Factory-build PBRs, ship, and assemble on site

A pebble bed reactor unit can be assembled within a containment building approximately 25 x 40 meters in size. Such a unit would generate approximately 100 megawatts, at a capital cost of roughly $200 million. Most large coal or nuclear electric power plants in the US are much larger, about 1,000 megawatts. An advantage of the relatively smaller PBR unit is that the modules can be grouped and added to as demand increases. From a business investment point of view, a utility company would be making a series of $200 million investments, adding to them as profitability is proven, rather than making a $2 billion investment all at once.

Distributed 100 MW power plants would have advantages over centralized 1,000 MW plants. About 8% of all generated US electrical power is lost in transmission. Having generation closer to consumption reduces these costs. Such localization of power during disruptions can lessen the chance of massive power blackouts caused by cascading failures of the transmission system. Today's electric power grid has become more strained and lossy with deregulation, which can encourage customers to buy power generated a long distance away. Utility companies have difficulty getting site approvals for new nuclear power plants. However, gaining ten times more approvals for smaller plants would not be possible today. Therefore ganged, centralized PBRs would be the most attractive to uilities. Public attitudes about the safety of PBR power plants will have to change before the nation can take advantage of the distributed power generation benefits.

Prefabricated units can be shipped by truck and rail



Marc Berte and Andrew Kadak of MIT have published a presentation Modularity Approach to the Modular Pebble Bed Reactor. The conceptual design of the heat exchangers, turbines, intercoolers, recuperators, and manifolds have been constrained to sizes that can be put into boxes and shipped standard truck and rail facilities. The large reactor vessel, approximately 14 feet in diameter by 50 feet long, will require special transportation. All units can be brought together on site and assembled. One of the design goals is that the units bolt together, without the requirements for specialized welding at the site. Central, factory production of standardized PBR components allows improved quality control, lower costs, and rapid on-site assembly.

MIT drives much pebble bed reactor research

MIT, the Massachusetts Institute of Technology, is a great institution. For example, the MIT Department of Nuclear Science and Engineering has led the study of basic nuclear processes and nuclear engineering for a half century. In light of the fact that the US has not built a nuclear power plant since the 1970s, we are fortunate that the science lives on with in faculty of 28 and senior research staff, 101 graduate and 48 undergraduate students.

Andrew Kadak has the unusual title of Professor of the Practice in this department. He has served the US government looking into issues of nuclear waste and safety. His pre-MIT career within the nuclear power industry included being the CEO of Yankee Atomic Electric. This nuclear power plant in Rowe, Massachusetts was decommissioned in 1992. Click Yankee Rowe to see before and after pictures and learn the fate of the spent fuel.

Much of the current interest in pebble bed reactors sprang from an MIT student summer project in January 1998 advised by faculty advisors Ronald Ballinger and Andrew Kadak. Nuclear Power Plant Design Project involved 6 students and 10 guest lecturers. The students reviewed:

1. Westinghouse AP600 advanced pressurized light water reactor (LWR)
2. ABB System 80+ LWR
3. GE Advanced Boiling Water Reactor (ABWR)
4. General Atomics high temperature gas reactor (HTGR)
5. German AVR pilot pebble bed reactor (a HTGR)
6. Lead bismuth reactor
7. Thorium breeder reactor
8. Liquid metal breeder reactor

They evaluation process considered 26 criteria such as safety, economics, construction time, modularity, efficiency, and lifetime. The process selected the small, modular pebble bed high temperature gas reactor. Some of the identified unique features of the PBR are:

• Inherent safety: no operator actions nor automated systems needed
• Proliferation resistance: reprocessing spent fuel pebbles is impractical
• Short, 36 month construction time
• Modular growth: 100 MW units
• Refueling: no shutdown
• Fuel disposal: spent fuel ready for disposal on removal from core
• High thermal/electric efficiency: no water cooling needed
• On-site assembly/disassembly: components shipped intact to/from factory

Subsequently a number of research projects were conducted and documented at the MIT Pebble Bed Reactor web site. Marc Berte and Andrew Kadak prepared the interesting Modularity Approach of the Modular Pebble Bed Reactor which contains a physical design for a PBR, including designs for all component parts that permit construction in a factory, shipment by rail and truck to a job site, and on-site assembly with no welding required.

I am pleased that in the recent three-decade dark ages of US nuclear power plant construction the US government has funded and MIT has continued research and development in nuclear engineering. Unfortunately it appears that US-sponsored funding of pebble bed reactor research projects at MIT has dropped off in recent years. China and South Africa are both developing pebble bed reactors, attracting the attention of university scientists and engineers. MIT is cooperating and sharing information with Tshinghua University in China. The university now has an operational pilot PBR. Kadak has traveled to China to observe the plant's fail-safe testing, and he is now consulting with the South African project.

PBR passive safety comes from basic physics

The uranium fuel in the pebble bed reactor can not melt down. The nuclear chain reaction slows down as the temperature rises.

Naturally occurring uranium contains two isotopes, 0.7% U235 and 99.3% U238. The fissionable U235 in the fuel spheres is enriched to exceed 3% so that a chain reaction can be sustained. As a U235 atom nucleus decays it releases neutrons and creates heat. The neutrons bounce off moderating carbon atoms and slow down enough to be captured by other U235 atoms, continuing the chain reaction. If unchecked this process could overheat a conventional reactor, so control rods and other moderators are introduced to prevent a melt-down such as happened at Three Mile Island.

