MELCOR Accident Consequence Code System Research Articles

Integration of Level 3 probabilistic risk assessment for nuclear power plants with transportation simulation considering earthquake hazards

This research improves the realism of Level 3 probabilistic risk assessment (PRA) for nuclear power plants (NPPs) to avoid subjective expert judgement by modeling the evacuation behavior of the residents and making two advancements. First, evacuation speed taken from a transportation simulation code was used as an input to the Level 3 PRA code, i.e., the MELCOR Accident Consequence Code System (MACCS). The transportation simulation code uses the Multiagent Transport Simulation (MATSim) evacuation extension software. MATSim is interfaced with MACCS, by generating input-output communication through evacuation routes and speed distributions. Integrating the agent-based transportation simulation with the Level 3 PRA code enables the explicit incorporation of spatiotemporal evacuation processes into NPP offsite risk assessment. Second, this research is the first to explicitly incorporate the probability of seismic damage to roadway bridges into Level 3 PRA of NPPs. The likelihood of seismic bridge damage, given an earthquake, is estimated using the HAZUS program developed by the Federal Emergency Management Agency (FEMA). The road closure due to seismic damage, estimated by HAZUS, is used as input to the agent-based transportation simulation, where evacuation routes and speed distributions are calculated considering the impact of seismic damage. These advancements can improve the realism of Level 3 PRA for NPPs by enhancing the evacuation analysis resolution and reducing the reliance on subjective judgment to characterize evacuation routes, speed, and seismic impact on Level 3 PRA. Integrating the transportation simulation and the natural hazard risk assessment with the Level 3 PRA code can also provide more in-depth causal insights for offsite risk management, such as recommending optimal evacuation routes in an emergency response plan. These contributions are demonstrated using the Sequoyah NPP case study adopted from the state-of-the-art reactor consequence analyses (SOARCA) conducted by the U.S. Nuclear Regulatory Commission (NRC).

State-of-the-Art Reactor Consequence Analyses Project: Uncertainty Analyses for Station Blackout Scenarios

This paper provides an overview of the U.S. Nuclear Regulatory Commission’s (NRC’s) project to develop a technical report summarizing the most important insights from its three State-of-the-Art Reactor Consequence Analyses (SOARCA) project uncertainty analyses (UAs). The NRC, with Sandia National Laboratories, has completed three UAs as part of the SOARCA project, for three different operating reactor types in three different locations in the United States. The SOARCA UAs included an integrated evaluation of uncertainty in accident progression, radiological release, and off-site health consequence projections. These three UAs are currently documented in three detailed reports. The NRC is currently developing a technical overview report summarizing the important insights from the three SOARCA UAs. The purpose of the NRC summary is to provide a useful reference for regulatory applications that require the evaluation of off-site consequence risk from beyond-design-basis-event severe accidents. Examples include regulatory and cost-benefit analyses that rely on off-site consequence projections using the MELCOR Accident Consequence Code System (MACCS) code, in conjunction with MELCOR for source term characterization. This paper provides an overview and discusses the overall scope and methodology of the SOARCA UAs and the approach for the summary report currently under development.

Performing a multi-unit level-3 PSA with MACCS

MACCS (MELCOR Accident Consequence Code System), WinMACCS, and MelMACCS now facilitate a multi-unit consequence analysis. MACCS evaluates the consequences of an atmospheric release of radioactive gases and aerosols into the atmosphere and is most commonly used to perform probabilistic safety assessments (PSAs) and related consequence analyses for nuclear power plants (NPPs). WinMACCS is a user-friendly preprocessor for MACCS. MelMACCS extracts source-term information from a MELCOR plot file.The current development can combine an arbitrary number of source terms, representing simultaneous releases from a multi-unit facility, into a single consequence analysis. The development supports different release signatures, fission product inventories, and accident initiation times for each unit. The treatment is completely general except that the model is currently limited to collocated units.A major practical consideration for performing a multi-unit PSA is that a comprehensive treatment for more than two units may involve an intractable number of combinations of source terms. This paper proposes and evaluates an approach for reducing the number of calculations to be tractable, even for sites with eight or ten units. The approximation error introduced by the approach is acceptable and is considerably less than other errors and uncertainties inherent in a Level 3 PSA.

