Fundamental security problems

in the high-temperature reactor and particular deficits in the THTR-300

Lothar Hahn - June 1986

For the supposed "inherent" safety of the HTR

Since the beginning of high-temperature reactor development, interested parties have tried to suggest to the public that the HTR is "inherently" safe. This cleverly engineered advertising strategy has undoubtedly had some success, because it has led to unprecedented disinformation, even in the atomic energy debate. Like hardly any other assertion by the nuclear industry, it is based on scientifically untenable assumptions and incorrect conclusions.

In technology, in particular nuclear technology, a system is referred to as inherently safe if it remains in its design state solely on the basis of physical and chemical laws and if it does not depend on the functioning of active safety devices when dealing with accidents the intervention of personnel is instructed (according to the definition by Alwin Weinberg).

As is well known, the light water reactor does not have these properties. However, it is also completely clear that practically all HTR concepts that have been seriously pursued up to now are not inherently safe and that the THTR-300 in particular does not have this property. For example, two of the central safety-related requirements, shutdown and residual heat removal (and thus ultimately also the retention of fission products) are dependent on active safety devices and / or handles if serious accidents and significant releases of the ) radioactive inventory should be prevented.

As proof of the alleged inherent safety, the HTR industry usually cites some properties in which the HTR differs from the light water reactor and which are said to have advantageous effects in terms of safety. However, the HTR is far from inherently safe from this, because in addition to supposedly favorable ones, the HTR also has disadvantageous safety-related properties that other reactor types do not have. The most frequently cited alleged advantages of the HTR are presented and commented on below:

  • Property: Low ratio of power density to heat capacity, ie slower temperature rise compared to (compared to the light water reactor or breeder) in the event of a cooling failure.
  • Comment: This is not correct, but only applies to events with certain cooling failures. In the case of the HTR-specific accidents of water ingress, air ingress and reactivity accidents, this property is of lesser importance. If rapid cooling is required, the high heat capacity is rather disadvantageous.
  • Property: High temperature resistance of the ceramic fuel elements and core structure materials, no core meltdown such as B. possible with the light water reactor.
  • Comment: The statement is correct, but ignores the real problem. It is not primarily about the possibility of a core meltdown, but rather the question of whether and how radioactive fission products can be released. At temperatures above 1600o C noticeable proportions of fission products are released from the fuel particles and from the fuel assemblies. This effect increases at even higher temperatures, and at the latest at approx. 2500oC there are massive releases into the primary circuit. Temperatures at which dangerous releases occur can be reached in the core of all large and large high-temperature reactors due to accidents without the graphite losing its mechanical consistency. The statement that core meltdowns are not possible with the HTR is therefore misleading and not relevant for the release mechanisms.
  • Property: Negative temperature coefficient of reactivity, ie decrease in power generation with increasing temperature.
  • Comment: This property is not specific to HTR, but is also present in light water reactors; without this property, neither the HTR nor the light water reactor would be approved. The HTR in particular needs a negative temperature coefficient of reactivity, since in the event of accidental heating - unlike in the case of the light water reactor - the moderator effect is retained. Furthermore, it can be stated that the temperature coefficient becomes less and less negative with increasing temperature, that at the same time the uncertainties in the knowledge of its course become greater and greater and that above approx. 1200oC its values ​​are not experimentally verified. Another particular disadvantage of the HTR is that reactivity accidents are possible with rapid cooling.
  • Property: Inner, phase stable, neutron physical neutral coolant helium.
  • Comment: It is correct that the cooling gas contains impurities which can lead to corrosion phenomena on the fuel assemblies; therefore a gas cleaning system had to be provided specifically in order to reduce these impurities, among other things. The other two properties of helium (phase stability, neutron physical neutrality) are of little relevance. Otherwise, only helium can be used as a coolant.

The outlined apparent safety advantages of the HTR must of course also be compared with its specific disadvantages and safety problems. Some of the allegedly positive properties mentioned are based on the choice of graphite as moderator and structural material. The properties of graphite are also responsible for HTR-typical and HTR-specific accident possibilities, namely graphite-water reactions after water ingress accidents (caused by steam generator leaks) and graphite fires after air ingress accidents. In the event of additional failure of the required safety functions (e.g. in the event of water ingress: steam generator shut-off, residual heat removal, reactor shutdown), these incidents are not controlled and can lead to uncontrolled releases with considerable damage in the vicinity of the reactor. For the reason, among other things, that these releases take place earlier than after a pure core heating-up accident, it can be assumed that accidents caused by water and air ingress initiate the risk-dominating accident processes at the HTR.

