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NRC Staff Presentation on SAND2011-0128 Report on Source Term in Support of Aug 24, 2022 Public Meeting
	ML22229A071
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Issue date:	08/24/2022
From:	Michael Salay; NRC/RES/DSA
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SAND2011-0128 Report on Source Term Michael Salay Sr. Reactor Systems Engineer Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission August 24, 2022

Topics

Origin

Previous containment source terms

SAND2011-0128 Objective

Source Term development process

Summary of SAND 2011 Source Term

Changes in Releases

Choice of Sequences

Comments by 2011 Peer Review panel

Origin

Footnote to 10 CFR 100.11(a) (Siting: Exclusion Area, Low population zone, )

The fission product release assumed for these calculations should be based upon a major accident hypothesized for purposes of site analysis or postulated from consideration of possible accidental events, that would result in potential hazards not exceeded by those from any accident considered credible. Such accidents have generally been assumed to result in substantial meltdown of the core with subsequent release of appreciable quantities of fission products.

Origin: Defense in Depth

This is a classic performance-based rule to evaluate the defense in depth provided by the containment:

Applicant hypothesizes a release of radioactivity to the containment

Considering the design basis leak rate, and engineered safety features (but not ECCS) assesses the dose magnitude at the site boundary, control room and low population zone.

Origin: Interface with Risk-informed Regulation

Structural defense in depth

What protections are afforded to the public health and safety even if low probability events occur and there is a major release of radioactivity from the reactor core.

Previous Containment Source Terms

TID-14844 (1962)

Based on furnace heating of irradiated fuel chips

Instantly available in the containment at the start of an accident

NUREG-1465 (1995) (& RG1.183 rev 0)

Result of post-Three Mile Island Accident severe-accident research effort (>$0.5b)

Source Term Code Package (STCP)

Developed alongside NUREG-1150 - Risk Analysis for Five Nuclear Plants

Separation of radionuclides into chemical groups that behave similarly

Timing and release fractions of chemical groups in different phases:

gap release, in-vessel release, ex-vessel, late in-vessel (revaporization).

First two phases used in RG1.183

Aggregate of results of simulation of representative set of unmitigated accident sequences

Based on analyses of plants with fuel used to burnups < 40 GWd/t

Regulatory limit is 62 GWd/t

Rim effect and changes in pellet/clad interactions found for burnups in excess of about 45 GWd/t

Objective of SAND2011-0128

Define a representative source term to the containment for the evaluation of defense-in-depth capabilities:

Must be characteristic of accidents involving melting of a substantial fraction of the reactor core with HBU or MOX fuel

Not to be deliberately bounding

HBU result generically applicable to PWRs or BWRs

MOX result specific to Catawba

Not necessary to be the result of a self-consistent accident sequence

Conform as closely as possible to the structure of the NUREG-1465 Source Term

Containment Source Term Development 8

Summary of HBU Source Term

NRC sponsored analysis at Sandia National Laboratories (SNL) to calculate source terms for low burnup and high burnup fuel (SAND2011-0128)

LBU: 26-38 GWD/MTU discharge burnup, which varied depending on the plant analyzed

HBU: 59 GWD/MTU max assembly-averaged discharge burnup (~ 62 GWD/MTU peak rod-averaged burnup)

SNL calculations used MELCOR 1.8.5 for accident progression and ORIGEN for radionuclide and decay heat inventories

Cs diffusion coefficient

LBU based on Phebus FPT (fuel at 23 GWD/MTU)

HBU based on VERCORS test RT-6 (fuel at 72 GWD/MTU)

Sandia National Laboratories, Accident Source Terms for LWR NPPs using High-Burnup or MOX Fuel, SAND2011-0128, 2011 (ADAMS Accession Number ML20093F003)

Evolution of Core Modeling - Major Effect

SAND2011-0128 Overall Findings

Little difference between durations of release phases and release fractions for HBU and LBU, or MOX and LEU

Major differences from NUREG-1465 attributed to improved understanding of

Core degradation modeling

Radionuclide chemistry under accident condition

Fission Product Release Fractions

Substantially greater validation data available for the quantities of noble gases, iodine, cesium, and tellurium released under molten fuel conditions

PHEBUS-FP, VERCORS, VERDON, and others

Experimental evidence indicates that tellurium will not be chemically bound to the Zircaloy cladding, as assumed in NUREG-1150

The peer review committee concludes that the larger release fractions for the tellurium group appear to be warranted.

