Thursday, February 12, 2009

The Pressurized Water Reactor and the LFTR: Some Comparisons

In 1948 exploration of reactor technology was well underway. Most reactors had cores made of solid materials, for example uranium metal clad in aluminum. A second line of reactor development, the which began with the original chain reactor experiment at Cavendish Laboratory and continued with a reactor experiment at Los Alamos, involved the use of uranium compounds dissolved or suspended in water. The reactor was called the Aqueous Homogeneous Reactor. In 1948 reactors were cooled by air, some other gas, or water. Research was underway involving the use of molten sodium metal as a reactor coolant. Alvin Weinberg had proposed the use of water under pressure as a reactor coolant. This concept had the potential to control the heat produced by the reactor and put it to useful work powering ships, or driving electrical turbines. This technology attracted the attention of the United States Navy, and eventually led to the development of the nuclear-powered submarine. Naval reactor technology also had potential for electrical production, and the Navy set up the first project to demonstrate civilian electrical production at Shippingport, Pennsylvania.

Meanwhile the Air Force, which was also interested in its own reactor technology to power bombers, sponsored aircraft reactor research in Oak Ridge. The original aircraft reactor concept explored by engineers at the K-25 facility in Oak Ridge involved the use of liquid sodium as a coolant. The original K-25 aircraft reactor concept had a very significant safety defect, and in 1947 three K-25 engineers, V.P. Calkins, Kermit Anderson, and Ed Bettis began to explore a radical reactor concept involving the use of hot liquid fluoride salts. This was a natural concept, because in 1947 K-25 was the largest industrial facility using fluoride chemistry. The three engineers researched the possibility of using liquid fluoride salts as a reactor moderator, fuel carrier and reactor coolant. The K-25 research lead to the Molten Salt Reactor concept. When the aircraft reactor project was transferred to ORNL in 1950 and assigned to the brilliant chemist Ray C. Briant, Ed Bettis pitched the Molten Salt Reactor to Briant. Briant and Bettis pitched it to Weinberg, and it was agreed that the defective K-25 sodium-cooled aircraft reactor concept should be scrapped, and the promising liquid-salt reactor concept become the focus of ORNL Aircraft Nuclear Propulsion research.

During the next few years a radically different reactor concept was to emerge in Oak Ridge. Conventional reactors are much-evolved versions of Alvin Weinberg's water-cooled reactor. They feature complex cores which contain a ceramic uranium dioxide clad with zirconium metal. This fuel system prevents the escape of radioactive fission products into the cooling water, but creates considerable difficulties for processing the fuel for fuel recycling and the extraction of fission products. The UO2 fuel is also a very poor heat conductor, and the fuel pellets inside conventional reactors become very hot, so much so, that there is a danger if the reactor cooling system fails that the UO2 fuel could melt at 2800°C and create an unholy mess.

The water-cooled reactor is just that, water cooled. A system of pipes carry the water through the core where it extracts heat from the fuel pellets. Water boils at 212 degrees under atmospheric pressure, but scientists had long ago discovered that if water can be kept under high pressure, its boiling point goes up. Engineers had discovered that power conversion becomes more efficient if water is pressurized and prevented from boiling at 212 degrees F. In order to prevent the water from boiling inside the sort of pressurized reactor that the Navy uses, the reactor is placed inside a massive steel pressure vessel, and water is pressurized in the reactor. The pressurized water is superheated, and because it is under pressure it does not turn to steam inside the reactor. A second type of reactor, the boiling water reactor, operates under a little lower pressure, and pressurized water begins to turn into steam in the upper part of the reactor.

Reactors that are cooled with pressurized water are quite complex and can be quite large and pose a number of problems. The presence of pressurized water leads to the danger of a steam explosion. Pressurized water can also leak from pipes outside the reactor, creating a danger that the reactor might not receive coolant water. Coolant failure can lead to core meltdown as it did at Three Mile Island. Core meltdown can lead to containment breach, either by melting through the steel pressure vessel, or by releasing hydrogen gas which in turn can explode with enough force to rupture the pressure vessel.

Pressurized water reactors and their cousins boiling water reactors can be made safe, but at a significant price in terms of complexity and weight. Light water reactors have control issues. Chain reactions may not be uniform throughout the reactor. Operators may need to employ control rods to prevent excess reactivity in parts of the reactor which can lead to local overheating and core damage. This necessitates an elaborate system of internal sensors inside the reactor along with an equally elaborate instrumentation, designed to provide operators detailed information about core conditions. During the 1970's reactor operators could be swamped with information leading to confusion and operator errors. This happened at during the Three Mile Island accident. Computer systems are now in place to manage the flow of information from inside the reactor, and to assist human operators in managing pressurized water reactors. Recent designs of pressurized water reactors have impressive safety features and can be described as demonstrating revolutionary improvements in safety over earlier generations of water-cooled reactors. They are also very expensive, and still use enriched uranium dioxide fuel that is expensive and difficult to reprocess. Pressurized water reactor technology is stuck with once-through fuel technology and the problem of nuclear waste.

