Group 2 Systems

DOE-STD-6003-96

potentially be safety-significant relative to the worker safety function if the inventory level and

proximity of workers result in this designation.

6.5.2 Group 2 Systems

The important design consideration for these systems is that they are typically experimen-

tal in nature (magnets, plasma, divertor, breeding blanket) and their performance in a future

fusion reactor environment is not known a priori. Environmental conditions that these systems

potentially experience include high heat fluxes, high neutron and gamma irradiation, high-energy

particle flux including very high energy runaway electrons, cyclic loading, and significant thermal

gradients. Typically these conditions result in design decisions to use materials for plasma-

facing components such as carbon and beryllium that do not have an extensive nuclear industry

data base. Thus, it is strongly recommended that decisions be made early in the design phase

to preclude these systems from being designated as safety-class. Some of these systems have

significant stored energy as shown in Table 6.3, and innovative strategies should be developed

to ensure that this energy is locally contained within these systems and their support structure

during off-normal events. An example of design guidance for a typical Group 2 system, the

divertor, follows.

a. Divertor system

The divertor (more specifically referred to as a poloidal divertor in the case of interest

here) for a tokamak device consists of a set of structures that, taken together, form a toroidally

continuous element(s). The plasma-facing surfaces of the divertor are configured to intercept or

enclose the magnetic flux surfaces that lie outside the last closed flux surface that contains the

confined plasma. This surface, referred to as a separatrix, is formed by one or two nulls in the

poloidal magnetic field and separates the confined plasma from that diverted toward the exhaust

region. A configuration with a single null in the poloidal field is referred to as a single-null diver-

tor configuration, whereas a double-null arrangement is referred to as a double-null divertor

configuration. Poloidal divertors are usually located on the top and/or bottom regions of the

plasma chamber. Since the plasma exhausts most of its heat and particles to the divertor, active

cooling and vacuum pumping are required in the divertor region for long-pulse operation.

The divertor system consists of targets (plasma-facing structures), coolant piping, support

structure, and nuclear shielding. The surfaces of the divertor target are directly exposed to the

particle and energy fluxes resulting from the plasma exhaust processes. These surfaces are

usually constructed from two separate materials, each with different functions. The plasma-

facing or armor tile material is selected for its plasma interface characteristics, such as sputter-

ing erosion and thermal shock capability. Plasma-facing material candidates include metals

such as beryllium or tungsten, carbon-containing materials such as graphite or carbon-carbon

composites, or ceramic materials such as silicon carbide. The plasma-facing material is

attached to a structural metal, whose primary functions are to contain the coolant and to act as a

heat-conducting element between the coolant and the plasma-facing material.

The primary function of the divertor system is to protect the vacuum vessel from direct

interaction with the plasma while providing a means for plasma particle and energy exhaust.

136