SCWR Preliminary Safety Considerations

1
SCWR Preliminary Safety Considerations

• Cliff Davis, Jacopo Buongiorno, INEEL
• Luca Oriani, Westinghouse Electric Co.

April 29, 2003 Madison, Wisconsin
2
Introduction

• Safety concept and classification of the events
• Parametric thermal-hydraulic calculations of the
  SCWR during loss-of-feedwater and turbine-trip
  transients to determine the required response
  time and capacities of safety systems
• Calculations used the RELAP5 computer code, which
  has been recently improved for SCWR applications
• Analysis was performed for a design with solid
  moderator rods, but the results are expected to
  be more generally applicable
• Transient cladding temperature limit of 840?C was
  used to evaluate the thermal-hydraulic response

3
Safety Concept

• Active, non-safety systems have passive,
  safety-related back-up to perform nuclear safety
  functions
• Safety functions automatically actuated, no
  reliance on operator action
• Passive features actuated by stored energy
  (batteries, compressed air)
• Once actuated, their continued operation relies
  only on natural forces (gravity, natural
  circulation) with no motors, fans, diesels, etc.
• Common approach with the most advanced LWR
  concept proposed by the main NSSS vendors
• Westinghouse AP600/AP1000, IRIS and System 80
• Framatome-ANP SWR-1000
• GE ESBWR and ABWR
• Design Goal Achieve a degree of safety at least
  comparable to the more advanced plant concepts
  being currently proposed.

4

• ANS Classification of Events
• Classification of Accident events per ANSI
  N18.2-1973 (industry standard based on ANS
  committee)Condition I Normal operation and
  operational transientsCondition II Faults of
  moderate frequencyCondition III Infrequent
  faultsCondition IV Limiting faults
• Classification according to expected frequency of
  occurrence
• Less frequent events may have more severe
  consequences

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The loss-of-feedwater and turbine-trip transients
were evaluated because

• SCWR is a once-through direct cycle without
  coolant recirculation in the reactor vessel
• Loss of feedwater is important because
• It results in rapid undercooling of the core
• It is a Condition II event that must not result
  in any significant damage to the fuel
• Average coolant density is low in the SCWR core
  and pressurization events result in significant
  positive reactivity insertion
• Turbine trip without steam bypass has the
  potential to cause a significant increase in
  reactor power

6
Parametric calculations for loss of feedwater
investigated the effects of

• Main feedwater (MFW) coastdown time (0 to 10 s)
• Scram (with and without)
• Auxiliary feedwater (AFW) flow rate (10-30 of
  rated feedwater)
• Steam relief (20-100 capacity)
• Step changes in MFW flow rate (25-100)
• Coolant density reactivity feedback (nominal and
  high)

7
Transient temperature limit met when AFW flow
exceeded 15

• 5-s MFW coastdown
• Scram
• Constant pressure

8
Temperature limit met for 50 step change in MFW
flow

• No scram
• No AFW

9
Fast-opening 100-capacity turbine bypass system
helps significantly

• 5-s MFW coastdown
• Scram
• No AFW

10
Higher coolant density reactivity feedback lowers
cladding temperature

• 5-s MFW coastdown
• Scram
• No AFW

11
Parametric calculations of a turbine trip without
steam bypass investigated the effects of

• Scram
• Safety relief valve (SRV) capacity (0 - 90)

12
Pressure response following a turbine trip is
acceptable

• Instant control valve closure
• Continued MFW at rated flow

13
Small increase in reactor power following turbine
trip

• Instant control valve closure
• Continued MFW

14
Conclusions

• SCWR with solid moderator rods can tolerate a 50
  step change in MFW flow without scram
• Transient temperature limit can be met following
  a total loss of MFW if AFW flow exceeds 15 of
  initial MFW flow
• AFW flow requirements can be reduced by
• Fast-opening 100-capacity turbine bypass
• Higher feedback coefficients typical of designs
  with water rods
• Acceptable pressure response following turbine
  trip without steam bypass if the SRV capacity is
  greater than 90
• Power increase following turbine trip without
  steam bypass and with full MFW flow is much
  smaller than in comparable BWRs