Direct containment heating

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. 

Scenarios of core melt with high primary pressure direct containment heating (DCH - due to the violent expulsion of part of the molten core from the vessel and to its fragmentation in the atmosphere with consequent combustion and heat production) and destructive forces on the vessel (due to the expulsion of molten material from the vessel at high pressure). 

A super-strong pressure containment, passively cooled in order to sustain without failure slow over-pressurizations, hydrogen detonations and over-pressurizations from direct containment heating (DCH). 

The primary depressurization eliminates at the source, all the severe accident sequences with a pressurized primary system (i.e. direct containment heating, destructive reaction forces due to perforation of the vessel, etc.). Moreover, in case of malfunction of the high pressure cooling systems, it allows the cooling of the core by intermediate pressure accumulators and low pressure systems. 

Physical phenomena specific to severe accidents (attack of the container bottom, direct containment heating, steam explosions, production and behaviour of hydrogen, behaviour of the fission products in the form of aerosols or of gases and vapours, loading of the reactor vessel by the molten core and its coolability, coolability of the molten core outside the pressure vessel, etc.). 

Voluntary primary depressurization has also been considered as the best means to stop possible Direct Containment Heating (DCH) and to eliminate severe accident sequences with a vessel at high pressure. 

High pressure melt ejection/direct containment heating ... 

Direct Containment Heating (DCH) involves the ejection of the melt from the vessel at high pressure, thus spraying the molten material into... 

At or Soon After Vessel Breach Steam Spike Steam Explosion Combustion Direct Containment Heating Debris Contact with Containment ... 

R. W. Ostensen, et al.. Models and Correlations for Direct Containment Heating, Letter Report to the NRC. Sandia National Laboratories. March IS, 1991. 

H.D. Allen, M. Pilch, R.O. Griffith, R.T. Nichols and T.K. Blanchat, Experiments to investigate the effects of 1 10 scale Zion structures on direct containment heating (DCH) in the Surtsey test facility The lET-1 and lET-lR tests. SAND92-0255, Sandia National Laboratories (July 1992). 

M.D. Allen, T.K. Blanchat, M. Pilch and R.T. Nichols, Experimental results of an integral effects test in a Zion-like geometry to investigate the effect of a classically inert atmosphere on direct containment heating The IET-5 experiment. SAND92-1623, Sandia National Laboratories (November 1992). 

M. Pilch, Adiabatic equilibritim models for direct containment heating. SAND91-2407C, presented at the 19th Water Reactor Safety Information Meeting, Washington, DC (October 1991). 

Separate Effect Simulation Experiment on Corium Dispersion in Direct Containment Heating... 

RESULTS OF DIRECT CONTAINMENT HEATING INTEGRAL EXPERIMENTS AT 1/40TH SCALE... 

Allen, M. D., M. Pilch, R. T. Nichols, and R. 0. Griffith, "Experiments to Investigate the Effect of Flight Path on Direct Containment Heating (DCH) in the Surtsey Test Facility," NUREG/CR-5728, 1991. 

Pilch, M. and M. D. Allen, "A Scaling Methodology for Direct Containment Heating with Application to the Design and Specifications of an Experiment Program for Resolving DCH Issues," SAND 91-2784, 1991. 

However, in many of these areas, the assessment to date does not cover all phenomena of interest, or is based on a limited number of experiments and analyses which may be insufficient to cover the scales of interest and which may be insufficient to allow identification of experiment-specific problems vs generic code problems and deficiencies. Furthermore, there has been no assessment at all of MELCOR for ex-vessel melt phenomena such as core-concrete interactions, debris bed coolability or direct containment heating (although some assessment of the new MELCOR DCH model is planned, and the core-concrete interaction model has had some inherited validation from the standalone CORCON assessment activities). And, although SNL has assessed the new ice condenser model, there has been no assessment against test data for hydrogen burns or for other engineered safety features such as containment sprays and/or fans. 

The phenomena associated with natural circulation of mixtures of steam and noncondensibles is being evaluated to better understand their influence on the temperature of the vessel internals and the ex-vessel structures Including the hot leg, pressurizer surge line, and steam generator tubes. High temperature creep rupture failure of the ex-vessel structures prior to failure of the reactor vessel lower head has the potential to reduce the likelihood of containment failure as a result of direct containment heating. This paper provides results from studies that have been completed in two areas ... 

Early containment failure resulting from direct containment heating (DCH) has been identified as a potential contributor to the risk of operating a pressurized water reactor (PWR). One important factor needed to evaluate the contribution of DCH to risk is the conditional probability that, given a core melt, the primary system will be at high pressure when the reactor vessel lower head fails. 

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