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.


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 ?