leaving the price below $10 per pound for yellowcake by year-end 1989. As uranium prices fell, producers began curtailing operations or exiting the business entirely, leaving only a few actively involved in uranium mining and causing uranium inventories to shrink significantly. Since 1990 uranium requirements have outstripped uranium production. World uranium requirements have increased steadily to in 2017. Several factors are pushing both industrialized and developing nations to seek alternatives to fossil fuels. The increasing rate of consumption of fossil fuel is a concern for nations lacking in reserves, especially non-OPEC nations, as is the pollution produced by coal-burning plants. On the other hand, it is still difficult to tap economically into the world's vast solar, wind, and tidal energy reserves. Uranium suppliers hope that these factors will drive an increase in uranium production due to demand for nuclear power generation. Uranium prices reached an all-time low of US$7/lb in 2001. This low was followed by a period of gradual rise, followed by a bubble culminating in mid-2007, with the price to peaking at around US$137/lb, the highest price (adjusted for inflation) in 25 years. The higher price during the bubble has spurred new prospecting and reopening of old mines. In 2012 Kazatomprom and Areva were the top two producing companies (with 15% of the production each), followed by Cameco (14%), ARMZ Uranium Holding (13%) and Rio Tinto (9%). Following the shutdown of many nuclear power plants after the Fukushima Daiichi nuclear disaster in 2011, demand fell to about per year in 2015 with future forecasts uncertain. Because of the improvements in gas centrifuge technology in the 2000s, replacing former gaseous diffusion plants, cheaper separative work units have enabled the economic production of more enriched uranium from a given amount of natural uranium, by re-enriching tails ultimately leaving a depleted uranium tail of lower enrichment. This has somewhat lowered the demand for natural uranium. Market operations Unlike other metals such as copper or nickel, uranium is not traded on an organized commodity exchange such as the London Metal Exchange. Instead it is traded in most cases through contracts negotiated directly between a buyer and a seller. Recently, however, the New York Mercantile Exchange announced a 10-year agreement to provide for the trade of on and off exchange uranium futures contracts. The structure of uranium supply contracts varies widely. Pricing can be as simple as a single fixed price, or based