 Before we start, I've got to let you all know that this is the 100th episode of Plainly Difficult. To mark this event, I've got a new t-shirt out on my Teespring store. When you think of nuclear accidents in the former Soviet Union, Kushtym or Chernobyl come to mind. However, there was a lesser known incident in the newly formed Russian Federation, which involved an explosion and a release of radiation. If you like what we do here at Plainly Difficult, consider helping the channel grow by liking, commenting and subscribing. Let's get started. It has been a while since we've covered an incident in Russia, and this subject has been on my list for quite a long time. It is a very interesting event to look at, as it happened fairly recently, well in comparison to some of the other things I've covered on this channel. Of course, today we are looking at the explosion at Tomsk 7 reprocessing facility. Today I'm rating this subject here my disaster scale, not too bad, not too good. Nuclear fuel reprocessing is a vital link in the fuel supply chain for both nuclear actors and atomic weapons. On both sides of the iron curtain, many facilities were employed in the recycling of spent fuel for reuse. One such facility in the former Soviet Union, based in Tomsk 7, was a complex used in the nuclear technological cycle for the creation of nuclear weapons components based on fissile materials. Tomsk 7 was the name given to current day Svesk, a secret city in Tomsk Oblast Siberia. The city was founded in 1949 and quickly became a main hub for the Soviet atomic industry. The city was home to Siberian chemical enterprises. A complex involved in a large-scale production and reprocessing of uranium and plutonium fuels. Construction at the complex started in 1949 and began highly enriched uranium and plutonium production between 1952 and 1955, but plutonium at the site was originally used for commercial uses until two reprocessing lines in the radiochemical works came online in 1961. The site had a varying number of uses, including experimental reactors, an isotope separation plant, a plant for producing uranium oxide, protoxide and hexafluoride, a uranium and plutonium production plant, and storage facilities for radioactive wastes. With such a strategically important site, the USSR kept the plant and the city under tight controls, with the city not appearing on public maps and the movement of residents severely restricted. However, today's incident actually happened after the fall of the USSR in the new Russian Federation in April 1993. The city had been able to use its real name after President Boris Yeltsin to clearly 1992 that such cities could use their historic names. Although no longer a state secret, the facility in the newly named Svesk was still vital to the country's nuclear industry and, as such, was still heavily guarded. After the end of the Cold War, the Tomsk 7 complex had moved away from weapons materials production to more applications. Due to the activities on the 192km square site, an area surrounding it of 1560km2 was designated as a supervision zone, where routine measurements are taken to monitor the environmental impact of nuclear operations. Located around 16km from the regional capital of Tomsk, in 1993 the city which houses personnel working at SCE and their families had a population of over 100,000 people. The countryside around the SCE was sparsely populated apart from Tomsk and the former secret city, Svesk. The regional capital had a population of around 500,000 people. There were several smaller villages in the region, mainly agricultural communities with only a few hundred inhabitants. Reprocessing of uranium and plutonium was carried out in building 201 odd radiochemical works at the SCE. Let's have a look at the process employed at Tomsk 7. Reprocessing started with loading of the radiated standard uranium blocks into the plant where they were dissolved in concentrated nitric acid. The solution was then transferred to another vessel and was prepared for extraction by adjusting acidity and the temperature. Sometimes additional solution was added which had already gone part the way through the process before. To avoid a chemical reaction in the solution, compressed air was used to stop it from separating out. After this point the solution passed along the reprocessing line for settlement extraction using tributyl phosphate in a light hydrocarbon dissolver. A series of mixed settlers were used for selective transfer of uranium and plutonium to the TBP leaving behind unwanted products such as cesium and strontium in the aqueous solution. The two liquids then separate with the TBP sent on for further processing to separate the uranium and plutonium from one another. Installation AD6102-2 was used as part of the reprocessing process. It was a vessel used to prepare the solution prior to reprocessing. The vessel was made out of stainless steel with a volume of 34 meters cubed enclosing a steam heating and cooling sleeve. It was also fitted with a number of sensors to help with process control. These sensors included two level indicators, a thermometer, a pressure transducer, a pit open warning device, a flow rate transducer and some radiometric control sensors. It was placed underground with two meter thick concrete walls and a concrete lid below the ground in building 201 in one of its two reprocessing lines. This leaves us onto the disaster on the 6th of April 1993. In the preceding days, installation AD6102-2 was completely emptied. The facility was then recharged in preparation for new solution. Before the 6th, one batch had been extracted. After extraction, around four meters cubed of residual solution was left behind. In the morning of the 6th of April, two batches of product 166 with a volume of around 19.5 meters cubed were added to AD6102-2. At 10.30 am, 1.5 meters cube of nitric acid was also added in preparation for extraction. At 12pm, sensors within the vessel started to show a pressure rise. 40 minutes later, a new shift arriving at the building noticed red smoke coming out of the vent tube, something that was not a usual sight during this type of extraction. 