Frequency response measurements have been used extensively in almost all of the fast reactors that have been operated so far. This included EBR-I, EBR-II, Fermi, SEFOR, LAMPRE, and Dounreay. Tests are also planned for the fast flux test facility (FFTF) in the United States and for the prototype fast reactor (PFR) in Britain.

In commercial liquid-metal-cooled breeder reactors, suitable dynamic behavior will be achieved by designing to achieve a satisfactory balance between stabilizing and destabilizing effects while maintaining good economic performance. Dynamics tests are useful to determine whether stability margins have decreased due to changes in core characteristics because of aging and whether the models and parameters used in dynamics and safety studies are accurate. These needs are well recognized and account for the wide use of frequency response measurements in fast reactors.

Almost all of the frequency response measurements in fast reactors have used the oscillator method. The sole exception is the measurement at Fermi (69), which used single reactivity pulses for frequency response determination as a supplement to the oscillator tests. The oscillator rods used in fast reactors have been of the rotating type and the linear (in-out) type. Both have been notoriously unreliable. A great deal of work has gone into the design of oscillator mechanisms that give pure sine waves (no harmonics) and permit accurate determination of the time-varying reactivity at all frequencies of interest. These systems can be installed in new fast reactors, but the cost will be several hundred thousand dollars. This cost can be eliminated if newer testing methods that utilize standard rods can be used.

The suitability of standard control rods for frequency response measure­ments in fast reactors is largely determined by the ability of the rods to move fast enough to give the highest frequencies of interest. Since there is little uncertainty in the zero-power kinetics of the system, the frequency range of interest is determined by the frequency range over which feedback effects are significant. In all operating and planned fast reactors, the highest frequency of practical interest for observing feedback effects is about 10 rad/ sec. Standard control rods will probably suffice for measurements up to this frequency.

Experiences with EBR-1 (45) provide an interesting example of the use of frequency response results. EBR-I was a small (1.2 MW) reactor that began operation in 1951 with the main purposes of demonstrating that a fast reactor could have a breeding ratio greater than unity and that liquid-metal- cooled fast reactors are feasible for producing electric power. Two dynamic effects were apparent in the first core (Mark 1) that caused concern. The first was evidence that the system had a prompt positive reactivity coefficient and a (larger) delayed negative reactivity coefficient. The second was a dependence of system stability on coolant flow rate.

A number of frequency response and transient response tests were run on the EBR-1. During a transient response test on the Mark 11 core, the power rise was excessive and part of the fuel melted.

Further study suggested that bowing of the fuel because of temperature gradients was responsible for the prompt positive coefficient. To confirm this, rigid cores with little bowing were constructed. Frequency response tests showed that this reduced the prompt positive coefficient. The delayed negative coefficient was found to be due to thermal expansion of core structural members. In the interpretation of the results, the feedback frequency response was determined using Eq. (5.2.1). These results clearly showed that rigidizing the core increased the stability of the system.

The analysis of the EBR-1 system indicated that the stability problems were due to the design peculiarities of that system, and that other systems could readily be designed without those features. However, the possibility of other peculiarities suggests that experimental confirmation of stability margins during core life will continue to be useful.

Interesting tests were performed at Fermi in which the frequency response was measured using the pulse technique (69). These tests were performed to show the suitability of a standard control rod for frequency response measure­ments. Measurements were made over the range from 0.0001 to 0.15 Hz and were found to agree favorably with results from oscillator measurements.

So far, no frequency response tests on fast reactors have used periodic, binary input signals. However, they will be used in the FFTF and in the PFR.