Reactor-grade plutonium is found in spent nuclear fuel that a nuclear reactor has irradiated (burnup) for years instead of weeks or months, leading to transmutation of much of the fissile, relatively long half-life isotope 239Pu into other isotopes of plutonium that are less fissile or more radioactive.
Thermal-neutron reactors (today's nuclear power plants) can reuse reactor-grade plutonium only to a limited degree as MOX fuel, and only for a second cycle; fast-neutron reactors, which are uncommon today, can use this or any other actinide material indefinitely.
The degree to which reactor-grade plutonium is less useful than weapons-grade plutonium for building nuclear weapons is debated, with many sources saying it is difficult or impossible, and others saying it is relatively easy with modern technologies like fusion boosting to overcome predetonation, remote manipulation for assembly of highly radioactive components, and cooling of the pit during storage to offset accumulation of decay heat.
Classification by isotopic composition <1976 >1976
<7% Weapons grade
7-19% Reactor grade Fuel grade
>19% Reactor grade
The difference is important in assessing significance in the context of nuclear proliferation. Reprocessing of LWR (PWR or BWR) spent fuel recovers reactor grade plutonium (as defined since 1976), not fuel grade.
The DOE definition of reactor grade plutonium changed in 1976. Up until 1976, two grades were recognised: Weapons grade, less than 7% Pu-240 and Reactor grade, 7% or more Pu-240. From 1976, three grades were recognised: Weapons grade, less than 7% Pu-240, Fuel grade, 7% to 19% Pu-240 and Reactor grade, more than 19% Pu-240.
Reactor-grade plutonium nuclear test 
The reactor grade plutonium nuclear test was a low-yield (under 20 kilotons) underground nuclear test using non-weapons-grade plutonium, conducted at the US Nevada Test Site in 1962. Some information regarding this test was declassified in July 1977 under instructions from President Jimmy Carter as background to his decision to prohibit nuclear reprocessing in the USA.
The plutonium used was manufactured in a Magnox reactor in the United Kingdom, and provided to the US under the 1958 US-UK Mutual Defence Agreement. Its isotopic composition has not been disclosed, other than the description reactor grade and it has not been disclosed which definition was used in describing the material for this test as reactor grade. The plutonium was apparently sourced from the military Magnox reactors at Calder Hall or Chapelcross. The content of plutonium-239 in material used for the 1962 test is estimated to have been at least 85%, much higher than typical spent fuel from currently operating reactors.
Reuse in reactors 
Fast neutron reactors can use plutonium of any isotopic composition.
Reprocessing was planned in the 1960s when planners expected the uranium market to become tight and fast breeder reactors to be needed to efficiently use uranium supplies. This became less urgent with reduced demand forecasts and increased uranium supplies, and commercial deployment of fast reactors was postponed.
Today's thermal reactors can reuse plutonium to a limited degree as MOX fuel. Some reactors limit MOX fuel to a fraction of the total fuel load for nuclear stability reasons. Only the odd-mass isotopes of plutonium are fissile with thermal neutrons, and the even-mass isotopes accumulate. Plutonium-240 is a fertile material like uranium-238, becoming plutonium-241 on neutron capture, but plutonium-242 both has a low neutron capture cross section, and would require 3 neutron captures before becoming a fissile nuclide.
- Reactor-Grade Plutonium Can be Used to Make Powerful and Reliable Nuclear Weapons, FAS, Richard Garwin, CFR, Congressional testimony, 1998
- Reactor-Grade and Weapons-Grade Plutonium in Nuclear Explosives, Canadian Coalition for Nuclear Responsibility
- Nuclear weapons and power-reactor plutonium, Amory B. Lovins, February 28, 1980, Nature, Vol. 283, No. 5750, pp. 817–823
- Garwin, Richard L. (1999-06-15). "The Nuclear Fuel Cycle: Does Reprocessing Make Sense?". In B. van der Zwaan. Nuclear energy. World Scientific. p. 144. ISBN 978-981-02-4011-0. "But there is no doubt that the reactor-grade plutonium obtained from reprocessing LWR spent fuel can readily be used to make high-performance, high-reliability nuclear weaponry, as explained in the 1994 Committee on International Security and Arms Control (CISAC) publication."
- Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium
- Why You Can’t Build a Bomb From Spent Fuel
- Plutonium Isotopics - Non-Proliferation And Safeguards Issues
- International Panel on Fissile Materials, Global Fissile Material Report 2011: Nuclear Weapon and Fissile Material Stockpiles and Production (see Appendix 1), retrieved on October 1, 2012.
- "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium". US Department of Energy. June 1994. Retrieved 2007-03-15.
- WNA contributors (2009-03). "Plutonium". World Nuclear Association. Retrieved 2010-02-28.
moderator Light water
by coolant Water
Gas: carbon dioxide