An approach for radiological consequence assessment under unified temporal and spatial coordinates considering multi-reactor accidents

The current approaches and computer code systems for radiological consequence assessment of nuclear accidents have still gaps in assessing consequences of nuclear accidents involving multiple reactors, since multi-reactor accidents should be treated in one unique temporal and spatial system. This paper presents an approach to unify and generalize temporal and spatial coordinates for the consequence assessment of multi-reactor accidents. A code system named Advanced Radiological Consequence Assessment Toolkit (ARCAT) was developed accordingly. ARCAT is intended to support in particular emergency preparedness studies at the nuclear power plant level.To verify ARCAT, an analysis was carried out comparing ARCAT with MELCOR Accident Consequence Code System (MACCS), and the Java-based Real-time Online Decision Support System (JRodos). The results indicate that ARCAT is comparable to MACCS. This means that ARCAT is suitable for offsite radiological consequence assessment and able to support emergency preparedness including cliff-edge effects. Further to this, the effects of a unified temporal and spatial coordinate system are discussed to better recognize the impact of such special characteristics of multi-reactor accidents. A preliminary application based on ARCAT is presented, in which the plume emergency planning zone (PEPZ) for a hypothetical site with two reactors is developed. Finally, potential challenges and further research plans are discussed.

GIS-Based Integration of Social Vulnerability and Level 3 Probabilistic Risk Assessment to Advance Emergency Preparedness, Planning, and Response for Severe Nuclear Power Plant Accidents.

In the nuclear power industry, Level 3 probabilistic risk assessment (PRA) is used to estimate damage to public health and the environment if a severe accident leads to large radiological release. Current Level 3 PRA does not have an explicit inclusion of social factors and, therefore, it is not possible to perform importance ranking of social factors for risk-informing emergency preparedness, planning, and response (EPPR). This article offers a methodology for adapting the concept of social vulnerability, commonly used in natural hazard research, in the context of a severe nuclear power plant accident. The methodology has four steps: (1) calculating a hazard-independent social vulnerability index for the local population; (2) developing a location-specific representation of the maximum radiological hazard estimated from current Level 3 PRA, in a geographic information system (GIS) environment; (3) developing a GIS-based socio-technical risk map by combining the social vulnerability index and the location-specific radiological hazard; and (4) conducting a risk importance measure analysis to rank the criticality of social factors based on their contribution to the socio-technical risk. The methodology is applied using results from the 2012 Surry Power Station state-of-the-art reactor consequence analysis. A radiological hazard model is generated from MELCOR accident consequence code system, translated into a GIS environment, and combined with the Center for Disease Control social vulnerability index (SVI). This research creates an opportunity to explicitly consider and rank the criticality of location-specific SVI themes based on their influence on risk, providing input for EPPR.

Comparative Analysis of Calculation Possibilities of MACCS and RODOS Computer Codes for Emergency Response and Analysis of Radiation Consequences of Severe Accidents at NPPs

The paper presents a brief description of the Real-time On-line Decision Support System (RODOS) and MELCOR Accident Consequence Code System (MACCS) and evaluates the application of these codes for assessment of radiation impact on the public and environment in case of a severe accident at NPP in real time mode. The experts performed comparative analysis of results obtained from calculations using RODOS, WinMACCS and HotSpot codes. In the framework of the research, the chosen comparative criteria were assessed: total effective dose, thyroid equivalent dose, skin equivalent dose, I-131 and Cs-137 ground concentration at different distances up to 50 kilometers from the source of release.

