This subject is logically treated here because of the radioactive waste problem of fission reactors. However, fusion reactors have not yet been described. This section can be best understood if Chap. 9 on fusion engineering is read first. The reason for combining fusion with fission is that it could benefit both systems. Fission reactors can be run subcritically for better safety, and their high-level wastes can be trans­muted into fuel and a much smaller amount to be sequestered. Fusion reactors, on the other hand, can be run subcritically also, without producing all the energy of the reactor, greatly accelerating the time for their development. Many plasma theorists have advocated fission-fusion hybrids, notably Jeffrey Freidberg at M. I.T. and Wallace Manheimer at the Naval Research Laboratory in the USA. The idea was first proposed by none other than Hans Bethe. However, their arguments do not include specifics on how a hybrid reactor might be designed. A group at the University of Texas has proposed a reactor based on a spherical torus (see Chap. 10), a new fusion device that has not been extensively tested. The most detailed engi­neering design has been done by a group at the Georgia Institute of Technology (Georgia Tech) under the leadership of W. M. Stacey. Their subcritical advanced burner reactor [45] will be described here. A diagram of it appears in Fig. 3.61.

Within the D-shaped toroidal-field coils is the plasma of a fusion reactor, shown in yellow. Surrounding that is the fission fuel core, which is divided into four

Central solenoid Vacuum vessel Blanket and shield Reactor core Plasma

Plasma first wall Toroidal field magnets

concentric rings (gray). Surrounding both is a neutron absorbing blanket which breeds tritium from Li4SiO4 for DT fuel. The fission part is an LMFBR designed at Argonne National Laboratory. The fuel is 36 tons of transuranic waste from LWRs consisting of 40% Zr, 10% Am, 10% Np, and 40% Pu. It is in the shape of 7.3 mm diameter fuel pins, 271 of which form a fuel assembly. The fuel pins include a channel for the liquid sodium coolant. Their complete design and manufacturing process have been specified [46]. The fuel rings (batches) contain 918 assemblies. The tokamak part is a scaled-down ITER operating with conservative parameters lower than the maximum values needed for energy production. These include factors which will be explained in Chap. 9: the Greenwald limit, normalized beta, big Q, and the bootstrap current fraction.

The operating characteristics of this reactor have been extensively calculated. The fission part will generate 3 GWth (gigawatts thermal). It runs subcritically, generating fewer neutrons than is necessary to maintain a chain reaction. The missing neutrons are generated by the fusion part. Since its mission is not to generate power, it can be designed to contribute only 250-500 MWth of energy. The fission fuel is burned in 750-day cycles. Each batch spends one cycle in each position, for a total exposure of four cycles or 3,000 days. After that, it is removed to storage, and its decay heat over the next million years has been reduced by a factor of 2, and thus the storage facility requirements have been halved. The total time of exposure is limited by the life of the fuel cladding under neutron bombardment, set at 200 dpa (displacements per atom).

This amount of burnup of actinides can be greatly improved by reprocessing. If the fuel from the hybrid after four burn cycles is reprocessed, then mixed with
“fresh” waste from LWRs and sent through the hybrid again, the decay heat of the ultimate product can be reduced by 99%. High-level storage facilities can be reduced by a factor of 100. If the 200-dpa limit on neutron damage can be relaxed so that the fuel can be burned for four 3,000-day burn cycles for a total of 12,000 days (25 years), 91.2% of the transuranic waste can be removed after only once through the hybrid reactor. Such a fission-fusion hybrid can treat the waste from four 1,000-MW LWRs.

e

It is possible for the fission reactor to go critical. Zirconium is added to the fuel so that there is negative feedback: when the temperature rises; the reaction slows down. However, if this does not work and there is a runaway reaction, there is less time available for control rods to be inserted than in a normal LWR. Fortunately, there is a simple solution. The reaction cannot run without neutrons from the fusion reactor. The plasma producing these neutrons can be shut off within a second or so by a massive injection of gas.

Proponents of hybrids see that they can make fission safer and at the same time let fusion get online faster. Skeptics see that these would be extremely expensive and difficult reactors to design and construct and would detract from the main objective of developing pure fusion. In any case, this subject is still in its infancy compared with Generation III fission reactors or with tokamak fusion reactors.