Off-site Consequences Research Articles

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 Outline of COSYMA, A New Program Package for Accident Consequence Assessments

COSYMA (COde SYstem from MARIA) is a new program package for assessing the off-site consequences of accidental releases of radioactive material to atmosphere, developed as part of the Commission of...

Approaches that use seismic hazard results to address topics of nuclear power plant seismic safety, with application to the Charleston earthquake issue

Plant seismic safety indicators include seismic hazard at the SSE (safe shut-down earthquake) acceleration, seismic margin, reliability against core damage, and reliability against offsite consequences. This work examines the key role of hazard analysis in evaluating these indicators and in making rational decisions regarding plant safety. The paper outlines approaches that use seismic hazard results as a basis for plant seismic safety evaluation and applies one of these approaches to the Charleston earthquake issue. This approach compares seismic hazard results that account for the Charleston tectonic interpretation, using the Electric Power Research Institute (EPRI)–Seismicity Owners Group (SOG) methodology, with hazard results that are consistent with historical tectonic interpretations accepted in regulation. Based on hazard results for a set of 21 eastern U.S. nuclear power plant sites, the comparison shows that no systematic “plant-to-plant” increase in hazard accompanies the Charleston hypothesis; differences in mean hazards for the two interpretations are generally insignificant relative to current uncertainties in seismic hazard.

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.

An analysis of containment venting as a severe accident mitigation strategy for the BWR Mark II containment

An evaluation of a BWR/4 reactor with a Mark-II containment identified the effects of containment venting on core damage frequency and containment failure mode. A limited evaluation of the effects on the off-site consequences was also performed. The analysis was founded upon an existing probabilistic risk assessment (PRA) with the addition of a proposed filtered containment venting system, based on the Swedish Filtra system installed at the Barseback Nuclear Power Station in southern Sweden. Three different containment venting strategies were examined for their effects on plant risk: (1) venting using the proposed Filtra system, (2) venting utilizing existing equipment, and (3) no venting. The consequences of containment venting were examined in conjunction with two sets of assumptions about the effects of a harsh reactor building environment, produced by venting via the containment and reactor building HVAC systems, on the equipment located there. The consequences when a harsh reactor building environment is assumed to have either no adverse effect on equipment or to fail all equipment were analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Assessment of Uncertainties in Early Off-Site Consequences from Nuclear Reactor Accidents

A simplified approach has been developed to calculate uncertainties in early off-site consequences from nuclear reactor accidents. The consequence model (SMART) is based on a solution procedure that uses simplified meteorology and involves direct analytic integration of air concentration equations over time and position. This is different from the discretization approach currently used in the CRAC2 and MACCS codes. The SMART code is fast running, thereby providing a valuable tool for sensitivity and uncertainty studies. The code was benchmarked against both MACCS version 1.4 and CRAC2. Results of benchmarking and detailed sensitivity and uncertainty analyses using SMART are presented.

Consequence evaluation for pressurized thermal shock

In order to estimate the risk associated with Pressurized Thermal Shock (PTS), a sample calculation of the core melt frequency and offsite consequences has been performed for Oconee Unit 1, a Babcock and Wilcox pressurized water reactor located in the United States. Core melt frequency was derived from through-wall-crack frequency estimates based on thermal-hydraulic and fracture mechanics analyses performed by Oak Ridge National Laboratory and Pacific Northwest Laboratory. The mode and timing of containment response was estimated from previous risk studies for Oconee Unit 3 and other plants with large dry containments. The core melt frequency was calculated to be 6 × 10 −6 per reactor year for operation at the PTS screening criterion. Operation of redundant and independent containment heat removal systems results in low probability of containment failure. The risk dominant scenario involves overpressure failure of containment due to failure of containment heat removal. Prompt containment failure was assigned a very low probability (10 −4), and hydrogen burn failure was not considered. The central estimate of annual risk was 5 × 10 −7 early fatalities, 2 × 10 −4 latent cancer fatalities and 0.7 person-rem. These values are minimal compared with other severe accident scenarios. Uncertainties and sensitivies to important parameters are discussed. The response of other types of plants is briefly described.

Risk-Oriented Analysis of the German Prototype Fast Breeder Reactor SNR-300: Off-Site Accident Consequence Model and Results of the Study

Accident off-site consequence calculations and risk assessments performed for the ''risk oriented analysis'' of the German prototype fast breeder reactor SNR-300 were performed with a modified version of the off-site accident consequence model UFOMOD. The modifications mainly relate to the deposition and resuspension processes, the ingestion model, and the dose factors. Consequence calculations at the site of Kalkar on the Rhine River were performed for 115 weather sequences in 36 wind directions. They were based on seven release categories evaluated for the SNR-300 with two different fueling strategies: plutonium from Magnox reactors only and plutonium from light water reactors and Magnox reactors. In parallel, the corresponding frequencies of occurrence are determined. The following results are generated: 1. complementary cumulative frequency distribution functions for collective fatalities and collective doses 2. expected values of the collective fatalities and collective doses as well as distance-dependent expected values of individual fatality 3. contributions of the different exposure pathways to fatalities with respect to the various organs. For comparison with the risk of a PWR-1300, calculations for the PWR-1300 of the ''German Risk Study'' were repeated with the same modified consequence model. Comparison shows that smaller risks result for the SNR-300. However, the confidence intervalmore » bandwidths obtained for the frequencies of the release categories for the SNR-300 are larger than those of the PWR-1300.« less

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Field Implementation of an Emergency Assessment System

A computerized Emergency Assessment and Response System (EARS) was developed as a means to characterize and evaluate the offsite radiological consequences of potential accidents at the Diablo Canyon Power Plant (DCPP). The system consists of a central computer at the Technical Support Center, provisions for live-time collection of meteorological and monitor data, and a distributed network of minicomputer display stations at selected plant and offsite locations. During use in two full-scale emergency exercises at DCPP, the system demonstrated the capability to collect information, perform dose projections, and communicate the information effectively to remote participants. Problems observed during the first exercise, such as slowness in defining release rate terms and unavailability of certain data as needed by participants, were corrected prior to the second exercise. EARS is now a proven, integral part of the emergency response capabilities of DCPP, and is being extended for routine use in training, meteorological data processing, and gaseous effluent control programs.