DEVELOPMENT OF RADIOLOGICAL SOURCE TERMS FOR NON-REACTOR ACCIDENTS

Dale M. Petroff and Wendy C. Swygert

Washington Safety Management Solutions

DEVELOPMENT OF RADIOLOGICAL SOURCE TERMS FOR NON-REACTOR ACCIDENTS

Dale M. Petroff and Wendy C. Swygert

Washington Safety Management Solutions

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Within the DOE complex, there are a large number of non-reactor facilities that process or experiment with radioactive material and a number of facilities are undergoing D&D operations.

This has required the development of techniques that can estimate source terms for accidents involving these facilities based on an analysis of the event, the method of confinement, and the material involved.

Within the DOE complex, there are a large number of non-reactor facilities that process or experiment with radioactive material and a number of facilities are undergoing D&D operations.

This has required the development of techniques that can estimate source terms for accidents involving these facilities based on an analysis of the event, the method of confinement, and the material involved.

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A methodology was developed to determine radiological source terms for accidents involving:

Decontamination and Decommissioning (D&D) operations

Natural Phenomena (earthquake, tornadoes) Radioactive Waste Fires Transportation of radioactive material

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The methodology is based on the guidance in DOE Handbook 3010-94 Airborne Release Fractions/Rates and Respirable Fractions for Nonreactor Nuclear Facilities

(This handbook can be downloaded as pdf file at http://www.directives.doe.gov/)

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The handbook, provides guidance on the selection of applicable Airborne Release Fractions (ARFs) and Respirable Fractions (RFs) associated with various materials and accident initiators. The handbook is organized by types of materials.

Materials in the Gaseous State Liquids Solids Surface Contamination Inadvertent Nuclear Criticality Application Examples

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The handbook uses a multi-component formula to develop source terms.

ST =(MAR)(DR)(ARF)(RF)(LPF)

The various terms are defined as follows:ST = Source TermMAR = Material at Risk (inventory)DR = Damage Ratio (fraction)ARF = Airborne Release FractionRF = Respirable FractionLPF = Leak Path Factor (fraction)

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Material at Risk or inventory is the total amount of radioactive material that could be released by the initiator. If the initiator can not affect release of the material due to it’s physical form or storage configuration then the that material is removed is consideration. Such as:

Material in a fire proof container would be released by a fire.

Material in substantial solid form such as ingots would not be affected by impact or being dropped.

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The damage ratio is based on an assessment of the physical form of the radioactive material, how is it is contained, location, and how the initiating event breeches the containment. Such as:

Vehicle crash would impact 25 drums of 250 drums stored in a staging area for a DR of 0.1

The event is localized and would not impact a full facility inventory such as a room fire vs. a facility fire.

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Airborne Release Fraction and the Respirable Fraction are material and event specific.

Spill of radioactive material in Solution:

Free Fall Spills Heavy Metal Solutions. For a 3 meter spill height of concentrated heavy metal solution an ARF 2E–05/RF 1.0 is indicated.

Fire involving radioactive Waste:

Contaminated combustible materials heated/burned in packages with largely non-contaminated exterior surfaces (e.g., packaged in bags, compact piles, pails, drums), an ARF 5E-4/RF 1.0 is indicated.

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The leak path factor is based on factors that may filter or reduce the release. This includes filtered ventilation or building holdup. Example leak path factors are:

99.9% HEPA Filter = (1-0.999) = 1.0E-03

Building up hold up factor = 0.1

Transportation accident = 1.0

Filters in train are not additive as the first filter removes most of the filterable material.

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Example Scenarios

Waste Tank Spill/Breach Transportation Accident Building Fire Process Explosion

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Spill of one full waste processing tank in a building – ventilation functioning

Failure of the Primary Barriers: From an operational accident standpoint, it is reasonable to assume that the maximum spill would be the volume of one tank. This event is based on the physical failure of a tank or operator error that spills the contents.

Effects of Other Barriers/Mitigative Features: The cells provide a secondary confinement for the waste tanks. The primary mitigating feature for release is the ventilation exhaust system and associated HEPA filtration. Operational events (i.e., pipe failure, overflow, etc.) will not fail the secondary barrier or mitigative features. The HEPA filter has an efficiency of 99.9%.

