ML25202A192

Assessment of Radiological Dispersal Device (Rdd) Planning Basis, Revision 0

ML25202A192
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Issue date:	07/31/2025
From:	Keith Compton, Salman Haq, Amy Sharp, Shockley S NRC/RES/DSA/AAB
To:	Cindy Rosales-Cooper Office of Nuclear Security and Incident Response
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ASSESSMENT OF RADIOLOGICAL DISPERSAL DEVICE (RDD) PLANNING BASIS Revision 0 July 2025 Keith L. Compton, Steven Shockley, and Salman Haq Division of Systems Analysis Office of Nuclear Regulatory Research United States Nuclear Regulatory Commission

i TABLE OF CONTENTS Table of Contents...........................................................................................................................i List of Tables.................................................................................................................................ii List of Figures................................................................................................................................ii 1

INTRODUCTION.....................................................................................................................1 2

BACKGROUND.......................................................................................................................2 3

ASSESSMENT........................................................................................................................5 3.1 Methodology...................................................................................................................8 3.2 Results............................................................................................................................9 4

SUMMARY

AND CONCLUSIONS........................................................................................12 5

REFERENCES......................................................................................................................13

ii LIST OF TABLES Table 1: Summary of source terms (source term characteristics).................................................6 Table 2: Summary of source terms (release fractions).................................................................6 Table 3: Base case dose criterion (25 rem ICRP60ED in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, all exposure pathways).....10 Table 4: Scaled activity released (Ci), base case dose criterion.................................................10 Table 5: Alternate dose criterion (5 rem ICRP60ED in 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, groundshine only)....................11 Table 6: Scaled activity released (Ci), alternate dose criterion...................................................11 LIST OF FIGURES Figure 1: Time-dependence of release for source terms evaluated 7

Figure 2: Relative chemical group release fractions for source terms evaluated 8

1 1

INTRODUCTION Regulatory Guide (RG) 1.242 provides methods acceptable to the NRC for meeting emergency planning (EP) requirements under 10 CFR 50.160 and 50.33(g). RG 1.242 was written to be risk-informed and technology inclusive and provides a framework for scoping the planning effort as opposed to defined methodologies. With the interest in microreactors (ADVANCE Act) and certain small modular reactors (SMRs) and advanced reactors, there are opportunities to provide refined guidance specific to certain bounding assumptions. Specifically, applicants and staff would benefit from guidance on preparedness and response to security-related events, particularly when security events are not bounded by the safety case.

Emergency planning is informed by considering a spectrum of accidents to scope the planning efforts for the distance, time, and materials released. Specific plans are not developed for each accident scenario, rather, capabilities are developed and maintained to be able to respond to a variety of events, including security-related events. The consequences of security-related events are often assumed to be bounded by other internal and external events. However, this assumption may not always hold true. When security is not bounded by safety, there is no specific guidance on how to consider such events in the planning, including the emergency planning zone (EPZ) determination. As such, additional guidance is needed to provide acceptable methods for meeting EP requirements. The Part 53 proposed rule includes a request for public comment on methods to address security in EP and EPZ determinations, and there are options available to consider now for Part 50 and Part 52 applicants.

In addition, Section 206 of the ADVANCE Act directs the NRC to consider existing emergency preparedness organizations and planning. FEMA has developed science-based response planning guidance for State, Local, Tribal, and Territorial governments to prepare for RDD emergencies; this includes, Planning Guidance for Responding to and Recovering from Radiological Dispersal Device (RDD) Incidents [1] and "RDD Response Guidance: Planning for the First 100 Minutes [2]. Portions of this guidance may be applicable in preparing for and responding to a security event at a microreactor or facilities with similar potential consequences.

Research is needed to assess the planning basis for RDD guidance against potential consequences of security events at microreactor facilities and facilities for which the design is likely to support a near-site, site-boundary, or no-EPZ determination.

In assistance request NSIR-2025-004-IAR [3], the Office of Nuclear Security and Incident Response (NSIR) requested that the Office of Nuclear Regulatory Research (RES) identify and review published consequence analyses, including LWR, non-LWR, and RDD planning-basis consequence analyses, focusing on the range at which dose levels used to develop the RDD planning basis could be exceeded, and to identify a range of core thermal powers and source terms analogous to RDD planning basis accidents.

