Design basis event (DBE) definition and analysis is an ongoing and integrated activity among the design and analysis groups of the Yucca Mountain Project (YMP). DBE`s are those that potentially lead to breach of the waste package and waste form (e.g., spent fuel rods) with consequent release of radionuclides to the environment. A Preliminary Hazards Analysis (PHA) provided a systematic screening of external and internal events that were candidate DBE`s that will be subjected to analyses for radiological consequences. As preparation, pilot consequence analyses for the repository subsurface and surface facilities have been performed to define the methodology, data requirements, and applicable regulatory limits

Haas, M. N.

Martin, J. H.

Orvis, D. D.

UNT Digital Library

m Design Basis Event Consequence Analyses for the Yucca Mountain Project Authors D. D. O M S  and M. N. Haas, Yucca Mountain Project, Morrison Knudsen Corporation, Las Vegas, Nevada J. H. Martin, Yucca Mountain Project Fluor Daniel Corporation, Las Vegas, Nevada In t roduct ion Design basis event @BE) definition and analysis is an ongoing and integrated activity among the design and analysis groups of the Yucca Mountain Project (YM?). DBE's are those that potentially lead to breach of the waste package and waste form (e.g., spent kel  rods) with consequent release of radionuclides to the environment. A Preliminary Hazards Analysis (PHA) provided a systematic screening of external and internal events that were candidate DBE's that will be subjected to analyses for radiological consequences. As preparation, pilot consequence analyses for the repository subsurface and surface facilities have been performed to define the methodology, data requirements, and applicable regulatory limits. m e  of Pilot Analvsa Pilot Analyses for Subsu dace Fac ilities - A single limiting non-mechanistic event was evaluated. Implicit in this analysis is the assumption that the WP design precludes a failure by any credible event. Category 2 design basis events are defined by the revised 10 CFR 60 to be credible events expected to occur with a frequency of at least 1 Ob per year. Although the non-mechanistic scenario is deemed incredible, the offsite consequences were compared to the 10 CFR 60 limits for Category 2 DBEs'. For purposes of consequence evaluation, the WP is assumed to be breached non-mechanistically such that the WP confinement barrier and the cladding of 100% of the &el rod contents are concurrently breached. The repository subsurface drifts and ventilation shaft are presumed intact and provide no restriction to release of radionuclides to the environment; the ventilation system is assumed to hnction normally. A given WP was assumed to contain either a) 21 Design Basis PWR spent fuel assemblies (SFAs), b) 44 Design Basis BWR SFAs, or c) 5 Defense High Level Waste Canisters (DHLWCS). Pilot Analysis for Surface Facilities - The analysis addresses five scenario categories: handliig drop scenarios in the Waste Handling Building (WHB) involving, respectively, a) Spent Fuel Assembly (SFA); b) Spent Fuel Canister (SFC); or c) Defense High Level Waste Canister (DHLWC). These three categories are deemed credible events based on preliminary estimates of frequency of handling operations and industry data for drops during kel  handling operations. Two non-mechanistic release scenarios were analyzed: a) release fiom a WP inside the WHB; and b) release fiom the WP outdoors. These five scenario categories are expected to be representative of the bounding internal events. w aSTRl8UpON OF THIS DOCUMENT I$ UNLIMITED DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty? expm or implied, or assumes any legal liabili- ty or mporrsibiiity for the accuracy? completeness, or usefdneq of any information, appa- ratus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof. Portions of this document may be itlegible in electmnic image products. Images are produced from the best available original dOl?trment, Offsite Dose Ana lvses The offsite dose consequences for the respective pilot analyses were performed with spreadsheet models. The models are those routinely used and accepted by the NRC and EPA. Two basic expressions are used to calculate receptor doses respectively due to inhalation of respirable releases and to submersion in the cloud of released radionuclides: Inhalation = Source@) * Release * Mitigating * Breathing * Atmospheric Dose [Rem] [Re-I Fraction Factors Rate [m’lsec] Dispersion Factor [sedm’] Submersion = Source(S) * Release * Mitigating * Atmospheric Dose [Rem] [Rem/sec/WP/m3] Fraction Factors [m’/sec] Dispersion Factor [sedm’] The data and assumptions used for each term in the dose model and representative results are described below. Source Terms - A YMP design support document provides a review of isotopic inventories of pressurized- and boiling-water reactor (PWR and BWR) &el assemblies and defense high-level waste canisters (DHLWCs). The analysis identified