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Can Pebble Nuclear Reactor Technology Break the Barrier for a Safer and Cheaper Nuclear Power?

March 18, 2011

Pebble Bed Reactor Technology (PBMR)
This posting presents the Pebble Bed Modular Reactor (PBMR), a new type of meltdown-proof high temperature helium gas-cooled nuclear reactor, which builds and advances on world-wide nuclear technology. A reactor small enough to be assembled from mass-produced parts and cheap enough for customers without billion-dollar bank accounts.

The PBMR reactor promises to be a better way to harness the atom. The PBMR is a new type of meltdown-proof high temperature helium gas-cooled nuclear reactor. A reactor small enough to be assembled from mass-produced parts and cheap enough for customers without billion-dollar bank accounts. A reactor small enough to be assembled from mass-produced parts and cheap enough for customers without billion-dollar bank accounts. A reactor whose safety is a matter of physics, not operator skill or reinforced concrete. And, for a bona fide fairy-tale ending, the pot of gold at the end of the rainbow is labeled hydrogen.

PBMR is a new type of high temperature helium gas-cooled nuclear reactor, which builds and advances on world-wide nuclear operators’ experience of older reactor designs. The most remarkable feature of these reactors is that they use attributes inherent in and natural to the processes of nuclear energy generation to enhance safety features.

PBMR’s are designed to produce 110 MW each which means that 30,000 average homes could be sustained by one such reactor. More than one PBMR can be located in a facility thus creating energy parks. It is possible for a PBMR energy park to be made up of a maximum of 10 modules which share a common control center. This system allows sequential construction of modules to match users’ growth requirements; as the area grows, so more modules can be added to meet the industrial and domestic needs for electricity in an area

A single PBMR reactor would consist typically of a single main building, covering an area of 0.3 acres (165 x 85 ft). The height of the building would be 140 ft, some of it below ground level, depending on the bed rock formations as the building would sit on bed-rock. The part of the building that would be visible above ground is equivalent to a six story building. There would be a unit control room, a high voltage switch yard, and a cooling tower for inland facilities and a sea pump-house for coastal facilities.

These relatively small power stations would be versatile and flexible. They could be erected anywhere there is a steady and ready supply of water. They could be used as base-load stations or load-following stations, and could be adjusted to the size required by the communities they serve.

Concerning safety, the PBMR is walk-away safe. Its safety is a result of the design, the materials used and the physics processes rather than engineered safety systems as in traditional nuclear type reactors.

The peak temperature that can be reached in the reactor core is far below any sustained temperature that will damage the fuel. The reason for this is that the ceramic materials in the fuel such as graphite and silicon carbide – are tougher than diamonds.

Even if a reaction in the core cannot be stopped by small absorbent graphite spheres or cooled by the helium, the reactor will cool down naturally on its own in a very short time. This is because the increase in temperature makes the chain reaction less efficient and it therefore ceases to generate power. The size of the core is such that it has a high surface area to volume ratio. This means that the heat it loses through its surface (via the same process that allows a standing cup of tea to cool down) is more than the heat generated by the decay fission products in the core. Hence the reactor can never (due to its thermal inertia) reach the temperature at which a meltdown would occur. The plant can never be hot enough for long enough to cause damage to the fuel.

In terms of radiation leakage, the helium itself, which is used to cool the reaction, is chemically and radioactively inert: it cannot combine with other chemicals, it is non-combustible, and non-radioactive.

Because oxygen cannot penetrate the helium, oxygen in the air cannot get into the high temperature core to corrode the graphite used in the reaction or to start a fire. If, through some accident, the helium gas duct (inlet and outlet lines) is ruptured, it would take some nine hours for natural air to circulate through the core. Even if this could happen, it would only lead to less than 10-6 (one millionth) of the radioactivity in the core being released per day. That means that the amount of activity released in 24 hours under this very severe (and recoverable) situation would be some 10,000 times less than that requiring any off-site emergency actions. To avoid such a total failure of the main gas ducting it is designed to leak before it breaks, so that the depressurization will be gradual and cannot lead to such a rupture.

The helium pressure inside the closed cycle gas turbine is higher than the air pressure outside it, so nothing can get inside the nuclear circuit to contaminate it.

