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Page Title: Pressure Suppression Strategies - Continued
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1. Limit temperature of first wall--As pointed out above, 650C is a critical temperature
with respect to hydrogen-producing reactions. Therefore, the operating temperature
and the rapid plasma shutdown (if required, see Section 6.3.2) should be designed to
prevent the first wall from reaching this critical temperature in the reference off-normal
event sequences defined in the safety analysis. In low-frequency severe accidents
where plasma-facing component (e.g., beryllium) reactions can not be excluded, a
backup strategy is to provide sufficient passive heat transfer between the reacting
material and the structures to avoid self-sustained reaction. Segmentation of the
shield coolant loops and of the vacuum vessel coolant loops should be performed in
such a way to optimize the likelihood of heat transfer from the shield.
2. Prevent ingress of air (see Section ingress of air is a necessary condi-
tion for the forming of an explosive mixture with hydrogen (the detonation limits of
hydrogen-air mixtures range from about 14% to 70% H2). Therefore, prevention of air
ingress by maintaining the cryostat vacuum boundary and by providing inert atmo-
sphere around the cryostat are possible strategies to avoid hydrogen explosion.
3. Limit inventory--For the steam generation scenarios, the total amount of steam pro-
duced can be limited by limiting the amount of water spilled in the vacuum vessel in
case of a LCE. One way of limiting this amount is to segment the coolant loops for the
shield and for the vacuum vessel.
4. Limit chemical reactivity--The chemical reactivity of beryllium is dependent on its
form: porous or dense. Characterization of beryllium coating should be performed and
if possible the existence of porous beryllium in the vacuum vessel should be limited.
5. Provide adequate afterheat removal in all scenarios--The afterheat is the driving force
for the medium-term overpressurization scenarios. The strategies to provide adequate
afterheat removal are covered in Section 6.3.1.
The mitigative strategies have the goal to limit the pressures that are caused by steam
production and hydrogen explosion.
1. Blowdown volumes--The expansion volume provided by the vacuum vessel itself may
be insufficient to limit the pressure to reasonable design values from the blowdown of
the coolant circuit in case of an in-vessel LCE. If the vacuum vessel vents to a
suppression pool or an adjacent larger volume such as the cryostat, peak pressures
can be reduced.
2. Vacuum vessel draining--Another process to mitigate peak pressures is one in which
the water from a LCE in the vacuum vessel is drained and led over cold surfaces to
reduce the pressure. The same cold surfaces are used as condensation surfaces for
the steam formed in the LCE.

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