 Criticality accidents are a technician's worst nightmare, although a rare occurrence they can and do happen, and can result in severe radiological exposure and an expensive cleanup process. An event would unfold in the Russian Federation in 1997 and would have some similarities to two of the most infamous criticality accidents, those being the deaths linked to the Demon Corps. Although not identical, the accidental prompt criticality of some fissile material due to accidental dropping of a reflector does sound familiar to Lewis Slotkin's exposure. The accident would result in a dead, very experienced technician and a six and a half day excursion. I'm going to rate this event here four on my Painted Plain Difficult Disaster Scale. This is due to the event being relatively localised and didn't result in significant contamination. Tsarov is a city in Nyshny Novgorod of last Russia, originally founded in 1691, but our story starts quite a while after that. In 1946, the town became a closed city and it was removed from all unclassified maps. The reason was, well, but it found itself the centre of the Soviet nuclear weapons industry. The All-Union Scientific Research Institute of Experimental Physics was set up in the town, which was a nuclear weapons design facility. The institute over the years had set up testing grounds and hundreds of experimental facilities, leading it to become the country's largest research and development organisation. As a quick side note, the city became twinned with Los Alamos in the early 90s. Post-collapse of the USSR, the nuclear centre was still the heartbeat to the Russian nuclear industry, conducting experimental studies in the fields of physics, radiobiology and the health effects of radiation set over 12 facilities. One of the purpose-built facilities was the critical assembly, which was designed to be an experimental tool to study fission reactions relevant for design and development of power, research and impulse reactors. The assembly was placed on a cart on tracks inside a room surrounded by concrete walls, with a separate control room. There were also other rooms for storage and workshops, and each room had air samplers and detectors for measuring gamma radiation. The critical assembly was essentially a piece of fissile material surrounded by two reflectors, although the actual implementation was a little more complicated. The assembly had the ability to move the fissile material inside a half-sphere reflector up and down through the use of hydraulic pressure towards or further away from a fixed upper half-sphere reflector. For the construction of the assembly, thorough calculations are used to ensure the fissile material remains subcritical. The construction process involves successive layering of various materials, including copper, steel and uranium, which are prepared in the form of sets of machined hemispherical shells of standard sizes, which the technician can assemble into various configurations. The process was broken into two parts, first constructing the lower half of the critical assembly, then the second top half. The lower half of the research assembly is constructed on a table that can move vertically up and down using hydraulics. For the first part, the table had to be in the up position during construction of the assembly. For the second part, the lower assembly is lowered and the upper part is placed on a stand consisting of a ring where it can be positioned horizontally over the lower part of the system. Once the whole assembly is constructed, the operator is then meant to go to the control room to undertake the experiment. Also, I should say that during the construction process, it's meant to be a two-person job, the operator and the supervisor, who should be double checking all of the work. During the experiments, the assembly was essentially a small fast reactor that can regulate itself in a rather clever way. If the mathematics are correct, then as the assembly heats up, the materials expand, reducing the effect of the reflectors and as it cools down, the components return to their previous position. The behavior is seen as sharp fluctuations of the measured neutron flux, which stabilizes after several cycles, after which the critical assembly operates at a constant level of neutron yield. Sensors placed inside the assembly room tell the operators the state of the materials, which allows them to move up and down the lower sphere to change the power levels. If the assembly went out of control, it would melt, thus ending the chain reaction, and was an important safety feature. All experiments using the assembly were strictly controlled via regulatory and technical framework set out by the Russian Federation Ministry for Atomic Energy. The procedures precisely outlined the management requirements for work with critical assemblies. They require that this type of work be conducted only by a group of trained technicians, with each person being responsible for strictly defined actions within a sequence of operations which are closely supervised. Obviously, this is important as a mistake can result in a criticality event, which you know is not great, but we can probably predict what is going to happen next. Roll on the morning of the 17th of June 1997, a 41 year old male named Alexander Zakharov set to recreate a 1972 experiment using uranium with a copper reflector assembly. The technician was a very experienced worker with several hundred experiments under his belt. He was familiar with the process for the particular assembly, but contrary to the rules he