2 2

BACKGROUND In its guidance for the immediate response to an RDD incident, Tactic 4 (Issue Protective Actions to the Public) of [2] recommends an initial shelter-in-place region extending 500 meters in all directions from the point of release, with the area within 250 meters from the point of release being used to define the safety perimeter for the initial hot zone. According to Tactic 7 (Secure and Manage the Scene) of [2], this safety perimeter will protect the public and responders who are not involved in lifesaving rescue operations from the potential of acute internal or external exposure. Once radiological measurements are available, the Hot Zone perimeter should be re-established at contamination levels that exceed 10 mR/hr (0.1 mGy/hr) or 60,000 disintegrations per minute (dpm) / cm2 at 1.5 cm (~0.5 inch) above the ground for beta and gamma, or 6,000 dpm/cm2 at 0.5 cm (~0.25 inch) above the ground with an alpha probe. Reference [2] goes on to state Until radiological measurements are available, enforce the initial Shelter-in-Place Zone set at 500 m (~1600 ft) in all directions from the point of detonation. When the direction of the contamination is confirmed by radiological measurements, extend the Shelter-in-Place Zone out to 2000 m (~1.2 miles) in the direction of the contamination to protect the public from low-level contamination and external radiation from fallout on the ground. According to Annex 9 of [2], these recommendations are based on Harper et al. (2007) [4], Musolino and Harper (2006) [5], and Musolino et al. (2013) [6].

Reference [6] was published to provide definitions of the initial Hot Zone that provide an initial conservative stand-off distance from an incident that does not necessitate having specific radiation measurements. This concept is similar to the existing guidance offered by the Emergency Response Guidebook for transportation incidents. Once radiation measurements are used to define the Hot Zone, this guidance adopts the National Council of Radiation Protection and Measurements (NCRP) Hot Zone boundary definition of 1,000 Bq cm-2 (60,000 dpm cm-2) for beta-and gamma-ray surface contamination; it is 100 Bq cm-2 (6,000 dpm cm-2) for alpha surface contamination, or less than 0.1 mGy h-1 (10 mR h-1) measured at 1 m from the ground (NCRP 2010).. The NCRP describes the hot zone as follows [7]: NCRP also adopts the American Society for Testing and Materials and National Fire Protection Association terminology of the hot zone, which is defined as the zone immediately surrounding a HAZMAT incident that extends far enough to minimize deterministic effects and reduce the risk of stochastic effects from the HAZMAT to personnel outside the zone and is demarcated by the hot line. and is recommended to be established as where any of the following exposure rate or surface contamination levels is exceeded:

• 10 mR h-1 exposure rate (~0.1 mGy h-1 air-kerma rate);

• 60,000 dpm cm-2 (1,000 Bq cm-2) for beta and gamma surface contamination; and

• 6,000 dpm cm-2 (100 Bq cm-2) for alpha surface contamination [7].

Reference [6] states that The initial recommendation was 500 m for setting a hazard boundary to control the potential for acute radiation exposure, assuming there was no knowledge about the device design nor any coherent radiation measurements (Harper et al 2007; Musolino and Harper 2006). Based on results of experiments conducted after 2004, this recommendation is

3 lowered to 250 m. These recent experiments afford higher confidence that debris from radioactive ballistic fragments would fall within 250 m of the explosion..

Review of Harper et al. 2007 [4] and Musolino and Harper 2006 [5] suggest that the 500 m initial hot zone distance is based on Table 3 of [5], reproduced from Table 4 of reference [4]. Musolino and Harper [5] clarify that While the guidance discussed in this paper is appropriate for the probable effects of an explosive RDD, it recognizes that there are some scenarios that exceed the hazard boundaries assumed in this paper, but are much less likely to occur, i.e., RDD geometries that are of sophisticated engineering with a very large source as defined by Harper et al. (2006). Therefore, it appears that the basis for the recommendation of an initial 500 meter hot zone is consistent the column in Table 4 of reference [4] reflecting a Very large size source 7.4 x106 GBq (200,000 Ci), basic engineering. This is consistent with the statement in reference [4] that The area of highest concern is limited to the area within 500 m of the release in the more probable scenarios. The area of highest concern is defined as the area in which acute effects, a lifetime inhalation dose of 1 Sv (100 rem), or a 50 mSv (5 rem) groundshine dose (5-h exposure) might occur. This implies that the initial response should be to set up a Hot Zone within 500 m (if nothing is known about the release) or at 0.01 Gy h-1 (1 rad h-1) if exposure rate measurements are available. There are two dose levels in Table 4 of [4] that seem to support this observation: a groundshine dose of 1 Gy (100 rad), 24-h exposure assumed, with approximately a 300-meter exceedance range, and a 50 mSv (5 rem) groundshine dose (5-h exposure assumed) with a 600-meter exceedance range. Assuming that dose rates are approximately constant over a 24-hour period (i.e., no significant contribution from short-lived radioactivity),

