Context: Plutonium is a transuranic radioactive chemical element with symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous., A scram or SCRAM is an emergency shutdown of a nuclear reactor , though the term has been extended to cover shutdowns of other complex operations , such as server farms . In commercial reactor operations , this emergency shutdown is often referred to as a `` SCRAM '' at boiling water reactors ( BWR ) , and as a `` reactor trip '' at pressurized water reactors ( PWR ) . In many cases , a SCRAM is part of the routine shutdown procedure as well . The term is usually cited as being an acronym for safety control rod axe man , which was supposedly coined by Enrico Fermi when the world 's first nuclear reactor was built under the spectator seating at the University of Chicago 's Stagg Field , but NRC Historian Tom Wellock calls the axe - man story `` a bunch of baloney . '' It could also stand for `` Safety Control Rods Activator Mechanism '' or `` Safety Control Rods Actuator Mechanism '' , both of which are probably backronyms . ' Scram ' is also a verbal command used to tell someone or something to leave quickly and urgently , although its similarity to the technical term is most likely coincidental ., District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating. The heat is often obtained from a cogeneration plant burning fossil fuels but increasingly also biomass, although heat-only boiler stations, geothermal heating, heat pumps and central solar heating are also used, as well as nuclear power. District heating plants can provide higher efficiencies and better pollution control than localized boilers. According to some research, district heating with combined heat and power (CHPDH) is the cheapest method of cutting carbon emissions, and has one of the lowest carbon footprints of all fossil generation plants. CHPDH is being developed in Denmark as a store for renewable energy, particularly wind energy, that exceeds instantaneous grid demand via the use of heat pumps and thermal stores., A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. A marine propeller of this type is sometimes colloquially known as a screw propeller or screw, however there is a different class of propellers known as cycloidal propellers - they are characterized by the higher propulsive efficiency averaging 0.72 compared to the screw propellers average of 0.6 and the ability to throw thrust in any direction at any time. Their disadvantages are higher mechanical complexity and higher cost., In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay., A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., U). The nuclear chain reaction releases several million times more energy per reaction than any chemical reaction., A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884., A weapons-grade substance is one that is pure enough to be used to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples. (These nuclear materials have other categorizations based on their purity.), A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid (water or gas), which runs through steam turbines. These either drive a ship's propellers or turn electrical generators. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research. Today there are about 450 nuclear power reactors that are used to generate electricity in about 30 countries around the world., A nuclear power plant or nuclear power station is a thermal power station in which the heat source is a nuclear reactor. As is typical in all conventional thermal power stations the heat is used to generate steam which drives a steam turbine connected to an electric generator which produces electricity. , the IAEA report there are 435 nuclear power reactors in operation operating in 31 countries. Nuclear power stations are usually considered to be base load stations, since fuel is a small part of the cost of production. Their operations and maintenance (O&M) and fuel costs are, along with hydropower stations, at the low end of the spectrum and make them suitable as base-load power suppliers. The cost of spent fuel management, however, is somewhat uncertain., Pressurized water reactors (PWRs) constitute the large majority of the world's nuclear power plants (notable exceptions being the United Kingdom, Japan and Canada) and are one of three types of light water reactor (LWR), the other types being boiling water reactors (BWRs) and supercritical water reactors (SCWRs). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary water as both coolant and neutron moderator., Isotopes are variants of a particular chemical element which differ in neutron number. All isotopes of a given element have the same number of protons in each atom. The term isotope is formed from the Greek roots isos ( "equal") and topos ( "place"), meaning "the same place"; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table., A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events., The supercritical water reactor (SCWR) is a concept Generation IV reactor, mostly designed as light water reactor (LWR) that operates at supercritical pressure (i.e. greater than 22.1 MPa). The term "critical" in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor., Subject: scram, Relation: instance_of, Options: (A) aircraft (B) atomic pile (C) base (D) chain (E) chemical element (F) class (G) coating (H) concept (I) cost (J) device (K) direction (L) district (M) electric generator (N) energy (O) fluid (P) gas (Q) grade (R) handling (S) hydrogen (T) industrial process (U) may (V) method (W) motion (X) nuclear power plant (Y) nuclear reactor (Z) nucleus ([) operation (\) physics (]) plant (^) position (_) power station (`) process (a) production (b) sequence (c) single (d) six (e) station (f) store (g) symbol (h) term (i) turbine (j) volume (k) water (l) weapon (m) work
Output: concept