Not moon rovers and not jokers. What do we know about robots in Fukushima

    One of the most serious technological disasters in the history of mankind occurred on April 26, 1986. And then it almost repeated on March 12, 2011. As you might have guessed, we are talking about accidents at the Chernobyl nuclear power plant in the USSR and the Fukushima-1 nuclear power plant in Japan. The series "Chernobyl", shot by HBO, once again fueled interest in the history of the Chernobyl accident and recalled how hard it was to stop the release of radionuclides from the destroyed reactor into the atmosphere. Separately, they talked about the unsuccessful use of robots and the forced appeal to the help of people. Japan conducts numerous experiments with a variety of robots that can delay the need to attract people to eradication.

    The accidents at the Chernobyl nuclear power plant and Fukushima have different causes and a different chronicle of events, but they have one thing in common - the vast area around the stations is infected, which makes it impossible for a long-term safe stay at it, and there is no question of a complete clearing of the power units of the stations. In both cases, humanity was not ready for liquidation of the consequences - hundreds of thousands of people in the USSR worked on the decontamination of the exclusion zone and the construction of a protective sarcophagus over the reactor. In Japan, they went the other way.

    Reference: exposure standards

    The phrase about the radiation background from the Chernobyl series managed to become a meme on the foreign Internet: "3.6 x-rays are nothing good, but not terrible." Indeed, is it worth it to be afraid of these same 3.6 x-rays per hour, or is this really not a terrible background? In order to better understand the measurements from Fukushima given in the post below, you must first understand the scale of doses and their effect on the body.

    X-ray (P), which is well known to Russians, is an obsolete unit of measurement of radiation background that is not included in the SI system. Now Sievert (Sv) is used instead. Simply put, 1 Sv is equal to 100 R. That is, 3.6 R / h is 0.036 Sv / h or 36 mSv / h. In Russia, at some research institutes you can see a panel with the current background, measured in x-rays. On average, the urban background in our country fluctuates at the level of 12-20 μR / h or 0.12-0.2 μSv / h. A person receives a dose of about 2.4 mSv per year from the natural radiation background on our planet (depending on the country and altitude). This radiation comes from space, comes from soil, water, is present in the air, thanks to radon gas.

    And now a small table with the doses that we receive throughout our lives (the single exposure indicated below is the receipt of the appropriate dose for up to 4 days):

    Let's go back to the 3.6 R / h mentioned in the series. 0.036 Sv / h (36 mSv / h) is really not a terrible background in the event of an accident, for the development of mild radiation sickness it is necessary to stay in the danger zone for more than a day, and repair work for several hours is quite safe and acceptable. And now we recall how a little later they said in the series that the background at the reactor is not 3.6, but 15,000 x-rays (150 Sv / h). Four minutes with such a background entail inevitable death. After two minutes, only the fastest qualified medical care can give a chance of salvation, and 25 seconds is enough to get radiation sickness.

    Toshiba in Fukushima

    The partnership between Toshiba and the Tokyo Energy Company (TEPCO) began as early as the construction phase of the Fukushima-1 nuclear power plant. Toshiba was responsible for the construction of BWR-type reactors for the third and fifth power units, commissioned in 1976 and 1978, respectively. Among the advantages of BWR reactors is lower vapor pressure in the primary circuit than, for example, the Soviet VVER, and lower operating temperatures. The disadvantages include the complexity of manufacturing, the need for a very large-sized case, the complexity of control and radiolysis in fuel elements, which entails the generation of explosive hydrogen.

    If the accident at the Chernobyl nuclear power plant was caused by the lack of design of the RBMK reactor, which made itself felt during the violations during the tests, then two natural disasters led to the Fukushima incident. An earthquake with a record magnitude of 9.0–9.1 led to the shutdown of the reactors, and the ensuing tsunami flooded the Fukushima-1 territory and emergency generators supplying the reactor cooling pumps. The reactors of the station, which even in the event of an emergency stop, do not immediately cool down, were left without an influx of cold water, which caused it to boil away, increase the vapor pressure and temperature inside, generate hydrogen due to the contact of zirconium vapor in the fuel elements (steam-zirconium reaction) and its subsequent explosion . In the first, second and third power units, reactor core melted and nuclear fuel leaked.

    Chronicle of what happened at three power units. The fourth power unit was also affected, but there was no nuclear fuel in its reactor, so there was only a fire. Source: Roulex_45 / Wikimedia

    To eliminate the accident at Fukushima, it is necessary to compile a damage map, find molten fuel and its leak points, remove spent nuclear fuel from reactors and holding pools, disassemble the rubble - that is, carry out tremendous work under an equally enormous radiation background. Now this work at the FAES is done by robots and remote-controlled cars - sending people to the accident zone is too dangerous, in some places of the station the background can kill a person in just a few minutes.

