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SA’s nuclear pebble bed reactor could get second chance

Mar 03 2016 06:44
Dr Kelvin Kemm

Cape Town - A private South African team has been working on its own version of the Pebble Bed Modular Reactor, which it could export overseas, says nuclear expert Dr Kelvin Kemm. “We missed the first boat, but hopefully not the second,” he said in the below opinion story, which first looks back at the history of nuclear reactors.

Dr Kelvin Kemm:

There is some confusion in the public mind about types of nuclear reactors.

Of course this is not surprising; it is a highly technical subject. Interestingly, the first ever nuclear reactors built to produce power, were built for military use, to power submarines.

After the WW2 the navies of the world had realised just how useful submarines were to a naval fleet, but there was one big snag. Submarines could not stay submerged for more than a few hours before they had to resurface to recharge batteries with diesel engines. On the surface they were very vulnerable to detection and attack.

So a naval dream was to be able to keep a submarine submerged for days or weeks at a time. Nuclear power was the answer.

The world’s first nuclear powered submarine was the USS Nautilus, launched in 1954. In 1957, the then Soviet Union launched the first satellite into space, called Sputnik. This was seen by the western world as a huge potential military threat because, in principle, satellites could carry nuclear weapons which could be brought down anywhere.

In response, the United States decided to send USS Nautilus on a mission to pass under the North Pole. This secret mission was dubbed Operation Sunshine. The plan was that after the mission was successful, the US would then announce it and make a major public fuss about the scientific achievement.

The military significance was that the Soviet Union would clearly get the message that nuclear-armed submarines could approach the northern coast of the Soviet Union, and so be in missile range of important targets. The USS Nautilus trip was successful and achieved its objectives with great publicity.

Nautilus was powered by a small Pressurised Water Reactor abbreviated as a PWR. The hot steam coming from the nuclear reactor steam turbine cycle needs to be cooled after it has been through the cycle, and submarines have all the free water you want...the whole ocean.

Light bulb moment

A few years’ later people started to wonder if humankind could build reactors on land to produce electricity. So they tried. In essence, they brought the submarine reactor design ashore and adapted it.

First attempts were encouragingly successful, so more reactors were built. Significantly, one design factor that was maintained was to have all the free water you want for cooling. So all the land-based reactors were built next to large volumes of water, either the sea, or on large lakes or rivers.

During the earlier years of nuclear reactor development a number of similar designs were tried. All of these reactors are today collectively known as Generation One or Gen I.

If you want to cook potatoes in water on the stove you put them in a pot of water and boil them for a while. They boil at about 100°C, depending how high you are above sea level. Johannesburg potatoes boil at a lower temperature than Durban potatoes.

Another way to boil the potatoes is to use a pressure cooker. What happens is that the pressure-sealed lid causes the water to rise above 100°C without boiling. Some folks say that spuds taste far better if cooked in a pressure cooker.

A law of physics states that in a thermal system, the larger the difference between the temperature at the hot end and at the cold end of the thermal system the more efficiently one can get the heat out.

Great, so if you build a highly pressurised heat engine system you can more easily get more heat out, and in principle make more money. The downside is that you are dealing with pressurised systems, which can go bang if you are not careful; just like a pressure cooker will blow its safety valve if you overheat it.

So there is a science trade-off and a financial trade-off in deciding to use a pressurised system.

So with nuclear reactors the early designers tried both. Boiling Water Reactors (BWR’s) were built, which crudely speaking are large tanks of water with nuclear fuel in them (uranium-containing fuel elements). When a nuclear reaction is initiated it causes the fuel to get hot and it, in turn, causes the water to boil. The steam, at 100°C is then led off, to extract the heat to drive turbines.

Pressurised Water Reactors (PWR’s) are like the pressure cookers. By not letting the water boil, by keeping it under pressure, you can get the temperature to go much higher than 100°C. So you can get the heat out more effectively.

The next series of reactors, which were built, are known as Generation Two or Gen II reactors. The Gen II designs are typically 40 to 60 years old. Fukushima was a Gen II BWR system. Koeberg is a Gen II PWR system, which is newer than Fukushima. As time went by, the PWR system showed itself to be preferable to the BWR system and as a result world designers swung towards PWR’s.

When reactors were designed, designers would calculate what was known as a Maximum Credible Accident (MCA). In many instances, MCA’s were not really ‘credible’. They were way-out theoretically possible accidents, which really stretched the imagination. But, nevertheless, the designers designed for such imaginary incidents and then built what is known as “defence in depth”.

This is expensive. For example, some reactors have five sets of cooling pumps, so if the main pumps break down there is a second set, and third, fourth, fifth, to use as backup emergency. Mostly, a reactor would not ever be expected to use its second set, let alone a fifth.

As time passed and a vast vault of nuclear reactor design experience was built up internationally, the design philosophy started to change. The critical change was to make use of the natural laws of physics as a natural safety system. For example; if a reactor core were to melt, the design was such that the molten goo would naturally run downwards under gravity into a pre-prepared trough; or if emergency cooling was required then the emergency water was stored uphill so that it would naturally run down to where it was needed. There would be no need for an electrical system to run pumps.

These types of reactors are called Generation Three or Gen III. The latest, more advanced reactors are Generation Three Plus or Gen III+.

Fukushima was not built like this. Its dramas started when the tsunami washed away the electricity lines bringing power in to the primary cooling pumps. Another one of the Fukushima problems was that used fuel elements were stored in a large tank of water, but rather foolishly the tank was built above ground. So, when the tank cracked from the impact of the tsunami, the water ran out leaving hot fuel elements exposed. The Fukushima crew spent a lot of desperate effort trying to pump more water into the storage pool. Had the pool been built below ground, the water would not have run out, and any subsequent rain would have naturally run into the pool, helping matters.

