Afterheat Removal Systems

DOE-STD-6003-96

c. controlling coolant energy (e.g., pressurized water, cryogens);

d. controlling chemical energy sources;

e. controlling magnetic energy (e.g., toroidal and poloidal field stored energy); and

limiting airborne and liquid releases to the environment.

The above functions have been identified as "potential safety concerns" if their failure

could threaten the public safety function of confinement of radioactive and hazardous material.

However, the ultimate impact of these safety concerns on the public safety function can only be

judged in the context of a specific design of the fusion facility. Evaluation of these safety con-

cerns will normally be an iterative process. Fusion facilities contain a number of systems that

may interact in a complex way to sustain the fusion reaction. Identification of safety require-

ments for such systems requires a systematic methodology to ensure that, for a given facility,

each hazard is properly identified, that its impact on safety is assessed, and that the require-

ments to protect the worker, public, and environment from those hazards are balanced and

integrated into the facility design. Because of the range of potential hazards in fusion facilities

and the design options available, functional analysis combined with results from recent safety

studies of conceptual fusion power plants were used to identify the potential safety concerns

noted above for fusion plants. Because the hazards and their impact on public and worker

safety are facility design-specific, development of detailed prescriptive system-level safety

requirements is felt to be inappropriate. Instead, an approach has been used to develop broad

functional safety requirements that can be used by fusion facility designers to integrate safety

into the design up front in a cost-effective way. Design measures (as opposed to administrative

measures) are the primary means to deal with these potential safety concerns, and such mea-

sures are discussed in the following paragraphs.

6.3.1 Afterheat Removal Systems

The safe removal of afterheat (decay heat) is an issue to be evaluated in D-T fusion facili-

ties. Typically the afterheat amounts to several percent of the normal operating fusion power.

One day following shutdown, it decreases by a factor of 3 to 10 depending on the materials and

the operating scenario (pulsed vs continuous operation). Unlike fission cores, the decay heat

relative to thermal power level is smaller and distributed over large surfaces, and large heat

sinks are available in the fusion island structures. The design of fusion facilities should provide a

reliable means to remove any undesirable afterheat generated by activation products produced

by neutron absorption in structures such that the confinement public safety function is ensured.

The need for and reliability of afterheat removal systems should be commensurate with the role

of afterheat removal in complying with evaluation guidelines. Passive afterheat removal (i.e., no

major hazard or component melting can be expected even when all active cooling capacity is

lost, and removal is accomplished by only heat conduction and thermal radiation) is preferable

to active systems. For fusion facilities with high levels of afterheat (i.e., levels where active

cooling is required), the concepts of redundancy, diversity, and independence should be con-

sidered in the design of afterheat removal systems.

107