Rieselfilm Notkühlung für Forschungsreaktoren vom Typ des FRJ-2

A new, efficient emergency cooling procedure based on liquid film cooling was developed for FRJ-2 type research reactors, which allows a higher power generation in the tubular fuel elements used and which represents an improvement of the engineered safeguards of the reactor. The problem of producing coherent liquid films on the outer surfaces of the four concentrically arranged thin fuel tubes without obstructive modifications of the fuel element design was solved by using radial water jets. These jets discharge into the drained fuel elements from the outside thereby crossing the upper edges of the fuel tubes. In hydraulic experiments the influence of the geometry, of the jet velocity and of the water viscosity on the water supply to each fuel tube was measured and the conditions were evaluated where by eachfuel tube in the reactor obtains sufficient cooling water taking account of variations in the various parameters. The behaviour of the liquid film on the surface of the fuel tubes was measured in stable and unstable thermodynamic experiments using electrically heated test tube. The formation of dry spots was found to be the critical film instability. However it could be demonstrated that the combination of good wetability of the fuel tube surface, the high thermal conductivity of the tube wall, the small tube length and the high water temperature limits the breadth of a dry spot to subcritical values. The so called flooding effect caused by counterflow of steam in the narrow coolant channels turned out to be advantageous for fuel tube cooling since the violent whirl processes associated with the flooding effect revent the formation of dry spots. Detailed accident analyses showed that the developed emergency cooling procedure together with the protection system and the engineered safeguards of the reactor guarantee a sufficient and continuous cooling of the fuel tubes even in hypothetical loss of coolant accidents. In the worst case only 4.5 s elapse after shut down of the reactor before the emergency cooling system must take over the cooling of the fuel element. At this time the mean heat flux of a highly rated fuel element has only decreased to 12 W/cm$^{2}$ and the fuel tube temperature reaches a maximum value of about 114°C when the surface are completely covered by the liquid films. If dry spots are formed immediately after initiation of emergency cooling the temperature in the centre of a dry spot may increase to a maximum value of 250°C . No fission product release is to be expected below 500°C