Coolant Energy

DOE-STD-6003-96

6.3.3.1 Coolant Energy

For fusion facilities that use fluids for active cooling of in-vessel components (e.g., high-

pressure water or steam, liquid metals) or cryogenic liquids inside the cryostat, the design

should incorporate a means to accommodate the accidental release of the fluids to ensure that

confinement barriers such as the vacuum vessel or cryostat are not breached in a manner that

could result in exceeding evaluation guidelines. Consideration should be given to the effect of

large spills of cryogenic liquids inside the cryostat on the structural integrity of affected SSCs

due to loss of ductility at lower temperatures.

a. Discussion of Sources of Coolant Energy

For water coolant systems, the overpressure depends on mass and energy. Energy

sources include the stored energy in the water, energy from plasma operation if the water is not

being adequately cooled (overpower transient, loss of flow, loss of heat sink), energy from

chemical reactions, decay heat, and heat transferred from surrounding surfaces. Heat transfer

to energy sinks takes place via vaporization of water, conduction, and condensation on

surfaces.

For typical water-cooled designs, the potential sources for overpressure of the confine-

ment barriers (vacuum vessel and cryostat) are

1. release of cryogenic fluid in the cryostat;

2. steam production from leakage of coolant in the vacuum vessel, or, if applicable in the

cryostat; and

3. hydrogen production, with ingress of air and explosion in the vacuum vessel (see

Section 6.1.3.9).

The dynamics of the scenarios involving overpressure are different and lead to consid-

eration of two time scales. Short-term scenarios lead to overpressure in a time span of minutes

following the release of coolant or cryogenic fluid in the vacuum vessel or in the cryostat.

Medium-term scenarios are driven by decay heat and chemical reactions and lead to overpres-

sure in a time span of days if no sufficient decay heat removal can be provided.

The short-term scenarios include the release of cryogenic fluid in the cryostat and the

short-term pressurization of the vacuum vessel due to ingress of coolant with and without

hydrogen production.

The release of the cryogenic fluid from the magnet systems in the cryostat leads to typical

pressures in the cryostat of the order of several atmospheres. Note that in the absence of blow-

down volumes or venting, this pressure has to be supported by the cryostat as an internal load

and by the vacuum vessel as an external load.

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