Control element calibration by means of reactivity dedicated computer

To calibrate control elements by 'chronometer method' the Inhour relation and time dependence of the reactor period on positive reactivity insertion are used. Practically the time interval during which the reactor power increases 1.5 times is measured while from the Inhour table one determines the period and the reactivity inserted in the reactor core. Hence a control element can be calibrated by lifting it from fully introduced to completely extracted position in the core. Group constants for delayed neutrons characteristic for TRIGA - SSR 14 Mw reactor are given. To avoid uncertainties generated by time measurements with chronometers and large time consumption implied by this method, a dedicated computer was programmed for calculating the reactivity on the basis of Inhour equation. As input data the signal from the linear channel measuring the reactor power is used. The following steps are done to achieve a control element calibration: - I. reactivity computer is connected to the linear channel measuring the reactor power; - II. power is raised to get the reactor critical (10% of 3 Kw) with the element to be calibrated in down position while the other elements are in the same position and the following three conditions are fulfilled: 1, the reactor poison is removed (three days at least from the last shutdown); 2, fuel temperature equals the one of water in the reactor pool; 3, over the entire period of control elements calibration the reactor power is maintained at values lower than 3 Kw; - III. one waits a few minutes to get the power reactor stabilized (around the value 10% of 3 Kw); - IV. the element to be calibrated is lifted continuously by a given number of units providing the inserted reactivity remains lower than 0.5 $ (in TRIGA SSR 14 Mw reactor the element to be calibrated is lifted by amounts ranging from 25 to 200 units, as a function of the element's value and the portion which is calibrated); - V. After the reactivity insertion is terminated the reactivity is recorded and the largest value as indicated by the reactivity computer is registered till the reactor power indicated by the linear channel reaches the 3 Kw value (scale end); - VI. reactor power is lowered to 10% of 3 Kw by introducing inside the reactor core the other control elements; - VII. steps IV to VI are repeated till the element to be calibrated reaches the completely raised (extracted from the core) position; - VIII. from the relation between control element position and Σ (Δk/βeff) (where βeff is the effective fraction of delayed neutrons) the calibration curve is obtained for the examined element. A plot presenting the reactivity dependence on the position of element no. 1 of the TRIGA reactor is given