dose rates decrease at a rate proportional to r1/n, where r is the downwind distance (i.e. the Gaussian plume equation is a reasonable approximation for short range dispersion), and n 1.5 (i.e., the dispersion parameters are consistent with stable, low dispersion atmosphere),

it can be estimated that both dose exceedance distances correspond to a groundshine dose rate on the order of 1-2 rad/hr at 500 meters. This dose rate is consistent with the recommendation in [4] to, inter alia, base the initial hot zone distance at 0.01 Gy h-1 (1 rad h-1) if exposure rate measurements are available.

It may be noted that while reference [4] appears to base the hot zone distance at 0.01 Gy h-1 (1 rad h-1) if exposure rate measurements are available, the more recent reference [6] bases the hot zone distance in part on the NCRP 2010 [7] recommendation that a hot zone distance be based on a dose rate of 0.1 mGy h-1 (10 mR h-1) measured at 1 m from the ground once radiation measurements can be used. Assuming that dose rates are relatively constant, the dose rate of 1 rad/hr cited in reference [4] would yield a dose of 24 rad in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and 96 rad in four days, which represents a dose and dose rate where deterministic effects may be induced.

However, the 10 mR/hr dose rate used in the NCRP recommendation would yield a dose of 0.24 R in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and 0.96 R in 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />, which is in the very low to low dose range and is comparable to the lower EPA early phase protective action guideline of 1-5 rem in 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />. In keeping with the goal of identifying a zone in which acute effects might occur, the higher dose rate of 1 rad/hr cited in [4] is used in the subsequent analyses in the current report to identify a

4 range of core thermal powers and source terms analogous to RDD planning basis accidents.

Furthermore, it may be noted that reference [4] focuses on the groundshine exposure pathway, stating The inhalation hazard boundaries for the selected dose levels were minimal for the basic engineering scenarios (including the large source scenarios). There could be localized areas with high concentrations of respirable aerosol during plume passage (models tend to under-predict the maximums and over-predict the minimums). However, for the basic engineering cases, any significant inhalation dose would come from plume passage within a few hundred meters of the release. In this area, the plume would probably arrive within 10 minutes (and would be gone by the time most of the early first responders arrive). Because a reactor source term may be more prolonged than an RDD source term arising from an explosive event, inhalation exposures may persist longer. It therefore seems reasonable for purposes of this analysis to use a dose from all pathways (cloudshine and inhalation from a passing plume and groundshine and resuspension from deposited material). Finally, because consequence analysis codes like MACCS generally do not estimate time-dependent dose rates as a function of distanceand because reactor-derived source terms may be more likely to contain short-lived radioactivity, potentially resulting in a much more time-dependent dose ratea dose criterion of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> from all pathways is recommended as a surrogate for the approximately 1 rad/hr dose rate that appears consistent with the dose levels of concern in developing, in part, the RDD planning basis.

5 3

ASSESSMENT Staff reanalyzed one of the cases developed for the AERI sensitivity analyses [8] to inform a discussion to identify a range of core thermal powers and source terms analogous to RDD planning basis accidents. It should be noted that unlike RDD incidents, the source terms of which are assumed to arise from explosive aerosolization of a radiological source consisting of only a few isotopes [4], releases from reactor accidents are generally modeled as arising from thermally-induced fuel failures resulting in fuel melt and vaporization of many isotopes including fission products, activation products, and actinides. In contrast to the rapid release from an RDD, releases from reactor accidents can occur over a prolonged period. However, security-related events may differ in both timing and magnitude from other internal and external events.

