switch” logic in mind, i.e. power failure triggers a moderator flood, but in practice there are so many things that require power (remember humans can only survive for a few minutes in the vicinity of the core) that a total power failure is tantamount to catastrophe.
  6. Physically speaking, NPPs are small, orders of magnitude smaller than equivalent plants burning gas, oil or coal. This is a major design flaw of plants put in operation in the 60s and 70s (the majority), as in case of accident, the entire plant area quickly becomes unsuitable for human operation. If the same plants were designed today, probably the big backup diesels would be miles away to ensure that a disaster (such as a tsunami) would not cut power, but in those days the big worry was a terrorist attack and the ensuing need for military-grade security, much easier to achieve with a smaller plant footprint. So Fukushima’s diesels were meters away from the reactor; when the tsunami struck the reactor building weathered the hit without problems, but the diesels didn’t, power went off and BAM!
  7. The catastrophic scenario of a NPP diverges, while that of a traditional power plant converges: accidents typically involve faulty valves or lines, leading to leakages and/or fluid pressure losses. Temperature rises, steam pressure builds and in the worst case, an explosion may occur. This typically self-destroys a traditional plant, while in a NPP it leads to the loss of control of the core which continues to burn albeit not in the highly ordered chain reaction: look at Chernobyl whose now ceramized corium lava is still happily burning 28 years after the accident.
  8. The history of nuclear power generation is marred by the fact it was initially funded by the military who were interested in the well-being of humanity, but also <*ahem*> in the Pu239 (plutonium) by-product with which they built the most destructive (and luckily least used) weapons arsenal in the history of humankind. Were this not the case, research could arguably have explored the alternative Thorium cycle: Th232 is much more abundant, cheaper and better distributed. While its cycle is not without problems, it produces far less plutonium and actinides making it almost worthless for military purposes.
  9. Unlike traditional power plants, an NPP cannot be installed just anywhere: it needs water (LOTS of water: a water-cooled reactor needs 50 to 80,000 tons of water PER HOUR), seismically stable flat land and as little population as possible living within a 30 kilometers radius from the plant. Unfortunately the water abundance and people scarcity are usually at odds with each other; that does not mean these conditions are not met anywhere, but it means there’s plenty of locations (entire countries in some cases) where a NPP does not make any sense at all.

Happy birthday, Three Mile Island!

32 years ago, this very day, the reactor at the Three Mile Island power station initiated a series of events and mishaps which brought about the worst nuclear accident in U.S. history.

If you care to read the simplified Wikipedia account of the accident linked above, you will see that it, too, SCRAM-med in about 8 seconds; but as this does not stop decay heat, since the turbine had stopped working, heat built up in the core leading to a partial meltdown which was stopped before it broke through the main steel vessel.

Putting my Nuclear Engineering degree to work (sadly) !

I thought not many people knew I am Nuclear Engineer by background, but evidently they are enough to have a few of them asking what is my opinion about the situation in Japan. And knowing how complex such plants are, and to avoid showing disrespect for a wonderful nation hit by such a cruel catastrophe by treating these matters superficially, I will refrain from any analysis of a technical nature.

I will instead state four simple and – I believe – objective facts, all of which I learned in my university years.

Fact 1 – Nuclear Power Stations are small.

MUCH smaller than other power stations. A 1000 MW nuclear station is maybe 250k square meters; a photovoltaic plant of similar output would be around 400 square KILOMETERS ! This is good news (little land usage, securing is less expensive) but is also inherently more dangerous: should a catastrophe occur, it is likely to hit a nevralgic point.

UPDATE 22/3: especially when you build six reactors within walking distance from each other

Fact 2 – Nuclear plants contain all their fuel.

If a gas powered station blows up or is kidnapped by terrorists, all you have to do (in theory) is walk 5 kilometers upstream and close a valve for the plant to become inert (this is an approximation, as most power stations do have local fuel storage, but you get my point).

This is impossible in a Nuclear station, as the fuel rods are IN the plant at all times, enough to keep it going for a long time.

UPDATE 22/3: on my flight back yesterday someone was (rather loudly) saying: “Thanks God, the explosions in Fukushima were chemical, not nuclear like in Chernobyl”. Not so: all explosions in a nuclear accident are chemical, typically when release hydrogen oxydizes (Fukushima) or when moderating graphite burns (Chernobyl). These explosions happen in an environment contaminated by radioactivity and therefore carry it out, but no core ever becomes a bomb, although it reaches a temperature so high it melts destroying containment systems, (which in the case of Chernobyl were not so performing to start with) therefore polluting the surroundings with radioactivity.

Fact 3 – Removing fuel makes things worse.

In a gas, coal or oil furnace no fuel = no fire. Simple as that. In a Nuclear Power Station, pulling out the rods from the moderating medium makes them burn faster and uncontrollably. The nuclear fission reaction in fact is so furious that the only way to make it manageable is to have fuel immersed in a moderating medium that slows it down; in some reactors this moderating medium is pressurized water, in some is liquid sodium or graphite. Move the bars, leak some moderating fluid and the reactor immediately heats up.

UPDATE 15/3: While press reports speak of reactors being “automatically turned off” as soon as the quake hit, this is not technically what happens. The procedure, called SCRAM, quickly inserts more than a hundred neutron absorbing rods in the fissile core, stopping the chain reaction. The fuel rods are still active, however; if this were a car, it would be like putting the shift in neutral and applying the brake: the engine still runs, but the car does not move.

Fact 4 – Those catastrophes you cannot prepare for, you must avoid.

My university professor of Nuclear Plant Design used to say that, given all the above three facts, you must minimize the chances of a catastrophic event that you cannot prepare for (like a magnitude 9 earthquake followed by a 10 meter tsunami) by carefully selecting the location which, ideally, must have:

  • plenty of water
  • stable soil
  • nobody (or as little people as possible) living in a 30 kilometers radius

At the end of the day, nobody would place a hydro power station in the desert, right ?