# Presentation: 275112

## Risk Assessment as a Decision Tool

- Terry Sullivan
- Environmental Research and Technology Division
- Brookhaven National Laboratory
- July 18, 2006

## Role of Risk Assessment	

- Definition:
- Risk = Probability of occurrence * consequence of the event.
- Risk Assessment is a tool
- Rigorous Protocol to
- Manage global and local environmental threats
- Increase Scientific Understanding
- Integrate knowledge from different processes
- Increase communication
- Prioritize Actions

## Steps in Risk Assessment

-  Problem Definition
- Conceptual Model
- Data Collection
- Measures of Risk (move to more holistic risk assessment)
- Human Health
- Ecological
- Economic
- Societal

## Steps in Risk Assessment

-  Analysis
- Quantify Risks
- Address Uncertainties
- Integration
- Fold Risk Assessment information into the decision process
- Communication
- Explain results of risk assessment to the other stakeholders (regulatory agencies and the public)

## Risk Assessment Example –Low-level Radioactive Waste

-   Issue:  Human health impacts through exposure from disposal of radioactive waste
- Are safety measures protective of the public?

## Steps in Human Health Risk Assessment

-  Calculate potential radionuclide releases from disposal site.)
- Using site specific data (rainfall rate,  consumption rates, estimate range of exposure
- Using toxological data estimate consequence of exposure (Dose).
- Calculate risk

## Example:  Low-level disposal safety assessment 

- Problem Definition:  Safety Assessment
- Waste Container and Waste Form Components of a Safety Assessment
- Study Objectives

## Example

- Example Problems on the role of Modeling in Safety Assessment
- Moving from extremely conservative assumptions to more realistic calculations
- Waste Containers
- Distributed Container Failure Time
- Waste Forms
- Role of Diffusion on Controlling Release

## Safety Assessment

- Iterative process focused on regulatory compliance involving site-specific prospective model evaluations of the post-closure phase of waste disposal.
- Key Points:
- Goal is to demonstrate conditions for which compliance can be obtained.  Uncertainty Analysis
- Develop sufficient understanding of system behavior to support decisions.

## Disposal Schematic

- Infiltration

**Schematic diagram of a LLW Disposal Unit**

## Safety Assessment Components

- Multi-disciplinary effort requiring analysis of :
- Infiltration and cover/engineered barrier performance
- Waste container performance
- Waste form performance
- Vadose zone flow and transport
- Groundwater flow and transport
- Biotic transport
- Atmospheric transport
- Exposure Assessment
- Health Impacts (dose/risk)

## Source Term Calculation

## Waste containers

- Prevent water from reaching the wastes and therefore prevents release or radioactivity.
- Many different types (carbon steel, cement, high-density polyethylene, stainless steel)
- Different failure times
- First iteration often assumes all containers fail at the same time.

## Illustrative ExampleDistributed Container Failures

- Calculated using the DUST-MS computer code
- Comparison of three container failure models.
- Instant failure at 115 years after emplacement.
- Gaussian failure mean failure time 115 years, standard deviation 25 years
- Uniform failure from 65 - 165 years.
- Decay chain with Species A half-life = 44.7 years, Species B half-life = 77,000 years.
- Wasteform release controlled by diffusion (D=1E-8 cm2/s).  Release rate decreases monotonically.  Release continues for several hundred years.

## Species A Cumulative Fractional Release

## Species B Cumulative Fractional Release

## Species A Concentration

## Species B Concentration

## Lessons Learned from Example

- If regulatory compliance limit is total mass released, distributed failure is unimportant.  No need to collect data on distribution of failures.  Only need mean failure time.
- If regulatory compliance limit is peak dose, distributed failure of containers is important.  In the example, almost an order of magnitude lower peak concentration for distributed failure.
- Caution:  Need site-specific analysis.  Results depend on many interactive parameters (failure distribution, half-life, waste form release rates, etc.)

## Waste Form Performance

- Barrier to release
- Waste Form Types
- Cement
- Bitumen
- Glass
- Untreated (resins, lab trash, etc.)
- Polyethylene
- Paraffin
- Impacts of Multiple waste form types on system performance (unique release models for each waste form)

## Waste Form Release Models

- Instant Release upon contact with water (rinse)
- Partitioning between waste form, water, and surrounding media (soil)
- Untreated wastes
- Diffusion controlled release
- Solidified wastes (concrete, bitumen, polyethylene, etc.)
- Dissolution controlled release
- Solid wastes (glass, activated  metals)
- Solubility limited release
- Primarily actinides

## Illustrative ExampleEffect of Diffusion Coefficient

- Calculated using the DUST-MS computer code
- Diffusion release form a 200 liter waste form.  Radius = 28 cm, Height = 81.2 cm.
- Comparison of release from three different waste forms.
- D=1E-10 m2/s.   Leach Index of 6
- D=1E-12 m2/s.   Leach Index of 8
- D=1E-14 m2/s.   Leach Index of 10

## Effect of Diffusion Coefficient on Release

## Lessons Learned from Examples

- Diffusion may substantially limit release.  Need to examine effects of release on dose and then decide if data collection is warranted.
- Main Point:  By doing the modeling we provide a technically defensible rationale for data collection.

## Exposure Pathways

**Exposure Pathways**

## Concentration Example – H-3 in Well Water

**Concentration Example – ****H-3 in Well Water**

## Dose Example – H-3 in Drinking Water

## Example – H-3 Cancer risk

## Conclusions

- Risk assessment is a complex multi-disciplined process.
- In Radioactive Waste Disposal, the Waste package (container and waste form) performance is central to a safety assessment.
- Modeling can be used to:
- Support demonstration of compliance with regulatory limits
- Define data and modeling needs
- Examples focused on definition of data and modeling needs to improve safety assessment  (Container Failure and Waste Form Release).

## Conclusions

- As modeling complexity increases, data needs increase as well as sources of uncertainty.  Keep modeling as simple as possible while still meeting performance objectives.
- Keep the End Goal in Mind for modeling and data collection.