Loss-of-Coolant Event

DOE-STD-6003-96

description of each class of accident that can be used as a starting point for a detailed machine-

specific hazard analysis.

B.4.1 Loss-of-Coolant Event

Loss-of-coolant events (LCEs) refer to the actively cooled components that remove the

fusion power (e.g., blanket, shield, vacuum vessel, or divertor cooling systems). The serious-

ness of the event depends on the coolant being used in the design (e.g., water, liquid metal,

and helium) and details of the design (e.g., segmentation of cooling loops, material, and length

of piping).

Two types of LCEs have generally been considered in fusion conceptual design studies:

in-vessel LCE and ex-vessel LCE. The in-vessel LCE would spill coolant into the torus that

could cause pressurization and potential chemical reaction with hot PFC surfaces. The magni-

tude of the pressurization is a function of the spill size, the coolant being used, the surface tem-

perature of the PFC, the internal energy of the coolant, and for water the presence of condensa-

tion surfaces. The introduction of coolant into the plasma chamber would result in a plasma

disruption and terminate the plasma.

Ex-vessel LCEs generally tend to be larger in terms of coolant loss than in-vessel LCEs

because of the size of the ex-vessel piping that transports coolant to the heat removal systems

(e.g., steam generator and heat exchanger). Rapid detection of ex-vessel LCE may be required

so that the plasma shutdown system can terminate the plasma before damage would occur to

the divertor and first wall. The time scale for such detection and shutdown is a strong function of

the heat loads on the PFCs and could be on the order of seconds.

B.4.2 Loss-of-Flow Event

Both in-vessel and ex-vessel loss-of-flow events (LFEs) have been considered in past

conceptual design studies for fusion machines. The consequences of such events are a strong

function of the coolant material, the heat loads on the divertor and first wall, and the design of

the heat transport systems. LFEs can lead to an in-vessel LCE because of the possibility of

tube burnout if plasma shutdown is not accomplished quickly (in seconds).

Ex-vessel LFEs tend to be dominated by loss of off-site power, which results in pump

coastdown. Loss of pumping power would need to trigger the plasma shutdown system to pre-

vent propagation of the LFE into an in-vessel LCE. For an in-vessel LFE, the concern is tube

plugging or coolant channel blockage. Because of the small tubing in most in-vessel compo-

nents, an in-vessel LFE would result in burn-through of the tube or channel wall and a small in-

vessel LCE. The subsequent injection of coolant into the plasma chamber would terminate the

plasma probably due to a plasma disruption. The system would then have to be cooled down

and the failed tube or channel isolated and plugged to recover from the event.

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