In addition to these types of accidents, so-called reactivity accidents, ie accidents that are triggered by malfunctions in the control and shutdown rod systems, contribute significantly to the risk of accidents in high-temperature reactors.

It can be considered certain that the HTR lobby will refer to the incident investigations as part of the approval process for the THTR-300 and the HTR safety analyzes of the KFA (nuclear research facility) Jülich in order to substantiate their claim that the incidents mentioned are controlled or do not lead to relevant damage in the vicinity of the system even if other safety systems fail. It should be noted that the studies presented so far on the accident risk of high-temperature reactors are provisional, incomplete, largely unsecured and scientifically inconsistent. Before a consensus would even be conceivable or a dissent even narrowed down, essential elements and prerequisites of a scientific-technical discussion process are still pending. B. the critical and independent review, the traceability and the accessibility of the sources.

In addition, it is strange that up to now risk studies have only been carried out on HTR concepts that either will never be realized (HTR-1160) or have only existed on paper (HTR-500, module), but are the only ones in Germany existing large-scale HTR system, the THTR-300, except for a superficial brief study, there is no risk investigation.

Features of the THTR-300 that are disadvantageous in terms of safety

A safety-related assessment of the THTR-300 based on its design features and construction principles - regardless of any negative surprises during commissioning - reveals a number of safety-related disadvantageous features. A comprehensive assessment of the safety-related design of the THTR-300 is not to be carried out at this point. Only three design features are to be addressed here as examples, which not only appear questionable from a critical position, but also collide with the nuclear rules and regulations and the so-called safety philosophy in nuclear technology. Also taking into account the differences between light water reactors (to which the nuclear regulations are mainly based) and the THTR-300, the violation of fundamental principles of reactor technology in the THTR-300 becomes evident on the basis of the following examples.

1 example:

The two shutdown systems are not sufficiently independent, not diverse and do not meet the requirements placed on them in all operating states and malfunctions. Thus, contrary to the opinion of the Reactor Safety Commission, the shutdown systems do not meet the BMI safety criteria for nuclear power plants (criterion 5.3.). There have been shutdown concepts for a long time that are clearly and far superior to that of the THTR-300 in terms of diversity, shutdown balances and reliability and which are also technically feasible.

2 example:

The THTR-300 does not have an independent emergency cooling system, as is prescribed and implemented for the light water reactor. The residual heat is removed with the help of the operational fan and the steam generator. Incidentally, the proposed successor reactor HTR-500 is to be equipped with two independent units for residual heat removal.

3 example:

The THTR-300 has no containment like the light water reactor, which consists of a gas-tight safety container and a concrete shell. The THTR-300 is only equipped with a (not airtight) so-called reactor protection building (industrial hall concept)

Construction defects that have come to light so far

In addition to the safety deficits that are justified in the design of the THTR-300, a number of design flaws and design errors have come to light in the previous commissioning phase, some of which are responsible for incidents and additional safety problems.

1 example:

The pebble is more compact than assumed in the projections. This has a number of consequences:

  • When the core rods are moved into the pebble for the purpose of long-term shutdown, increased forces, which are at the limit of the design, act on the rods.
  • The reliability of the core rod system, which is already unfavorable, deteriorates even further. B. showed the event of November 23, 11 (see Chapter 1985).
  • The result is the need to loosen the pebble pile by circulating it, which, however, does not provide any remedy, since the pebble pile is repeatedly compressed by moving the rod in.
  • The ball breakage rate is much higher than calculated. While in the "Atomwirtschaft" (atw) from December 1982 in an article by employees of the high-temperature reactor construction GmbH it was said that "in two years of operation on average only one fuel element is crushed by the core rods", the power plant director Glahe now 800 crushed balls added. According to other information, so many balls have already broken that one of the two containers provided for holding the broken ball is full; Both tanks together are designed to accommodate the ball breakage that occurs during the entire service life of the system. (The "Westfälische Anzeiger of May 19, 5 reported:" Almost one and a half years after the start of the trial operation, 1987 (!) Fuel elements the size of a tennis ball had to be removed ... "; Horst Blume).
  • The unexpectedly high accumulation of radioactively contaminated graphite and fuel dust as well as metallic abrasion was responsible for the accident on May 4, 5. In addition, problems arise from contamination and the accumulation of dust at numerous points in the system. Among other things, it increases the likelihood of valve and other equipment failure. 

