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off-normal conditions to ensure continuity of the required safety function. The design should
incorporate sufficient redundancy and/or capability for safety-class systems. The electric power
to operate safety class instrumentation should meet the requirements of IEEE 1980. More infor-
mation is provided in Section 6.4.1, Instrument and Control Systems, and Section 6.4.2,
Electrical Power Systems.
e. Ventilation and Exhaust Systems
The design of cryostat confinement ventilation systems should ensure the ability to main-
tain desired airflow characteristics when personnel access ports or hatches are open. When
necessary, air locks or enclosed vestibules should be used to minimize the impact of this on the
ventilation system and to prevent the spread of airborne contamination within the facility. The
ventilation system design should provide the required confinement capability under all normal
operations and off-normal conditions with the assumption of a single failure in the system. If the
maintenance of a controlled continuous confinement airflow is required, electrical equipment
and components required to provide this airflow should be supplied with safety-class electrical
power and provided with a backup power source. Air cleanup systems should be provided in
confinement ventilation exhaust systems to limit the release of radioactive or other hazardous
material to the environment and to minimize the spread of contamination within the facility as
determined by the safety analysis. Guidance for confinement systems is included in DOE 1989.
System Maintenance
Opening a confinement system such as the cryostat requires prior removal of tritium,
radioactive dust, and loose toxic materials (if any) to the maximum extent feasible. If required by
the safety analysis, cleansing steps that exhaust to atmosphere should exhaust through a tri-
tium removal system to limit the release of tritium and other radioactive and toxic materials to
the environment consistent with release limits and ALARA principles. The safety analysis should
prescribe limits for tritium and other radioactive and toxic material releases to the environment
during cryostat maintenance openings. The exhaust from the cryostat may be through a dedi-
cated tritium removal system or through a confinement subsystem that has a tritium removal
system. The tritium removal systems should have capacity to recover from a design-basis tritium
release from the cryostat. If the cryostat is part of the biological shield, maintenance planning
should consider the effect of planned tasks on the shielding integrity. Confinement/Heating, Ventilating, and Air Conditioning (HVAC) Systems
Confinement/HVAC systems include SSCs designed to serve as barriers against the
spread or uncontrolled release of radioactive or other hazardous materials throughout the facility
or to the environs. The facility confinement strategy may consist of successive confinement bar-
riers based on the hazards present. The successive barriers are defined by the facility safety
The confinement/HVAC system boundary is generally defined for each confinement bar-
rier and includes the contiguous structural barrier and its associated ventilation and filtration

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