Pressure Suppression Strategies

DOE-STD-6003-96

The ingress of coolant in the vacuum vessel would lead to rapid pressurization of the

vacuum vessel to pressures that, for large quantities of water released, come close to saturation

pressure (about 1.6 MPa at 200C). Venting to the cryostat or a blowdown volume (suppression

pool) would lead to significantly lower maximum pressures.

Another possible source is hydrogen production (see Section 6.1.3.9) and energy release

from chemical reaction between the water coolant or air and the plasma-facing components

(e.g., beryllium tiles and coatings on the first wall). Although the reaction rates between water or

air and beryllium are uncertain and need further analysis, it is known that the form of beryllium

(porous or dense) has a big impact on these rates.

The 600650C range is critical with respect to beryllium reactions. Since the beryllium

water and berylliumair reactions are exothermic (377 kJ/gmole beryllium), the major concern

for short-term hydrogen production comes from reactions that are self-sustained. Self-sustained

reactions require that the heat production (from the reaction) exceeds the heat loss (from

cooling). Scenarios including short-term hydrogen production start with overheating of the beryl-

lium in the first wall or divertor to the 650C range. The following scenarios are examples.

Loss-of-Flow Events (LFEs) in shield lead to Loss-of-Cooling Events (LCEs) by local

penetration of the overheated first wall and subsequent self-sustained berylliumwater reaction.

Preliminary calculations show that this scenario (without mitigation) gives rapid production of a

few kilograms of hydrogen (in the solid beryllium case) to tens of kilograms of hydrogen (if the

beryllium is in porous form).

Another possibility is an LFE with ingress of air in the vacuum vessel (LVE). Worst-case

scenarios of this type (without mitigation) with a porous berylliumair reaction could lead to pro-

duction of hundreds of kilograms of hydrogen in time scale of minutes.

The medium-term scenarios involve extensive steam production from the coolant inven-

tory by energy sources like decay heat, stored heat, and heat produced by chemical reactions

as well as hydrogen production due to chemical reactions. Examples of such scenarios are in-

vessel LCEs in the vacuum vessel or shield, combined with reduced or no decay heat removal.

b. Pressure Suppression Strategies

The strategies with respect to pressure limitation and suppression can be divided into

preventive and mitigative strategies. In both preventive and mitigative strategies, use can be

made of passive or active means to implement the strategies. Whenever possible, the priority

should be given to preventive strategies and to passive means.

Preventive strategies have the goal to reduce the amounts of steam and hydrogen pro-

duced. Prevention of pressure buildup can be achieved by acting upon the phenomena that lead

to hydrogen production (see Section 6.1.3.9) and by limiting the amounts involved in chemical

reactions or in steam formation.

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