Excerpts from the report by Lothar Hahn - June 1988

Safety problems and accident risks

Chapter 6.) Safety problems and accident risks of the HTR module and other high-temperature reactors

to chapter 8.) Proliferation problems with the HTR line

In terms of safety, the HTR, especially the small high-temperature reactors HTR-Modul and HTR-100, are said to be miracles. Interested parties make claims that do not stand up to scrutiny. Propaganda campaigns dominate the security debate in public, the necessary differentiated consideration has so far been omitted.

In principle, the same approach is chosen by the nuclear industry that was introduced at the beginning of the 70s in the safety debate about the light water reactor. Such a style, in which trivialization and concealment, misinformation and half-truths take the place of open discussion, is favored by an unprecedented isolation of the security debate from the public specialist discussion. The amalgamation of interests and the at least ideal interdependencies between the actions of the authorities, experts (e.g. TÜV, Gesellschaft für Reaktorsicherheit = GRS), advisory services (e.g. Reactor Safety Commission), large-scale research institutions (e.g. nuclear research facility) and industry mean that there is no really independent monitoring body exists and effective democratic control is disabled.

The activity of an ad hoc discussion group "Basic safety issues of future high-temperature nuclear power plants (HTR-500 / HTR module)" set up by the Federal Minister of the Interior (BMI) previously responsible is to be assessed as a typical consequence of such conditions. This committee, made up of representatives from authorities, experts and industry, discussed security issues relating to the HTR module behind closed doors until 1984. The actual task of this uncontrollable secret body was obviously to develop a common strategy and interpretation of the safety criteria in anticipation of later approval procedures in order to prepare the smooth approval of the HTR module and the HTR-500.

The technical background for the alleged safety advantages of the HTR is usually the lower power density of the reactor core compared to the light water reactor, the higher heat capacity of core and structural materials and their high temperature resistance. Building on this, it is argued that an HTR behaves good-naturedly and sluggishly in the event of a coolant failure; in the event of incidents with failure of the residual heat removal, the heating process runs so slowly that there are still a large number of intervention and correction options to restore the incident control. In addition, a core meltdown like in the light water reactor is excluded, since graphite does not melt, but at around 3500 o C sublimes, i.e. at temperatures that could not be reached in small and medium-sized high-temperature reactors anyway. Generally speaking, it is then asserted that at the HTR no accident sequence is possible, as a result of which there would be radioactive releases that make disaster control measures necessary outside the facility.

Such an argument must be rejected as false and dubious because it - consciously or unconsciously? - bypasses the actual security problems of the HTR. It is partly based on an incorrect and uncritical transfer of safety considerations in the light water reactor to the HTR and thus to overestimate the importance of cooling failures in the HTR.

As in the case of the light water reactor, the hazard potential is also determined by the inventory of radioactive fission products as well as by their natural release mechanisms.

The total radioactive inventory of fission products depends primarily on the thermal capacity of the reactor and less on the type of reactor. With the HTR module it is therefore approx. 5% of that of a light water reactor of the Biblis class. Accordingly, this inventory is still so large (approx. 2 x 1019 Becquerel) that the release of a percentage of this inventory is sufficient to cause massive damage to the health of the population. This is all the more true as small high-temperature reactors should preferably be built close to settlements.

With regard to the release mechanisms in the HTR, it is irrelevant whether core meltdown is possible or not, but it depends on whether and when the fuel element particles (("coated particel") and the fuel elements lose their retention effecto C and goes down at temperatures between 2000 and 2500 o C practically lost. However, these are exactly the temperatures that are reached in the THTR-300 and in the HTR-500 if the residual heat removal fails. In the event of a leak in the primary circuit, releases into the environment can occur, especially since the THTR-300 has no containment.

The HTR module was designed from a safety point of view in such a way that, in the event of heating-up accidents, the maximum temperature in the fuel assemblies exceeds the critical temperature of 1600 due to passive heat dissipation oShould not exceed C. However, this can only be guaranteed under certain conditions, including the effectiveness of passive heat dissipation and successful shutdown. If the systems required for this are not available when they are needed, accident sequences can also develop with the HTR module, during which the fuel element temperatures above 1600 oC increase. This means that massive fission product releases from the fuel assemblies are also possible with the module.

What is decisive, however, is that the slower behavior of the HTR in the event of a cooling failure was bought, among other things, with a measure that is the potential cause of HTR-specific accidents: the use of graphite as a moderator and structural material. Despite precautionary measures, it cannot be ruled out that there will be large ingress of water (from the secondary circuit via steam generator leaks) and air ingress into the primary circuit. If there is an additional failure of safety systems, serious accidents with graphite-water reactions and graphite fires are the result. These types of accidents are also among the risk-dominating processes in the HTR module.

In addition, there are a large number of other accident sequences with the HTR module, of which only a few causes should be mentioned here without further discussion:

  • External influences, e.g. B. plane crash, explosions, sabotage, acts of war,
  • Failure of passive components, e.g. B. of pipelines, pressure vessels, surface coolers.

Other influences that can have a direct or indirect negative impact on the safety of the HTR module are:

  • the security concept, which has been scaled down for cost reasons (e.g. the lack of containment),
  • the (combined with numerous setbacks) little operating experience with high-temperature reactors,
  • the (compared to the light water reactor) lower penetration depth in the safety analyzes,
  • the lack of a comprehensive risk analysis for the HTR module.

