JEN research programme on organic moderator-coolants. Status report from Spain

The interest displayed by the Spanish Nuclear Energy Board (JEN) in DON-type reactors (moderated with heavy water and cooled with organic liquids) has already been discussed at various international meetings. The Board has embarked on a research and development programme under which it is planned, within the budgetary limits laid down, to build a 30-MW(e) prototype in the near future; this facility will be used to test the concept, to train staff and to test fuel elements (a central channel will be added for full scale fuel element irradiation). In physics and technology the research and development programme has been mainly concerned with the construction of an exponential facility, the testing in the facility of UO2-UC lattices with D2O and organic substances, the production of UC, the fabrication of fuel elements and the technological testing of components in contact with the organic coolants. In all these fields progress has been considerable and experiments are now being carried out with a large experimental loop to test the hydraulic characteristics of fuel elements, vibration in fuel elements and also the behaviour of pumps, valves, static and dynamic joints and other components. The present DON project is based on the utilization of OMP coolants like Santowax H or Santowax OMP. However, in view of the complexity of the problems posed by the heating of the tubes and other components and the difficulty of the initial preheating of the reactor pressure tubes, it seems advisable to make use of OM-type coolants, which have a lower melting point. It has been estimated that for the DON reactor the additional coolant cost would be $35000 a year; the initial saving on heating equipment could amount to $500000. The basic investigations carried out with direct reference to the technological programme have been concerned with the kinetics of pyrolytic decomposition under experimental conditions at various controlled times and temperatures and also with the degradation and polymerization effects as associated with the pyrolysis of the coolants tested. The influence of radiation on stability has been studied from the point of view of the effect of accelerated electrons on the various organic materials. The influence of out-of-pile fluid dynamics has been studied in a test circuit and this has made it necessary to study the problems of purification and degassing and to investigate the structural modifications associated with the dynamic pyrolysis of these terphenyl mixtures. Particular importance has been attached to the development of control methods, viz, analytical methods in the widest sense and the problem of determining physical constants. The object here has been to obtain maximum information on the changes occurring in the organic coolant with a view to arriving at conclusions on the probable chemical and physical properties of the coolant as well as its purity under extreme reactor conditions. The results of these basic and applied investigations and information on the control techniques developed are reported in detail in the body of this report