    Toshiba together with the International Research Institute for the Decommissioning of Nuclear Reactors (IRID) is engaged in the development of robots for specific tasks - inspection of Fukushima power units and decontamination (cleaning) of rooms from radioactive contamination. We will talk about some of the most notable Toshiba robots that have made their small but important contribution to the cause of the accident.

    By the way, there is an erroneous opinion that the liquidation of Fukushima did not take into account the valuable experience of Chernobyl. This is fundamentally wrong. Immediately after the accident, a group of Japanese-Russian experts in the field of nuclear energy was created to deal with the consequences of the Fukushima-1 accident, among which was Larion Aleksandrovich Lebedev, a direct participant in the Chernobyl nuclear power plant since the summer of 1986, who was a member of the Moscow Engineering Physics Institute, who made a huge contribution in the study of radiation conditions and the construction of the sarcophagus. After the accident in Japan, Larion Lebedev improved the technology for the separation of tritium from radioactive water, which allowed us to start cleaning the huge volumes of heavy water left after cooling the destroyed reactors. On behalf of the Government of Japan, Larion Alexandrovich was awarded the Order of the Rising Sun,

    Four-legged Scout

    The first Toshiba robot to enter the FAES was the nameless four-legged drone, the development of which began immediately after the accident. His mission, like all robots launched at the power plant in the early years, was radiation reconnaissance and damage assessment inside power units.

    A dosimeter and six cameras were installed on the chassis to inspect the station premises. Protected electronics was designed to operate with a radiation of 100 mSv / h for a year (with a 10-hour working day) and with a much larger background for short periods. True, one battery charge was only enough for 2 hours of autonomous existence. The robot moved at a speed of about 1 km / h, which is enough when examining a destroyed power unit. Management was carried out over the air with redundancy in case of interference.

    At the bottom of the device, a small reconnaissance drone was attached, which could detach from its carrier and crawl into narrow places where a tall robot could not get through. It was supposed to be used to search for leaks of cooling water under the piping of the reactor.

    This is not to say that high hopes were pinned on the robot: even at the stage of the demonstration, the journalists pointed out the slowness of the machine - it took up to one minute to climb each step of the ladder, and when setting the foot on an unstable surface, the robot could roll over on its side.

    However, the first experimental Toshiba mechanism for the FAES still managed to penetrate the building of the second power unit and conduct a little reconnaissance. TEPCO published a report on December 11, 2012. The robot took pictures of one of the pipes, confirming the absence of leaks. In March 2013, he went inside five more times. But soon, while trying to further inspect the premises, the robot lost balance on the stairs and fell to its side. Due to the lack of a turning mechanism, the four-legged scout remained lying in the second block.

    Folding Explorer Scorpion

    The next robot of the company, which took into account all the shortcomings of the previous model, was the Toshiba Scorpion, so named for its shape (title photo). It was developed for a very difficult task - to study the bottom of the reactor and search for fuel rods, and this mission involves working with a huge radiation background. Scorpion was spared the legs, which were replaced by tracks, and for compactness, the design was folding - the car was supposed to be thrown into the reactor through the passages for loading fuel rods. In the operating mode, the robot lifts its tail manipulator, moving along three axes in the manner of a scorpion, at the end of which a camera and backlight LEDs are adapted instead of the sting. Another camera is mounted on the front and always looks forward.

    Scorpion is controlled by the operator via cable, so there are no problems with power and signal transmission. In addition, the robot is made returnable, it should not remain in the reactor after the testimony is transmitted. Tipping him is not afraid, the “tail” with the camera returns the robot to its normal position.

    For several years, Scorpion was finalized in order to finally get inside the FAES - over time, he received a small water cannon to clear the path, a bucket and a cutter for working with corium. In February 2017, the robot went to the reactor, where he measured the background and shot a video. The dosimeter showed an impressive 210 Sv / h (21,000 R). In the next room, where there were workers loading the robot into the pipe, the background was 6 mSv / h.

    ROV: small submarine

    A small submarine with video cameras, which have high expectations. Source: Toshiba A

    remotely controlled underwater vehicle, or ROV for short, is the first Toshiba floating robot built to study the reactor of the third power unit, the lower part of which is hidden under a six-meter water column. This small submarine with dimensions of 30 x 13 cm and weighing 2 kg carries two cameras and a backlight, is controlled by cable and can move underwater in any direction with high accuracy. The device is controlled by the operator, and the signal and power are supplied via a long cable. To prevent the wire from tangling and clinging to debris, Toshiba developed a special coating with minimal friction, and two powerful engines were installed on the ROV itself, so that the robot could easily drag the cable along with it.

