Chemical Energy

DOE-STD-6003-96

6.3.3.2 Chemical Energy

Fusion facilities should be designed such that chemical energy sources are controlled

during normal and off-normal conditions to minimize energy and pressurization threats to

radioactivity and toxic material confinement barriers. Design measures should ensure that

evaluation guidelines are met. Chemical reactions should be prevented from releasing energy

that threatens a confinement boundary, either by preventing the reaction or by accommodating

the additional energy and pressure.

Additional design guidance for chemical energy sources is provided.

a. Chemical Reactions

Much of the chemical energy source term in a typical fusion device is from plasma-facing

components made of beryllium or carbon. Examples of chemical reactions include beryllium

steam (or carbonsteam), H2-air, and berylliumair. The lower flammability limit for H2 is about

4% volume in air. Berylliumsteam reactions are exothermic, and the energy release will tend to

increase overpressures, cause higher accident structural temperatures, and volatilize some

chemically toxic beryllium. Unlike berylliumsteam, carbonsteam cannot become chemically

ignited because it is an endothermic reaction. Both berylliumsteam and carbonsteam reac-

tions will mobilize the tritium in these materials.

Several scenarios could lead to berylliumsteam (or carbonsteam) reactions, such as

the following:

In-vessel LCE (water ingress) triggers a plasma disruption; the disruption heats the first

wall or divertor surface above operating temperature; a thermal gradient starts to relax but

berylliumsteam reactions may be sufficient to either ignite the beryllium or generate an unde-

sirable amount of H2 (plus mobilize tritium and beryllium)--depending on operating temperature

and temperature rise from the disruption. Even if short-term temperatures are too low for signifi-

cant berylliumsteam reactions, afterheat (without adequate decay heat removal) could raise

temperatures sufficiently for berylliumsteam reactions to start later. Similar scenarios start from

in-vessel LCE (flow blockage), overpower transient, or ex-vessel LFE or LCE.

At present, it appears difficult to argue that much of the beryllium on the first wall and

divertor surface will not be porous (8590% of theoretical density) because of possible effects

from neutron irradiation, ion irradiation, and redeposited and accumulated beryllium dust. Addi-

tional research on this is needed, as well as further testing of berylliumsteam and berylliumair

reaction rates as a function of porosity, temperature, and gas pressure. Another potential chem-

ical reaction is liquid metal-water reactions; for example, this reaction should be evaluated if a

liquid metal such as lithium is used in the blanket with pressurized water used for in-vessel

cooling.

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