On 14.05.2020, an insignificant excess, as specified under the terms of the operating licence, was detected in the value of the discharged nuclide C-14 via the chimney into the atmosphere at the FRMII.
C-14 is formed naturally by cosmic radiation in the earth's atmosphere. At the FRM II, C-14 is formed in a routine operation by the nuclear reaction of neutrons with oxygen in the water of the reactor pool. During the continuous purification of the water, C-14 is bound in the ion exchange resins. These must be dried for further treatment. An isolated error during the installation of the mobile drying equipment used for this purpose led to the discharge of the C-14 over a short period of time. As soon as the emission values had been determined, all drying processes were immediately halted. Due to COVID-19-related restrictions, the FRM II has been out of operation since 17 March 2020.
Regarding the release of C-14, the operating licence of the Heinz Maier-Leibnitz Research Neutron Source (FRM II) stipulates a limit of 2 x 1010 Becquerel per year. This is a value that was originally defined for reasons of radiation minimisation. From a radiological point of view, the release of this annual approval value for C-14 amounts to a theoretical radiation exposure of a reference person to max. 3 microsievert. This is equivalent to 1 % of the annual radiation exposure of 300 microsieverts for the population from radioactive substances discharged into the air, as permitted by the Radiation Protection Regulation (Strahlenschutzverordnung). The annual licensed value is only 20 % of the value specified in the Radiation Protection Ordinance for the handling of C-14, which is present in the form of carbon dioxide (CO2), without a licence.
In accordance with the reporting criteria of the FRM II, the relevant supervisory authority was notified of the event on 15 May 2020. The notification was classified as "Category E" according to the Nuclear Reporting Ordinance. The evaluation according to the seven-level international evaluation scale (INES) is performed according to level 0 (no or very low safety significance).
The event has no impact on the population, the environment or any area of the FRM II plant.
C-14 is a radioactive isotope of carbon. It is formed continuously in the atmosphere by cosmic radiation.
Natural formation of C-14: The natural nitrogen isotope (N-14) captures a neutron and transforms into radioactive C-14 via a nuclear reaction. This in turn decays, with a half-life of 5730 years, to nitrogen. In archaeology in particular, the C-14 method is used to determine the age of organic materials.
In reactors, C-14 is formed during routine operation by, among other things, a nuclear reaction with the oxygen isotope (O-17) in the water of the reactor pool. O-17 is not radioactive and occurs naturally with a frequency of 0.038 percent. During the nuclear reaction, O-17 captures a neutron and, through the emission of an alpha particle, C-14 is formed.
The licensed annual limit value of the research neutron source Heinz Maier-Leibnitz (FRM II) for the discharge of C-14 via the chimney into the atmosphere is 2.0 x 1010 Becquerel.
The C-14 discharged at the FRM II is in the form of carbon dioxide (CO2). The value specified in the Radiation Protection Ordinance for the handling of C-14 in the form of carbon dioxide (CO2) without a licence is 1.0 x 1011 Becquerel, which is 5 times higher than the annual licensed value limit of the FRM II.
This makes it clear that the annual licensed value for C-14 is set at an extremely low level.
A theoretical upper exposure limit for a reference person was established by means of a model calculation. This resulted in a maximum dose of 3 microsievert (for comparison, less than 1x X-ray of the teeth - approx. 5 microsievert). This corresponds to 1 percent of the annual radiation exposure of 300 microsieverts permissible for the population from the discharge of radioactive substances into the air under the Radiation Protection Ordinance.
The radiological effect of the discharge of 2.3 x 1010 Becquerel is not harmful.
Due to the fact that C-14 is a weak beta emitter, the discharged activity cannot be measured directly in the exhaust gas stream. Instead, a sample is taken regularly (approx. every 3 minutes) from the exhaust gas stream over a 3 monthly period and collected. The evaluation is carried out by the FRM II's own chemical laboratory and, as part of the control of the self-monitoring process, by the Federal Office for Radiation Protection (BfS).
The total discharge is calculated from the activity concentration determined in the laboratory and the exhaust air quantity discharged via the chimney.
The evaluation of the sample for the 1st quarter 2020 in April resulted in an emission of 1.85 x 1010 Becquerel, taking into account possible sources of error. Based on the results of the 1st quarter, the sample collection period was changed to a monthly evaluation. The evaluation of the sample for the month of April resulted in an emission of 4.5 x 109 Becquerel on 14 May 2020, taking into account possible sources of error.
Due to the measurement procedure, results are available with a delay of about two weeks
The sum of the values thus resulted in 2.3 x 1010 Becquerel. This corresponds to a slight excess with respect to the value specified in the operating permit for the discharge of C-14 via the chimney into the atmosphere.
The first five day drying cycle was carried out with a scheduled one day interruption from 20 March 2020 to 26 March 2020. Deviating from the operational regulations, the CO2 separator unit was not connected and the pump exhaust air was fed directly into the ventilation system.
After a scheduled waiting period, the second drying cycle of 5 days was carried out from 2 April 2020 to 7 April 2020 in compliance with the operational regulations with the CO2 separation unit connected.
Online monitoring, e.g. KfÜ, is not metrologically possible due to the low radiation energy of C-14. For this reason, the C-14 emission can only be determined by means of a collection over a longer period of time.
Since the emission was over a period of 5 days, the weather situation cannot be clearly identified. Due to the prevailing wind direction (NW to E) and strength, the calculated main impact point is on the FRM II site and the immediate vicinity.
The FRM II is responsible for monitoring C-14 emissions. The Federal Office for Radiation Protection is responsible for the control of the self-monitoring, and on this account carries out its own measurements.
During the continuous cleaning of the water from the reactor pool, among other things, the C-14 is bound in ion exchange resins. These must be dried for further treatment, which is a multi-stage process.
One of the drying units used for this purpose is mobile and can be used at the FRM II for various operational purposes.
To reduce the C-14 emissions during the drying of the ion exchange resins from the heavy water system, a special CO2 separation unit has been in use since 2013. The chemically washed CO2 and thus the C-14 bound in it is disposed of as radioactive waste after further treatment.
Due to an operating error, the first drying process was carried out without a downstream CO2 separation unit, contrary to the operational regulations.
When the emission values had been identified, all drying processes were immediately stopped.
Furthermore, the FRM II has changed the previously quarterly examinations to a monthly rhythm.
In order to monitor C-14 emissions even more closely, an additional internal monitoring system with weekly evaluation was put into operation on 20 April. These are washing bottles filled with caustic soda (NaOH), through which a small portion of the stack air is passed. The CO2 contained in the chimney air reacts with the caustic soda (over several steps to sodium carbonate). The sodium hydroxide is changed weekly. In the operational chemistry laboratory of the FRM II the NaOH is examined for C-14 after a wet-chemical processing.
The latest evaluation of the washing bottles shows that the actions taken by FRM II are having an effect.
The FRM II has already provided the authorities with full information on the event to facilitate a detailed explanation of the causes.
The nuclear regulatory authority (StMUV) will examine the current situation in cooperation with the LfU and the experts consulted and decide on future procedure.