So currently, the world’s most advanced water reactors are the Gen III+ PWR’s. These are the reactors which South Africa will build. They have considerable natural passive safety built into them.

However, in principle, all these reactors are still the great grand children of the original submarine reactors, built to assumptions like ‘all the free water you need.’

SA’s Pebble Bed Modular Reactor

Then, as design philosophy matured, various teams around the world said: “Why don’t we start from scratch, and design a reactor which has no hereditary link to submarine reactors.”

In South Africa, a great team did just that, and designed a high tech reactor that did not need large amounts of water. It was cooled by gas, and it did not have to be built on the coast. It used natural laws of physics as a main design feature. The reactor was called the Pebble Bed Modular Reactor (PBMR). The ‘modular’ was the concept that initially a single reactor was built, driven from one control room. Then later, as demand increased, additional reactors were built next to the first one and then joined to the single control room. In other words new modules would be added as the requirement increased. They would all be attached to one main control room, much like extra locomotives are added to big trains, but all driven by one driver. This provided building flexibility and price flexibility.

The ‘pebble’ referred to the fuel which was a black graphite ball, as big as a cricket ball, and containing nine grams of uranium. In contrast, Koeberg fuel is four metre long assemblies of metal tubes containing uranium. If you drop a Koeberg fuel element from a crane it is probably a very expensive write-off, like dropping a motorcar engine three metres onto concrete: Very expensive, and very difficult to explain to the boss. With PBMR fuel you could throw a fuel ball against a wall and it would bounce, like a cricket ball, no problem.

The South African PBMR team, at its height, consisted of about 2 000 people, probably the largest nuclear design team in the world.

As far as I am concerned, the PBMR was virtually complete and we should have started building it years ago. Test systems, as large as a house, were built in Potchefstroom, and all worked perfectly. Some world firsts were achieved. But they didn’t tell the public. They didn’t even tell the minister of Energy. I know that because one day after the PBMR project had been closed down, I walked down a passage with a previous minister of Energy and mentioned the existing operating systems. The minister did not know that they had been built. That was not the fault of the minister.

Meantime, anti-nuclear propaganda was hitting the media, almost daily, saying that one of the finest nuclear design teams in the world was a bunch of incompetent idiots and that nothing had been achieved. The team ground their teeth in frustration, but said nothing. They were polite and well-mannered; they did not fight in public.

These PBMR designs, which are a radical departure from the water-based PWR’s, are known as Generation Four or Gen IV systems.

We South Africans should have had the courage to start building a prototype PBMR, but it was easier to keep delaying one more year to ‘polish off the edges’ to try to get the final design perfect on paper.

Nobody gets a final design perfect for a new major development. There is always learning ‘on-the-trot’ as the device is built. A few months ago, I was invited to be a guest speaker at a nuclear conference in Hanoi in Vietnam, and for the first time I flew in a Boeing 787, known as the Dreamliner.

I recalled that when the Dreamliner first flew with passengers there was an incident of an overheating battery and smoke came drifting into the cabin. This hit the news headlines. Boeing’s PR machine hastily said: “don’t worry, it is a small fault, we will fix it.” They did.

Believe me, there were probably dozens of other small faults detected in the Dreamliner during the first year, which were quietly fixed without the public ever knowing. Even Boeing can’t build a perfect new plane one-shot in the first attempt. It is the nature of the game of complex technology that there are teething issues to fix on the way.

We should have built the PBMR and fixed the teething issues along the way. So what if the first one was only 75 or 80% perfect. The second one would have been much better, but we tried to design to 90% plus before digging the first trench. Also, the team did not brag to the public or to the politicians as to what they had achieved. So, foreign nuclear scientists knew more about the South African PBMR than our own cabinet. I gave a PBMR lecture in London in a 500 person hall next to the Houses of Parliament, and there was no standing room left in the hall. I gave another at the Nuclear Energy Institute in Washington DC, and it was packed. Meantime, back home in South Africa, some people had heard the letters PBMR, but had no idea what they meant, and the media spent its time smashing the spirit of the team anyway.

Other teams in other countries then started developing their own PBMR’s, largely based on what the South African team had developed. A couple of these foreign reactors are now on the verge of going into commercial operation.

Hope for SA’s PBMR?

Some good news is that a private South African team has been working on its own version of the PBMR, so hopefully South Africa will still build PBMR’s. The export potential for these reactors is huge. The political influence that goes with being a supplier is huge. We missed the first boat, but hopefully not the second.

PBMR’s could also be used to just produce high temperature heat, like near 1 000°C, and not produce electricity at all, so one could efficiently use them to melt metals or run heat-intensive industrial operations without first making electricity, which you then just turned back into heat anyway.

PBMR’s were deliberately designed to be small, only about 5% the size of the big Gen III power stations. So right now, Gen III+ and Gen IV are two different design approaches. A comparison is like buying a bulldozer or 20 tractors equipped with earth moving buckets. Both options have their place.

Right now, there is an international set of Gen IV specifications and general operating principles, largely based on the original South African input, so our PBMR team has already written itself into the nuclear history books.

We also actually produced PBMR fuel, which was tested in other countries and declared to be equal to the best nuclear fuel ever built by anybody in the world.

Other Gen IV designs are now being proposed, at the same time as more advanced Gen III+ designs come out of the world design systems. Both will exist side by side, for decades to come. Modern nuclear reactor systems are now evolving much faster than other newer power technologies such as wind and solar.

* Dr Kelvin Kemm is a nuclear physicist, and CEO of Nuclear Africa. He is a member of the Ministerial Advisory Council on Energy (MACE).

* His views do not necessarily reflect those of Fin24.

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kelvin kemm  |  nuclear



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