Safety Benefits Assessment for Accident Tolerant Fuels in Consideration of Steam Generator Tube Degradation Using Dynamic Event Tree Analysis

Figures & data

TABLE I SG Tube Parameters

Tubing Material	Alloy 600MA
Ultimate tensile strength, ${\sigma }_u$	713 MPa (Ref. 22)
Yield strength, ${\sigma }_y$	362 MPa (Ref. 22)
Tube outside diameter, $D$	22.23 mm
Tube thickness, $t$	1.27 mm
Primary-side pressure, $P_{in}$	15.5 MPa
Secondary-side pressure, $P_{out}$	8.0 MPa

TABLE II SCC Crack Growth Parameters*

Parameter	Minimum	Average	Maximum
$\alpha$ ($\times {10}^{-12})$	2.5	2.8	3.1
$K_{th}\left(MPa\sqrt{m}\right)$	8	9	10
$\beta$	1.07	1.16	1.25
$\mathrm{\Delta }P\left(MPa\right)$	6.5	7.5	8.5
$k$	0.5	0.55	0.6
$t\left(mm\right)$	0.95	1.27	1.59
$D\left(mm\right)$	21.73	22.23	22.73

*References 26 and 28.

Fig. 1. State transition model for SG tube degradation (adapted from Ref. 31).

Fig. 2. State probability calculation flowchart.

Fig. 3. SG tube state probability over cycle without SG replacement for Alloy 600MA.

Fig. 4. Plugged tube fraction over cycle with SG replacement for Alloy 600MA.

Fig. 5. Simplified event tree for a 2-in. LOCA.

TABLE III Steady-State Initial Condition of the Reference Plant

Plant Parameter	Value
Core power [MW(thermal)]	2 815.0
Core flow rate (kg/s)	14 687.0
RCS pressure (MPa)	15.5
Hot leg temperature (K)	606.0
Hot leg flow rate (kg/s)	7 361.5
Cold leg temperature (K)	5 75.4
Cold leg flow rate (kg/s)	3 758.8
Steam pressure (MPa)	8.1
Steam flow rate (kg/s)	807.9

Fig. 6. RELAP5 nodalization of the reference plant.³⁸

TABLE IV PCT Failure Criteria

Fuel Type	Failure Criteria	Reference
Conventional Zr	PCT > 1477 K	Ref. 42
Cr-coated Zr	PCT > 1578 K	Refs. 43 and 44
FeCrAl	PCT > 1640 K	Ref. 45

TABLE V Branching Parameters and Conditions for DET

Branching Parameter	Value	Distribution	Reference
SG cycle	1	Uniform distribution (minimum = 1,maximum = 13)
	3
	…
	13
ADV opening time (s)	5	Lognormal distribution (mean = 900, error factor = 5)	Ref. 49
	10
	…
	3600
SIT availability	Unavailable	Bernoulli distribution (unavailability = 7.45E-6)	Ref. 50
	Available
LPSI availability	Unavailable	Bernoulli distribution (unavailability = 3.92E-4)	Ref. 50
	Available

Fig. 7. Example of accident sequences generated using the DET approach (OK: no core damage, CD: core damage, S: success, and F: failure).

Fig. 8. PCT over time for different fuels without operator action.

Fig. 9. Coping time for different fuels without SG tube degradation: (a) with SIT and (b) with LPSI (OK: no core damage and CD: core damage).

TABLE VI CCDP for Different Fuels Without SG Tube Degradation

	Conventional Zr	Cr-coated Zr	FeCrAl
CCDP	2.75E-3	2.25E-3	1.77E-3
CDF reduction with respect to Zr cladding (%)	–	18.30	35.76

Fig. 10. Coping time for different fuels with SG tube degradation: (a) with SIT and (b) with LPSI.

TABLE VII CCDP for Different Fuels with SG Tube Degradation

		SG Cycle
Fuel Type		1	3	5	7	9	11	13
Conventional Zr	CCDP	2.75E-3	2.83E-3	2.94E-3	3.15E-3	3.61E-3	4.21E-3	4.76E-3
	CDF increase with respect to cycle 1 (%)	–	2.74	7.01	14.57	31.32	52.89	73.06
	CDF reduction with respect to Zr cladding (%)	–	–	–	–	–	–	–
Cr-coated Zr	CCDP	2.25E-3	2.31E-3	2.40E-3	2.54E-3	2.71E-3	2.98E-3	3.33E-3
	CDF increase with respect to cycle 1 (%)	–	2.72	6.95	12.88	20.76	32.79	48.07
	CDF reduction with respect to Zr cladding (%)	18.30	18.32	18.35	19.51	24.87	29.04	30.10
FeCrAl	CCDP	1.77E-3	1.79E-3	1.82E-3	1.86E-3	1.99E-3	2.19E-3	2.47E-3
	CDF increase with respect to cycle 1 (%)	–	1.34	2.69	5.47	12.74	23.79	39.70
	CDF reduction with respect to Zr cladding (%)	35.76	36.64	38.35	40.86	44.85	47.99	48.14

Fig. 11. CCDP for different fuels with and without considering SG tube degradation.

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