Containment severe accident

L.E. Herranz and F. Robledo, A Potential Strategies to Control Iodine Released into the Containment in the Case of a Severe Reactor Accident , In the Proceedings of OECD Specialist Meeting on Selected Containment Severe Accident Management Strategies,... 

C-8. H. Tuomisto, et al, External spray cooling of the Loviisa containment . Proceedings of the Specialist Meeting on Selected Containment Severe Accident Management Strategies, CSNI Report NEA/CSN1/R(95)3, Committee on the Safety of Nuclear Installations, Paris, 1995. 

In the structural failure characterizations for the reactor and spent fuel pool Level 2 PSA modeling, NRC is relying on a combination of (i) past assessments, (ii) new calculations using three-dimensional finite element analysis in conjunction with statistical data for the failure strain of the liner materials, as well as models for tearing of the liner, and (iii) assessments using engineering criteria. Of particular interest here are the characterizations of containment severe-accident-induced failure and spent fuel pool seismic failure. 

On August 8, 1985, the U.S. Nuclear Regulatory Commission (NRCf requested the operators of nuclear power plants in the U.S. to perform Individual Plant Examinations (IPE) on their plants. IPEs are probabilistic analyses that estimate the core damage frequency (CDF) and containment performance for accidents initiated by internal events (including internal flooding, but excluding internal fire). Generic Letter (GL) 88-20 was issued to implement the IPE request to identify any plant-specific vulnerabilities to severe accidents and report the results to the Commission. ... 

BWR results, grouped by containment type, follow expected trends and indicate that, in general, Mark I containments are more likely to fail during a severe accident than th c II... 

Risk-based information provides a foundation for regulation of severe accidents. Early PRAs, with large uncertainties, indicated risk that was above or below the Safety Goals depending on containment performance. Consequently the NRC developed an Integration Plan for Closure of Severe Accident Issues (SECY-88-47) with six main elements to this plan 1) individual plant examinations (IPE), 2) containment performance improvements, 3) improved plant oper itions, 4) severe accident research, 5) external event considerations, and 6) accident management. 

The severe accident research program improved public risk assessment, reduced uncertainties, and the reliance on subjective expert opinion. To close two severe accident issues in NRC s Severe Accident Research Plan (NUREG-1365) Mark I Liner Attack and Direct Containment Heating (DCH) were addressed with a new approach using the Risk Oriented Accident Analysis Method (ROAAM) (Theofanous, 1994, 1989). The resolution of the Mark-I Liner Attack issue constitutes the first full demonstration of ROAAM. It emphasizes the determinism and provides a basis for synergistic collaboration among experts through a common communication frame. 

Answer Use the plant s PSA to determine the risk of accidents that include containment failure from overpressurization. Then make a preliminary design of a vented containment that has sufficiently low impedance to the gas at the pressure predicted for the most severe accident sequences such that the containment is not damaged. This containment bypass will include iodine and HEPA filters as well as scrubbers and a discharge through a stack. Estimate the dose that the population would get using this bypass for comparison with the PSA result for ruptured containment sequences. 

OSHA stands for the Occupational Safety and Health Administration of the United States government. OSHA is responsible for ensuring that workers are provided with a safe working environment. Table 1-2 contains several OSHA definitions applicable to accident statistics. 

A part of the US Nuclear Regulatory Commission s (NRC) severe accident research program was dedicated to hydrogen issues in LWR containment designs under core meltdown conditions. The analysis included the in-vessel and ex-vessel hydrogen generation as well as its mixing and distribution in the containment. 

The development of the computer code RALOC at the German GRS for the simulation of the distribution of hydrogen, air, and steam in a LWR containment after severe accidents has started in 1974. RALOC is based on a lumped parameter approach with a differential equation system which describes the composition of the containment atmosphere, temperature, and the transport phenomena of convection and diffusion. Validation... 

BRETTUNG, W., REDLINGER, R., Containment Pressure Loads from Hydrogen Combustion in Unmitigated Severe Accidents, Nucl. Tech. Ill (1995) 395-419. 

All nuclear power reactors outside the previous USSR and CMEA countries have a reactor containment building, the purpose of which is to contain steam and released radioactivities in case of a severe accident, and to protect the reactor from external damage. The contairunent is designed (and tested) to withstand the internal pressure from a release of the water in the entire primary cooling circuit (and in the case of PWRs of the additional loss of one of the steam generators), corresponding to excess pressures of 0.4 MPa. The containmrat is provided with a spray, which cools and condenses the steam released and... 

Special system is provided for filling the containment with water and its subsequent sealing to exclude destruction of containment at FPU sink. Main trends of severe accident control strategy limitation of core damage size ... 

There are many opportunities for aerosols to be generated through normal laboratory procedures. Studies have been conducted of the average number of droplets created by many typical operations and some procedures are prolific generators of aerosols. Each droplet often contains several organisms. There are far more of these daily releases than there are accidents, and where the potential exists for many thousands of infectious organisms being released. 

What are the possible causes, the typical phenomena and the possible course of events in a severe accident Here, a concise and necessarily incomplete description will be attempted. The typical sequences entail damage and melt of the core, interaction of the molten core with the pressure vessel and afterwards with the containment floor and, finally, perforation of the containment itself... 

The most severe hazard caused by hydrogen release is that it will be released, sooner or later according to the conservative assumptions made in severe accident studies, into the primary containment atmosphere where it may cause, in the presence of air, explosions or relatively slow combustion. In both cases, the internal pressure in the primary containment will increase and its integrity will be endangered. The containment safety margins against internal pressure are, however, normally high. ... 

Unfortunately, the abundant information supplied by the designers does not allow us to conclude that the corrective measures adopted in other reactors of the same type (about 20) are sufficient to rule out the danger of another severe accident, possibly with different modalities. The accident, indeed, has highlighted a dangerous vulnerability of this type of reactor, which is generic in nature, and which is not specifically tied with the sequence of events that happened at Chernobyl in 1986. In particular, a weak point of the reactor is its upper closure plate, to which 1700 fuel channels and the control rods are fastened. There is no containment present above the plate a major hazard during possible accidental internal over-pressurization of the reactor. 

In order to comply with this stringent goal, it is understandable that attention has been mainly switched to future reactors which now include substantial design modifications. Moreover, the importance of a perfectly leak-proof containment in case of severe accident is now clear. 

---