Estimation of Intervention Distances for Urgent Protective Actions Using Comparative Approach of MACCS and InterRAS

Distances for taking evacuation as a protective measure during early phase of a nuclear accident have been approximated using MELCOR Accident Consequence Code System (MACCS). As a reference data, the source term of Pakistan Research Reactor 1 (PARR-1) and meteorological data of Islamabad, Pakistan, have been considered. Based on comparison with published data and international radiological assessment (InterRAS) code results, it is concluded that MACCS is a rational tool for estimation of urgent protective actions during early phase of nuclear accident by taking into account the variations in meteorological and release concentrations parameters.

Development of web-based Off-Site Consequence Analysis Program (OSCAP) for extending ILRT intervals and its application

For an offsite consequence analysis, MELCOR Accident Consequence Code System (MACCS) II code is widely used as a tool. In this study, the algorithm of web-based Off-Site Consequence Analysis Program (OSCAP) using the MACCS II code was developed for an integrated leak rate test (ILRT) interval extension and Level 3 probabilistic safety assessment (PSA), and verification and validation (V&V) of the program was performed. The main input data of the MACCS II code are meteorological data, population distribution data and source term data. However, it requires lots of time and efforts to generate the main input data for an offsite consequence analysis using the MACCS II code. For example, the meteorological data are collected from each nuclear power site in real time, but the formats of the raw data collected are different from each other as a site. To reduce efforts and time for risk assessments, the web-based OSCAP has an automatic processing module which converts the format of the raw data collected from each site in Korea to the input data format of the MACCS II code. The program also provides an automatic function of converting the latest population data from Statistics Korea, the National Statistical Office, to the population distribution input data format of the MACCS II code. In case of the source term data, the program includes the release fraction of each source term category resulting from Modular Accident Analysis Program (MAAP) code analysis and the core inventory data from ORIGEN code analysis. These analysis results of each plant in Korea are stored in a database module of the web-based OSCAP, so a user could select the defaulted source term data of each plant without handling source term input data. For V&V of the web-based OSCAP, the following analyses were performed. First of all, an offsite consequence analysis for reference plants was carried out using only the MACCS II code, and using the web-based OSCAP based on both methodologies of NUREG-1493 and NEI interim report. Secondly, the results of the offsite consequence analysis were compared with each other. Lastly, a sensitivity analysis was accomplished for the main input data of the meteorological data and the population distribution data using the web-based OSCAP. It is considered that the program can remarkably reduce man-power on performing a periodic risk assessment according to extending ILRT intervals, and Level 3 PSA. Also, the program can be applied to making accident management plan (AMP) and quantitative health objective (QHO), and to the field of public acceptance (PA).

Reevaluation of the emergency planning zone for nuclear power plants in Taiwan using MACCS2 code

According to government regulations, the emergency planning zone (EPZ) of a nuclear power plant (NPP) must be designated before operation and reevaluated every 5 years. Corresponding emergency response planning (ERP) has to be made in advance to guarantee that all necessary resources are available under accidental releases of radioisotope. In this study, the EPZ for each of the three operating NPPs, Chinshan, Kuosheng, and Maanshan, in Taiwan was reevaluated using the MELCOR Accident Consequence Code System 2 (MACCS2) developed by Sandia National Laboratory. Meteorological data around the nuclear power plant were collected during 2003. The source term data including inventory, sensible heat content, and timing duration, were based on previous PRA information of each plant. The effective dose equivalent and thyroid dose together with the related individual risk and societal risk were calculated. By comparing the results to the protective action guide and related safety criteria, 1.5, 1.5, and 4.5 km were estimated for Chinshan, Kuosheng, and Maanshan NPPs, respectively. We suggest that a radius of 5.0 km is a reasonably conservative value of EPZ for each of the three operating NPPs in Taiwan.

