at lower pressure with wider diameter piping. The world's first commercial Brayton cycle solar power module (100 kW) was built and demonstrated in Israel's Arava Desert in 2009. Removal of fission products The LFTR needs a mechanism to remove the fission products from the fuel. Fission products left in the reactor absorb neutrons and thus reduce neutron economy. This is especially important in the thorium fuel cycle with few spare neutrons and a thermal neutron spectrum, where absorption is strong. The minimum requirement is to recover the valuable fissile material from used fuel. Removal of fission products is similar to reprocessing of solid fuel elements; by chemical or physical means, the valuable fissile fuel is separated from the waste fission products. Ideally the fertile fuel (thorium or U-238) and other fuel components (e.g. carrier salt or fuel cladding in solid fuels) can also be reused for new fuel. However, for economic reasons they may also end up in the waste. On site processing is planned to work continuously, cleaning a small fraction of the salt every day and sending it back to the reactor. There is no need to make the fuel salt very clean; the purpose is to keep the concentration of fission products and other impurities (e.g. oxygen) low enough. The concentrations of some of the rare earth elements must be especially kept low, as they have a large absorption cross section. Some other elements with a small cross section like Cs or Zr may accumulate over years of operation before they are removed. As the fuel of a LFTR is a molten salt mixture, it is attractive to use pyroprocessing, high temperature methods working directly with the hot molten salt. Pyroprocessing does not use radiation sensitive solvents and is not easily disturbed by decay heat. It can be used on highly radioactive fuel directly from the reactor. Having the chemical separation on site, close to the reactor avoids transport and keeps the total inventory of the fuel cycle low. Ideally everything except new fuel (thorium) and waste (fission products) stays inside the plant. One potential advantage of a liquid fuel is that it not only facilitates separating fission-products from the fuel, but also isolating individual fission products from one another, which is lucrative for isotopes that are scarce and in high-demand for various industrial (radiation sources for testing welds via radiography), agricultural (sterilizing produce via irradiation), and