In closing, it is unclear why Pebble Bed Modular Reactor technology is not widely known or discussed in the U.S. Bill Gates has given a good deal of money for research in a new type of reactor that addresses many of the concerns/objections about nuclear power. It would be advantageous to see a more diverse mix of energy solutions in the future including nuclear, renewables, as well as responsible use of more traditional sources (fossil). It would be really nice to see oil’s near monopoly in the transport sector broken up for lots of reasons besides the environment.

9 Comments leave one →
  1. Rainier permalink
    March 18, 2011 10:08 AM

    the PBMR was just abondoned in South Africa after spending about 1.5 bn $. They had lost all customers and investors because of substantial technical and safety problems. The South African government recently announced to construct new NPP, but conventional ones, not PBMRs.
    In case of a leak in tghe pressure vessel air can easily penetrate into the core and a graphite fire as in Chernobyl will immediately start.

  2. March 18, 2011 11:08 AM


    You are correct and am aware that:
    1. The South African government closed down the PBMR project almost completely. Westinghouse Nuclear left the PBMR project in May this year.

    2. There are many arguments against the PBMR, particularly based on bas experience with the 2 PBRs operated in Germany: It is an undisputable fact that the German PBR AVR ist most heavily contaminated nuclear facility worldwide and thus cannot be dismantled for the next 60 to 70 years. Because the contamination is also in the soil, the whole reactor vessel was filled with concrete and will be transported in 2012 by airlifters to an intermediate storage (in order to clean the soil under the reactor from Strontium).

    3. PBMR is far from new 1 has been in operation over 15 years in Texas and there are quite a few in China. Pebble bed reactors can be scaled and are able to consume the spent fuel of our 2nd generation reactors. Pretty simple grind up the spent fuel and encapsulate the grains in graphite. Allow the fuel to react at full temperature. The resulting waste is hot for centuries instead of millennia.

    4. As a technical person, it does appear but would like to validate that the proper attention and technical resources have been used to circumvent current problems. Not sure South Africa had the wherewithal and capabilies to conduct such work.


  3. March 18, 2011 4:08 PM

    Hands up from those, who want an installation like this in their neighbourhood, please!

  4. March 20, 2011 7:31 PM

    six inherent problems;
    1.) an earthquake can rip open the pipes letting out helium much quicker than pinhole leaks enabling the graphite coated balls to catch fire like the graphite rods in the Chernobyl plant.
    2.) cold air hitting carbide wrapped balls will cause them to crack and enable graphite fires.
    3.) terrorists can steal the balls, crack them open and make dirty bombs and low yield atomic bombs.
    4. ) terrorists occupying a plant can cause a meltdown.
    5.) terrorists can hit nuclear plants with multiple hijacked jets.
    6.) the mindset that says plan for nuclear plants of the next generation, full steam ahead will let inferior alternative deoisgns that are unsafe multiply. There’s a french company that is desingning nuclear subs with cables to supply electricity to onshore cities.
    greed heads promoting that might contaminate the harbors of cities located near accidents worse than any other nuclear pollution incident ever.

  5. March 20, 2011 7:42 PM

    proponents of inherently unsafe nuclear plants like
    1.)hydrogen manufacturing plants employing nuclear splitting of water that puts huge amounts of hydrogen in contact with the nuclear fuel and the core are super unsafe! They will put much bigger amounts of hydrogen near the core than the 5 reactors in japan that exploded. Its like designing a nuclear triggered bomb!
    2.) The submarine nuclear plants invented by the French company
    will cause unprecedented amounts of water polution that will ruin coastal fisheries for thosuands of miles. dont expect the beaches near it to be usable either.

    The people who design plants like that and promote them despite the danger should be psychoanalyzed. Are they sociopathically malignant and so evil they dont care how many people are killed, like greedy traitors who dont care even if they are in the same country or region?

  6. Tom S permalink
    March 30, 2011 6:50 PM

    Maybe I’m being overly simplistic: Why not build robust smaller modular units capable of generating a few MW apiece, each encased in multiple layers of redundant concrete-steel-etc. and buried well underground to avoid terrorist activities. Engineer it so that most of the plumbing, ductwork etc. is fully encased in the heavy unbreakable walls of the module, and work out a simple leakproof way to circulate inert fluid and/or gas into and out of the module so as to extract a great quantity of usable heat without getting near the source of radiation.

    Build a redundant safety factor into the fuel compartment which would allow common gravity to pull the fuel elements a safe distance apart – and lock them there – in case of an emergency, catastrophe or seismic event, thus bypassing this stupid reliance on backup generators, pumps, batteries etc.