Use of Post-Chernobyl Data from Norway to Validate the Long-Term Exposure Pathway Models in the Accident Consequence Code MACCS

This paper describes a task performed for the U. S. Nuclear Regulatory Commission (NRC), consisting of using post-Chernobyl data from Norway to verify or find areas for possible improvement in the chronic exposure pathway models utilized in the NRC’s program for probabilistic risk analysis, level 3, of the MELCOR accident consequence code system (MACCS), developed at Sandia National Laboratories, Albuquerque, New Mexico. Because of unfortunate combinations of weather conditions, the levels of Chernobyl fallout in parts of Norway were quite high, with large areas contaminated to more than 100 kBq/m2 of radioactive cesium. Approximately 6% of the total amount of radioactive cesium released from Chernobyl is deposited on Norwegian territory, according to a countrywide survey performed by the Norwegian National Institute for Radiation Hygiene.Accordingly, a very large monitoring effort was carried out in Norway, and some of the results of this effort have provided important new insights into the ways in which radioactive cesium behaves in the environment. In addition to collection and evaluation of post-Chernobyl monitoring results, some experiments were also performed as part of the task. Some experiments performed pre-Chernobyl were also relevant, and some conclusions could be drawn from these.All the long-term exposure pathways routinely treated by MACCS were considered. The Chernobyl accident brought no new insights to the cloudshine exposure pathway, but understanding of the groundshine, soil-grass-milk, soil-grass-beef, and the freshwater exposure pathways was considerably improved. Much new valuable information on exposure pathways not routinely included in MACCS has also been gathered, but this aspect is not discussed in this paper.In most connections, the data available show the models and data in MACCS to be appropriate. A few areas where the data indicate that the MACCS approach is inadequate are, however, also pointed out in the paper. This concerns in particular root uptake to grass from soil and the freshwater exposure pathways. Both of these areas ought to be revised. It is also pointed out that MACCS’ inability in the present version to distinguish between chemical forms of cesium with different bioavailability may lead to conservative results.The task is limited to radioactive cesium, which proved to be by far the most important post-Chernobyl radionuclide in the Norwegian area.

Comida

A dynamic food chain model and computer code, named "COMIDA," has been developed to estimate radionuclide concentrations in agricultural food products following an acute fallout event. COMIDA estimates yearly harvest concentrations for five human crop types (Bq kg-1 crop per Bq m-2 deposited) and integrated concentrations for four animal products (Bq d kg-1 animal product per Bq m-2) for a unit deposition that occurs on any user-specified day of the year. COMIDA is structurally very similar to the PATHWAY model and includes the same seasonal transport processes and discrete events for soil and vegetation compartments. Animal product assimilation is modeled using simpler equilibrium models. Differential transport and ingrowth of up to three radioactive progeny are also evaluated. Benchmark results between COMIDA and PATHWAY for monthly fallout events show very similar seasonal agreement for integrated concentrations in milk and beef. Benchmark results between COMIDA and four international steady-state models show good agreement for deposition events that occur during the middle of the growing season. COMIDA will be implemented in the new Department of Energy version of the MELCOR Accident Consequence Code System for evaluation of accidental releases from nuclear power plants.

Simplified methodology for assessing the risk impact of potential performance improvements for the dry pressurized water reactor containment

This paper presents a simplified methodology for calculating the risk benefit associated with potential performance improvements for the dry pressurized water reactor (PWR) containment. The analysis is based on the June 1989 draft NUREG-1150 results for the Zion commercial nuclear reactor. Simplified containment event trees and the large accident progression event trees from draft NUREG-1150 are used to evaluate the effects of potential improvements on the response of the Zion containment to dominant severe accident sequences. Source terms are generated parametrically using the ZISOR code and offsite consequences are calculated with the MELCOR Accident Consequence Code System (MACCS). These results give point estimates of the risk reduction associated with each containment improvement identified by Brookhaven National Laboratory in their draft Issues Characterization Report.