To examine the impact of these differences, the source terms selected for evaluation in Reference [8] are also used in this assessment. These source terms are based on a selection of those developed for NRC site Level 3 PRA [9]. Although these source terms are based on analyses of reactor risk for a large pressurized water reactor, they are selected to illustrate the complexity of potential realistic source terms. These three source terms represent a variety of source term characteristics such as timing of the onset of the release, the release duration and the relative isotopic composition. The summary of the characteristics of these source terms that follows is adapted from Section 2.2.4 of Reference [8].

The base case source term is based on MELCOR Case 5D, which represents an interfacing systems loss of coolant accident (ISLOCA) with no scrubbing and failure of the auxiliary building (V-F). This source term is a relatively fast and large source term, with over 80% of the total iodine, tellurium, and cesium release occurring within one hour of the onset of release.

However, a residual tail of releases continues for several days after the initial release. It therefore represents a pulse type release, with little opportunity for wind shifts over the course of the majority of release. As a sensitivity case, an alternate source term is based on MELCOR Case 1B, which represents a late containment failure (LCF) in which containment fails tens of hours after the time of vessel breach due to long-term quasi-static overpressure. This is a very prolonged source term, with releases occurring over several days. It therefore represents a prolonged release, with ample opportunity for wind shifts over the course of most of the release.

A final sensitivity case source term is based on MELCOR Case 2R1, which represents a smaller release because containment is not bypassed or failed and radiological release to the environment occurs via design-basis containment leakage only (NOCF). This source term is notable in that the release is more predominately associated with noble gases rather than radiocesium or radioiodine. The release is relatively prolonged, with the major period of release occurring over a period of several days after the onset of release. Tables 1 and 2 provide a brief tabular summary of the three source terms.

6 Table 1: Summary of source terms (source term characteristics)

RC Case Release Category Description NUMREL PDELAY (hr)

PLUDUR (50%)

(hr)1 PLUDUR (100%)

(hr)2 PLHITE (m)

PLHEAT (MW)

VF 5D Release occurs from the reactor coolant system to the auxiliary building via interfacing systems loss-of-coolant accident. The break point is not submerged. The auxiliary building fails.

86 3.2

<1 hr 69 11 19 LCF 1B Containment fails tens of hours after the time of vessel breach due to long-term quasi-static overpressure. Releases to the environment are not mitigated significantly by sprays or water pools 179 48 103 120 0.36 5.9 NOCF 2R1 Containment is not bypassed or failed, and radiological release to the environment occurs via design-basis containment leakage only.

This release may or may not benefit from any aerosol scrubbing.

199 13 26 155 32 0.0026 Source: reproduced from Table 3 of [8], which was adapted from Tables 3.1-1 and A.1a in [9]

1.

Based on the average of the time since the initial release needed to reach half of the maximum release of the Cs, I, and Te chemical groups 2.

Based on the average of the time since the initial release needed to reach the maximum release of the Cs, I, and Te chemical groups Table 2: Summary of source terms (release fractions)

RC Case Xe Cs Ba I

Te Ru Mo Ce La VF 5D 8.6E-01 1.3E-01 2.1E-03 1.4E-01 1.3E-01 2.6E-03 3.3E-02 9.3E-05 2.7E-06 LCF 1B 9.1E-01 9.9E-03 3.0E-04 1.2E-02 1.1E-02 6.6E-06 4.0E-02 1.4E-06 5.8E-07 NOCF 2R1 1.0E-02 7.4E-05 2.4E-06 8.5E-05 7.9E-05 3.7E-06 2.0E-04 2.3E-08 2.0E-08 Source: reproduced from Table 3 of [8], which was adapted from Table A.1a in [9]

7 Table 1 shows the number of plume segments (NUMREL), the time at which the first plume segment is released (PDELAY), the time at which the plume reaches 50% and 100% of its cumulative release (PLUDUR), the height from which most of the material is released (PLHITE),

and the weighted average1 plume segment heat content (PLHEAT). The VF source term begins early (3.2 hrs) and releases most of its material within 4.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, whereas the LCF and NOCF source terms are delayed by 48 and 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> respectively and release most of their material over a period of more than a day. This is more clearly seen in Figure 1, which shows the time-dependence of the cesium release fraction for the three source terms. These curves are normalized to the maximum cumulative release fraction shown in Table 2 to highlight the time-dependence of the release. The VF source term is relatively early (less than a few hours) and rises relatively quickly over a period of several hours, suggesting a limited time available for wind shifts during most of the release. In contrast, the LCF source term is a much more gradual release, starting after a few days and gradually increasing over a period of several days. This suggests a high likelihood of wind shifts over the source of the release.

