Inherent incidents and radioactivity emissions from the HTR line!

August 2008

A new investigation (1) by the scientist Rainer Moormann about the operation of the thorium high-temperature reactor (THTR) AVR in Jülich, which was shut down in 1988, not only calls into question the entire previous official safety architecture of this reactor line, but also shakes the statements of the international nuclear community about the advantages of the new Generation IV reactors in theirs Foundations.
Remarkably, this criticism comes from a scientist who has been doing regular research on the HTR line at Forschungszentrum Jülich for many years and has published on this (2). With an unprecedented degree of openness, this "safety-related reassessment" is the first to reveal significant problems in the operation and the current dismantling of the general test reactor (AVR) in Jülich and to address considerable radioactive contamination. Here are the results in detail:


1. Many security problems in the AVR have so far been concealed.
"This work mainly deals with some inadequately published but safety-relevant problems of the AVR operation."

 

2. The dismantling brings it to light: There was significantly higher contamination within the facility than predicted. Radioactive graphite dust is "mobile".
"The AVR cooling circuit is heavily contaminated with metallic fission products (Sr-90, Cs-137), which leads to considerable problems with the current dismantling. The extent of the contamination is not exactly known, but the evaluation of fission product deposition experiments suggests that this contamination reached a few percent of a core inventory at the end of operation and is thus orders of magnitude higher than preliminary calculations and also considerably higher than the contamination in large LWR. A significant proportion of this contamination is bound to graphite dust and is therefore partially mobile in pressure relief accidents, which must be taken into account in safety assessments of future reactors is."
 
3. Inadmissibly high core temperatures are the cause of high releases.
"The result was that the contamination of the AVR cooling circuit was not primarily caused by inadequate fuel element quality, as previously assumed, but rather by impermissibly high core temperatures, which accelerated the releases considerably. The impermissibly high core temperatures were only discovered 1 year before the final AVR operation ended, since a pebble cluster core has not yet been instrumentable. The maximum core temperatures in the AVR are still unknown, but they were more than 200 K above calculated values
not possible."


4. The steam generator was damaged during operation.
"In addition, azimuthal temperature differences of up to 200 K were measured at the core edge, which can probably be attributed to a performance imbalance. Strands of hot gas with temperatures above 1100 ° C, which could have damaged the steam generator, were occasionally measured above the core."

5. AVR operation was unsafe and unreliable. As a result, these negative safety properties can also be expected in future Generation IV reactors.
"There was therefore no safe and reliable AVR operation at gas outlet temperatures suitable for process heat, as assumed as the basis of the pebble bed VHTR development in the Generation IV project."

6. HTR spherical fuel assemblies cannot prevent radioactivity from escaping. A myth is exposed as a lie.
"The AVR contamination problems are also related to the fact that intact HTR fuel assemblies cannot be viewed as an almost complete barrier for metallic fission products as they are for noble gases. Metals diffuse in the fuel core, in the coatings and in the graphite. A breakthrough through this Barriers occur in long-term normal operation when certain temperature limits specific to the fission product are exceeded. This is an unsolved weak point in HTR that does not exist in other reactors. "

7.
There is an uncontrolled (!) Distribution of radioactive nuclides over the entire cooling circuit.
"Another HTR weak point that contributed to the AVR contamination is due to the fact that the nuclides released from the fuel elements in the HTR are distributed in an uncontrolled manner over the entire cooling circuit. Because of the high deposition rates of chemically reactive fission products in HTR cooling circuits namely, the activity released from the fuel assemblies cannot be removed using a cleaning system, as is the standard in the LWR. "
 
Comment: So now we know why the operators of THTR Hamm resisted our request for a nuclide register so violently after its shutdown. An additional disaster would have become obvious and public!

8.
Water ingress took place. These must be eliminated in the future by additional devices.
"In the event of water ingress, the penetration of liquid water into the pebble, as happened in an AVR accident, must be structurally excluded in order to prevent a possible positive void coefficient of reactivity with reactivity excursion."

9.
A gas-tight containment (safety container) is completely missing, but is absolutely necessary.
"Criteria for a maximally tolerable accumulated activity in the HTR cooling circuit were developed on the basis of German ordinances for design accidents as well as on the basis of requirements from maintenance and dismantling. The application of these criteria to pebble bed reactors leads to the conclusion that gas-tight containment is necessary even if no excessive core temperatures are assumed. "

10. In his study, the author discusses whether, in the interests of safety, one should generally refrain from hot gas temperatures in the future. In other words: The Very-High-Temperature Reactor (VHTR), which was particularly favored in Generation IV, creates a particularly large number of problems that have yet to be solved. A "very extensive R&D program" would be indispensable for this before further steps are taken.


11. The further development of the pebble bed reactor will be very expensive and therefore economic risks should be precisely estimated beforehand. Is the huge effort even worth it?
"An extensively instrumented experimental pebble bed reactor would be indispensable to solve these problems. Before an R&D program of this size is started, a feasibility study including an estimate of the costs should be carried out in order to quantify the economic risk of this development."

12. All previous HTR safety studies have been inadequate and far too optimistic in their conclusions.
"With regard to beyond-design-basis accidents, safety problems in the case of air ingress / core fire have not yet been adequately resolved. A comparative safety study of pebble-pile HTR, block-HTR and generation III LWR would be helpful to get a more reliable statement on the security of current pebble-pile HTR concepts : From today's perspective, earlier safety studies for pebble bed reactors must be viewed as too optimistic. "
 
After the publication of this critical study within the framework of the Jülich Research Center, there can only be one demand: No more euro for HTR and Generation IV research; no construction of the PBMR in South Africa, which would have exactly the mentioned problems!
 
Horst Blume

 


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Notes:

1. Rainer Moormann: "A safety-related reassessment of the operation of the AVR pebble bed reactor and conclusions for future reactors". Reports from Forschungszentrum Jülich, 4275. ISSN 0944-2952.
2. Previous publications by Rainer Moormann on the HTR problem:
1999: Moormann, Hinssen, Latge: "Oxidation of carbon based materials for innovative energy systems (HTR, fusion reactor): status and further needs". Article in a book. 11 pages.
1999: Moormann, Schenk, Verorden: "Source term estimation for small sized HTRs; a German approach Proceedings of the 1st Meeting Survey on Basic Studies in the Field of High Temperature Engineering (including Safety Studies)". Article in a book. 9 pages.
2004: Kühn, Hinssen, Moormann: "Differences between the oxidation behavior af A3 fuel element matrix graphites in air and in steam and its relevance on accident progress in HTRs". Proceedings of the ICAPP 04, Pittsburg, USA
2004: Moormann, Hinssen, Kühn: "Oxidation behavior of an HTR fuel element matrix graphite in oxygen compared to a standard nuclear graphite". In: Nuclear Engineering and Design, 277 (2004), pp. 281-284

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(Release of atomic radiation since the early 1940s: see INES - The international rating scale and list of nuclear accidents worldwide)

*

What is Generation IV? FZ Karlsruhe, February 2004 (.pdf file)

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