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Rubbias Project (file A11)
by Ludwik Kowalski, Sylvie Leray and David Whittal
A hybrid system can be designed to generate electricity and to incinerate its own spent fuel at the same time. Thus it can possibly become a replacement for today's reactors which produce long-lasting radioactive substances and which are controlled by mechanical rods. Such a device, partially inspired by an instrument of fundamental research (23), has been designed by the Rubbia's team. Named energy amplifier, EA, it has many unique features. First, the proton accelerator will be a cyclotron, rather than a linac. Second, its chain reaction is to be based on fast, rather than on slow neutrons; neutrons are called slow when their energies are less than 1 eV. The initial version of the design (15) was based on slow neutrons but a recent analysis concluded (16) that considerable improvements are possible using neutrons of higher energies. Thus, unlike the ATW, the EA has no moderator.
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Third, the fuel elements in the blanket are solid, as in today's power plants. The elements are surrounded by molten lead, as illustrated in Figure 8. The fuel is to remain in the core for approximately five years before the replacement. After that time, and after the usual preliminary storage of ten years, the remains are expected to be no more radioactive than the byproducts of incineration; they must be isolated from the biosphere for several hundred years, as described in the previous section. (Solid fuel is also going to be used in one of the two hybrid systems designed by the Japanese researchers. Their other system is based on the liquid salt concept similar to ATW.)
The lead in the Rubbia's system serves two functions, it is a spallation target and the medium which removes heat from the fuel elements and delivers it to the water-steam system. The core is located at the bottom of the vessel where the lead temperature is close to 600 oC. The temperature at the top, where the first heat exchangers are located, is lower. Under such conditions the transfer of heat is accomplished by natural convection. The all-convective approach add significantly to the operational safety. A melt-down accident is virtually impossible because, unlike a pump, convection never fails; it will always take away an excess of heat from the lower part of the container. Recall that in the ATW system the impossibility of the melt down is guaranteed by the continuous removal of short-lived fission products from the molten salt; these products do accumulate in solid fuel elements and they would generate heat, even after the accelerator is turned off.
Lead was chosen for the EA because it has a low probability of capturing neutrons and because its melting temperature is low. Approximately ten thousand tons of that material are needed for a device designed to generate electricity at the rate of 600 MW. The thermal expansion of lead is expected to shut down the cyclotron automatically when the temperature becomes excessive. Cooling of the EA by lead can be contrasted with cooling of the ATW core by salt where pumps must be used to support circulation. The idea of using natural convection is not limited to hybrid systems; a smal conventional reactor based on that idea, EP600, has been designed by Westinghouse (24). According to a preliminary estimate, (25) the cost of electricity produced by a large commercially oriented plant could be as low as 2 cents per kWh. This is less than what is possible by other methods.
Generation of electricity is not the only task which can be assigned to an EA device. The unit can be designed as an incinerator of wastes from today's reactors. To do this, capsules loaded with the waste must be introduced into the subcritical core together with fuel elements. Another visionary application (26) is to produce "large amounts of H2 by direct transformation of nuclear heat into chemical energy, thus extending the potentials of nuclear energy beyond the production of electricity". Hydrogen can be as useful as natural gas; it has often been considered as an ideal medium for storing energy from renewable resources such as solar-thermal and solar-voltaic. The gas can be transported through a pipeline or liquefied for a container. The only pollutants from the burning of hydrogen in air are nitrogen oxides; their amounts can be reduced to a negligible level with catalytic heaters. Internal combustion engines can be designed to run on hydrogen. The estimated cost, roughly 1 dollar per 1000 MJ of stored energy, is 20 times less than the cost of obtaining the same amount electrolytically. The pollution-free liquid hydrogen has already been used to power internal combustion engines.
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