 It is the 15th of October 1958 and a little known nuclear disaster is about to unfold in Finca, Yugoslav Republic, modern day Serbia. In its wake one would be dead and four would be seriously injured. Although largely forgotten now, in its aftermath an uncharacteristic cooperation agreement between east and west would result in several medical and nuclear research first. The desire to explore the radiological consequences of the accident resulted in French, British and Yugoslav cooperation. The results would benefit all sides of the political spectrum and prove the usefulness of a budding international nuclear organisation. My name is John and this is a brief history of the Vinca reactor excursion and dosimetry experiment. Our story starts in the late 1940s and the establishment of the Vinca Institute of Nuclear Sciences. Yugoslavia had become a communist country in 1946, after the country had been liberated at the end of WW2 and after the abolishment of its monarchy in November 1945. In its early years Yugoslavia was tentatively aligned with the USSR and it was amongst this background that the Vinca Institute was formed. Politically the country would start to become more independent from the Soviet Union in 1948 with the Tito Stalin split. Needless to say this brought the fear of war between the two communist countries. Rumours were floating around that the USSR was close to its own atomic bomb and as such Tito wanted to match his new rivals. 75 billion dinars, about $35 million at the 1953 exchange rate was spent on building and operating the country's fledgling nuclear industry. This constituted a significant part of the country's GDP. In the early 1950s several organisations were set up at Vinca in preparation to develop an atomic weapon. A department for spent fuel reprocessing was created in 1956 and a heavy water-moderated zero-power critical assembly RB Reactor was constructed in 1958. This RB Reactor was the first to be built inside Yugoslavia and strangely for the political landscape was of a Soviet design. The reactor made use of roughly 4 tons of natural uranium in the form of 216 aluminium clad fuel rods. Each rod was 2.5 cm in diameter and 210 cm long. The fuel rods, with a lattice spacing of 12 cm, were contained in a cylindrical tank made of aluminium of 1 cm thickness. One end of each fuel rod rested on the bottom of the aluminium tank. The assembly used heavy water as a moderator of the neutrons created during chain reaction. Interestingly, instead of control rods, the system made use of the level of the water in the tank to control the reactor. Two safety rods were provided and were held out of the core by electromagnets and if power is cut, then the rods fall into the core by gravity, stopping the chain reaction and is thus failsafe. A storage tank was provided under the reactor for excess heavy water. The design was to also not have any form of neutron reflection. As such, it was mounted on a supporting rack to ensure the tank was always at least 4 metres away from any potential reflection surfaces. As well as no reflective surfaces, the reactor had virtually no shielding, which the operators would find out later was not such a good thing. Now I need to quickly mention the reactor's name, as it was considered a zero power installation. You see, a zero power nuclear reactor is capable of sustaining a stable fission chain reaction, with no significant increase or decline in the reaction rate. This type of installation is essential to gain practical experience of reactor operation, but can still be deadly if the delicate balance isn't maintained. It is the 15th of October 1958 and an experiment is being undertaken at the Venture Reactor at the Boris Kedrick Institute. Six operators are working inside the reactor hall on a subcritical foil counting experiment. The workers rely on BF3 counters to monitor the reactivity of the reactor. On the day, the reactor has 3,995kg of aluminium clad natural uranium fuel inside its core. Because the liquid level was normally used to control the system reactivity, and the operators wanted to obtain as much activation of the foils as possible, the heavy water level was raised in increments, but it was intended to keep the assembly subcritical. To do this, at each level the BF3 counters needed to be closely monitored. Two of the BF3 counters maxed out at the maximum saturation level, but the third was acting erratically. This was disconnected and technical assistance was requested, but what the operators didn't know was that the reactivity of the assembly was increasing ever closer to supercriticality. After the assembly had been at this D20 level for around 5-8 minutes, one of the operators started to smell ozone. Originally it was thought that maybe a valve had been leaking, but after investigation it was realised that the system was supercritical at some unknown power level. Upon realising this, the reactor was scrammed with its safety rods and the hall was evacuated. A paper recorder based 540 metres away recorded an increased background gamma ray level for around 10 minutes. The heavy water level was at 183cm and the reactor was in an uncontrolled state for roughly 433 seconds. During the excursion the total energy release was around 80 million joules, or 2kg of TNT equivalent. A terrific amount of energy to be exposed to, especially in an unshielded assembly, definitely somewhere you would not want to be standing next to. Six personnel had received varying doses of radiation. Initially they received first aid on site before being sent to Paris for specialist care. The hospital they were transferred to was the World Famous Curie Foundation and they were placed under the care of Dr H Jammay. Of the six exposed individuals, one was to be treated conventionally with blood transfusion. This was due to his exposure being significantly lower than the others, but it was quickly realised