Nuclear chain reaction. A series of nuclear reactions in which one of the agents necessary to the series is itself produced by the reactions so as to cause similar reactions. Depending on whether the number of reac­tions so caused directly by one reaction is on the average less than, equal to, or greater than unity, the

reaction is convergent (si/btntical) self-sustained (спи cal), or divergent (supercritical)

Nuclear rcactm A device in which a self-sustaining nuclear fission chain reaction can be maintained and controlled (fission reactor). The term is commonly called “reactor” or “pile ”

Iast reactor A reactor in which fission is induced predominantly by fast neutrons (Also called fast- neutron reactor.)

Thermal reactoi A reactoi in which fission is induced predominantly by thermal neutrons.

Multiplication factor The ratio of the total number of neutrons produced during a time interval (excluding neutrons produced by sources whose strengths are not a function of fission rate) to the total number of neutrons lost by absorption and leakage during the same interval When the quantity is evaluated for an infinite medium or for an infinitely repeating lattice, it is referred to as the infinite multiplication factor (k»,) When the quantity is evaluated for a finite medium, it is referred to as the effective multiplication factor (kepf) (The term is also called multiplication constant.)

Critical Eulfilhng the condition that a medium capable of sustaining a nuclear chain reaction has an effective mult’phcation factor equal to unity (A nuclear reactor is critical when the rate of neutron production, exclud­ing neutron sources whose strengths are not a function of fission rate, is equal to the rate of neutron loss )

Delayed critical Identical with critical, the term is used to emphasize that the delayed neutrons are necessary to achieve the critical state

Prompt critical fulfilling the condition that a nuclear chain-reacting medium is critical utilizing prompt neu­trons only.

Prompt neutron fraction. The ratio of the mean number of prompt neutrons per fission to the mean total number of neutrons (prompt plus delayed) per fission.

Delayed-neutron fraction. The ratio of the mean number of delayed neutrons per fission to the mean total number of neutrons (prompt plus delayed) per fission.

Effective delayed neutron fraction. The ratio of the mean number of fissions caused by delayed neutrons to the mean total number of fissions caused by delayed plus prompt neutrons. (Note The effective delayed-neutron fraction is generally larger than the actual delayed — neutron fraction )

Reactivity. A parameter, p, giving the deviation from criticality of a nuclear chain-reacting medium such that positive values correspond to a supercritical state and negative values to a subcritieal state Quantitatively, p = 1 — (l/keff), where keff is the effective multiplica­tion factor.

Excess reactivity. The maximum reactivity attainable at any time by adjustment of the control members.

Built-in reactivity The reactivity of a system as a function of design excluding the experimental and control inserts of the system.

Reactor control The intentional variation of the reaction rate in a reactor or adjustment of reactivity to maintain a desired state of operation.

Reactivity coefficient. The change in reactivity caused by inserting a small amount of a substance in a reactor The reactivity coefficient of a substance may depend upon the amount and distribution of the substance inserted, but is usually quoted as the reactivity change per unit mass of the substance at specific positions in the reactor or as a uniform distribution.

Void coefficient The partial derivative of reactivity with respect to a void (i. e, the removal of the material) at a specified location within a reactor. It is equal to the reactivity coefficient of the material removed.

Isothermal temperature coefficient of reactivity. The change of reactivity caused by a one-degree increase in the uniform temperature of a reactor at zero power

Powei coefficient of reactivity The change of reactivity per unit change of reactor thermal power when other variables are not independently changed