Fukushima Daiichi: Chernobyl experience

Post-Accident Management

Building the Chernobyl "sarcophagus" (object "Shelter") was necessary to reduce emissions of radioactive substances from the fragments 4 reactor. The main reason of problems is the use of temporary structure as permanent [1-2].
Previously in the "clean" area were tested methods of remote connection of large constructions, remote control for concrete pumping equipment, designed a system of television and radio construction control, created special cabins-capsules that allow visually inspect poorly accessible places with the help of cranes. To reducethe overall level of radiation dose was shot and buried radioactive ground, fragments of building constructions and equipment from the area around the reactor, after which the entire area was covered with a layer of gravel and sand thickness of 50 cm and a concrete layer. At this concreting had spent more than 100 000 m3 of concrete. [3]
For building of the Chernobyl sarcophagus been used elements of Block 4 destroyed by reactor explosion. This was done to reduce construction period (6 months). But even this variant required the use of more than 200 000 tons of concrete and more than 10 000 tons of steel. The principal disadvantages of this approach are:
- huge collective dose of radiation exposure obtained by builders and fitters;
- necessity to build new structures very close to destroyed reactor in large radiation fields;
- impossibility to fit exactly to each other large metal structures by using remote installation;
- impossibility to assess residual strength of structures after an explosion and fire;
- large masses of concrete were not included in the designated area because of the
imperfections of remote methods of concreting;
- flow of concrete in the damaged areas have limited access to them;
- impossibility accurate and precise work near a strong source of radiation has led to the main
problem of construction - leakage;
- total cracks area was estimated as 1000 m2. [4-8]
Through the holes every year leaks around 2000 m3 of water, which eventually coalesce at rooms of the lower level Block 4. Yearly volumes of condensate at Block 4 is about 1650 m3. The 180 m3/year are the remnants spray for dust suppression. All of this water passes through the debris Block 4 and is going to basement rooms. Every year about 2100 m3 of collected water evaporates, and about 1300 m3 seeps through the foundation at ground under the Block 4 [9].
Water, under the sarcophagus is a source of several kinds of dangers:
- falling on the accumulation of fuel-containing materials, it leads to an increase in the effective neutron multiplication factor in the system;
- water slowly destroys the fuel, and contributes to the uncontrolled movement of radioactivity in the inner rooms and the removal of radionuclides beyond the "Shelter";
- transferring the dissolved salts of enriched uranium, the water can increase the potential danger of a nuclear Object;
- water promotes corrosion destruction of building constructions "Shelter".
- it prevents research to improve the electrical safety of object;
- water violates the normal operation of diagnostic systems. After strong rains have taken place 2 "anomalous" neutron events (significant increase generation of neutrons) recorded by 12 and 16 September 1996. Neutron radiation, which initially increased, after spraying with a solution of gadolinium decreased to usual levels [2].
Carried out in 1995 - 1997 years studies have shown that the main source of water pollution by 90Sr are oxidised fuel particles U3O8. Their rate of dissolution is much higher than that of the fuel particles UO2
In the first moments of the accident takes place the destruction of the fuel tablets of uranium dioxide on grain boundaries. During the Chernobyl accident was found that the over time takes place crushing large particles at a more dangerous form for humans [10-14]. 
Because of the possibility of the collapse of the "Shelter" and the release of radioactive dust into the environment, under the roof was installed dust suppression system, which periodically spray dust suppression solutions. This system has been in operation since 1990, and during this time it was sprayed over 1000 tons of solutions [15].

After the accident comparable with Chernobyl, finely dispersed solid radioactive substances can penetrate into the constructions at a depth of several cm, and the radioactive water at cement screed, plaster, brick at a depth of 10 cm or more. Repeated treatment of decontamination solutions does not usually lead to lower radiation dose rate to acceptable levels. To reduce the level of radiation, it is necessary to cover the surface by lead.
During the Chernobyl accident, the best medical care was assisted for KGB officers. Techniques of enterosorbtion by zeolite and hemodialysis with activated carbon proved their efficiency for removing of radionuclides from the body. Studies have shown that the amount of radionuclides in the body can be reduced by 3-4 times before they produce significant internal exposure [16-20].
Enough quality, to protect from external radiation and radioactive dust, nowadays have only spacesuits for astronauts. For the work at nuclear power plants are most suitable Hard-shell suits. However, they are very heavy and it is necessary to movement a mobile bigger vehicle for one person. 

Among other space-based technology may be useful robot arm for different manipulations.

 The most expensive and most universal way to protect and cool the reactor is filling it by Indium (In). Indium's high neutron capture cross section for thermal neutrons makes it suitable for use in control rods for nuclear reactors, typically in an alloy containing 80% Silver, 15% Indium, and 5% Cadmium [21-27] In the reactor, together with the indium, need to place thermoelectric elements (based on the Peltier effect) as a coolers. All other variants of cooling require mass transfer of cooling fluid. Radioactive water at the reactor is best to use for the concreting of the internal spaces. In the absence of information about the location of molten uranium fuel, it is better in the water and concrete solutions add boron compounds (B4C for example). 
For fast cutting armature rubble, and welding sections of new constructions is best to use plasma torches. Cracks will be formed in construction in any case. The main task is the localization of cracks. In laminated structures, cracks increase mostly within the layer. It must be a sufficient viscosity of the material to resist earthquakes and radiation embrittlement. Lead is consistent most of the requirements. Supporting function should be assigned on the welded, not removable formwork of stainless steel. All other options require additional corrosion protection, which near the source of radiation is difficult to make. The design should be formed layer by layer of lead and concrete. To prevent the formation and movement of radioactive dust by wind, all external surfaces of buildings must be made of steel. Protective structures of steel and concrete, lead, will have considerable weight, and therefore need more support construction for the roof. Temporarily this function can perform reliably fixed crane. On the crane line, fixing the building, must be mounted dampers for the localization of vibrations during earthquakes. It is necessary to mount geodetic marks which allow to determine the sediment and deformation structures in time. For the isolation of radioactive groundwater is necessary under the base lay several layers of rubber. This is usually made by an underground cutter with large number water cutting nozzles. However, all existing models of such units are small in size and are designed for small depth (few meters). To create the underground layers of rubber or heat-resistant materials under nuclear power plants need to design an exclusive technology.
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