AVR prototype pebble bed reactor: a safety re-evaluation of its operation and consequences for future reactors

Abstract

R. MoormannAVR prototype pebble bed reactor: a safety re-evaluation of its operation and consequences for future reactorsThe AVR pebble bed reactor (46 MWth) was operated 1967 -1988 at coolant outlet temperatures up to 990°C, which are suitable for process heat applications. Also because of a lack of other experience the AVR operation is a basis for future HTRs. This paper deals with insufficiently published but unresolved safety problems of AVR and draws conclusions for future pebble bed HTRs: Although the AVR was operated only for about 4 years at coolant temperatures > 900 8C its primary circuit is heavily contaminated with dust bound metallic fission products ( 90 Sr, 137 Cs), which create major problems in current dismantling. The end of life contamination reached several percent of a single core inventory. The AVR contamination was mainly caused by inadmissible high core temperatures, and not -as presumed in the past -by inadequate fuel quality only. The high core temperatures, which cannot be easily measured in pebble beds, are probably caused by the insufficiently examined pebble bed mechanics. Safety of future reactors requires a gas tight containment and in addition elaborate R&D or reduction of requirements concerning temperatures and burn-up.

Upgrading (V)HTR fuel elements for generationIV goals by SiC encapsulation

“…Further, because of the much larger yield of Silver in Plutonium based fuel, Pu-burners based on PBRs seem to be particularly problematic for present TRISO fuel. These results of the EU1bis experiment are in line with predictions performed in [1] and are due to the before mentioned weak point of the current PBR fuel concept: Due to the comparably high diffusion coefficients of Cs and Ag in SiC, the SiC layer cannot act as a long term diffusion barrier for these nuclides at fuel temperatures > 800 8C (Silver) respectively > 1100 8C (Cesium) because of its small thickness of only 35 lm. Accordingly an additional diffusion barrier, as a thick SiC layer on the outer surface of the fuel elements, is worth to be examined more detailed.…”

“…It becomes especially attractive when core outlet temperatures higher than about 1000 8C would enable nuclear heat application to such processes as hydrogen, steel and aluminum production. Recent re-evaluation of AVR and THTR results however emphasizes once more that the claimed advantages of pebble bed reactors (PBR) strongly depend on the properties of the fuel elements and their behavior under operational conditions [1]. Additionally, safeguards, waste management and disposal aspects gain increasing importance for Generation IV facilities [2].…”

"Advanced" isn't Always Better: Assessing the Safety, Security, and Environmental Impacts of Non-Light-Water Nuclear Reactors

“…Depending on the number of wires that melted, operators could determine the maximum temperature-as long as the peak melting temperature was not exceeded. But in a significant number of monitor pebbles all of the wires had melted, indicating the coolant temperature had exceeded 1280°Cmore than 200 degrees above 1070°C (Moormann 2009). It was estimated afterward that the actual peak core temperature could have reached 1420°C, or 350 degrees above the calculated maximum.…”

“…Rainer Moormann, a German HTGR researcher who has become a leading skeptic of the technology, concludes that future pebble-bed HTGRs should include leak-tight containments, given the many unresolved safety issues including the potential for fuel temperatures and fission product releases to greatly exceed expected values (Moormann 2009). In a recent critique of the HTR-PM commercial demonstration pebble-bed reactor that is under construction in China, Moormann and collaborators proposed a number of safety upgrades to compensate for the absence of a leak-tight containment at the reactor, such as improving the confinement vent filtration system (Moormann, Kemp, and Li 2018).…”

Phenomenology of Graphite Burning in Air Ingress Accidents of HTRs

“…Small HTR concepts are designed in a manner that the risk dominating accident in conventional reactors, failure of forced cooling followed by a core meltdown, can virtually be excluded as catastrophic source term contributor [1]. However, there are accident scenarios with severe consequences in HTRs, not existing for conventional reactors [2]: one weak point of HTRs is the small oxidation stability of graphite, the main component in the core, at high temperatures. It leads to potentially severe accidents in case of accidental air or steam ingress into the coolant circuit.…”