Figure 1: Time-dependence of release for source terms evaluated Source: reproduced from Figure 6 of [8]

Table 2 provides the cumulative release fractions over the course of the accident. Figure 7 shows the chemical group release fractions relative to the noble gas (Xe) release fraction for all 1 The weighted average plume segment heat content is obtained by weighting each plume segment by its relative contribution to the total iodine release. Other weighting methods could produce different results.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 24 48 72 96 120 144 168 V-F LCF NOCF Fraction of Maximum Cs Release Time since accident initiation (hr)

8 source terms. The values are normalized to the Xe group release fraction for each source term to highlight the differences in the relative releases of different chemical groups. While the relative release of the Cs, I, and Te source terms is similar across source terms, the relative release of noble gases generally is higher for the LCF and NOCF source terms and the relative release of the Ru, Mo, Ce, and La chemical groups varies across source terms.

Figure 2: Relative chemical group release fractions for source terms evaluated Source: reproduced from Figure 7 of [8]

3.1 Methodology For this assessment, Case 8 of Reference [8] was adapted to evaluate the amount of core scaling needed to yield doses of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Case 8 of Reference [8] was designed to examine the effect of meteorological variability coupled with a prolonged release during which wind direction can vary. Because the effect of release prolongation differs between source terms, three source termsVF (5D) / LCF (1B) / NOCF (2R1)were modeled. These source terms were modeled with hourly plume segments. The SOARCA Peach Bottom meteorological file was used. The spatial grid was modified to allow a dose to be computed at 100 meters (at the midpoint of an interval from 50 to 150 meters) and at 500 meters (at the midpoint of an interval from 450 meters to 550 meters). The plume was modeled as a point source using an initial plume dimension of 0.1 m. A ground level, non-buoyant release was modeled by setting plume heat content to 0 W and plume release height to 0 m. An exposure duration of 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> from the arrival of the first plume segment was modeled by setting the MACCS parameter ENDEMP to a value of 86400 seconds (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) with hotspot and normal relocation disabled 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 Xe Cs Ba I

Te Ru Mo Ce La V-F LCF NOCF Release Fraction (relative to Xe Chemical Group)

Chemical Group

9 by setting TIMHOT and TIMNRM equal to ENDEMP and setting DOSHOT and DOSNRM to their maximum values. MACCS Type A (peak dose) values were generated for both the L-ICRP60ED and A-RED MARR dosimetric quantities.

As discussed in the previous section, a dose criterion of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> was selected as a surrogate for the approximately 1 rad/hr dose rate identified as consistent with the dose levels of concern in developing the RDD planning basis. Because the initial core inventory is based on a large light water reactor with a thermal power of 2,546 MWt (see Appendix B of [10]), the release is scaled using the MACCS core scaling factor CORSCA to result in a maximum early phase dose of 25 rem at 500 meters. This scaling was done in an iterative fashion, with an initial core scaling factor of 0.02 assumed and then subsequently adjusted by the ratio of the computed maximum early phase dose to the 25 rem dose criteria. The resulting scaled core power levels are assumed to inform an identification of the range of core thermal powers and source terms analogous to RDD planning basis accidents.

As a sensitivity, an alternate dose criterion that more closely aligns with the criterion of 5 rem in 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> from groundshine only was also modeled. Because MACCS 4.2 does not allow a value for ENDEMP to be set to less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, the exposure period was modeled by setting DOSHOT and DOSNRM to their minimum values (0 seconds) and the values of TIMHOT and TIMNRM were set to 18000 seconds (5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />). This ensures relocation would occur 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after plume arrival at any non-zero dose level. The cloudshine, inhalation, and skin deposition exposure pathways were eliminated by setting the MACCS shielding parameters CSFACT, PROTIN, and SKPFAC to 0, thereby eliminating exposure from these pathways. As in the base case, the groundshine shielding factor was set to 0.82, reflecting groundshine exposure associated with an irregular outdoor surface (as opposed to groundshine exposure from a flat infinite plane, as reflected in the groundshine dose coefficient).