10 minutes later, the engineer in charge was notified of the pressure increase, which was at 2 atm and still rising. The order to reduce pressure in the vessel was issued however no discernable effect was seen as the pressure raised to 5 atm. At 12.58pm, the pressure exceeded that of the vessel's design specifications and ruptured. Within seconds, an explosion engulfed building 201, knocking down walls on two floors of the complex. Start within the building, donned breathing apparatus, which was located near their workstations, and mustered in a safe area where they were informed of the incident. The radiation monitoring system was activated and within a few minutes of the explosion, the on-site fire department arrived. Within 10 minutes, the fire on the roof and equipment room was extinguished by the swift response on site. Dose rates at the site of the explosion meant that first responders couldn't get direct access however significant damage was clearly visible. The slabs covering the cell had been displaced and the ceiling of the equipment room had been partially destroyed. The surviving walls showed signs of blackening from the intense fire. Due to the damage, no immediate assessment of the vessel could be undertaken and the compromised building meant that no passive containment of radiation was available. This meant the deadly radioisotopes were escaping into the atmosphere. Around 250 metres cubed of radioactive gases, 8.7kg of uranium and 500g of plutonium were released into the environment. This amounted to around 30 tB of beta and gamma emitters. At the time of the explosion, there was a light wind and snow limiting the spread of the release, however contamination did make its way into the environment. Almost straight away, aeroplanes were sent out to get a scope of the radioactivity and an aerial survey was undertaken. The accident released a plume of fallout into the surrounding countryside. This was both from the damaged building and from the 150m high stack on site. The main area affected by fallout from the accident was the village of Gorgievka, with a population of about 200. Doses up to as high as 30 microceverts an hour were recorded. The weeks after the accident, the snow contained radioactive elements, such as caesium 137 and strontium 90, both known carcinogens. For the cleanup efforts, lead shielding was erected to protect workers. To decontaminate the area, snow and soil was removed, roads around the SCE site were washed down and any solid waste, for example debris from the buildings, were disposed of on site. The damaged building was rebuilt by mid 1994. In the village of Gorgievka, 380 tonnes of soil and snow were removed for burial. Contaminated planks of wood, firewood and garbage were also removed from the village. The final stages of decontamination consisted of deep plowing of vegetable gardens and the use of mineral fertilizers and asphalting the main street. For the remainder of 1993, residents were encouraged not to eat any local produce. The government provided free fruit and vegetables, bought any livestock to be slaughtered and mixed feed was imported to enable food for farm animals to not be contaminated. The children of the village were voluntarily relocated in total 18 children were kept away from the village for around 2 and a half months. The doses estimated between 0.15 to 0.37 millisieverts for villagers during the event and were estimated to be between 0.016 and 0.049 millisieverts one year post event. At the SCE, around 1900 personnel were exposed to varying levels of radiation during the event and subsequent clean up, with the highest being received at 7 millisieverts. Amongst the firefighters, the dose was thought to be around 2 millisieverts, mainly from exposure during the 10 minutes that they quelled the blaze. But how did this disaster happen, you might ask? Well, let's look at this next. In a 1998 IAEA report, it was thought that the cause of the incident was poor flow of the compressed air. This meant that the solution was not properly mixed, causing separation. This was thought to have been caused by one of two reasons, operator error or mechanical fault. The disaster started when nitric acid was being introduced to adjust the acidity. Because of the poor airflow, this was not properly mixed into the solution. The chemicals within the vessel settled into different layers in which the nitric acid settled at the top of the vessel, oxidizing with organic materials inside, releasing gases from the reaction. Eventually the pressure ruptured the vessel. The gases released were inflammable and ignited on either a spark caused by the rupture or on a 450 degree centigrade hot vessel. After the vessel was ruptured, disaster was inevitable. The site continued to see use how the similar reactors on site were shut down in 2008 as part of a Russian-US agreement in winding down of weapons-grade plutonium production. Today the facility produces fuel for foreign customers and stores low and intermediate level nuclear wastes from reprocessing. 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