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Los Alamos Project (file A10)

by Ludwik Kowalski, Sylvie Leray and David Whittal

In the US spent fuel has been accumulating since the 1970s at the steady rate of 2000 tons per year. A large number of hybrid systems, perhaps one for each reactor, will be needed to destroy the wastes at the same rate at which they are being accumulated. The incinerator of nuclear wastes proposed by the Los Alamos team is expected to function as in Figure 6. According to that figure most of the material, uranium and zirconium, is at once separated from fission products and actinides. That material is reused in the fuel fabrication loop which is not shown in the flowchart. The rest of the material, including radioactive fission products and actinides (about 1.3% of the total mass of waste) is sent to the blanket and remains there as long as necessary, perhaps several months or years, to be transmuted. Then the blanket is reloaded and the cycle is repeated.

click to see Figure 6 (use the back button to return later).

The Los Alamos investigators refer to their proposed hybrid system as ATW which stands for the accelerator-driven transmutation of waste. The target/blanket vessel of that machine is shown in Figure 7 . It is a cylinder whose height and diameter are equal to 10 and 7.5 meters, respectively. The proton beam, delivered from a linear accelerator, arrives from above and enters the spallation target located along the axis. The blanket contains a graphite moderator through which a molten salt is forced to circulate by a set of pumps. The chain-reaction fuel, and the radioactive substances to be transmuted, are dissolved in the salt which is an essential component of a subcritical reactor. The composition of the salt can be regulated by adding or removing chemicals involved in the operation. Due to the high level of radiation these operations must be performed remotely, as often as necessary, in a chemical plant near the vessel. The salt circulating through the blanket also removes heat which can be used to generate steam and electricity.

click to see Figure 7 (use the back button to return later).

or click here to see details).

The continuous removal of short-lived radioactive products from the salt is an effective meltdown protection. Suppose that the chain reaction is stopped suddenly due to a malfunction. From that moment the amount of heat produced in the blanket is directly proportional to the radioactivity of fission products circulating through the system. But this radioactivity is very low due to the frequent cleansing of the salt. Some aspects of this approach must still be tested but the technology of the proposal is not necessarily more complicated than what has already been mastered. In the past, however, the processing of spent fuel has deliberately been discouraged (illegal in the U.S. during Carter's administration) on the basis of the politics of nonproliferation (21,22). The policy was not shared by other industrial powers. Prior to the U.S. decision to abandon fuel processing, an experimental molten salt reactor was constructed in the Oak Ridge National Laboratory. It was used between 1965 and 1969 as a tool for mastering the new technology. The experience of working with a molten salt circulating through a graphite moderator has become an important component of recent American and Japanese efforts (13,18).

What should be done with the byproducts of transmutation after they are discharged from the processing plant? These byproducts contain many short-lived radioactive substances and they must be stored in isolated areas for several hundred years. After that time the concentration of radioactivity is expected to be nearly as low as that found in common rocks and soil. The task of storage is simplified by the fact that the volume of the byproducts is considerable smaller than the volume of the spent fuel from which they were originally extracted. All the byproduct of incineration can thus be stored in near-surface cement containers, for example, in a desert. An area smaller than a football field would be sufficient for that purpose. This is more reliable, and potentially less dangerous, than a geological depository where the unprocessed spent fuel remains radioactive for tens of thousands of years. Predictions of geological stability on a relatively short time scale are more reliable than those referring to very long time intervals.

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