Monday, April 8, 2013

Pebble Beds

I recently began to think again about the Pebble Bed Reactor. In the early days of Nuclear Green, I was encouraged by what I thought was the potential of the Pebble Bed Reactor. The Pebble Bed Reactor is cooled by a gas; most likely helium and the pebbles resemble tennis balls in size. They are made from graphite and other carbon based materials. In the Pebble based reactor, the helium coolant blows the pebbles up into the reactor core until they reach a critical geometry and begin to support a chain reaction. This heats the helium which exits the reactor core drawing off the surplus heat. The helium passes through a cooling system that transfers the heat to a secondary gas which then drives a turbine or in some cases the heat is transferred to water which turns to steam and the steam drives the turbine.

When I first began to think about post carbon energy, the Pebble Bed Reactor ranked high on my list of coal replacement candidates. Later on I was to discover there were problems. The Molten Salt Reactor always ranked high on my list and continues to do so, but I believe one of the primary functions of fourth generation nuclear technology is to lower energy costs. I held that the Molten Salt Reactor had real potential for doing so and I continue to hold this view. The Pebble Bed Reactor does not hold as much potential for lowering energy costs.

The core of the Pebble Bed Reactor must be quite large and strong enough to withstand the pressure of heated helium passing through the reactor core. The core of the Pebble Bed Rector may not be as heavy as the core of the commercial Light Water Reactor, but it is quite large. Larger in fact than the core of conventional reactors. There are other parts of the Pebble Bed Reactor that will be quite large. All of these large parts add up to a considerable cost disadvantage when it is compared to the Molten Salt Reactor.

Seven or eight years ago reactor scientists at Oak Ridge National Laboratory and at the University of California at Berkley came up with a solution to the problems of Pebble Bed Reactors. That solution was to replace the helium core coolant with a Molten Salt Reactor coolant. The core of a molten salt cooled Pebble Bed Reactor would be quite small compared to a helium cooled Pebble Bed Reactor yet the molten salt cooled Pebble Bed Reactor would have many of the advantages of a helium cooled Pebble Bed Reactor.

The molten salt cooled Pebble Bed Reactor could take advantage of research conducted in West Germany and South Africa on Pebble Bed Reactors as well as research done on molten salt coolant and molten salt cooling systems conducted at ORNL. The resulting reactor would be quite inexpensive and could be build in factories and transported by truck, railroads, and river barges. Most of the safety advantages of Molten Salt Reactors would apply to a molten salt cooled Pebble Bed Reactor. Pebble bed safety technologies could be used when molten salt technologies were not appropriate.

Small Pebble Bed Molten Salt Reactors could be build in factories and buried underground. Larger Pebble Bed Molten Salt Reactors could be constructed in factories through modular design and then assembled on site. Many of the safety regulations applied to light water cooled reactors are redundant for Pebble Bed Molten Salt Reactors. A nuclear regulatory agency ought to recognize the superior safety characteristics of molten salt nuclear technology and regulate accordingly.

Tuesday, April 2, 2013

What I Would Do, If I Were Kirk Sorensen

Kirk Sorensen has kept his cards very close to his chest since he and a business partner established Flive Energy. Officially, Kirk is as committed to the LFTR, Liquid Fluoride Thorium Reactor, as much as always. Long term, I would not expect that to change, but in the short term, if I wanted to make money, I would not focus much attention on LFTR development.

Before my illness, I focused a good deal of attention on uranium fueled Molten Salt Reactors with or without added thorium. Such reactors might require much less research and development than the LFTR. Uranium fueled Molten Salt Reactors have significant commercial potential. They probably can be build at a lower cost than commercial Light Water Reactors and can be housed underground saving on building costs. Small Molten Salt Reactors can be factory built and can largely be based on technology that has already been successfully tested by Oak Ridge National Laboratories' Molten Salt Reactor experiment.

Using successfully tested technology is the key to getting a new reactor concept on the market quickly and at a reasonable cost. Were I Kirk, I would be working towards the development of a uranium fueled Molten Salt Reactor that has potential for commercial sales. The uranium fueled Molten Salt Reactor has the potential for factory production and underground siting. This in turn could lead to a low cost reactor option that could be cost competitive with Light Water Reactors. The aim would be a relatively small reactor, something between 60 Mw and 250 Mw, that would be produced in factories and could be housed in abandoned salt mines,  underground silos, and other underground facilities and would be safe enough to site close to large cities.

Production of the first generation uranium fueled Molten Salt Reactor would lead to greater understanding of the MSR and could serve as a launching point for a more advanced Molten Salt Reactor design such as the LFTR. A successful profit making reactor business could finance its' own LFTR research through reactor sales, but the goal is not to produce LFTRs as much as to make nuclear power more competitive as a replacement for coal and natural gas in stationary generating plants. Therefore, were I Kirk Sorensen, I would not be working on the development of the LFTR, but on the development of uranium fueled Molten Salt Reactors.

Should Kirk then abandoned the use of the LFTR as the symbol of the Molten Salt Reactor? I think not. The uranium Molten Salt Reactor is likely to be only a bridge between the commercial Light Water Reactor and the LFTR. The LFTR offers several advantages over Light Water Reactors. One of the most significant being a solution to the problem of nuclear waste. Uranium fueled Molten Salt Reactors do not solve the problem of nuclear waste, but LFTRs can largely solve it. In addition, the most promising form of uranium fueled Molten Salt Reactor, the Denatured Molten Salt Reactor, operates with many times more thorium than uranium and is in effect a thorium-uranium hybrid. It produces a lot of nuclear waste, but less nuclear waste than a pure uranium Molten Salt Reactor.

At one time, I did not think that the DMSR was a good idea, but I now think that it is. The supply of thorium in the earth's crust is virtually unlimited and thus people can rely on energy from thorium for a very long time to come. This means in the discussions of the sustainability of nuclear power the supply of thorium is not likely to run out before the sun passes into the stage of solar evolution that does not support life on earth. The problem will in no way ever be a threat to the biosphere or human existence.

A number of years ago, Kirk and a couple of his fellow students in nuclear engineering at the University of Tennessee developed a pre-design of a small truck mobile Molten Salt Powered Reactor that could be used as a mobile electricity generation source. The design included the building of an underground silo in which the reactor would be lowered. Truck mounted generation units could  be located on the surface above the reactor. I believed, up until yesterday, that the small mobile reactor concept was the path that Kirk was following, but yesterday, April 1st 2013, I learned to my astonishment, that Kirk and NASA are designing small Molten Salt Powered drones which will be America's primary weapon in any future war.


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