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Range of Possible Releases: As the release height is less than 10 m, the event is analyzed as a ground level release. The inventory of a single tank is 3.06E+00 Ci Cs-137. As this is a single tank failure, a DR of 1.0 is selected. A median ARF*RF of 3.00E–07 (1E–6/RF 0.3) was selected from the DOE 3010, Summary Free Fall Spills Heavy Metal Solutions.” The median value was selected to compensate for the other conservatisms in the analysis. The LPF for the HEPA filter is 1.00E–03.

MAR Cs-137 Ci DR ARF*RF LPF Source Term3.06E+00 1 3.00E-07 1.00E-03 9.18E-10

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Derailment of UO3 Drums

Failure of the Primary Barrier - A transportation accident (i.e., railcar, truck) results in an airborne release of UO3. The primary barrier preventing the release of UO3 is the drum. Failure of the drums releases the entire contents. The amount of material impacted by the event directly influences the source term and the release rate.

Effects of Other Barriers/Mitigative Features – The drums are banded in groups of four. This protects interior drums.

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Range of Possible Releases

The event causes 3 railcars to be impacted. Each railcar is carrying 100 drums each. The number of drums ruptured per railcar is assumed to be 25 based on the banding of the drums in groups of four, the MAR is 75 drums of waste. There is 1.22E-03 Ci of U-238/drum. Total activity of 9.15E-02 U-238 is available for release.

A per drum DR of 0.5 is assumed reflecting an estimate that 50% of the material remains within the damaged drums due to agglomeration of UO3. The UO3 is in the form of small particles and is assumed to behave as a powder.

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From DOE 3010 for the situation where the material is

packaged in a relatively robust container (e.g., hard pail, drum) that is opened or fails due to impact with the floor or impaction by falling debris (shock-vibration induced by impact), an ARF 1E-3/RF 0.1 is selected

As the drums are in the open an LPF of 1 is assumed.

MAR U-238 Ci DR ARF*RF LPF Source Term9.15E-02 0.5 1.00E-04 1.00E+00 4.58E-06

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Single Room Fire in Laboratory Failure of the Primary Barriers: Combustibles are

ignited inside a room. The heat of the fire causes containers containing Technetium 99m (Tc-99m) to fail. The MAR is limited to the inventory in the room, 2.0 Curies of Tc-99m.

Effects of Other Barriers/Mitigative Features: Fire analyses determine that the room fire does not

spread beyond the room. As the fire is a single room fire, the ventilation remains operable for the facility. The efficiency of the HEPA filtration is 99.9%.

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Range of Possible Releases: A conservative DR of 1.0 is selected to assess the worst case impact of the room fire. The Tc-99m is assumed to be in a powder form. The containers are assumed to be pressurized and failed by the heat of the fire. An ARF*RF of 7.00E-02 (ARF 1E-1/RF 0.7) was selected from 3010 for Venting of pressurized volumes. The LPF for the HEPA filters is 1.00E–03.

MAR Tc-99m Ci DR ARF*RF LPF Source Term

2.00E+00 1 7.00E-02 1.00E+00 1.40E-01

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Mixed Waste Process Explosions

Failure of the Primary Barriers: An explosion occurs inside a storage room caused by a chemical reaction in the waste. The explosion is assumed to breach any container holding the material. The majority of the radioactive inventory is 2.5E-8 Ci of Cs-137 and 5.4E-12 Ci of Co-60.

Effects of Other Barriers/Mitigative Features: The explosion is assumed to damage only those barriers or mitigative features in the immediate vicinity. The ventilation remains operable for the facility. The efficiency of the HEPA filtration is 99.9%.

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Range of Possible Releases: It is conservatively assumed that 100% of the maximum MAR is affected by the explosion for DR of 1.0. The ARF*RF of 2.0E–03 (ARF 5E–3/RF 0.4) was selected based on Explosive Stress for Powders. The low pressure venting, ARF*RF was selected as the drums are sealed with ring device.

Isotope MAR Ci DR ARF*RF LPFSource Term

Cs-137 2.50E-08 1 2.00E-03 1.00E-03 5.00E-14Co-60 5.40E-12 1 2.00E-03 1.00E-03 1.08E-17

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The DOE Handbook 3010 is an excellent source of

information. When detailed analysis of material behavior is available that should be applied.

The key is determining the impact of the initiating event on the primary barrier and then identifying any mitigating features. Sometimes the damage ratio is based on the judgment of the scenario developer when evaluating the overall event.