GC:

HIC:

CC:

SCC:

Monte Carlo Simulations of the Degradation of the Engineered Barriers System in the Yucca Mountain Repository Using the EBSPA code

Z. Qin and D.W. Shoesmith, The University of Western Ontario, London, Ontario, Canada

Introduction

Failure Model

EBSPA Code

Conclusions

Acknowledgements

Yucca Mountain (Nevada, USA) is the proposed site for a geologic repository for the disposal of high-level radioactive waste.

The nuclear waste will be stored within an engineered barriers system (EBS) that combines a drip shield (DS) and a waste package (WP). The performance of the EBS is primarily controlled by the corrosion performance of the titanium Grade 7 (Ti-7) DS and the Alloy 22 (C-22) WP.

Corrosion commences once aqueous conditions are established on the DS and the WP surfaces. The degradation modes considered in the model are general passive corrosion (GC) and hydrogen induced cracking (HIC) for the DS, and GC, crevice corrosion (CC), and stress corrosion cracking (SCC) for the WP.

The temperature profile and Ti-7 GC rates were provided by Y. Sun (Lawrence Livermore National Laboratories) and F. Hua (Bechtel SAIC Company), respectively.

Responsibility ratios of various failure mechanisms and individual components to failure

Responsibility ratio Conservative Worst-case

HIC to DS failures 1 1

CC to WP shell failures 0 8.8018x10-7

SCC to WP outer lid failures 0.333645 0.333221

DS to EBS failures 0.195058 0.095747

Based on the probabilistic model proposed, a Monte Carlo simulation code (EBSPA) has been developed to predict the lifetime of the EBS within the Yucca Mountain nuclear waste repository. The degradation modes considered are GC and HIC for the DS, and GC, CC and SCC for the WP. Two scenarios have been simulated using the EBSPA code. The results show that the EBS should not fail until 255,000 years under conservative conditions, and even for the worst-case scenario, the EBS would likely survive for more than 115,500 years. The simulations also predict that the DS will always fail due to HIC, and CC of the WP is a remote possibility even for the worst- case scenario.

EBSPA simulation pathway

The calculated CPF (10-4) shows that the EBS should not fail until 255,000 years under conservative conditions, and even for the worst-case scenario, the EBS would likely survive for more than 115,000 years after emplacement.

00`)( ddRtd

t

GCGC ≥= ∫ τ

HICGC

Ti

HH C

dtd

MftC ≥⎥

⎦

⎤⎢⎣

⎡−−=

0

)(1ln4)(

nCC Bttd =)(

Simulation Results

flawCCGC ddd ≥+

Based on a probabilistic model, a Monte Carlo code, EBSPA (currently in its third version), has been developed to very conservatively predict the lifetime of the EBS.

Two scenarios were simulated using the EBSPA code: a) a conservative scenario for the conditions thought likely prevail in the repository; b) a worst-case scenario in which the impact of degradation processes is overstated.

The EBSPA can calculate a) the cumulative probability of failures (CPF) of the DS, the WP (including the shell, the

inner lid, and the outer lid), and the EBS; b) the responsibility ratio of individual components to the overall EBS failure; andc) the contributions of different degradation modes to failure.

WP outer lid(GC+CC+SCC)

WP inner lid(GC+CC)

WP lidsWP shell(GC+CC)

WP lifetime Drip shield(GC/HIC)

EBS lifetime

Sum of the two

Whichever fails first

Whichever fails last

DS intact / leaks

T <= Taq

The simulations demonstrate that the DS will always fail due to HIC, and CC of the WP is only a remote possibility even for the worst-case scenario.

104 105 10610-4

10-3

10-2

10-1

100

DS (conservative) DS (worst-case) WP (conservative) WP (worst-case) EBS (conservative) EBS (worst-case)

Cum

ulat

ive

prob

abili

ty

Lifetime (years)