Appendix . Background for DOE-STD-6002-96, Safety of Magnetic Fusion Facilities

DOE-STD-6002-96

in fusion, careful attention to design can prevent accidents or can minimize the

threats posed by the energy source to liberate the inventories. These differences

indicate that a unique approach is needed in the development of fusion safety

requirements.

One detailed example of the difference in hazards is that fusion facilities are

expected to contain no fissile or fertile materials or fission products.1 Nuclear critical-

ity with its associated energy release cannot occur in fusion facilities. For the fusion

reaction to take place, controlled and difficult-to-achieve conditions must be main -

tained. Any event that disturbs these conditions results in a quenching of the plasma

and the cessation of fusion reactions.

A second example is the difference in the hazard associated with the radionuclide

inventories. Fission, by its nature, results in long-lived, highly radioactive fission

products. In fusion facilities, however, radionuclide inventories will be dominated by

tritium fuel that collects on internal structures and activation products in the struc-

tures, depending on the fuel cycle, the stage of operation, and the specific mission

and operating profile of the machine. Tritium is also a highly mobile gas, relatively

difficult to contain. Fusion activation products will be principally solids, not easily

mobilized except in an extreme accident scenario. Furthermore these inventories

may be reduced by proper selection of materials. Differences in the vulnerabilities of

the inventories of radioactive material will exist in fusion machines as compared with

those in other nuclear facilities. For example, early fusion machines will probably

operate in a pulsed mode where operation is only for relatively short periods. The

hazards tend to be more distributed spatially than in fission systems. Further, there is

no possibility for criticality related accidents in a fusion machine. There are also

differences in the relative biological risks of the radioisotopes because actinides,

radioactive noble gases, radioiodine, radiocesium, radiostrontium, or plutonium,

which are inherently associated with the fission-connected process, are not present

in fusion. These are more biologically hazardous than tritium (which is the most

significant releasable radionuclide in fusion).

A third difference is that while fission-related facilities usually can be operated only

as nuclear facilities (e.g., with fission reactions taking place in reactors), a fusion

facility may be operated using only protium and/or deuterium for comparatively

extensive periods during which the radionuclide production is below thresholds

requiring special handling as a nuclear facility.2 Hence, for that period, fusion

1Some designs have been proposed for hybrid fusion reactors that would contain fissile fuel, but there are no

specific plans to build machines of this type.

2The Tokamak Fusion Test Reactor and the DIII-D facility have operated with deuterium for years as non-

nuclear facilities.