BWR Results (SAND2011-0128)

Gap Release In-vessel Release Duration 0.16 0.20 0.5 8.0 8.8 1.5 Release Fractions HBU LBU NUREG 1465 HBU LBU NUREG 1465 Noble Gases (Kr, Xe) 0.008 0.008 0.05 0.96 0.96 0.95 Halogens (I) 0.002 0.003 0.05 0.47 0.54 0.25 Alkali Metals (Cs) 0.002 0.003 0.05 0.13 0.14 0.20 Alkaline Earths (Ba) 0.005 0.005 0.02 Tellurium Group 0.002 0.003 0.39 0.39 0.05 Molybdenum 0.02 0.03 0.0025 Noble Metals 0.0027 0.0026 0.0025 Lanthanides 1.4E-7 2.0E-7 2.0E-4 Cerium Group 1.3E-7 1.6E-7 5.0E-4

PWR Results (SAND2011-0128)

Gap Release In-vessel Release Duration 0.22 0.33 0.5 4.5 5.3 1.5 Release Fractions HBU LBU NUREG 1465 HBU LBU NUREG 1465 Noble Gases (Kr, Xe) 0.017 0.022 0.05 0.94 0.85 0.95 Halogens (I) 0.004 0.007 0.05 0.37 0.30 0.35 Alkali Metals (Cs) 0.003 0.005 0.05 0.23 0.23 0.25 Alkaline Earths (Ba) 0.0006 0.0014 0.004 0.004 0.02 Tellurium Group 0.004 0.007 0.30 0.26 0.05 Molybdenum 0.08 0.10 0.0025 Noble Metals 0.006 0.006 0.0025 Lanthanides 1.5E-7 1.1E-7 2.0E-4 Cerium Group 1.5E-7 1.1E-7 5.0E-4

Choice of Sequences

Accidents analyzed included station blackout (SBO) accidents, transients involving the loss of injection or loss of decay heat removal, Anticipated Transient Without Scram (ATWS), and breaks in the reactor coolant system.

When combined, the accidents consider nearly all of the core damage frequency based on the results of Individual Plant Examinations (IPEs) documented in NUREG-1560

Larger fraction of SBO sequences

Representative of greater set of sequences

BWR HBU In-Vessel Halogens Release Fraction LOCA ATWS The rest are SBOs CDF = Cumulative Distribution Function scenario

BWR Dominant Sequences (IPEs)

CDF = Core Damage Frequency Peach Bottom Initiator CDF LOCA 0.44x10-6 10 ATWS 0.32x10-6 8

Loss of offsite power 0.49x10-6 11 Transient 3.02x10-6 70 Coverage = 98% of CDF = 5.34x10-5 Grand Gulf Initiator CDF LOCA 0.38x10-6 2

ATWS 0.05x10-6 0.3 Station Blackout 7.45x10-6 43 Transient 9.34x10-6 54 Coverage = 99% of CDF = 1.72x10-5 SBO Loss of Feed Loss of DHR LOCAs ATWS

Suppression Pool Scrubbing

Consistent with prior regulatory positions, all radionuclides that reach containment are considered part of the source term

Typical licensing/siting source term calculations involve a generic, rather than sequence-specific, source term

Uncertainties in accident sequence

Uncertainties in scrubbing conditions (e.g. pool saturation at time of release)

This approach neglects scrubbing in Suppression Pool (SP)

SAND2011-0128 Peer Review*

The proposed source term satisfies the regulatory requirements of RG 1.183 pertaining to attributes of an acceptable alternative accident source term

Change in release fraction

The changes between the NUREG-1465 source terms result predominantly from advances in modeling and not from differences introduced by using MOX fuel or extensions to the burnup range.

The proposed source terms are more realistic than those provided in NUREG-1465 in terms of the timing, chemical forms, and release fractions; plutonium release fractions may be underestimated, but do not significantly impact the final source term.

Methodology

The representation of variabilities in release quantities and other characteristics associated with the proposed source terms are reasonable. However, there are significant uncertainties in release quantities and species for iodine, which are currently being investigated.

Choice of sequences

The peer review committee finds that this spectrum of accidents is sufficient to satisfy the following stated attributes of an acceptable alternative accident source term: The accident source term must be expressed in terms of times and rates of appearance of radioactive fission products released into containment, the types and quantities of the radioactive species released, and the chemical forms of iodine released.

Tellurium

The peer review committee concludes that the larger release fractions for the tellurium group appear to be warranted.

PEER REVIEW OF ACCIDENT SOURCE TERMS FOR LIGHT-WATER NUCLEAR POWER PLANTS USING HIGH-BURNUP AND MIXED OXIDE FUEL, ERI/NRC 11-211, ML12005A043, December 2011

Questions & Discussion

How did we do?

Link to NRC meeting feedback form:

https://feedback.nrc.gov/pmfs/feedback/form?m eetingcode=20220575

Abbreviations