2 example:

Above a certain power, the ball pile can no longer be circulated, since no more balls can be withdrawn because of the excessive flow forces of the cooling gas flow on the "separator" on the ball extraction pipe. This results in operational restrictions.

3 example:

Incorrect dimensioning of the insulation in the steam generator annulus as well as inadequate design of the ventilation system can lead to excessive temperatures occurring in parts of the system with certain outputs and with certain outside temperatures.

4 example:

Due to incorrect guidance of the primary cooling gas flows, the cooling throughput through the core is lower than planned due to the presence of a so-called bypass. As a result, it is not possible to achieve full load, which the operator will probably try to avoid through additional manipulations in the reactor core.

5 example:

The so-called reactor protection building is not airtight, so that the negative pressure intended to reduce possible radioactive releases from the reactor hall into the environment cannot be built up everywhere. One tries to get this error under control by means of provisional sealing measures.

In addition to these design flaws and deficiencies, there are a number of other deficiencies that are said to have been partially or completely eliminated, e. B. a leak in the liner cooling system and a fault in the loading system. At the moment it is not possible to assess whether these and other errors have really been finally and completely remedied.

Incidents in the THTR-300

Certainly, incidents are ultimately always unforeseen and unexpected events if they are assessed as individual events. Nonetheless, when evaluating the list of accidents of the THTR-300 that has been available up to now, one has to determine retrospectively that a number of incidents or types of accidents can be traced back to construction defects and almost inevitably occurred. The list of incidents includes the following events:

23.11.1985:

Seven of the forty-two core rods of the long-term shutdown system could not be driven into the full depth of the pebble cluster as planned. Only the use of the operational short-stroke drive led to full retraction. The actual cause of this partial failure of the core rod system lies in the increased rod forces as a result of the compressed pebble cluster. The information policy and attempts at explanation by the operator turned out to be implausible. (For example, the insertion of the core rods must of course also be guaranteed without feeding in ammonia as a "lubricant", since the ammonia feed is not a safety system according to the permit.)

04.05.1986:

The cause of this accident with increased radioactive release can be traced back to the increased accumulation of graphite and fuel dust and abrasion. After a valve on the low pressure side of the buffer zone of the charging system did not close due to contamination by dust and this error could not be remedied even with (non-radioactive) purge gas, the operator opened the valve on the primary side for the purpose of purging. A considerable amount of radioactively contaminated primary cooling gas with dust was released directly and unfiltered through the chimney into the environment via the pressure relief duct. In addition to the radiological aspects, what is particularly worrying about this incident is that the surgeon committed an obvious mistake and that due to the design and design (due to the lack of interlocks) it is at all possible that a single mistake can trigger a direct release of primary cooling gas, which is Otherwise, in the event of an additional error (e.g. due to a further operating error or failure of the closing function of the primary-side valve) to an almost complete loss of coolant into the environment.

In addition to these two more precisely described and publicly known, there were a number of other security-relevant incidents:

  • Error in the emergency power supply
  • Malfunctions in the measurement technology and in the control equipment
  • The NK 11 emergency cooling procedure has already been triggered 45 times; this would mean that the contingent of 45 such emergency cooling shutdown procedures for the entire service life of the system would already be used up to a quarter. 

Rating

The THTR-300-specific disadvantageous safety properties, the special design features, the construction defects known to date and the results of the commissioning phase so far make it urgently necessary not to start up the THTR-300 again. Otherwise, further negative surprises, difficulties and incidents are inevitable. From a safety point of view (but also due to economic considerations) the operator is requested to abort the dangerous large-scale test with the THTR-300. The conclusion can already be drawn that the pebble bed reactor technology has failed.

 

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


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