1. INTRODUCTION ................................................. 1
1.1. Background .............................................. 1
1.2. Objective ............................................... 1
1.3. Scope ................................................... 3
1.3.1. Events considered ................................ 3
1.3.2. Analysis types ................................... 3
1.4. Structure ............................................... 4
2. GENERAL SAFETY FEATURES OF MODULAR
HTGR DESIGNS ................................................. 4
2.1. Coated fuel particles ................................... 5
2.2. Helium primary coolant .................................. 6
2.3. Decay heat removal ...................................... 6
2.4. Core thermodynamic characteristics ...................... 6
2.5. Reactor shutdown ........................................ 7
3. CLASSIFICATION OF EVENTS ..................................... 7
3.1. General event classification guidelines ................. 7
3.2. Classification of events by frequency ................... 9
3.3. Grouping of events by type ............................. 10
4. CONSEQUENCE ACCEPTANCE CRITERIA FOR ACCIDENTS ............... 12
5. ANALYSIS METHODOLOGY ........................................ 13
6. ANALYSIS OF EVENTS RELEVANT TO MODULAR HTGRs ................ 15
6.1. Heat removal transients: Loss of forced circulation
under pressurized conditions ........................... 15
6.1.1. Initiating events ............................... 15
6.1.2. Consequences and safety aspects ................. 16
6.1.3. Modelling and data needs ........................ 16
6.2. Primary system rupture transients ...................... 17
6.2.1. Loss of forced circulation with
depressurization ................................ 17
6.2.1.1. Initiating events ...................... 17
6.2.1.2. Consequences and safety aspects ........ 17
6.2.1.3. Modelling and data needs ............... 19
6.2.2. Rupture with air ingress ........................ 19
6.2.2.1. Initiating events ...................... 19
6.2.2.2. Consequences and safety aspects ........ 20
6.2.2.3. Modelling and data needs ............... 21
6.2.3. Rupture with water ingress ...................... 23
6.2.3.1. Initiating events ...................... 23
6.2.3.2. Consequences and safety aspects ........ 23
6.2.3.3. Modelling and data needs ............... 24
6.3. Accidents associated with reactivity control ........... 24
6.3.1. Reactivity events ............................... 24
6.3.1.1. Initiating events ...................... 24
6.3.1.2. Consequences and safety aspects ........ 25
6.3.1.3. Modelling and data needs ............... 26
6.3.2. Anticipated transient without scram ............. 27
6.3.2.1. Initiating events ...................... 27
6.3.2.2. Consequences and safety aspects ........ 27
6.3.2.3. Modelling and data needs ............... 28
6.4. Accidents associated with pressure transients .......... 29
6.4.1. Initiating events ............................... 29
6.4.2. Consequences and safety aspects ................. 29
6.4.3. Modelling and data needs ........................ 30
6.5. Other sources of radioactivity: Loss of pebble bed
reactor spent fuel storage forced cooling .............. 30
6.5.1. Initiating events ............................... 30
6.5.2. Consequences and safety aspects ................. 31
6.5.3. Modelling and data needs ........................ 31
7. RECOMMENDED FEATURES OF ACCIDENT ANALYSIS CODES ............. 31
7.1. Neutronics and thermohydraulic codes ................... 32
7.2. Fuel performance codes ................................. 34
7.3. Plant system analysis codes ............................ 35
7.4. Confinement analysis codes ............................. 36
7.5. Codes dealing with chemical reactions .................. 37
7.6. Codes dealing with fission product release, transport
and dose ............................................... 38
REFERENCES ..................................................... 43
CONTRIBUTORS TO DRAFTING AND REVIEW ............................ 45
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