Nuclear reactors use uranium as their basic fuel. Uranium ore is mined in a number of countries, and must then be processed before it can be used as reactor fuel. Uranium exists in several forms or "isotopes", the principal ones being U-238 (99.3% of naturally occurring uranium) and U-235 (0.7%). U-235 has 3 fewer neutrons in its nucleus, and it is this isotope which splits ("fissions") in a nuclear reactor, giving off energy. There are three main types of nuclear reactor used in the UK — Magnox, Advanced Gas-cooled (AGR) and Pressurised Water (PWR). In Magnox reactors, uranium is used in its metallic form. However, it is more commonly used as ceramic uranium oxide enriched with additional U-235 (up to around 4%). Using uranium fuel in a nuclear reactor produces plutonium (around 1% by weight of used fuel) and waste products (around 2 or 3%). After some years use, the build up of these waste products makes it inefficient to continue using the fuel, although large quantities of fissile material remain. The "spent" fuel is removed from the reactor, and in the UK most spent fuel is taken to Sellafield for reprocessing. After storage under water, to cool the fuel rods and shield against radiation, the used fuel elements are broken up and dissolved in acid. Reprocessing separates out the waste products in used fuel from plutonium and uranium, which are theoretically capable of re-use in the nuclear fuel cycle. The high level waste is converted into a form of glass — "vitrified" — and stored, pending the availability of a national nuclear waste disposal route for such waste. The House of Lords reported on the management of nuclear waste in March 1999, recommending phased disposal in a deep repository; recognition of much separated plutonium as waste; and changes in the organisational structure of nuclear waste management.[146]