    If the ground starts shaking past a certain G-force it releases a latch, and gravity pulls the fuel plug. If the plant’s electricity output gets hit with an extreme overload it releases the latch, and gravity pulls the fuel plug. If the plant operator receives notice of an impending catastrophe he hits the red button which releases the latch, and gravity pulls the fuel plug, etc. etc. In all cases whenever the plug gets pulled, the module’s power output winds down all by itself, in a safe, predictable manner. What’s so hard about that?

    Will the fuel not generate any heat at all, unless it’s jammed into an almost-critical mass right at the edge of melt-down? Is the resultant heat (power) production linear, related to proximity of the fuel, or is it exponential? Why be greedy and insist upon maximizing the heat production, if excess heat is the only real problem? Dial the heat (and your profits) down a bit so as to make the modules more stout and reliable, and move on.

  7. Dah Bomb permalink
    February 21, 2013 2:30 PM

    A lot of the fear mongering that has been going around has to do with the misconceptions about the Chernobyl Disaster and how that particular reactor differs from how traditional American pressurized water reactors are made and designed.

    The first major difference is that graphite moderated reactors have a positive temperature coefficient. In layman terms this means that as reactor temperature goes up … the moderator becomes more effective and increases reactor power. Pressurized water reactors use water as a moderator which has a negative temperature coefficient which means that as reactor temperature increases the water becomes a less effective moderator and reactor power is reduced.

    This makes graphite moderated reactors inherently unstable because a rise in reactor temperature results in a rise in reactor power which therefore results in a further rise in reactor temperature. A water moderated reactor inherently stable because a rise in temperature results in a decrease in reactor power which means a lower reactor temperature.

    Granted, graphite moderated reactors are far more efficient than water moderated reactors. This efficiency in power generation is the trade off required to make an inherently safe reactor. I am very leery when people start promoting the efficiency of graphite moderated reactors. Especially when they start talking about adding systems that will automatically regulate temperatures etc.

    As a 12 year nuclear engineer for the US Navy I would personally want a less efficient nuclear plant next to my house that requires constant intervention to keep it operating (a pressurized water reactor) instead of a highly efficient nuclear plant that requires constant intervention to keep it from going into a meltdown condition.

    P.S. The reactions that are going on (while not as quite hot as the processes of fusion which occur inside stars such as our sun) are ultimately hot enough to melt any materials we have to date if runaway conditions occur while any materials are in direct contact with the fission process. Anyone who says that they have a meltdown proof reactor obviously underestimates Murphy’s Law and the sheer raw energy and heat capable of being released by a nuclear reaction in general.

    P.P.S. The last thing I would want to see (as a nuclear engineer) is a 30k home subdivision laid to waste because someone overlooked a slight material defect in the construction process of an inherently unstable highly efficient reactor. Not only would this be a huge set back in public confidence in nuclear technology in general, but it would be a feeding ground for environmentalists for decades to come.

  8. Dah Bomb permalink
    February 21, 2013 2:42 PM

    I have no problem with a pressurized water reactor nearby, I lived near Palo Verde Nuclear Power Plant in Arizona most of my adolescent life with little fear. I would not want to live anywhere near a graphite moderated reactor, regardless of the systems they employ. Reactors are subject to brittle fracture (catastrophic failure with little or no warning – think shattering glass) due to neutron impregnation after decades of operation. So for long term growth, I’ll take the less efficient inherently stable pressurized water reactor over a 4th gen inherently unstable highly efficient reactor.

    Brittle Fracture:–brittle-fracture.cfm (a technical explanation)

    Brittle Fracture: (Little easier to understand)

  9. Dah Bomb permalink
    February 21, 2013 3:43 PM

    Tom S. don’t take this the wrong way, but your ignorance on the subject is very pronounced. I understand that you have a simple way of looking at it and let me explain a few things in basic reactor construction that are will help you to understand why terms such as ‘heavy’ and ‘unbreakable’ have little meaning when talking about nuclear plant construction. This is not meant to demean you and I hope that you do not take it that way, I merely want to help educate you because there are a lot of fear and misconceptions about nuclear power/technology in general.