that the others would require some drastic treatment, as they had received in some cases close to and well over a potentially lethal dose. The radiation had destroyed tissues in their bone marrow, resulting in a number of white blood cells falling sharply. At the time the concept of bone marrow transplants was in its early stages. Great strides had been made in the field in the US, but in the late 1950s it was still very much an experimental science. The bone marrow grafts were performed by Georges Maffe, an oncologist, and made use of donors that were all French, including Marcel Pabion, Albert Byron, Raymond Castenair and Odette Draft. The fifth donor was Leon Schwarzberg, a member of Maffe's own team. The grafts and subsequent treatment helped further the concept of bone marrow grafts, immunotherapy and how it could be used to treat cancer. Treatment would start roughly 28 days post exposure, but for one of the men this was too late. He had received too high of a dose and the treatment didn't help reverse his outcome, and sadly he passed away. However for the other four the grafts worked and they gradually made a recovery, reportedly still being alive in 1980, 19 years post exposure, with at least one of the men becoming a father to a healthy child. You see what was interesting with the transplants was that graft versus host disease was not observed in the patient. One theory that was posed was that the high radiation exposure had actually prevented the creation of antibodies and in a happy side effect facilitated the incorporation of the grafted marrow. It was estimated that the doses of each man who was aged between 23 and 26 had received in descending order 433, 422, 415, 410, 320 and 205 rem respectively. For reference 500 rem without any treatment is fatal. It really depends on how much was received, where in the body and the health of the victim prior to exposure. Now the doses of the patients was very inaccurate due to the fact that the reactor was super critical for a rather long period of time, and studies into acute radiation sickness were not very abundant at the time. But the reactor now powered down offered an opportunity for the scientific community in which a study could be undertaken to further understand dose effects on humans. The situation presented to the international community and Yugoslav nuclear officials was that if the exact doses could be ascertained through an experiment then it could be linked up to the treatment and ARS symptoms in the patients. Post Stalin-Tito split Yugoslavia was more open to collaboration with the West and in February 1960 an agreement was signed between the Federal Nuclear Energy Committee and the IAEA where the reactor would be reactivated and prepared for an experiment. Part of the reactivation required around 6.5 tons of heavy water as the original stock had been reused in Vinch's other reactors. After negotiations with a number of IAEA member states the United Kingdom offered the required material for free for the length of the experiment. The French commission of atomic energy offered to make modifications to the reactor for free as well. The modifications consisted of detector and more comprehensive control equipment which allowed the reactor to be placed into different reactivity states more effectively. More obvious alterations were a wall of sandbags and concrete between the reactor hall and control room to protect the operators from any fast neutrons. For the experiment the reactor was to be operated in two power ranges low and high and this was to take place at the end of April 1960. The low power test ran the reactor at 5 watts and used multiple positions to measure fast neutrons and gamma doses. But the high power tests were where things really got interesting as this would involve these rather creepy characters named phantoms. These were designed to mimic a human body to allow greater dosimetric accuracy. The phantoms were filled with an accurate solution of sodium chloride at a concentration of 15.7 milligrams of sodium per gram of solution. After each experiment the sodium activation was measured. The high power experiment ran at two power levels one kilowatt and five kilowatts. For each run the phantoms were placed in different positions with the lower being closer to the reactor and the higher being further away. After the experiments the IAEA released its report in 1962 and it is definitely well worth reading. The study helped create a greater understanding of potential doses a person can receive in certain circumstances. This also helped in turn in the understanding of the types of shielding needed to protect staff from fast neutrons and gamma rays. However the experiment wasn't the magic catchall bullet but rather another tool available to the nuclear industry. Even in the IAEA's own report which made its concluding remarks hinted upon this. It can not be expected that data for any one accident can solve all the basic questions at once or even any of them completely. What the experiment did show was the benefits of cross border cooperation when it comes to radiological investigation. The disaster and subsequent experiment showed that the young IAEA which was founded in 1957 could work as a concept for interstate nuclear industry cooperation. After all it did successfully broker an agreement where an experiment in a Soviet design reactor based in Yugoslavia modified by the French and filled with British heavy water could be used for the greater good of dosimetric discovery. Now where would you rate the disaster on my scale? I'm going to give it a free due to the death of the young operator and the horrible pain the others must have gone through but at eight on my legacy scale because of the international cooperation valuable information gained from the experiment and the successful use of bone marrow grafts. 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