was working alone in the hall. Zakharov had also broken another rule, and that was by not properly filling out the appropriate paperwork for the experiment. He had taken the dimensions for all of the components from the original 1972 logbook, but he had mistakenly written down the wrong outside diameter. Instead of 205mm he noted 265mm. At 10.50am whilst placing the upper reflector on its stand above the fissile material, it slipped from the technician's rubber-gloved hand and fell onto the lower part of the assembly. This sent the subcritical assembly prompt critical, and a blue flash of light, and a heatwave was experienced. The lower part of the assembly was ejected downwards into the bottom of the stand. Knowing his mistake, Zakharov fled the room and reported to his superiors, explaining the situation he claimed to have received more than a fatal dose. Initially he was conscious, but within minutes his exposure began to take its effect. The radiation protection personnel performed an initial radiological survey of the technician. This detected the neutron-induced gamma radiation emitted by radionucleides in his body. It was estimated that he had received a whole-body dose of 10g. For comparison, between 4 and 6g whole-body dose has been lethal before. Zakharov was sent to hospital for treatment, arriving at Sarov Occupational Medical Service at 11.45am on the 17th June. By this time he was experiencing nausea and he began to vomit, increasing in frequency over the following two hours. The sickness was lessened by 2pm by anti-metric drugs. The situation was looking pretty dire as his condition deteriorated. It was decided in the evening that he be transferred for specialist care in Moscow. He was admitted to the clinical department of the Institute of Biophysics in the Ministry of Health at 9pm. He was fully conscious, but his fatigue and headache were persistent. His hands showed signs of severe exposure. They would become severely swollen during the first night between the 17th and 18th June. On the morning of the 18th June, the patient's general state was evaluated as critical and getting worse by the hour. His hand injuries continued to worsen with the spreading of the swelling over more areas of the forearms. On the 19th June, the condition of Zakharov was dire. By 2pm X-rays showed fluid on the lungs and his arms were deteriorating rapidly. Later on in the day, the decision to amputate both arms was taken and the surgery was performed around 4pm. Initially his condition was stable, but sadly not for long. At 2.45 in the morning of the 20th June 1997, his blood pressure dropped dramatically and at 3.20 in the morning, around 66 hours after exposure, the patient died. But what happened to the critical assembly after Zakharov left the room and was taken to hospital? As the technician passed away in the early hours of the 20th June, the chain reaction would still be ongoing. Back to 10.50 in the morning on the 17th June, the specialists arriving at the accident facility had to determine the status of the critical assembly and the possible consequences of the accident. At 11am evacuation of all staff from the building where the accident had occurred and the radiation survey outside the building was carried out. By the early afternoon it was confirmed that there was no airborne or surface contamination, but the assembly was still emitting significant neutron and gamma radiation, which would preclude anyone from entering the room. All but the dosimeter furthest away from the experiment showed readings off the scale. Another meter was entered into the room and showed the neutron flux was stable, hinting at the chain reaction being self-regulating. Obviously the chain reaction had to be stopped and this necessitated altering the assembly remotely. The first step taken was to remove containers of nuclear materials which had not been used in the construction of the assembly. This was conducted by using a robot. Once complete the same robot was used to alter the assembly. At 12.45 in the morning of the 24th June 1997 when a vacuum gripper was used to remove most of the assembly from the stand, leaving only the outermost lower copper hemicelle in place. The remainder of the assembly was disassembled bringing an end to the incident. During the excursion the radiation monitors in other rooms of the facility showed normal levels, but the technician's personal dosimeter read a neutron exposure of 45g and a gamma dose of 3.5g. It was thought that the uranium had reached temperatures of up to 865 degree centigrade during the excursion. Because the accident was solely down to Zakharov and his infringement of the rules set out by the facility. Although his death is a tragic accident some comfort can be taken that no one else received elevated radiation doses. Although the IAEA found the rules of the facility to be adequate they still did not prevent the accident. After the accident experiments with the test rig were stopped to implement safety upgrades to the research operations. This is a plain difficult production all videos on the channel are creative commons attribution share alike licensed. Plain difficult videos are produced by me John in a not so sunny southeastern corner of London UK. Help the channel grow by liking commenting and subscribing, check out my twitter for all sorts of photos and odds and sods as well as hints on future videos. I've got patreon and youtube membership as well so you can check them out if you fancy supporting the channel financially. And all that's left to say is thank you for watching.