In both cases, the released inventory was obtained from the MACCS tbl_outVector.txt file by importing the file into Excel, parsing the data into columns, and constructing a pivot table.

Because MACCS reports the released inventory for all modeled isotopes, the values for Cs-137, I-131, and the total across all isotopes were selected for tabulation. The release inventory within both the exposure period (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> in the base case and 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> in the alternate case) and over the course of the release was tabulated.

3.2 Results Table 3 shows the core scaling factor needed to yield a dose (L-ICRP60ED) of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters. The equivalent thermal power is obtained simply by multiplying the thermal power of 2,546 MWt, reflected in the core inventory CORINV by the core scaling factor CORSCA. Because the L-ICRP60ED dosimetric quantity reflects a dose commitment period of 50 years from inhalation exposures, the acute dose to the red bone marrow (A-RED MARR),

which is used by MACCS to estimate deterministic health effects, resulting from that core scaling factor is also provided. An approximation of the scaled thermal power needed to yield a dose (A-RED MARR) of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters is also provided by scaling the

10 thermal power by both CORSCA and the ratio of the peak A-RED MARR dose to the peak L-ICRP60ED dose.

Table 3: Base case dose criterion (25 rem ICRP60ED in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, all exposure pathways)

Source Term CORSCA PEAK DOSE (Sv),

L-ICRP60ED PEAK DOSE (Sv),

A-RED MARR Scaled Thermal Power (MWt) based on 25 rem L-ICRP60 ED Scaled Thermal Power (MWt) based on 25 rem A-RED MARR 5D 3.09E-04 2.50E-01 5.24E-02 0.79 3.75 1B 8.50E-01 2.50E-01 7.49E-02 2,164 7,223 2R1 3.25E+00 2.50E-01 4.21E-02 8,275 49,136 It can be seen from Table 3 that even a very small inventory (and commensurately, low thermal power/burnup) would yield doses greater than 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters if the source term is assumed to release a large fraction (on the order of 10-20% of the inventory) of volatile elements such as cesium and iodine within the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> exposure period. Even if the dose is limited to the acute dose (i.e., the A-RED MARR dose), the inventory from a core rated at only a few MWt could yield doses above the criterion for an aggressive release. In contrast, if the source term is more akin to the lower, delayed, and prolonged releases exhibited in a late containment failure (1B), even a fairly large inventory (in this case, approximately 85% of the Scaled Thermal Power) would not give rise to doses of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters. If the source term was more akin to the contained release exhibited in source term 2R1, even a very large core inventorylarger than that of a large light water reactorwould not give rise to doses of 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters.

Table 4: Scaled activity released (Ci), base case dose criterion Inventory released within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of initial release Total (entire release duration)

Source Term Cs-137 I-131 Total Cs-137 I-131 Total 5D 3.19E+02 3.16E+03 6.45E+04 3.20E+02 3.17E+03 8.57E+04 1B 3.72E+02 4.98E+03 3.33E+05 6.92E+04 4.89E+05 9.24E+07 2R1 1.51E+03 7.30E+03 2.59E+05 1.98E+03 1.76E+04 3.49E+06 Table 4 illustrates the magnitude of the release from the scaled inventory for both the 24-hour exposure period as well as the total amount released over the course of the accident. It can be seen, consistent with Figure 1, that source term 5D releases most of its activity during the exposure period. In contrast, source term 1B releases only a small fractionless than one percentof its activity during the exposure period. It may also be noted that while the total scaled inventory released varies greatly (over three orders of magnitude) across the three source terms, the total activity released during the exposure varies much less (by only a factor of 5) and is comparable to the magnitude of the Very large size source size of 200,000 Ci described in Table 4 of [4].

11 The previous results from the base case suggest that even a very small core inventory is capable of yielding doses below 1 R/hr at 500 meters if the source terms release a significant fraction of that inventory. To verify that this observation holds for a different implementation of the 1R/hr dose criterion, Table 5 shows the core scaling factor needed to yield a dose (L-ICRP60ED) of 5 rem in 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> at 500 meters from groundshine only.