In the pebble bed reactor high temperature U238 atoms absorb neutrons to prevent run away overheating. At normal operating temperatures the U238 atoms have a low probability of absorbing a neutron, because the relative velocities of the neutron and U238 must be within a narrow range. The process is said to have a low cross section for absorbtion of the neutron by U238. As the reactor heats the U238 atoms vibrate more rapidly, increasing the chance the neutron and U238 nucleus will have the right relative velocity to absorb a neutron. This is called Doppler broadening, much like the Doppler effect that raises the pitch of oncoming train whistles. The captured neutrons can not further the U235 chain reaction, so as the reactor heats up the chain reaction is checked and the temperature approaches a steady state.


The graph above results from a computer simulation of the temperature of a pebble bed reactor in a worst case scenario. The temperature rises to a maximum of 1500 degrees Celsius. The pyrolytic carbon and silicon carbide structures of the fuel pebbles maintain physical integrity to about 2000 degrees Celsius. The hot pebble bed reactor will remain stable and safe until corrective measures are undertaken. This intrinsic, passive safety has been verified by experiments with the 1980s German AVR reactor and the 2006 Chinese pebble bed reactor. Continuing reasearch and experimentation within the US will be key to calming public concerns about nuclear reactor meltdowns.

Operator errors are reduced.

Operator errors were causes of the meltdown at Three Mile Island and the fire at Chernobyl. The PBR design is much safer. Suppose that the operators were suicidal terrorists and they undertook to create havoc and they

• Shut off all cooling.
• Withdrew all control rods.
• Undertook no emergency actions.

Over a period of hours the PBR temperature would rise to its stable 1500 degree Celsius operating point, giving off heat by radiation and convection, until engineers could be brought to the site to correct the situtation.

PBR design has important safety factors.

1. High heat capacity of the graphite core.
2. High temperature capacity of the core components.
3. Chemical stability and inertness of the fuel, coolant, and moderator.
4. High retention of fission products within fuel coatings.
5. Single phase characteristics of the helium coolant.
6. Low energy density of the fuel core.
   

Pebbles circulate within the reactor

The reactor vessel is about 50 feet high and 15 feet in diameter, containing approximately 360,000 fuel pebbles. Pebbles are introduced at the top and flow down and out at a rate of 3,000 per day. As each fuel pebble is removed it is examined to determine how much of the fuel has been used up. After a few years to reach steady state operation about 350 pebbles will be replaced daily with fresh ones.

A nuclear chain reaction takes place within the reactor, creating heat. The heat is taken up by circulating helium gas. Helium is chemically inert and it does not form radioactive isotopes, so it can safely be pumped outside the reactor vessel and its containment building to directly power a gas turbine generator to produce electric power. The helium gas enters at 500 degrees Celsius and exits at about 900 degrees Celsius.

Helium cools the reactor and powers the generator

The helium circuit contains pressure modifying turbine-compressors and heat exchangers between the reactor and the generator. This results in a high, 44% transfer of thermal energy to electrical energy. In comparison, a typical coal or nuclear power plant has a 33% efficiency.

The mechanisms for removing and introducing the fuel pebbles are important to the overall safety of the reactor. In 1988 the German AVR pebble bed reactor experienced a pebble jam and released some radiation. Coming shortly after the Chernobyl disaster, public fears led to the AVR shutdown. Germany decided to turn off all its 19 nuclear power plants by 2025.

Pebbles contain the products of radioactive decay

Pebbles contain thousands of coated particles of UO2

Each fuel sphere, or "pebble" is a bit smaller than a tennis ball and contains many small particles of uranium oxide, which is the fuel for the nuclear reactor. Each fuel particle is wrapped in several layers of ceramic carbon and silicon carbide. The first layer of porous pyrolytic graphite absorbs the radioactive xenon gas emitted when the uranium splits. Next is a containment wrapping of high density nonporous pyrolytic carbon, a layer of fireproof silicon carbide, further contained by a layer of pyrolytic carbon. About 15,000 of the coated fuel particles are embedded in a graphite matrix that forms the fuel sphere, which is surfaced with a layer of pyrolytic graphite.

The carbon in the pebbles also acts as a moderator for the nuclear reaction. Carbon slows down U235 decay neutrons that bounce off the carbon atoms so that the slowed neutrons have a good chance of splitting another U235 atom.

A designed effect of these multiple layers is to contain the products of radioactive decay within the pebbles themselves. The pebbled radioactive waste can be safely disposed of in geological storage.

The carbides and pyrolytic carbon materials in the pebbles all maintain strength at temperatures well above the reactor's possible temperatures. The presence of carbon within the reactor should not lead to concerns that a fire would lead to a Chernobyl-like disaster. Pyrolytic carbon will not burn at temperatures as high as 2000 degrees C, well above the maximum possible 1650 degree temperature of a worst case criticality excursion. In any case, the UO2 is contained within a sealed silicon carbide firestop. The inherent safety of the pebble bed reactor is key to public acceptance of widespread use of the technology, so further research and experimentation will be an important confirming activity.

Wikipedia and MIT web sites have more information.