Efficacy assessment of independent severe accident mitigation measures in OPR1000 using MELCOR code

Figures & data

Figure 1. Evaluation methodology for SAM strategies.

Table 1. Probability of transition from initiating events to severe accidents for OPR1000.

Initiating event	Probability of SA (%)
SBLOCA without SI	22.4
SBO	14.4
TLOFW	13.8
SGTR	13.8
LBLOCA without SI	12.7
MBLOCA without SI	7.7

Figure 2. Flowchart of OPR1000 SAMG.

Table 2. Severe accident mitigation strategies of SAMG.

Mitigation strategy	Objective	Mitigation measure
Mitigation-01	Establish heat removal source	AFW
	Maintain integrity of steam generators
Mitigation-02	Prevent direct containment heating by high-pressure melt ejection	SDS
	Establish core cooling	ADV
		CDV
Mitigation-03	Establish core cooling	HPSI
	Prevent reactor pressure vessel failure	LPSI
	Reduce the release of FPs into containment	Charging pump
Mitigation-04	Establish core cooling	Spray
	Prevent reactor pressure vessel failure	RWT gravity feed
	Prevent molten core–concrete interaction
	Reduce the release of FPs into containment
Mitigation-05	Reduce the release of FPs	Spray
		Ventilation system
Mitigation-06	Maintain containment integrity	Spray
	Reduce the release of FPs	Fan cooler
		Containment purge
Mitigation-07	Prevent hydrogen explosion	Hydrogen PAR
		Igniter

Table 3. Selection of mitigation measures.

Mitigation strategy	Safety condition for mitigation strategy	Mitigation measure
Mitigation-01	SG water level > 63%	AFW
Mitigation-02	RCS pressure < 2.86 MPa	SDS
Mitigation-03	CET < 923 K	HPSI
Mitigation-04	Containment water level > 17%	Containment spray
Mitigation-05	Site Area Emergency	Containment spray
Mitigation-06	Containment pressure < 133.6 cmH2Og (0.131 MPa)	Containment spray
Mitigation-07	Hydrogen concentration < 5%	PAR

Figure 3. MELCOR nodalization for OPR1000.

Figure 4. MELCOR nodalization for reactor core of OPR1000.

Table 4. OPR1000 operating conditions: design values per FSAR and steady-state MELCOR modeling results.

Parameter	FSAR	MELCOR
Core thermal power (MWt)	2815	2815
RCS pressure (MPa)	15.5	15.5
Core inlet temperature (K)	569	573
Core outlet temperature (K)	600	603
Primary flow rate (kg/s)	15,306	15,546
Secondary side pressure (MPa)	7.37	7.37
Steam flow rate per SG (kg/s)	800	809

Table 5. Major accident sequences in the base cases.

Accident sequence	SBLOCA without SI	SBO	TLOFW
Accident start (h)	0	0	0
Reactor trip (h)	0.04	0	0.01 (28 s)
PSRV open (h)	0	1.36	0.42 (1517 s)
Steam generator dryout (h)	1.74	1.04	0.27
Oxidation start (h)	2.35	2.29	1.08
Cladding melting (h)	2.64	2.65	1.36
UO2 relocation to lower head (h)	2.89	2.83	1.56
SIT injection (h)	3.64	3.80	2.33
RPV failure (h)	5.82	3.78	2.30
FP leakage into the environment (h)	30.93	39.54	34.60
Pressure at RPV failure (MPa)	5.42	16.22	16.62

Table 6. Major accident sequences for SBLOCA without SI in base and mitigation cases.

Accident sequence	Base case	MS-01	MS-02	MS-03	MS-04	MS-05	MS-06	MS-07
Mitigation strategy begun (h)	–	2.36	2.36	2.36	2.36	2.39	2.36	PAR
Cladding melting (h)	2.64	9.32	4.11	23.45	2.64	2.64	2.64	2.64
RPV failure (h)	5.82	10.02	6.32	29.48	6.90	6.90	6.90	6.88
FP leakage into the environment (h)	30.93	34.67	29.07	38.80	48.57	48.57	48.57	29.03

^*MS: mitigation strategy.

Table 7. Major accident sequences for SBO in base and mitigation cases.

Accident sequence	Base case	MS-01	MS-02	MS-03	MS-04	MS-05	MS-06	MS-07
Mitigation strategy begun (h)	–	2.25	2.25	2.25	2.25	2.48	2.67	PAR
Cladding melting (h)	2.65	15.03	4.35	2.65	2.65	2.65	2.65	2.65
RPV failure (h)	3.78	17.91	6.60	3.78	3.78	3.78	3.78	3.78
FP leakage into the environment (h)	39.54	58.91	29.45	53.53	60.45	60.45	60.45	38.36

Table 8. Major accident sequences for TLOFW in base and mitigation cases.

Accident sequences	Base case	MS-01	MS-02	MS-03	MS-04	MS-05	MS-06	MS-07
Mitigation strategy begun (h)	–	–	1.08	1.08	1.08	1.26	1.08	PAR
Cladding melting (h)	1.36	–	3.54	1.36	1.36	1.36	1.36	1.36
RPV failure (h)	2.30	–	7.54	2.30	2.30	2.30	2.30	2.31
FP leakage into the environment (h)	34.60	–	27.14	50.79	52.62	52.62	52.62	33.26

Figure 5. Core exit temperature in Mitigation-01 cases.

Figure 6. Containment pressure in Mitigation-02 cases.

Figure 7. HPSI total flow rate in Mitigation-03 cases.

Figure 8. Cavity water level in Mitigation-04 cases.

Figure 9. Mass of radioactive fission product in Mitigation-05 case (SBLOCA).

Figure 10. Mass of radioactive fission product in Mitigation-05 cases (SBO and TLOFW).

Figure 11. Containment pressure in Mitigation-06 cases.

Figure 12. Hydrogen mole fraction in the containment in Mitigation-07 cases.

Figure 13. Shapiro diagram for SBLOCA without SI.

Figure 14. Shapiro diagram for SBO.

Figure 15. Shapiro diagram for TLOFW.

Figure 16. Containment pressure for Mitigation-06 cases, including hydrogen ignition.