    When you talk about nuclear power everything is about the rate of neutron generation. If the neutron rate is increasing … reactor power goes up. If the neutron rate is decreasing … reactor power goes down. The control rods are just that, to control the rate of nuclear fuel consumption to help limit the increases in reactor power. Most reactor designs that I have seen all use control rods that are held against spring pressure with safeguards in place that will automatically release the ‘latches’ (if you will) allowing the control rods to be fully inserted into the reactor vessel in a matter of seconds (even if they are fully withdrawn) if any serious fault is detected. This is normally how reactor power is controlled. There are also other means such as injecting a slurry of borate enriched material into the primary coolant which will absorb the neutrons and therefore shut the reactor down (regardless of reactor design).

    Another point is about earthquakes (presumably) which is one of the safeguards already built in to the control rod release mechanism. It is also important to note that the orientation of the reactor is irrelevant, the control rod springs are designed to insert the control rods from fully out to fully in in a matter of seconds even if the reactor is somehow upside down. Gravity is not a good way to put in the control rods because if somehow the reactor found itself upside down … the control rods would not get inserted and we really don’t want that condition to occur :)

    The materials do not need to be heavy, or bulky (robust). It is more about minimizing corrosion and brittle fracture rather than massive thick ‘plumbing’ as your post indicates.

    You also do not want a device that is deeply buried because at some point you have to change out the fuel rods, and normally reactors are designed to be as easy to change out fuel rods as possible because this is the greatest chance for nuclear waste to be spread under normal conditions.

    Also the systems that support a reactor … ie the coolant sampling, pressure control, temperature monitoring, power monitoring, power generation, heat exchange and extraction, etc … can only be made so small. This is generally why larger, higher MW reactors are generally built because the size of the reactor scales disproportionately with the size of the support systems (meaning you can vastly increase the size of reactor while making small relative increases in support systems). This allows a better and more efficient cost to be associated by building larger reactors over many smaller ones.

    I touched on the two basic different reactor designs in my previous point. In pressurized water reactors (which I know the most about) it is not about excess heat since we have heaters to keep temperate at the rates we want, coolant pumps to vary the flow through the reactor, and a heat exchanger to remove heat which is regulated by inserting water of different temperatures in to further help regulate the coolant temperature. Think of a nuclear fission reactor as a giant hot water heater. The only purpose of the nuclear fuel is to heat the coolant that is transferred to a heat exchanger (steam generator) that allows the heat from the pressurized coolant (usually water to keep it from boiling – which in excess is VERY bad) to be transferred to unpressurized (minimally pressurized) water to create steam which is used to turn turbines to generate electricity. This steam is then cooled back into liquid water, to be sent back into the heat exchanger to be heated and turned back into steam once again.

    The temperatures that is in contact with the nuclear fuel in a controlled nuclear reaction is on the order of thousands of degrees. This allows a reactor at full power (designed ‘full’ power output … you can go well beyond 100% reactor power) to rapidly heat the coolant and carry this coolant to the heat exchanger to be transferred to a secondary system and then sent back to be reheated. If there is a reactor meltdown condition (in the case with Chernobyl) nothing you can do will prevent the energy from being released, this is why constant attention and rapid insertion of the control rods is important to halt the increase in reactor power. Think of it this way, all of the energy that will be released over the course of years of constant power generation is already bottled up inside the reactor prior to the reactor becoming critical (which despite all of Hollywood, is not a bad thing. It means the reactor is online and producing power). If all of this energy is released suddenly, its not going to react like a nuclear ‘bomb’ would (fundamentally different designs – same principles though) but it is going to generate lots of heat and energy (think of the different of holding a lit blackcat in a closed fist versus an open palm) and it is going to melt the container that houses it because we simply cannot make a material that can withstand the intense heat generated by an uncontrolled nuclear chain reaction (emphasis on uncontrolled). This is why after the Chernobyl disaster they ended up dumping tons and tons of powered lead and earth on top of the meltdown site and the site is probably STILL producing heat to this day (although at a much lower rate since the fuel is vastly spread out).

    By the way, I would like to point out to the Author of this site … Chernobyl was a graphite moderated nuclear reactor.

    Hopefully this explanation gives you a better understanding of the basic principles of reactors and how they operate. I also hope that you (nor anyone else) did not take this post in a demeaning or condescending way as that was not the intent. Information is power and everyone needs to know a little bit more about nuclear technology even if it is just the basics of how it works. A better, more well informed public, can make better more rationale intelligent decisions when considering their energy needs in the future.

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