For the safety assessment of the HTR module, it also remains to be determined - without addressing all safety-relevant problems - that this type only exists on paper and that some of the claimed safety advantages cannot be specifically checked. Experience has shown that a large part of the safety-related problems only come to light when a system is set up and operated, as the example of the THTR-300 shows.

As a conclusion of the outlined safety problems it can be stated that the HTR - especially in its small version as an HTR module - has significant other design features than z. B. the light water reactor has, on the other hand, but also the small HTR has its special safety deficits, which can lead to major accidents.


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Chapter 8.) Proliferation problems with the HTR line

The question of the possibility of using fissile material for technical weapons purposes has so far been kept out of the discussion about the HTR with the utmost care.

The investigation of the technical aspects of the proliferation problem is necessary, however, if one wants to gain a complete picture of all aspects of the HTR line. A discussion of possible motives for a diversion of fissile material for military purposes as well as the possibilities and limits of the monitoring of the fissile material flows will be dispensed with here. For this, reference is made to other publications; at this point it should only be about technical issues.

With regard to the proliferation problems of a reactor line, the following questions should be asked from a technical point of view:

  • At which stations through which the fuel passes is fissile material in a form that is directly suitable for weapons, ie as plutonium (of any isotopic composition) or as highly enriched uranium 235?
  • At which of these stations can fissile material be diverted for direct military use?
  • At which of these stations can fissile material be branched in a form that requires physical and / or chemical treatment before it can be used for military purposes?

The answers to these questions should be outlined below for the three areas of supply, reactor operation and disposal.

On the supply side, there is always the possibility of access to enriched uranium 235 at some stations.

In the manufacture of the fuel elements for the THTR-300 and the AVR, U-235 is directly accessible in various process steps in a highly enriched form, namely from enrichment to completion of the fuel elements.

Each fuel element ball for the THTR-300 and approx. Half of the AVR fuel elements (Arbeitsgemeinschaft Versuchsreaktor GmbH, Jülich) each contain approx. 1 g of highly enriched U-235. The storage and processing quantity of this material at NUKEM is in the range of one ton (the requested handling quantity is 6 t of any degree of enrichment).

The disappearance of highly enriched uranium 235 in the 1 to 10 kg range could therefore go undetected.

Only low-enriched uranium is planned for future HTR plants. This can also be branched off at the stations mentioned, including the necessary transport processes; however, it must be further enriched for the purpose of military use, which in principle can be carried out in any type of uranium enrichment plant - albeit with different effort and time requirements.

With regard to the possibility of branching off the reactor operation, after the Chernobyl accident, the assertion was made on various occasions that the Russian RBMK reactor was used for the production of weapons plutonium and is particularly suitable for this because fuel elements are removed or added to it without interrupting the continuous power operation can be. However, it is precisely this property that the HTR has to a particular degree, and it is even mentioned as a particular advantage for the HTR module ("There are no downtimes for fuel element changes and no associated operating processes.") Because of the continuous adding and withdrawing and Due to the handiness of the fuel assemblies, it is technically possible at any time during their residence time on the reactor site to divert part of them.

The metrological and accounting recording of the fuel elements by the IAEA and EURATOM cannot offer complete protection against diversion due to the measurement methodology, measurement inaccuracies and the random sampling nature of the monitoring.

Even after its scheduled use in the reactor, the fuel contains fissile material suitable for use in weapons. The THTR and AVR fuel elements of the thorium / uranium strategy contain, in addition to the remainder of uranium-235, the high-quality nuclear fuel U-233, which in principle is also suitable for weapon purposes. The spent fuel of all future high-temperature reactors contains - similar to the light water reactor - plutonium and other actinides. The mixture of plutonium isotopes is basically suitable for weapons.

As long as the U-233 and the plutonium are enclosed in the fuel elements, these fissile materials cannot be accessed directly. You can only get access to them through a reprocessing process.

A civil reprocessing of HTR fuel elements - as mentioned above - has so far failed, among other things, due to unsolved safety-related and radiation protection problems (e.g. in connection with the combustion of graphite).

In contrast to the possible large-scale introduction of the reprocessing of HTR fuel elements for the purpose of producing nuclear fuel, technical and economic problems could be ignored with a military variant. Furthermore, aspects of radiation protection (both for employees and for the population) could be neglected. Finally, the size of the system could be determined purely from a military point of view and kept relatively small (e.g. like a laboratory system). 

A spent fuel element made from low-enriched uranium 235 contains approx. 0,1 g of plutonium. Consequently, the material for an atomic bomb could theoretically be obtained by processing 50.000 spent fuel element balls, ie with a throughput of 1000 balls per day in less than two months. From these points of view and in these scales, this route is only apparently more complex and technically more demanding than via the plutonium production from other reactor lines. In any case, it is easier to camouflage, especially since fuel elements branched off at any point can be replaced by dummy elements.

From this point of view, however, the HTR has a unique feature that can be used militarily: it can be used as an effective tritium producer. The generation of tritium for the purpose of use in atomic bombs can be controlled by means of a suitable fuel composition (e.g. by adding lithium) and can be of military interest to technically well-developed nuclear weapon states. An American HTR provider has even blatantly tried to penetrate the armaments sector with this military option.

In summary, it can be stated that the operation of high-temperature reactors including the stations for fuel supply and disposal represents a specific risk of proliferation. With regard to the diversion of materials for nuclear fission bombs (uranium, plutonium), situations arise that are qualitatively comparable with those of the RBMK reactor and the heavy water reactor. With regard to the production of tritium for use in bombs, the HTR is of particular military importance.


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

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