|Fig. 1: The cooling towers of the Jaslovske Bohunice nuclear power plant. A1 building lies beyond the left edge of the picture. (Source: Wikimedia Commons|
The A1 nuclear power plant (NPP) Jaslovske Bohunice, which lies about 60 kilometers northeast from Bratislava, the capital of Slovakia, was the first nuclear power plant in the former Czechoslovak Socialist Republic (CSSR). Its construction, which started in 1958, lasted fourteen years and the plant was finally connected to the electrical grid on December 25 1972.  In the late 1970s and 1980s, additional two blocks, V1 and V2, were built, that we will not discuss in this report. In the present day, the A1 and V1 blocks have been decommissioned, while V2 is operational. The current status of the plant can be found in the National Report of the Slovak Republic. 
The reactor in the A1 station was an experimental heavy-water gas cooled reactor (HWGCR) KS-150 of soviet design with power of 150MW, operating pressure 65atm and temperature 400 degrees Celsius.  The reactor used natural metallic uranium for fuel (supplied from the mines in Jachymov) - due to the heavy water, the enrichment of the fuel was unnecessary, similar to today's CANDU reactors. When running, the reactor contained 23 tons of natural uranium and 54 cubic meters of heavy water, used as the moderator. Next to the lack of enrichment, the design allowed the reactor to be refueled without shutting it down as further advantage.
Before its ultimate shutdown in 1977 and the decision on decommissioning in 1979, the reasons for which are described below, the reactor produced almost 1GWh of electric energy. 
During its operation, the experimental reactor KS-150 housed in the A1 station suffered multiple incidents, that were kept secret from the public at the time they occurred.  Two major events are discussed below. Even though their details are not widely known among the population to this day, they both had serious consequences, the second one ultimately leading to the plant's shutdown after only four years of operation.
First incident happened on January 5th 1976 during a refueling. The electronics mistakenly indicated that a new fuel element had been tightly connected to the reactor.  The gas coolant started leaking into the reactor hall, suffocating two workers that were not following the emergency instructions. The fuel element was subsequently ejected into the hall. Viliam Paces, the head of the work shift at the time of the incident, manually reconnected the refueling machine and stopped the leakage.
Even though the plant personnel was not immediately irradiated, this incident marks the first casualties due to the failure of a nuclear power plant (CO2 inhalation in this case).  It has not been assigned a level on the INES scale, although some authors consider it a level 3 event ("serious incident"). [1,5] The core of the reactor did not suffer any damage in the incident and the operation resumed in September of the same year, after a repair and inspection. 
The worst accident of the A1 power plant, which would be subsequently classified as INES level 4 event ("accident with local consequences"), occurred on February 22 1977. As opposed to the previous incident, human error of the refueling crew lead to this one. The fuel assembly would be covered in silica gel during transport to protect it from humidity. The gel was to be thoroughly cleaned off before the assembly could be put into the reactor. However, on this particular occasion, the staff did not sufficiently remove the gel. This hampered an easy access of the cooling gas to the newly inserted fuel. Subsequently, this lead to a local overheating and a breach of the barrier between the heavy water and the fuel with the cooling gas. Finally, the combination of water and carbon dioxide quickly corroded the coating and contaminated the primary and secondary circuits with fission products. 
After the accident the decision was made to shut down the operation of the A1 plant, both for economical reasons and in accordance with the government's decision of not building any more gas cooled reactors in Czechoslovakia. Soon afterward, in 1979, the authorities decided to decommission it, starting a decades-long and currently still ongoing effort.  The project consisted of several phases. At the moment of writing, only the first phase had been completed, in 2007, that mostly focused on transporting the spent nuclear fuel to the Russian Mayak facility. The delays were mostly caused by improper handling of the fuel on site of the plant. Storing the fuel in a pool caused corrosion and subsequent contamination of the pool through leaking. Further problems were caused by the damaged fuel assemblies from the 1977 accident that have also been stored at the same place. Transport of the fuel to Russia finally started in 1983 and last assemblies left the plant in 1999. Further phase I activities involved disassembling of the secondary components, such as turbines or pumps, and development of new techniques for treatment and storage of the radioactive waste. The second phase will include the complete decommissioning of the plant.
A significant amount of research effort targeting the impact of the A1 NPP on the environment and the population has been carried out in the region of the plant. This effort intensified after the accidents. The measurements predominantly focused on various kinds of environmental and health indicators that could be potentially influenced by the plant.
The impact of nuclear activities can be seen in the concentration of C-14 in the atmospheric CO2. Long-time measurements in two locations - village Zlkovce near the Jaslovske Bohunice NPP and the capital Bratislava - have been reported by Povinec et al..  Apart from semi-annual variations caused by the Suess effect (increase of the natural CO2 concentration in the atmosphere due to the higher usage of the fossil fuels in winter and thus an apparent decrease in C-14 concentration), the authors found a clear indication of the NPP's influence on the C-14 concentration. Its peaks measured in Bratislava (60km to the southwest from the plant) clearly correlate with the time periods when the wind predominantly blows from the direction of the plant. However, the air-borne C-14 is not expected to significantly influence the health of the population, due to the relatively low intake of carbon by breathing. Most of the isotope is ingested in food instead. The authors estimate an annual radiation dose from this source to be about 3μSv, an insignificant amount with respect to the dose limit for the public of 1mSv annually.
Due to poor water treatment (and flooding of the shutdown A1 plant), the banks of the water recipient from the NPP were contaminated by radiation. In total, on the 19km-long banks, 67000 m2 were found to contain Cs-137 with contamination levels above 1Bq/g of soil. The contamination occurred in patches, rising to values up to 322 Bq/g in some places. This is concerning, as the area lies in the vicinity of nearby villages, whose inhabitants have unrestricted access to the banks of the river.  The original plan of dealing with the contamination was to remove the upper layer of soil (about 13000 m3) in all the contaminated areas. However, this was later reduced to only 1100 m3 when it was found that a significant environmental improvement could be achieved by removing the upper layer of soil from only about 10000 m2 that were the most contaminated. 
The nuclear impact on the environment can be evaluated by studying plants in a given region, that absorb nutrients from the potentially contaminated soil. These techniques were employed to study the environmental status of the Chernobyl exclusion zone, for example. A study of the wild plants around the Bohunice region has been carried out in the 1990's and early 2000's, however no significant mutations or signatures of phytotoxicity were found.  The authors point out, that the lack may be caused by a relatively low levels of contamination around the Bohunice NPP with respect to Chernobyl. Lack of mutations could also be caused by long term adaptation of the plants to the environment, as it took 18 years for the first measurements to be taken.
Arguably the most important question is about the impact of the plant's presence and the accidents on the human population in the region.  studied several health indicators, such as the number of premature births or deaths, or deaths by leukaemias, in the vicinity of the Slovak nuclear power plants (30km radius for Jaslovske Bohunice and 20km for the other NPP, near Mochovce). The authors did not have access to data from before the Jaslovske Bohunice plant's operation. However, they were able to draw conclusions based on comparing the health status of the community to the Slovak average. Some of the indicators differ from the average due to the demographic structure of the population. Both NPPs lie in rural regions where people live predominantly in small villages, and the average age of the population tends to be higher. However, all the studied health indicators tend to be the similar, or better, compared to the Slovak average. The authors conclude that the plants (including the accidents in Jaslovske Bohunice) have had no negative impact on the people's health, even suggesting a positive influence due to the better life style of NPP employees, who have relatively high income, follow strict work rules and live in clean rural environment.
© Ondrej Urban. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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