Abstract
In this chapter, an overview on some important experiments regarding the safety of modular HTR is given. A major topic is the proof tests on the concept of self-acting decay heat removal. Relevant results for the transport of the heat in the pebble bed, in the reactor structure, and from the surface of the reactor pressure vessel are explained and show that today the different steps are well known and show good overestimation with calculations and experimental data gained from integral AVR experiments. In case of extreme assumptions on accidents, the decay heat is stored in concrete structures and therefore large concrete blocks have been tested at very high temperature with good success. The reactivity behavior of HTR core has been tested in practical operation and in accompanying tests on special facilities. The strong negative temperature coefficient has been verified by these, and especially the experiment in AVR (simulation of total loss of active shutdown system) and the ATWS experiment in HTR-10 have demonstrated this important characteristic of modular HTR. Many experiments regarding the behavior of water in the primary system and in the core of HTR plants have been carried out. These were the measurements of corrosion rates of graphite from the important parameters like temperature, pressure, and type of graphite. The transport of water, forming of steam and gases, and changes of materials have been measured in detail. Similar activities have been carried out for the accidents with ingress of air. Solutions to exclude the occurrence and consequence of large air ingress have been developed and qualified too, for example burst-safe vessels, which would allow just small openings in case of breaks. An important activity was the possible transport of air in primary systems too and thereby to qualify computer programs. A major experimental work of HTR development was directed toward the behavior of fission products in the reactor. The intention was to define source terms for the normal operation and for accidents. A central importance has the VAMPYR experiments in AVR, which gave information on the contamination of the circuit by solid fission products. Irradiated fuel elements have been heated up in hot cells (KŰFA tests), and these experiments delivered release rates for all relevant fission products dependent from burnup, operation temperatures, heating temperature, and time. The requirement to limit the maximal temperature of modular HTR with LEU TRISO fuel to a value of 1600 °C in all accidents is a result of these activities. Further experiments were done to study the transport, deposition, and remobilization of fission products from different reactor components. A broad basis of data today is available, to describe these processes. The earthquake behavior of reactor systems was a further topic of experimental research. The large test facility SAMSON has been used to test relevant components and especially pebble beds at large acceleration values. It was stated that reactor designs should be carried out with high safety factors for strength of earthquakes, which were assumed until now.
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Kugeler, K., Zhang, Z. (2019). Some Experimental Results on Safety Aspects of Modular HTR. In: Modular High-temperature Gas-cooled Reactor Power Plant. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-57712-7_14
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