Sunday, April 22, 2007

Nuclear power: build plants underground

More nuclear power, like it or not: It seems likely that many of the world's states will soon begin to build many nuclear power stations; some for the first time and others - like the US - after ending a long-frozen programme. The reasons cited centre on climate change as it is true that, once operational, nuclear reactors are largely carbon-neutral. Furthermore, they have high energy density (very high power output from a very small space) and operate continuously over lengthy periods. All they do is provide a framework in which a controlled fission reaction within its uranium fuel heats up a primary coolant (circulating water or inert gas, contained under pressure). The super-hot coolant then heats water via a heat exchanger to raise steam to drive turbines to generate electric power. Renewables have low energy densities and operate intermittently regardless of the source of energy. At present, there is no viable way to store energy produced on a large enough scale to keep power available at all times; something we have come to expect. These factors, among others, make it inevitable that many new reactors will be built.

Damage limitation: It is not my purpose here to argue for or against nuclear power or discuss other issues like uranium mining and reserves. It's more of a damage-limitation exercise. Given that reactors will be built whether we like it or not, how can we ensure that they are as safe as possible? Mention the word 'nuclear' to most people, and words like Chernobyl, Hiroshima, missiles, nuclear waste, Windscale and Three Mile Island trip into the mind. Nuclear power has not, over the years, had a good press. And with good reason given its sinister association with bomb-making and several serious accidents. Yet it could easily be made much safer.

Nuclear power hazards: These are well known so I'll just briefly review them. The hazards all stem from the radiation produced by the primary heat-generating fission reaction, spent fuel rods, irradiated reactor assemblies, reprocessing (if any) and the resulting radionuclides which are created in the fissioning of uranium-235 atoms. Chernobyl reactor 4 after the disaster on April 26, 1986The reactor is typically sealed in a primary containment vessel with radiation shielding surrounding it. These assemblies, in turn, are usually contained in a secondary reinforced concrete building which is designed to contain radiation products in the event of an accident in which the primary containment breaks down. [There was no secondary containment at Chernobyl and the results of the partial meltdown that followed the doomed 'experiment' are now grim history.]

  • Loss of Coolant Accident (LOCA) in which the fuel rods, normally cooled by water or carbon dioxide gas, become so hot that they melt down (as at Chernobyl)
  • Containment damage or breach due to warfare (bombing, missile, suicide using hijacked civil aircraft and so on), accident (aircraft crash), earthquake or climate change events (sea level rise, for example, since many existing plants are located by the sea which is used as a coolant in the secondary circuits)
  • Hazards associated with transport of fuel assemblies to and from reactors (attacks, hijacking, accidents)
  • Storage of irradiated fuel (attacks, leaks, accidents) in cooling ponds or air-cooled stores
  • Reprocessing - the fuel route chosen by the UK and France in particular - which yields highly radioactive acid liquid wastes with potential for explosion in poorly-maintained facilities. Reprocessing notoriously is intended to yield other fissionable products such as plutonium metal, the source for atomic bomb-making and trigger for fusion (hydrogen) bombs. Plutonium, along with enriched uranium (also used for atomic bomb-making), has been repeatedly stolen from former USSR facilities and most of it not tracked down
  • Decommissioning costs high and defunct installations will require monitoring and access protection for a century or more

What happens to the long-lived radioactive wastes? The best option at present - far from perfect - is monitored deep burial as vitrified blocks which must be protected from corrosive circulating groundwaters. Unprocessed irradiated fuel rods will likewise need to be stored deep underground (e.g. Yucca Mountain). Radioactive elements such as plutonium-239 in spent fuel will remain hazardous to living things for hundreds of thousands of years.

Scary list, isn't it?

And now, the answer? Almost all of these nuclear hazards become significantly reduced when you factor in a new possibility to the construction equation. If they must be built, why not build these facilities underground? To give you some idea of the relative sizes of the excavations needed, take a look at this drawing I made many years ago for UK examples.
Size comparison between Sizewell nuclear power station and underground hydro-electric scheme in North Wales

On the left, in my drawing, is a vertical section of the Sizewell B reactor and containment building (click for full size image). On the lower right is an equivalent section through the Dinorwig pumped storage hydropower station, located deep inside a mountain in Snowdonia, beneath defunct slate quarries. For good measure, I added (top right) an as-yet-unbuilt example of another reactor design, a high temperature gas reactor. The point of all is that these engineered structures are all at the same scale: see scale bar.

