[en] At Ringhals 1, a high resolution Ge(HP)-detector was installed for on-line monitoring of system activity contamination. A vertical RWCU-pipe, DN 300, was monitored during operation of the reactor. 30-minutes spectra were continuously recorded during one fuel cycle. The statistical scattering of Co-60 and Co-58 for 30 minutes measurement is 0.5 and 2 %, respectively. For 24 hours measuring time the statistical scattering is 0.1 % for Co-60, 0.4 % for Co-58, 3 % for Mn-54, 4 % for Cr-51 and about 10 % for Zn-65, Zr-95 and Sb-124. Thus, changes significantly less than 1 % for Co-60 and Co-58 of the surface contamination levels on primary system pipes, can be continuously recorded during normal operation of a BWR. The contribution to the net measured detector signal from background sources is very low. As an example, the relative contribution to the measured Co-60 (and Co-58) signal from activity in the reactor water in the monitored pipe is less than 4^*10^-6. The relative contribution to the Co-60 signal from all other sources was less than 6^*10^-3. With the on-line activity (OLA) measuring method it is possible with high time resolution and with significant sensitivity to follow-up tests of various kinds, for instance iron injection, zinc-injection, HWC, pH changes, and how chemical transients in the reactor water affects the activity uptake on the surfaces. The measuring system can provide an early warning if unexpected activity uptake will take place and also reveal operational condition, which may cause increased activity deposition. OLA-equipment will be installed in a number of Nordic BWRs by the start of the fuel cycle 1998-99. (author)