Core severe accidents

Precursors to Potential Severe Core-Damage Accidents... 

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. ... 

Its unique design suggests several accident scenarios that could not occur at other reactors. For example, failure to supply ECC to 1/16 of the core due to the failure of an ECC inlet valve. On the other hand, some phenomena of concern to other types of reactors seem impossible (e.g., core-concrete interactions). The list of phenomena for consideration came from previous studies, comments of an external review group and from literature review. From this, came the issues selected for the accident progression event tree (APET) according to uncertainty and point estimates. 

Minarick, J. W. and C, A. Kukielka, Precursors to Potential Severe Core-Damage Accidents 1969-1979 A Status Report, Vols. 1 and 2, ORNL, June 1982... 

Belles, R.J. etal., 1998, Precursors to Potential Severe Core Damage Accidents 1997, A Status Report, NUREG/CR-4674, Vol. 26, U.S. Nuclear Regulatory Commission, Washington, D.C. 

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. 

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 ... 

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... 

B - Maximum DBA (degraded safety systems) or accidents with partial core melt C - Severe accidents with quick intervention D - Severe accidents with delayed intervention E - Severe accidents without intervention... 

Although none of the Rasmussen report sequences replicated exactly the course of events in TMI, the report sequence TMLB was rather close to what happened there. TMI was certainly a severe accident, even if the degree of devastation suffered by the core was not clear from the start. 

The studies of this period led to a definition of severe accident protection criteria (see Section 1-2 and Chapter 18) similar to those already in force in Italy and to those developed in Sweden. In Italy, it was thought possible to provide a defence against severe accidents by accident management provisions and by some reasonable plant modification, up to the point of limiting iodine and caesium releases to 0.1 per cent with a probability higher than 95 per cent in the case of core melt (conditioned probability). 

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. 

The transfer of scientific knowledge on phenomena into actions and procedures is a difficult process (see the above quoted case of the pouring of water on a degraded core) research still plays an important part in the implementation of accident management. Moreover, additional work is needed in the field of severe accident management under low power or shutdown conditions. 

It is worth repeating that the source term has the purpose of replacing, using the modern research data now available, the releases given in the TID-14844 report in their specific applications, essentially of US interest. The new source term represents a reasonable average reference for severe accidents with extensive core melt. 

It is practically impossible to perform a probabilistic treatment of the vessel failure in severe accidents, that is where there is major damage and core melt, because of a lack of sufficient data on the phenomena and on their probabilities. 

It is deemed realistic to ensure, by additional provisions of accident management, with a confidence limit of the order of 95 per cent, that the external iodine or caesium releases, in situations which otherwise would lead to uncontrolled severe accidents (core melt, and so on), be kept within the limit of the 0.1 per cent of the core inventory. 

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.). 

The reactor normally contains a steam-water mixture so that any fast increase in pressure produces steam condensation, an increase of the water mass present and, because of the negative void coefficient for safety reasons, an increase in the core reactivity. It is easily seen that in a BWR the ATWS accident (transients with failure to scram) is particularly serious and represents one of the dominant severe accidents in overall risk evaluations. An accident caused by the spurious and complete closure of isolation valves on steam... 

Accident management includes pre-planned and ad hoc operational practices which, in circumstances in which the design basis specification of the plant is exceeded, would make optimum use of existing plant equipment to restore control. This applies to design extension conditions (i.e. to prevention of core damage and mitigation of severe accidents) (see Section 2-1-4). 

Severe accident Event(s) or event sequence capable of producing more serious consequences than those anticipated for design accidents (in particular, significant reactor core melt). 

TA-V installations that could potentially affect or be affected by the HCF include the Annular Core Research Reactor (ACRR), Gamma Irradiation Facility (GIF), Auxiliary Hot Cell Facility (AHCF), Radiation Metrology Laboratory (RML), and the Sandia Pulse Reactor III (SPR III). The GIF provides two cobalt cells for total dose irradiation environments. A new GIF is under construction in the northeast quadrant of TA-V. SPR III provides intense neutron bursts for effects testing of materials and electronics. The RML provides radiation measurement services to Sandia s reactors, isotopic sources, and accelerator facilities. The AHCF provides a capability to handle limited quantities of radioactive material in a shielded cell. These facilities have separate SARs that describe potential accidents. The most severe accidents for all of these facilities involve the release of radiological materials which could necessitate a site evacuation. No physical damage to the HCF could be induced by any of the postulated accidents, nor could any of the HCF accidents physically affect any of the other facilities. 

---