Session 3: Discussion

Dr D.J. Higson, Australian Atomic Energy Commission

Could Mr Kirk please explain how he can take credit for control system_grotection, with a failure probability of 10 , as well as shutdown system protection with a failure prob ability of 10 ? If the control system is expected to act as another protective system, it is subject to the same causes of failure.

It woujd be difficult tg argue a failure rate of 10 rather than 10 merely by adding another shutdown system, because of their common cause of failure.

Mr J. Kirk

The failure probability of 10 ^ referred to was that of the reactor guard line system, the function of which is to interrupt rod supplies. The guard line system is not involved in the action of the automatic control system which acts in response to changes in channel gas outlet temperature changes. In the AGR system it is anticipated that the automatic system will be in continuous operation to trim out changes in power caused by grid frequency fluctuations and refuelling penetrations. A common mode fault, due for instance to top dome or core movement which might prevent control rod movement, would therefore have a high probability of being detected.