the characteristics for bounding commercial spent nuclear &el (SNF) with respect to burnup and enrichment by which to define radionuclide inventories internal to the he1 matrix for the 10-year-old &el and in the solid DHLW. The radionuclide activities of each SFA or DHLWC were converted into dose units, e.g., RedSFA for inhalation dose, and Rem/SFA/m3/hr for submersion dose. The lists of radionuclides for each waste form were screened to include only those that contribute 99.9% of the total dose per unit waste form for at least one organ or the whole body; other radionuclides considered to be important by precedent were added to the inventory including a Co-60 source term representing surface crud on SFAs. For events involving entire WPs or SFCs, the source terms per SFA (or DHLWC) were multiplied by the number of SFAs (or DHLWCs) inside the WP. Release Fractions - The release fraction represents the respirable fraction of total radionuclide inventory available in waste form that escapes to the environment past the barriers posed by fbel or glass matrix, &el cladding or waste canister, and the WP. Release fractions for noble gases and volatile isotopes are well established by NRC precedents such Regulatory Guide 1.25 ’; e.g., 30% for noble gases and 10% for iodine. For the non-mechanistic events, the release fractions for volatiles and respirable quantities of particulates and crud were adopted fiom the NRC’s draft guidance for similar events for interim storage installations2, e.g., 5 x lo-’’. Implicit in this value is the assumption that the breach of the barrier provided by the WP is small, Le., one square-inch or less4. Release fractions for the mechanistic drop scenarios were estimated from available data and included a contribution from impact pulverization of SNF matrix or DHLW glass ‘. Breathing Rate - The breathing rate is defined in Regulatory Guide 1.25 as 3.47 E-4 m3/sec. Atmospheric Dispersion Factors - X/Qs for a ground release were obtained from Regulatory Guide 1.25 for assumed receptor distances of 100 meters, 1000 meters and 5000 meters. Mitigating Factors - Potential mitigating factors include plateout and deposition of radionuclides on surfaces and equipment, and HEPA filters in the effluent paths. No credit for mitigating factors was necessary to meet dose l i i t s  for the non-mechanistic scenarios. At the time of this writing, neither pilot analysis has been finalized so results are viewed as preliminary. For brevity, results for the non-mechanistic release from a WP containing 21 PWR Design Basis SFAs are presented in Table 1. Similar results are available for a WP containing 44 Design Basis BWR SFAs. Calculated doses for typical DHLWC are orders of magnitude less. For the non-mechanistic cases, offsite dose limits for Category 2 DBEs can be met without taking credit for any mitigating factors. Preliminary fiequency estimates indicate one drop scenario may be a Category 1 DBE. Preliminary results for offsite doses were noted to exceed the regulatory limits for Category 1 at 5000 meters, when no credit was taken for mitigation. Taking credit for reduction of releases through HEPA filters, however, showed that dose limits could be met. References 1. “Amendments to 10 CFR 60 on Disposal of High-Level Radioactive Wastes in Geologic Repositories -- Design Basis Events for the Geologic Repository Operations Area -- Final Rulemaking”, USNRC, SECY-96-136, June 24, 1996 (Released to Public Document Room 10/15/96) 2. “Standard Review Plan for Dry Cask Storage Systems”, USNRC, NUREG-1536, Draft Report for Comment (February 1996) 3. “Assumptions Used for Evaluating the Potential Radiological Consequences of a Fuel Handling Accident in the Fuel Handling and Storage Facility for Boiling and Pressurized Water Reactors”, Regulatory Guide 1.25 (USNRC Safety Guide 25), USNRC (March 1972) 4. H. R MAC DOUGALL, a, “Site Characterization Plan Conceptual Design Report, Volume 4, Appendix F, Preliminary Preclosure Radiological Safety Analysis,” SAND84-2641, Sandia National Laboratory Albuquerque, NM (September 1987) Table 1 Regulatory Dose Limits [4] TEDE = CEDE + DDE CDE* + DDE 50 Rem. 5 Rem. EYE 15 Rem. SKIN 50 Rem. Comparison of Calculated Offsite Doses for Non-Mechanistic Release With Regulatory Limits Results: PWR Design-Basis 10-Year Old Spent Fuel Calculated Doses (Rem) at Assumed Boundary Distance [l] 100 Meters 1000 Meters 5000 Meters 2.05 E+OO 3 .SO E-02 3.51 E-03 5.56 E+OO 1.03 E-01 9.54 E-03 Neglig. Neglig. Neglig. 3.26 E+O1 6.05 E-01 5.58 E-02 where: TEDE = Total Effective Dose Equivalent CEDE = Inhalation Whole Body DDE = Submersion Whole Body CDE = Maximum Internal Organ, Inhalation EYE = Eye Dose Equivalent SKIN = Skin Shallow Dose Equivalent 

Design basis event consequence analyses for the Yucca Mountain project

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Design basis event consequence analyses for the Yucca Mountain project

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