The experiences with frequency response testing on heavy-water reactors have been at the NPD and NRU reactors in Canada, the Agesta reactor in Sweden, and the Halden reactor in Norway. Each of these countries has devoted considerable effort to development of practical testing procedures. The experiences with all heavy-water reactors have been combined in this section, but the reactor characteristics and testing procedures were quite different.

The Canadian reactors NPD and NRU use fluid level in the core to control reactivity. In NPD, the level of D20 in the reactor calandria is controlled. In NRU, reactivity is controlled by adjusting the level of light water in special control compartments. In Agesta and Halden, conventional control rods are used for reactivity control.

Measurements on Canadian reactors have used the oscillator technique and the PRBS technique. Results were obtained at frequencies up to about 1 Hz in NRU and NPD. These tests provided valuable experience in on-line data analysis, since a PDP-8 computer with an FFT analysis algorithm was

t See the literature (85-93).

f See the literature (93-109).

used. Tests at NPD uncovered a need for changing the settings in the control system.

The measurements at Agesta were made to gain experience with testing methods and to study system dynamic behavior as a function of fuel burn-up. Standard control rods were used to introduce PRBS and MFBS reactivity inputs.

The Halden tests were also extensive and innovative. The testing program included PRBS and MFBS input signals. The reactivity perturbations were introduced using magnetic jack control rods. A significant aspect of these tests was that up to ten control rods were moved simultaneously by the introduction of a single signal from the control panel. This indicates that increased input amplitude can be obtained in systems with magnetic jack control rod drives simply by using enough rods. There is no need to wait until sufficient reactivity can be introduced by multiple steps of a single rod.