Radiation Protection Decision-Making Basis for Disaster Control Planning at the Hamm-Uentrop Nuclear Power Plant (THTR)

[Automatically extracted and translated] The following study provides an expert opinion on whether the contractually agreed 'conduct of extended investigations (updating) of accident scenarios and their extent of damage as a basis for disaster control planning for the THTR-300' between MWMT-NW and KFA can be dispensed with in view of the current state of the plant (reactor shut down) and the foreseeable plant states until after the establishment of secure containment. Ultimately, this raises the question of whether, according to the current state of science and technology and the newly formulated 'framework recommendations for disaster control in the vicinity of nuclear facilities', the existing disaster control planning, which was based on the operation of the THTR-300, is clearly sufficient for the shut-down plant without any doubt. This is affirmed and justified as follows: 1) The changes in the 'framework recommendations' significantly reduce the release values for the relevant nuclides, whose undershooting would no longer justify the initiation of disaster control measures (e.g., by a factor of 50 for strontium); however, the severity of maximum possible accidents from the shut-down plant decreases even more. 2) The previously considered worst-case scenario for disaster control planning, the total failure of core cooling, now leads, after a shutdown period of about 2 years, to a maximum core temperature of around 500 °C, even with conservative calculation allowances and boundary conditions, so that significant temperature-induced release of radioactive substances from the reactor core can be excluded; under otherwise identical assumptions, a core heating accident from operation would result in temperatures about 2000°C higher. 3) Other event combinations, including those with massive graphite corrosion, can be excluded with regard to disaster-relevant releases, also due to the low initial temperatures of about 50 °C. This also applies to criticality incidents during the 'unloading phase', based on the concept of inserted core rods. 4) Event combinations during which activity adsorptively bound to dust or the steam generator could be released gain relative importance but remain irrelevant for the initiation of disaster control measures: The amount of dust-bound activity was estimated by the operator to be about 12 GBq cesium-134/137 and max. 4 GBq strontium-90, from which even with a (mechanistically hardly conceivable) complete release during a pressure relief, radiation doses well below the lower reference values for initiating protective measures (here: staying indoors) result. Intruding water can only lead to the detachment of a small amount of adsorptively bound activity, which also remains safely enclosed in the primary circuit. 5) The previously specified path for disaster-relevant releases of radioactive substances via the chimney into the environment is also ensured for the out-of-operation THTR during the times when fuel elements are still present in the plant; a near-ground release can thus be excluded. 6) Events in plant areas outside the reactor would also not lead to disaster-relevant releases. The largest activity inventory in this regard is the storage for irradiated operating elements. It was not found to be penetration-proof against the aircraft type to be considered for today's design against aircraft crash. Therefore, it cannot be ruled out in principle that the crash of an aircraft of this type onto the storage, if associated with the destruction of operating element containers and a kerosene fire in the storage, could lead to the 'burning' of fuel elements; however, this can be prevented by simple internal emergency protection measures. The 'other' radioactive wastes, including absorber and moderator elements, which also remain in storage during the time of secure containment, have such a low activity inventory that even with complete, near-ground release, the previous disaster control planning proves to be sufficient. We hereby assure that the study was prepared impartially and to the best of our knowledge and belief, free from any directives.