    By the way, the ROV diameter of 14 cm was due to a narrow inlet in the reactor vessel of the third power unit, so Toshiba engineers had to work hard to fit electronics, protection and engines into such a compact case. At the end of a two-month intensive training course for operators, Toshiba ROV went to the FAES. The robot visited the reactor on July 19, 21 and 22, 2017 and successfully inspected the destroyed entrails.

    Magnetic SC-ROV

    Another inaccessible place in which water with radionuclides accumulated was toroidal pressure relief chambers located below the reactor. It was necessary to send a robot to search for leaks under the camera, but the matter was complicated by the fact that the containment was flooded - the robot was required to inspect pipes immersed in muddy water. Floating devices were not suitable for this, a probe was needed that would be fixed on the pipe and able to travel along it without sliding even at a large angle.

    How to fix the robot on a steel pipe so that it does not roll? With the help of magnets. Toshiba's SC-ROV was designed specifically for the pressure relief chamber. It is a chassis with wheels made of powerful neodymium magnets. Using four cameras and a marker, the operator can move along the surface of a pipe submerged under water and mark the holes found.

    During the demonstration, SC-ROV held magnets perfectly to the inclined surface. But the real conditions were much tougher. Source: IRID

    According to the results of SC-ROV in 2014, no leaks were found, but problems were found in the operation of the device. Firstly, because of the turbidity of the water, the range of visibility did not exceed 30-35 cm, which significantly slowed down the search. Secondly, due to some defects in the surface of the pipe, the robot nevertheless slid from it when tilted to 120 °.

    Corium Search Camera

    Previous search missions have shown a depressing picture - the nuclear fuel in the reactor of the second power unit has gone beyond the reactor vessel. The photographs of the melted grate at the bottom of the containment made it clear that the fuel was already somewhere at the bottom. But did the containment hold it back or did the corium sink into the ground? It was necessary to send a new robot to the very "hell" of the former reactor to incredibly fading fuel.

    The molten grate inside the containment of the second reactor is exactly above the drive of the control and protection system rods. Source: TEPCO

    Crawler robots work only on a flat floor, and a submarine needs a thickness of water. Toshiba came up with a telecontrol camera mounted on the end of a five-meter telescopic tube. Regardless of the angle of entry of the pipe into the containment, the camera takes a strictly vertical position - the main unit is held by a control cable, that is, the camera seems to be "hanging on the wire." Its lens rotates 360 ° horizontally and 120 ° vertically. In fact, there are two cameras on the device at once - directly working and a camera for orientation. In addition to the lenses, a backlight, a dosimeter and a thermometer are installed on the unit. The device withstands the absorbed dose of up to 1000 gray.

    The first version of the camera worked at Fukushima in January 2018, where it successfully found the corium and measured the background at 530 Sv / h (53,000 x-rays). But the work did not end there - it was required to take samples for analysis. In the updated version of the device, the backlight was enhanced and a sliding arm was added. In February 2019, the Toshiba remote-controlled camera first received a corium sample from the second power unit.

    Cleaning machine

    The premises of power units will have to be deactivated sooner or later, but first you need to draw up a map of pollution. The results of numerous research missions showed a very bleak picture: contrary to expectations, most of the radionuclides settled not on the floor or on the walls, but on the upper-level elements, such as pipelines and ventilation - they accounted for up to 70% of all radiation. Decontaminating the floor is relatively simple, the walls are a bit more complicated, but how do you get to the dirt on the ceiling and in the intricacies of pipes, given that the height of the ceilings on the first floor is 7-8 meters?

    Toshiba radionuclide “cleaner” - rises 8 meters in height and diligently scrape the walls. Source: IRID

    Toshiba has developed a robot operating a cannon with dry ice - ice powder covers the surface, binds radioactive particles, and the robot scrapes it and absorbs itself. The mechanism raises the structure to a height of 8 meters. Since this is another telecontrolled machine, its operator receives a picture from 22 cameras at the same time. The machine started cleaning power units in January 2016. Its productivity is small, but even a small contribution to the decontamination of the station is valuable.

    Help but not solution

    33 years have passed since the accident at the Chernobyl nuclear power plant, since then technology has made a tremendous breakthrough. Now dozens of robots and types of telecontrolled construction equipment are participating in the elimination of consequences in Fukushima. The TEPCO website has posted a lot of reports, the data from which as a whole form a very sad picture: there are robots and there are technologies, but they all did not come close to the effectiveness of the Chernobyl liquidators. The development and testing of robots takes months and years, and the result of their work is advancement by a dozen meters, collection of dosimetric data, a muddy video and the frequent termination of missions due to unforeseen problems.

    Every robot in Fukushima is a saved people, and every human life is worth it to engage in the development of robots. But TEPCO's current plans make it clear that, according to conservative estimates, it will take at least 30-40 years to deal with the consequences of the accident. And this only confirms the heroism and scale of the feat of the Soviet liquidators of Chernobyl.

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