A number of BWRs have been subjected to frequency response tests. All of these measurements used the oscillator method.

The BORAX reactor and the experimental boiling-water reactor (EBWR) were early boiling-water reactors that were operated in the 1950s and 1960s to evaluate the feasibility of the BWR concept. One of the major points of concern was the stability of this type of reactor. The EBWR experience has already been mentioned in Section 6.1.

As the BWR concept was developed into a commercial power reactor by the General Electric Company, continued emphasis was placed on plant stability and control. In the theoretical phase of this work a major effort has been devoted to techniques for computing the frequency response of the system. A great deal of theoretical work has been done, and a number of frequency response measurements have been made on test loops and on reactors to check the theoretical predictions. The standard method for theoretical dynamic analysis of large BWRs involves a frequency response analysis using a computer code called FABLE (43. 44).

The first large commercial BWR power plant in the United States was Dresden 1 (a 613-MW dual-cycle plant). The control rods in Dresden 1 and all subsequently designed BWRs to date are the bottom entry, hydraulically driven type of rod described in Section 7.3. In the Dresden 1 tests, a standard rod was slightly modified so that it could introduce sinusoidal reactivity perturbations with frequencies up to 0.27 Hz (25). Oscillator tests were performed at eleven power levels up to full power, and satisfactory stability performance was observed at all power levels.

Further control rod oscillator tests have been made at Big Rock Point, Garigliano, and Gundremmingen (42). The Gundremmingen test program also included measurement of the response of the system pressure to oscilla­tions in the steam control valve position (45). The tests at Garigliano (26) (a 506-MW dual-cycle plant) provide a good case study. These oscillator tests were performed using a special purpose hydraulic oscillator on the central control rod. In a discussion on the Garigliano plant, Mr. F. Santasilia (of Ente Nazionale per l’Energia Elettrica in Rome) indicated that it would have been much easier and less expensive to introduce trapezoidal reactivity perturbations than sinusoidal perturbations. He stated that the effort

required to achieve the sinusoidal input was justified because the results are easily correlated with theoretical frequency response measurements. This suggests that measurement techniques based on binary signals that have been developed since the Garigliano tests would have been valuable for that application.

The frequency response of a BWR depends on the operating conditions (power, flow, void content) and on the location of the detector. These features are accounted for in the FABLE model, permitting a direct compari­son between theory and experiment. Typical results from Garigliano are shown in Fig. 8.1.

In all of the newer BWRs that are being planned or built, step response tests are to be used to measure decay ratios (see Section 6.1) for certain process variables. These tests will suffice for checking stability margins and for rough comparisons with theoretical models. However, a detailed com­parison with the FABLE theoretical calculations would require a complete frequency response measurement.