Melt Wire Experiments: AVR Fuel Testing Analysis Explained

[Maokeng]

Can someone please help me understand the phrase "melt wire experiments" as used at the AVR fuel testing analysis and maybe why the such a choice of phraising. I would aso appreciate if I could be referred to either papers, journals or web links where one can get information and probably the results of the experiments.

I do know that AVR was used to test TRISO fuel particle (pebbles) for the modular HTR reactor.

Maokeng
(ZA)

 

[Astronuc]

The objective of the experiments was to determine local "hot spots" in the pebble bed during operation. Basically one takes a wire with a well calibrated melting temperature and places samples at different locations, primarily those locations where integrated thermal-hydraulic models indicate maximum local temperature. If the wire melts, then obviously the local temperature exceeds the melting temperature. Using wires of different melting points can then indicate the local temperature within the difference of melting temperatures.

Some basic references:

www.nrc.gov/reading-rm/doc-collections/commission/secys/2003/secy2003-0059/attachment2.pdf (page 40)

In advanced HTGR designs, the integrity of the coated particle fuel in its function as primary FP barrier depends strongly on the maximum fuel temperatures reached during irradiation and in accidents. These fuel temperatures are predicted by reactor system calculations using a combination of codes and models for core neutronics, decay heat power, and system T/Hs. So-called melt-wire experiments performed in Germany’s Arbeitsgemeinschaft Versuchsreaktor reactor (AVR) showed the unexpected presence of in-core hot spots, where maximum local operating temperatures were much higher than predicted with codes like those now being used by the PBMR developers. Moreover, the AVR’s true maximum local operating temperatures remain unknown due to measurement inadequacies in those experiments. For all advanced HTGR designs, significant uncertainties also exist in predicting the maximum fuel temperatures and vessel temperatures during heatup accidents. Such uncertainties relate to basic data like irradiation- and temperature-dependent thermal conductivities, as well as the integral effects of variable local power densities with conductive, radiative, and convective heat transfer through the core and surrounding structures. Appropriate data measurements and system analysistools will therefore be needed to support the staff’s understanding and assessment of factors that govern fuel temperatures and uncertainties in relation to fuel integrity and HTGR safety margins.

http://www.nea.fr/html/nsd/docs/2007/csni-r2007-6.pdf (page 90)

In-core hot spots. The results of melt-wire experiments conducted in the German AVR test reactor demonstrated the existence of unpredicted local hot spots under normal operating conditions in pebble bed cores. Such hot spots can be used to determine the maximum normal operating temperatures of the fuel. These hot spots may arise from a combination of higher local power density (e.g. due to moderation effects near the reflector wall or from chance clustering of lower burn-up pebbles), lower local bed porosity due to locally tight pebble packings, and reduced local helium flow due to the increase of helium viscosity with temperature.

ADVANCED REACTOR RESEARCH PLAN
www.ms.ornl.gov/programs/imet/PDF/June_2002_AARP.pdf

To obtain reports of the actual experiments, one has to find the FZK reports.