What's even more important is the size of the Sizewell reactor vessel in relation to everything else. It's small without its containment building. And it wouldn't need such a building underground because being located underground is far better containment than anything that could be built on the surface.

Safety underground: the advantages

  • immune to military attack from the air
  • containment unbreachable (given proper choice of ground conditions, hydrogeology and rock types) and so immune to attack from, say, a suicide bomber. Even major LOCAs would be better contained than anything above ground
  • no need ever to remove irradiated fuel assemblies. When the reactor reaches the end of its operating lifetime, the whole facility could be sealed, complete with its spent fuel. Monitoring would be needed but because nothing is above ground, access would only be minimal
  • planning consent more likely to be straightforward since there wouldn't be much surface infrastructure to object to. Most of the usual public fears and objections wouldn't be serious issues

Disadvantages: is there a flip side?

  • Cost: I have no idea how much underground siting would add to a budget. But if you take into account minimised decommissioning costs (not historically factored in to the cost of nuclear power as we are now finding out) and spent fuel disposal possibilities, I would guess that it would be completely viable. Anyway, what price security and safety?
  • Location: Finding suitable underground conditions, especially in flatter rainy areas with fast-moving groundwater circulation, could be a problem. The ideal would be mountain areas with relatively low rainfall.
  • Cooling: Like any steam-driven turbines, cool water is needed both for raising steam and for condensing it. There's no reason for the turbine and cooling systems to be located underground since these aren't in contact with radioactive parts of the circuit

A safe way forward: I've set out what seems to me an obvious way forward for nuclear power, if we are to have much more nuclear electricity as looks certain. (France, by the way, already generates 75% of its electricity this way - but above ground). If you, the reader, agree that making it mandatory to locate future nuclear plants underground is worthy of consideration, please help begin a real debate by contacting your government representative and, perhaps, your country's nuclear generating industry. The onus is on the industry to explain why underground containment is a bad idea, not a good one. If it is a good one, let them start digging!


Unknown said...

Interesting article. Perhaps we should go one step further and mandate thorium reactors, which are safer than uranium reactors. They produce less toxic waste and less weapons grade plutonium. See
Bryson Tait

Unknown said...

Actually, nuclear power is the safest of all controllable forms of power.

By far.

Lovelock pointed this out in his book "The Revenge of Gaia". When you compare the number of deaths per GWyr, nuclear power comes out on top, even with Chernobyl - one of the world's most primitve plants, abused to breaking point and then very badly handled after the event (which lead to most/nearly all of the deaths.)

I think that it would be much better for the World, if two nuclear plants were built above ground than one below.

Yes, yours is a good idea, but CO2, toxins from burning coal, spills from oil, explosions from gas and breaches of hydro dams are, in practice, far more dangerous than nuclear power and its waste.

Tim Skoglund said...

The walls of containment buildings are about 75% steel reinforcement rods by volume, a significant investment of installation time and material. If containment were underground in bedrock much of this reinforcement might be unnecessary, giving more cost savings.

Could spent fuel be reprocessed on-site eliminating transport offsite and perhaps reducing opportunities for accident or diversion?

Companies are promoting advanced passive designs were makeup water is stored immediately above containment and reactor pressure vessel so possibility for gravity flow make-up water.

Interesting website.

Anonymous said...

i just thought of this on my own and desided if anyone else had thouht of it

Dr.A.Jagadeesh said...

It is a good alternative. When there are trains running under sea in Japan Human Intelligence can certainly build nuclear plants underground. Another alternative is smaller nuclear plants up to 500 MW size.

Dr.A.Jagadeesh Nellore (AP), India

Dr.A.Jagadeesh said...

It is a good alternative. When there are trains running under sea in Japan Human Intelligence can certainly build nuclear plants underground. Another alternative is smaller nuclear plants up to 500 MW size.

Dr.A.Jagadeesh Nellore (AP), India