types of boiling water reactors (BWR) in Germany are comparable to PWR between 90% and 100% power, though limitation to 1% per minute can be necessary, and can be regulated down to 60% nominal power by changing the steam bubble quotient;

These numbers seem to originate from tests at the Isar-1 plant. The linked document tells of tests with both control rods and the regulation of the steam bubble quotient [sorry if that's a bad translation of the technical term]. The first proved problematic because the power distribution in the core and thus the thermal load on the fuel rods was uneven. The second describes regulation in the 60-100% band, addig that spending longer time in the lower part of the band is problematic due to xenon. Interestingly, 2002, the Isar-1 block was approved for participation in the "primary regulating power" regime (peak load with automatic response in 30 seconds) with +|- 2.5% nominal power while it is in the 60-97.5% band, and participation in the slower and higher amplitude regime is mentioned as plan for 2003, apparently not realised.

Now, this was a BWR, but I wondered: aren't the negative experiences with regulation using control rods independent of power plant type? The report on tests by the same engineers at the Isar-2 plant (a PWR) claims no problem, though differential load is experienced there, too. (Also, the test wasn't a contiuous load variation, more a daily up and down.) However, I found an example of similar problems elsewhere: the oversight authority explained fuel rod damage experienced at block 3 of the Cattenom power plant in France, which is a PWR, with stress in variable output operation.

_{*Lunatic*, n.
One whose delusions are out of fashion.}