When Ray C. Briant and Ed Bettis approached Alvin Weinberg in 1950 to discuss the Molten Salt Reactor concept, Weinberg was already aware of the shortcomings of his invention, the pressurized water reactor. Weinberg's mentor Eugene Wigner believed that the Aqueous Homogeneous Reactor was a better route to low-cost electrical energy than the Pressurized water reactor, and Weinberg was pushing Aqueous Homogeneous Reactor research at Oak Ridge. Ed Bettis' Molten Salt Reactor had many of the attractive features of the homogeneous reactor without some of its drawbacks, but it was to take Weinberg some time before he realized that the MSR represented the preferred route to the pressurized water reactor alternative.

Both the Aqueous Homogeneous Reactor and the Molten Salt Reactor featured a liquid fuel-coolant mixture. The mixture was pumped into and out of the core where moderation and geometry enabled criticality. Eugene Wigner had been attracted to the Aqueous Homogeneous Reactor because its fuel could be continuously run through chemical processors outside the core. This meant that neutron-eating fission products could be removed, making the neutron economy of the Aqueous Homogeneous Reactor so efficient that it could breed Thorium to U-233 advantageously. ORNL reactor designers were to design an Aqueous Homogeneous Reactor with a thorium-containing blanket surrounding core containing a heavy water with a dissolved uranium compound. Before his death Ray C. Briant suggested to Weinberg that a Molten Salt Reactor with a thorium blanket, similar to that designed for the Aqueous Homogeneous Reactor would have superior performance to the latter reactor. Thus Briant can be considered the father of the Liquid Fluoride Thorium Reactor, but in many respects the LFTR had many fathers at ORNL.

Compared to the Light Water Reactor the MSR/LFTR had many safety features, the most outstanding of which was its strongly negative temperature coefficient of reactivity. The liquid salt fuel mixture of the LFTR responds to slow and then stop chain reactions as heat within the reactor increases.

The liquid salt in the LFTR core expands as it heats. As it expands there is less liquid salt in the core, carrying with it fissionable fuel. As fissionable fuel leaves the core, the fission reaction rate slows. At maximum core heat, enough fissionable fuel leaves the core to bring the fissionable mass left in the core down below the amount needed to maintain criticality, The chain reaction stops. Core salts retain heat, and heat is also replenished by the radioactive decay of fission products within the core.

What first attracted Ed Bettis and his associates to the Molten Salt Reactor idea was the way it would respond to a pilot's throttle use.

When the pilot demanded more power for his jet engines, heat is drawn out of the reactor core and transferred into the jet engine where it produces jet power. Heat from the LFTR core can also power powers closed-cycle gas turbines in electrical generating systems. As core temperature decreases, core salts shrink, and more salt is in the core, thus increasing the fission reaction rate. The greater the demand for power for a jet engine or a generator the greater the amount of heat generated by the core, and as a consequence the reaction rate within the core increases. The limitation of power output is determined by the heat removal rate, which in turn is based on the limitations of the turbine generating system.

The reaction rates slow down and then stop as heat withdrawal is decreased, or as temperature increases in Molten Salt Reactors--they basically control themselves. Thus while Pressurized Water Reactors require constant operator monitoring and operator input into its control system, MSRs including the LFTR, basically control themselves. The potential instability of the PWR is simply not present in the LFTR.

Compared to PWR, the LFTR has superior peak load reserve and load-following capacities. Since a LFTR's salts are at maximum heat when a LFTR is on standby, the LFTR can produce maximum power as quickly as its turbines can go to full generating speed under load. Thus the LFTR can not only load follow but can serve as peak demand reserve.

In the case of decreased load demand, less heat is drawn from the core, and the fission reaction rate slows. Thus the same feature that gives the LFTR superior safety over the Pressurized Water Reactor also gives it superior flexibility in generating electricity.

10 comments:

Anonymous said...

It is unfortunate that pro nuclear folks feel the need to denigrate nuclear technology that is not their favorite. This is the same strategy the anti nuclear people use. All the engineering problems with light water reactors listed here have been analyzed and solved.

If we mass produced Gen III reactors they could meet the worlds energy needs for clean low cost reliable power with a simple once through fuel cycle using sea water uranium for 400 years. We should have been doing that for the last 30 years and should do it now.

We should have a massive R&D program to develop Gen IV reactors including LFTR. The detailed development will reveal that these plants have their own set of engineering problems that will be solved.

As for safety, existing reactors are too safe already. The money time and emotion we spend on safety leaves us dependant on far more dangerous technology. Claiming that the new technology will be safer, as if it reduces risk significantly, is false and plays into the hands of the anti nuc’s.

Existing nuclear technology is good not bad. Future technology can be better.

Bill Hannahan

Anonymous said...

Bill,

We need to greatly expand nuclear power. The status quo is not getting the job done. LFTR greatly improves the safety, proliferation resistance, waste management, and cost of current reactor technology.

Current reactor technology is very good. Good enough for you and me. But we can do much better. And we must, if we are to expand nuclear like we need to.

charlesH

Charles Barton said...

Bill the issues i discuss are not original with me. I feel that interest of nuclear technology is best addressed by acknowledging the problems and offering solutions. This is the route to progress.

donb said...

