Page 190 - Neutrons for Sciences and Society

Neutrons for Science
structure would be replaced by zircaloy which would have a longer lifetime. Already at this time samples taken from different nose-pieces taken from the reactor had enabled the dynamic evolution of the alloys under high flux to be studied. The changes in the aluminium would lead to planning a replacement of the reactor vessel. This possibility had been foreseen in the original design of the project. A note on this subject was written by Franzetti, the head of the reactor department. The new replacement vertical cold source mentioned above was installed during this long shutdown.
In early 1990 it was discovered that the actual reactor power had been nearly 10% greater than 57 MW from the beginning
of operations. This was indicated by the lifetime of the fuel element being somewhat shorter than that calculated. The error was due to using cooling flow rates based on light water, without further adjustment for the heavy water used. In addition pressure differences were measured using a mercury column, without taking into account the difference in density between light and heavy water. To re-establish the power that had been announced to the safety authorities the reactor power (and hence the neutron flux) had to be reduced. As a consequence the cycles could be extended from 44 to 50 days. The discovery of the over-running of the reactor explained a troubling fact. During the first years the used fuel had been sent back to the USA for reprocessing. The Americans reported that the remaining quantity of uranium-235 unconsumed was 10% smaller than expected; this was confirmed when reprocessing was later performed in France107.
107 I am grateful to Michel Jacquemain for this information.
181