The development of techniques for the surveillance of LMFBRs

The development of surveillance techniques of LMFBRs is determined by the interaction of three factors: the specification of requirements, improvements in technique and the physical analysis of the processes involved. The specification of requirements, which sets the structure for the discussion, is mainly concerned with public safety. Two main divisions are identified: those concerned with thermal events in the nuclear core and those concerned directly or indirectly with the mechanical integrity of components. The necessary developments are then discussed in terms of the signal analysis techniques to anticipate various modes of failures. The importance of an adequate understanding of the failure mode is emphasised in optimising the surveillance technique.

Core surveillance may be achieved by monitoring individual sub-assemblies or by monitoring bulk conditions. The important features of sub-assembly monitoring are discussed and the advantages of temperature analysis explained. The specification of the temperature-monitoring systems is identified and the conflicting requirements for the reactor sensor discussed, viz adequate band width as against a robust and reliable construction. A theoretical treatment using Monte Carlo techniques allows a full examination of the choice of method of temperature analysis. This shows that, although a filtered rms value has been the preferred choice for detecting either local blockage or sodium boiling, it may be possible to distinguish the temperature signals of blockages from those of power gradients by an amplitude probability density plot. The advantages of acoustic monitoring using the noise of boiling sodium to detect overheating, leading to core damage, are examined. An important consideration is the thermal-acoustic process of sodium boiling, and evidence is submitted from a range of out-of-pile experiments involving local sub-cooled boiling and bulk boiling in discussing the merits of pulse analysis and power spectral density techniques. An important factor in discriminating background from signal is the extent of cavitation in reactor components. Experiments are mentioned in which pulse techniques have been used to locate boiling sources by spatial correlation. The interpretation of reactor signals requires a detailed knowledge of the transmission of acoustic waves in reactor pools and structures and the effect of gas bubbles. Measurements in PFR and sodium loops have helped to lead to a more quantitative assessment of the sensitivity of the acoustic techniques.

Structural integrity depends on detecting failure modes, particularly those arising from crack propagation. Manufacturing defects or pre-existing cracks may be identified by ultrasonic inspection or by stress-wave emission. On-line monitoring for stress-loaded cracks by a stress-wave emission is seen as intrinsically difficult because of low signal strength and high attenuation but initial experiments have indicated possibilities for detecting stress-corrosion cracking. Mechanical failure from fatigue may be anticipated from a understanding of the vibrational modes of the sodium and its coupling with the structure. A one-eighth scale model of a LMFBR design has recently demonstrated the likely vibrational modes. A major handicap in supervising mechanical operation in sodium systems is the opacity of the sodium. Visualisation techniques of the major parts of the core structure are being developed. An important aspect is the study of the information processing required to present an image easy for the reactor operator to understand. Advances may be made using transform methods to improve object boundaries by modifying the spatial frequencies of the display or record.