Thursday, August 14, 2008

MSR/LFTR Development: Forsberg

Two American scientists, Drs. Ralph Moir and Charles Forsberg have attempted to assess the state of MSR/LFTR development. (Access to papers discussed can be found here.)

In this post, I will discuss Forsberg's views on MSR/LFTR development as stated in a 2006 paper, Molten-Salt-Reactor Technology Gaps (Proceedings of ICAPP ‘06, Reno, NV USA, June 4–8, 2006, Paper 6295), in which he lays out both the case for development and discusses the developmental research required to achieve a commercial LFTR product.

Forsberg pointed to two major advantages of MSR/LFTR technology:

"As a liquid-fuel reactor, the MSR has two sets of unique characteristics relative to solid-fuel reactors.

• Safety. Under emergency conditions, the liquid fuel is drained to passively cooled critically safe
dump tanks. By the use of freeze valves (cooled sections of piping) and other techniques, this
safety system can be passively initiated upon overheating of the coolant salt. MSRs operate at
steady-state conditions, with no change in the nuclear reactivity of the fuel as a function of
time. Last, the option exists to remove fission products online and then solidify those
radionuclides into a stable waste form. This minimizes the radioactive inventory (accident
source term) in the reactor core and potential accident consequences.

• Fuel cycles. The liquid fuel allows online refueling and a wide choice of fuel cycle options:
burning of actinides from other reactors, a once through fuel cycle, a thorium-233U breeder cycle,
and a denatured thorium-233U breeder cycle. Some of the options, such as a thermal-neutron-
spectrum thorium-233U breeder cycle require online refueling and thus can not be practically
achieved using solid fuels. The use of a liquid fuel also avoids the need to develop fuel or
fabricate fuel

Forsberg believes that recent advances in Brayton cycles gas turbines, actually solve many MSR developmental issues, and point theway to a significant improvement in MSR efficiency:

• "Efficiency. MSRs are naturally high-temperature reactors. Depending upon the choice of salt, the
freezing points are between 320 and 500°C. The heat transfer properties (viscosity, thermal
conductivity, etc.) improve rapidly with increasing temperature. Consequently, the
detailed 1000-MW(e) conceptual design of the MSR had a reactor-core fuel-coolant exit
temperature of 705°C. However, because of corrosion and other constraints in steam cycles,
peak steam cycle temperatures are between 500 and 550°C. In the 1960s designs, high-
temperature heat was inefficiently dumped to lower temperatures to match what the steam
cycle could tolerate. This process reduces heat exchanger sizes but has a large penalty in terms
of efficiency. In contrast, many Brayton cycles operate above 1000°C. The adoption of closed
helium or nitrogen Brayton power cycles enables the power cycle to efficiently use the high-
temperature heat generated by the MSR. This capability allows a 15% improvement in
electrical power output without changing the temperatures of the fuel salt exiting the reactor

According to Forsberg, the choice to use Brayton cycle power generating technology would be a considerable developmental resolve these developmental issues:

Freeze protection
• Tritium control. Adoption of a Brayton cycle provides an alternative tritium trapping option where the tritium is removed from the helium in the Brayton power cycle. This is potentially a high
performance low-cost option based on demonstrated inexpensive methods to remove tritium gas or tritiated water from helium. Helium-cooled high-temperature reactors produce tritium from nuclear reactions with 3He and from leaking fuel; consequently, these reactors are equipped with systems to remove the tritium from the helium
• Chemical reactions. "Changing from a steam cycle to a gas Brayton cycle eliminates this class of challenges."

Forsberg also reported that Pebble Bed Reactors are "being constructed in South Africa with a helium Brayton power cycle. Additional technology development would be required for an MSR; however, the closed Brayton cycle technology istransitioning to a commercial technology".)

Forsberg notes second developmental shortcut:

"In the last decade, compact plate-fin and printed circuit high-temperature heat exchangers have been developed for the aircraft, chemical, and offshore-oil industries. The adoption of compact heat exchangers drastically reduces the molten fuel salt inventory in the heat exchangers and may reduce the inventory of fuel salt in the reactor by up to 50%. There are major benefits in using such heat exchangers products used in industry. These heat exchangers are being considered for use in high-temperature helium-cooled reactors and in the transport of heat from high-temperature gas-cooled reactors to hydrogen production plants using liquid-fluoride-salt heat- transport systems. Additional work is required to fully evaluate their use in MSRs".

Forsberg reports the advantages of new technology heat exchanges to include:

* Fuel salt inventory. Reducing the fuel inventory reduces both fuel salt costs and nonproliferation
risks, because the total fissile inventory in the nuclear system is decreased

Fuel salt processing. "In an MSR, volatile fission products (including xenon) are removed continuously, which creates a large parasitic neutron sink in solid-fuel reactors. For nonvolatile fission products, the fuel salt is processed online or off-line, depending upon design goals. Reducing the salt inventory reduces the quantities of salt to be processed".

Heat exchanger size "The size of the heat exchangers is reduced by a factor of 3 or more.

