A containment within which a controlled chemical, biochemical, or nuclear reaction takes place. There are various forms of chemical reactors whose design and application is dependent on the form of the chemical reaction. They may be large vessels or pipes, operated continuously or as a batch. The design of the reactor used for contacting of the reactants is important in determining the yield of desired product. In general, batch reactors are used where chemical conversion is dependent on reaction time only, whereas tubular reactors are used where chemical conversion is dependent on the position. Continuous stirred-tank reactors are used where neither time nor position is critical. Batch reactors require a charge of reactants, whereas continuous reactors operate at steady state with a continuous flow of fresh reactants and the removal of products. The reactants and products may be liquid or gaseous. Catalysts may be required within the reactor or the presence of inert materials such as in fluidized bed reactors. Continuous stirred-tank reactors are commonly used in which liquid reactants are continuously charged to the reactor, which is equipped with an agitator to ensure homogeneity of the reactants. There is a continuous overflow to maintain a constant volume within the reactor. A plug flow reactor also operates continuously and consists of tubes that may be packed with a catalyst and used for gas- and liquid-phase reactions. They assume idealized plug flow of materials flowing through them and are used for both exothermic and endothermic reactions that require rapid heat transfer from or to the reaction. Fluidized bed reactors involve reactants to be injected under a bed of solid particles such as a catalyst and are effective at promoting heat and mass transfer as well as reaction rates. There are many other types of reactor such as pulsed baffle reactors and semi-batch reactors, which operate with both continuous and batch feeds and outputs.