Table 5: Alternate dose criterion (5 rem ICRP60ED in 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, groundshine only)

Source Term CORSCA PEAK DOSE (Sv),

L-ICRP60ED PEAK DOSE (Sv),

A-RED MARR Scaled Thermal Power (MWt) based on 5 rem L-ICRP60 ED Scaled Thermal Power (MWt) based on 5 rem A-RED MARR 5D 1.47E-03 5.00E-02 4.88E-02 3.74 3.83 1B 5.49E+00 5.00E-02 4.84E-02 13,978 14,440 2R1 3.48E+02 5.00E-02 4.88E-02 886,008 907,795 Similar to the results obtained using the base case criteria, it can be seen from Table 5 that even a very low inventory (and commensurately, low thermal power/burnup) would yield doses greater than 25 rem in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 500 meters if the source term is assumed to release a large fraction of volatile elements such as cesium and iodine within the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> exposure period.

Because the dose is limited to the groundshine dose, which does not involve a prolonged exposure to inhaled activity after plume passage, the scaled power does not differ significantly whether the L-ICRP60 ED or the A-RED MARR dosimetric quantity is used to scale the core power. The similarity between the scaled core power from the alternate groundshine-only dose criterion and the base case all-pathways dose criterion support the observation that the majority of the L-ICRP60ED dose arises from inhalation during plume passage.

Table 6: Scaled activity released (Ci), alternate dose criterion Inventory released within 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of initial release Total (entire release duration)

Source Term Cs-137 I-131 Total Cs-137 I-131 Total 5D 1.52E+03 1.51E+04 3.07E+05 1.52E+03 1.51E+04 4.08E+05 1B 2.42E+03 3.30E+04 2.43E+06 4.47E+05 3.16E+06 5.97E+08 2R1 2.77E+03 2.17E+04 6.47E+05 2.12E+05 1.89E+06 3.74E+08 Table 5 illustrates the magnitude of the release from the scaled inventory for both the 5-hour exposure period as well as the total amount released over the course of the accident. It can be seen, consistent with Figure 1, that source term 5D releases most of its activity during the brief 5-hour exposure period. The total amount released within 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of initial release is higher than the value shown in Table 4 for the amount released within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of initial release, showing that limiting the dose to groundshine exposure only results in a larger release (by a factor of 2-7) being needed to yield a dose rate comparable to the 1 R/hr. This observation is consistent with the observation that most of the effective dose represented by the L-ICRP60ED dosimetric measure arises from inhalation during plume passage.

12 4

SUMMARY

AND CONCLUSIONS The objective of the work was to identify a range of core thermal powers and source terms analogous to RDD planning basis accidents. Given the assumption that the RDD planning basis is consistent with a dose rate on the order of 1 R/hr at 500 meters following plume passage, it can be seen that a reactor accident source term involving the rapid release (on the order of an hour) of a large fraction (on the order of 10-20%) of the volatile fission products requires a core inventory corresponding to a very low (on the order of a few MWt) core power. On the other hand, if the reactor accident source term is very delayed and prolonged (e.g., akin to a late containment failure at a large pressurized water reactor) or very small, delayed, and prolonged (e.g., akin to the release from a core melt accident with an intact containment), even a large reactor with a thermal power on the order of 1000 MWt or more may not yield dose rates in the first hours after the onset of release comparable to the 1 R/hr dose rate consistent with the RDD planning basis initial hot zone distance of 500 meters.

A driver for this work was, in part, to evaluate the assumption that the consequences of security-related events are bounded by other internal and external events. It can be seen from the results in Section 3 that if the accident source term is comparable to those of an ISLOCA at a large light water reactori.e., a rapid (on the order of an hour) release of a large fraction of the core inventory (on the order of 10-20% of the inventory of volatile elements such as cesium and iodine)the consequences of a security event comparable to that used for the RDD planning basis may be bounded by those from other internal and external events if the reactor power is larger than a few MWt. In contrast, if the accident source terms are more comparable to a late containment failure or a release from an intact containment, even releases from a large (>1000 MWt) reactor may not bound the releases from a security event comparable to that used for the RDD planning basis.

13 5

REFERENCES

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ML25202A179; Memo ML25202A192 OFFICE RES/DSA/AAB RES/DSA/AAB RES/DSA/AAB RES/DSA/AAB NAME SShockley SHaq KCompton ASharp DATE Jul 21, 2025 Jul 24, 2025 Jul 23, 2025 Aug 5, 2025