Tritium control. "The aircraft and other industries have developed compact heat exchangers with buffer gas zones to separate different fluids that may react explosively—such as hot gases vaporizing fuels in aircraft. The same technologies enable trapping of tritium from the primary system in the heat exchanger".

Forsberg also addresses four further MSR/LFTR developmental issues:
* Fuel Storage
* Noble Metal Plate-Out
* High-Level Waste (HLW) Form
* Peak Reactor Temperature

The curious can read what Forsberg says about the first three by accessing Ralph Moir's MSR paper collection. I will briefly report of Forsberg's suggestions on peak reactor temperature.

According to Forsberg with current reactor construction materials, materials, the peak operating temperature is limited to around 750 degrees C. Forsberg suggests that there are a number of good reasons making higher temperature operations desirable. He states, "Carbon-carbon composites are potentially an enabling technology for very high temperature MSRs. However, there are major technical uncertainties including joining technologies". Indeed there are. The same radiation problem that effect core graphite will effect carbon-carbon composites. One possibility which Forsberg did not consider is the use of carbon nanotubes in MSR core construction. "Pound for pound, carbon nanotubes are stronger and lighter than steel . . ."

Boris Yakobson, a Rice University professor of mechanical engineering and materials science and of chemistry, noted a unique feature of some carbon nanotubes, there ability to self repair. When damaged, tiny blemishes crawl over the skin of the damaged tubes, sewing up larger holes as they go. Yakobson stated, "The shape and direction of this imperfection does not change, and it never gets any larger," "We were amazed by it, but upon further study we found a good explanation. The atomic irregularity acts as a kind of safety valve, allowing the nanotube to release excess energy, in much the way that a valve allows steam to escape from a kettle."

Yakobson and a research associate Feng Ding found that this mechanism had the power to heal damage to carbon nanotubes caused by radiation. Whether the nanotubes can self repair in a reactor core has yet to be tested, but carbon nanotubs have an interesting potential both as a MSR/LFTR core moderator, and as a heat and radiation resistant structural material.

1 comment:

jimcrea said...

Charles, in this post I would like to express some “out of the box thinking” to address some of the design challenges that you listed in your post. As a laymen, I would also like to express an up front apology for any technical or political offence that I am about to unintentionally express which is only due to inexperience and lack of knowledge of Molten salt reactors. With the courage of ignorance, let me begin.

I propose a hybrid reactor design that is a combination of a TRISO pebble bed reactor and a MSR/LFTR reactor.

This hybrid is a Pebble Bed Modular Reactor (PBMR) with a LFTR pot located in the center of the pebbles such that the pebbles provide a blanket for the LFTR pot. This blanket covers the pot on all sides and top and bottom. The pot is made of Hastelloy-N and separates the pebbles from the molten salt.

Description of the molten salt.

The molten salt contains only thorium and associated standard beryllium and fluorine additives.

Description of the pebble bed blanket.

At reactor startup, the pebble bed is standard TRISO 60MM fuel enriched to less then 20% with the amount of TRISO pebbles just enough to begin the thorium fuel ignition.

Once the thorium fuel has ignited, the TRISO fuel is gradually replaced with 60 MM silicon carbide coated graphite pebbles that serve as a moderator of the thorium chain reaction.

The standard PBMR automatic pebble inspection process replaces all deteriorated TRISO and/or graphite pebbles.

A mix of TRISO and graphite pebbles in any proportion is possible to optimize the thorium fuel chain reaction.

Both the pebble blanket molten thorium salt are helium cooled to 700C and provide for a light weight Brayton cycle gas turbine.

Design advantages.

High neutron fluxes and temperatures in a compact MSR core that rapidly change the shape of a graphite moderator element and the problem of SiC thermal stresses and de-bonding due to differential thermal expansion are mitigated by constant automatic inspection and replacement of the graphite pebbles.

The pebble bed provides for a heat exchange mechanism based on helium flow around the pebbles.

Freeze protection of the molten salt can be supported by the enrichment of the TRISO fuel proportion as required.

Tritium control is provided by standard “off the shelf” PBMR technology.

The use of TRISO fuel is minimized to keep the fuel cost low.

TRISO fuel can be deep-burned through the high neutron flux provided by the thorium cycle. This minimizes the waste problem.

Protactinium poisoning can be mitigated by increasing the TRISO fuel pebble ratio to avoid fuel reprocessing of this element.

This maintains the advantage of the small 100 MWe to 300 MWe sized reactor in that it can be mass produced in a factory, and transported in several modules to a final set up site.

Minimizes fuel cost by the primary reliance on the thorium fuel source.

TRISO fuel startup allows for rapid construction and deployment of hybrid reactors by eliminating dependence on U233 availability.

Keeps the thorium fuel load minimized to facilitate fuel reprocessing.

This approach takes advantage of the advanced design and deployment state of PBMR in terms of both technical and political acceptability.

Design disadvantages.

The devil is in the details and can’t be resolved until this idea is modeled to see if there is any instabilities and dangers in the operation of this hybrid.


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