An exploratory sensitivity study with the MACCS reactor accident consequence model

Abstract An exploratory sensitivity study with the MELCOR Accident Consequence Code System (MACCS) is presented. This study was performed to provide (1) an indication of the possible impact of consequence modeling uncertainties on the results of an integrated probabilistic risk assessment for a nuclear power station; (2) guidance on important parameters to direct data acquisition for future uncertainty analyses with MACCS; (3) a check that MACCS is operating as intended; and (4) insights on the effect of problem gridding on model resolution. The sensitivity results obtained in this study involve fatalities and injuries due to radiation exposure incurred within seven days of an accident and are based on the group of source terms (i.e. source term cluster) that made the largest contribution to early fatality risk in a probabilistic risk assessment for the Peach Bottom Atomic Power Station. The following consequence measures were investigated: early fatalities, prodromal vomiting, lung impairment, population dose (0–500 miles), population dose (0–10 m iles), mean early fatality distance, maximum early fatality distance, mean early fatality risk, probability of one or more early fatalities, and amount of central processing unit (CPU) time required to evaluate MACCS. The analysis used techniques based on Latin hypercube sampling and regression analysis. Results obtained in this study indicate that (1) the impact of consequence modeling uncertainty in an integrated risk assessment may be significant; (2) dominant variables involving early health effects include evacuation area, scaling for horizontal dispersion, population relocation time, dry deposition velocity, and groundshine shielding; (3) the models in MACCS related to early health effects are implemented correctly; and (4) model resolution may be improved by finer radial gridding.

Estimating the financial consequences of a severe nuclear accident in Canada

The offsite financial consequences of a severe accident at the Pickering Nuclear Generating Plant in Pickering, Ontario, are estimated with use of the Melcor Accident Consequence Code System (MACCS) developed at Sandia National Laboratories (Alberquerque, NM). Although the consequences of an accident have implicitly been included in developing the Canadian Nuclear Liability Act, no detailed study of these consequences has ever been conducted except by Ontario Hydro, a public utility and a branch of the Ontario Ministry of Energy with responsibility for nuclear power planning and development in the province of Ontario. The MACCS model was adapted to the southern Ontario case and run over a range of weather and source-code scenarios for a region with a 100 km radius around Pickering. Estimates were made of the health and property-damage consequences of an accident under these scenarios, with varying evacuation and dose threshold assumptions. While the results show a wide range of possible damages, they indicate that the liability limits set in the Nuclear Liability Act may be too low.

Cost-Effectiveness Analysis of Countermeasures using Accident Consequence Assessment Models

In the event of a large release of radionuclides from a nuclear power plant, protective actions for the population potentially affected must be implemented. Cost-effectiveness analysis will be useful to define the countermeasures and the criteria needed to implement them. This paper shows the application of Accident Consequence Assessment (ACA) models to cost-effectiveness analysis of emergency and long-term countermeasures, making use of the different relationships between doses, contamination levels, affected areas and population distribution, included in such a model. The procedure is illustrated with the new Melcor Accident Consequence Code System (MACCS 1.3), developed at Sandia National Laboratories (USA), for a fixed accident scenario. Different alternative actions are evaluated with regard to their radiological and economical impact, searching for an 'optimum' strategy.

Cost-Effectiveness Analysis of Countermeasures using Accident Consequence Assessment Models

In the event of a large release of radionuclides from a nuclear power plant, protective actions for the population potentially affected must be implemented. Cost-effectiveness analysis will be useful to define the countermeasures and the criteria needed to implement them. This paper shows the application of Accident Consequence Assessment (ACA) models to cost-effectiveness analysis of emergency and long-term countermeasures, making use of the different relationships between doses, contamination levels, affected areas and population distribution, included in such a model. The procedure is illustrated with the new Melcor Accident Consequence Code System (MACCS 1.3), developed at Sandia National Laboratories (USA), for a fixed accident scenario. Different alternative actions are evaluated with regard to their radiological and economical impact, searching for an 'optimum' strategy.