I think Bill Hannahan makes some good points. And as CharlesH says, safety well beyond that obtained with fossil fuels has been engineered into current LWR designs. I think we have every right to expect more safety from nuclear energy than from fossil fuels.

Engineering has brought us the blessing of safety, and has also brought us the curse of high cost. Regulation has been a significant contributor to high cost. A lot of regulation has been done in the name of safety. But a lot of regulation has simply added to costs with little benefit.

The goal now should be to come up with designs that are naturally safe to reduce the need for expensive materials and systems needed to "engineer in" safety. That is one of the promising features of the LFTR. As Bill Hannahan said, we need a strong development program for find out if the promises are true. A parallel design effort on a different design also needs strong support so that we don't put all our eggs in one basket. I personally would like to see the parallel effort working to make breeder reactor using uranium (we have LOTS of depleted uranium around, already mined, processed, and purified).

In the mean time, we should be doing what we know how to do now: building new LWR reactors as fast as we can in order to mitigate problems associated is burning fossil fuels. We need to build LWR fuel recycling plants to reduce waste, and to provide the start-up charge for LFTRs should they prove successful.

While there is no such thing as perfect safety or zero failures, future reactors should be designed so that failures are (as someone here has said previously) an economic event, not a safety event.

donb said...

I think Bill Hannahan makes some good points. And as CharlesH says, safety well beyond that obtained with fossil fuels has been engineered into current LWR designs. I think we have every right to expect more safety from nuclear energy than from fossil fuels.

Engineering has brought us the blessing of safety, and has also brought us the curse of high cost. Regulation has been a significant contributor to high cost. A lot of regulation has been done in the name of safety. But a lot of regulation has simply added to costs with little benefit.

The goal now should be to come up with designs that are naturally safe to reduce the need for expensive materials and systems needed to "engineer in" safety. That is one of the promising features of the LFTR. As Bill Hannahan said, we need a strong development program for find out if the promises are true. A parallel design effort on a different design also needs strong support so that we don't put all our eggs in one basket. I personally would like to see the parallel effort working to make breeder reactor using uranium (we have LOTS of depleted uranium around, already mined, processed, and purified).

In the mean time, we should be doing what we know how to do now: building new LWR reactors as fast as we can in order to mitigate problems associated is burning fossil fuels. We need to build LWR fuel recycling plants to reduce waste, and to provide the start-up charge for LFTRs should they prove successful.

While there is no such thing as perfect safety or zero failures, future reactors should be designed so that failures are (as someone here has said previously) an economic event, not a safety event.

P.M.Lawrence said...

An idle thought has occurred to me - just as something amusing to turn around in the mind, not as a serious practical suggestion. In theory, you could make a Gaseous Homogeneous Reactor just by having a large enough chamber with fluorine and some uranium hexafluoride in it.

Rod Adams said...

Though I cannot talk about the details, please understand that submarines and aircraft carriers need power plants that respond just as rapidly as those that power airplanes.

Negative temperature coefficients of reactivity are not a unique characteristic of LFTR technology.

I understand what Charles is saying - light water technology does have some inherent engineering challenges that can add a bit to the cost of the plant. Safety is clearly achievable, but not "easy" or inherently cheap. However, much of the cost associated with LWR plants is avoidable through the use of good project management, regulatory reform, and careful work force training. As Charles says, mistakes during construction can lead to very expensive rework - my answer is "then don't make those mistakes."

Like Bill, however, I do get a bit frustrated with the continuous bickering among fission advocates. As far as I can tell, every fission power plant design available today is hundreds of times BETTER than the fossil fuel power plants that provide more than 80% of the world's electricity and nearly 100% of the process heat and transportation markets. Those are the competitors that we need to attack, not each other.

Charles Barton said...

Rod, I see the Great energy debate of the 20th century as a continuation of a debate that was begun by Enrico Fermi, and Eugene Wigner during World War Ii. Even then there was not agreement among the founding fathers of reactor design about the best way to go about extracting electricity from Nuclear power. Wigner argued, correctly in light of experience, that capital costs for solid core reactors would be higher than coal fired power plants, but that the cost of liquid core reactor generating systems, especially if they used the thorium fuel cycle, would be competitive or lower than coal.

Agood case can be made that Wigner knew what he was talking about. Further, I feel that making that case is important, because there are impotant issues here that effect the future of the American economy, and indeed the world economy, as well as the cost of coping with peak oil and AGW. These issues which are impacted by choices we make about nuclear technology are important reasons for caring about our choices.

What you see as petty bickering, I see as an important debate among nuclear advocates. I believe that we ought to be confident enough about the future of nuclear power to lay the issues on the table and start to sort them out.

alex de maida said...

A question for Charles, or anyone wants to answer.
LWR are often (almost always) operated at 100% of full power, but what about the thermal efficiency (usually 30-35% at full power) of LWR at partial loads? I' ve never found anything about this point

Kaj Luukko said...

One big safety issue in LWR is the power generated in the core after the shut down. As we know, there is enough of power to melt the core if the cooling fails.

How is this taken care of on MSR? Is there the same amount of thermal power in the fuel after shutdown?

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