PIRK 2: a computer program to solve the inverse reactor kinetic equations

One of the most important physics parameters required for the operation of the Harwell reactors DIDO and PLUTO is the reactivity worth of the control absorbers. This was originally determined by making a series of measurements of the rate of change of reactivity with position, the differential curve, by means of the doubling time method in which the stable reactor period is obtained from a plot of the reactor power or flux occurring after a small disturbance from equilibrium. The reactivity is then given by the so-called 'Inhour' formula. This paper describes an alternative method which has been used for some years to determine reactivity changes by measuring the output from an ion chamber coupled to a data logger which gives a permanent record of the flux samples. A computer program is presented which will calculate the time dependent reactivity changes which cause any measured departure of reactor flux or power from equilibrium. Allowance is also made for the presence of a source of neutrons. The underlying point kinetic theory is described together with the numerical methods for solving the equations. PIRK 2 is an improved version of an earlier code PIRK. (author)