Severe core damage accidents

Precursors to Potential Severe Core-Damage Accidents... 

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. 

With respect to the redundant heat removal paths, the moderator can act as an emergency heat sink even with no water in the fiiel channels. Should the moderator heat removal system subsequently fail, the large water-filled reactor vault surrounding the calandria vessel provides an additional line of defence (Fig. 5.7.2). Its primary purpose is to provide shielding of the concrete reactor vault from neutrons and gamma rays. However it can also act as a passive emergency water reservoir in case of a severe core damage accident that is, should the primary coolant... 

Unresolved Safety Issue (USI) A-44 in NUREG-0933 (Reference 1), addresses the concern that the complete loss of all alternating current (AC) electrical power to the essential and non-essential switchgear buses in a nuclear power plant, referred to as a "Station Blackout", could lead to a severe core damage accident. 

The high neutron economy, and hence low reactivity hold-up, of HWRs means that the reactor is very unlikely to become critical after any postulated beyond-design-basis severe core damage accident. 

The HWR has an inherent prompt shutdown mechanism (besides the engineered shutdown systems [SDSs] and the control system) for beyond-design-basis severe core damage accidents. If steam is introduced into the moderator as a result of, for example, multiple... 

In the case of a channel failure, the moderator acts as an energy-absorbing cushion, preventing failure of the calandria vessel. Even for beyond-design-basis severe core damage accidents, where a number of channels are postulated to fail, the calandria may leak (pressure relief rupture discs) but would retain its gross structural integrity. 

In the following description of the reactions occurring during this stage of a severe core damage accident, three different topics will be discussed the release of fission products from the fuel, the release of constituents of the core structural and control rod materials (although these two sources develop almost simultaneously in the reactor pressure vessel so that the volatilized substances can be assumed to enter the gas flow as a mixture) and, finally, volatilization of substances during the molten core - concrete interaction phase. The current state of the art will be discussed with special emphasis on the important chemical phenomena no attempts will be made to establish numerical values of source terms from the results of these experimental and theoretical efforts. 

However, in the course of an actually occurring severe core damage accident the reactor core will not be at a uniform temperature at any given moment still-intact fuel rods, liquefied fuel and molten core materials will be present simultaneously within the reactor pressure vessel, though in different regions of the core. Therefore, releases of different elements from the core are not isolated in time but will in fact take place simultaneously in the different regions, with consequences for the potential chemical reactions in the steam—gas phase above the core. 

Summarizing the results obtained from the theoretical studies as well as from the experimental investigations, it can be expected that, in the course of a severe core damage accident, fission product iodine released from the fuel is converted to... 

The complete loss of AC electrical power to the essential and nonessential switchgear buses in a nuclear power plant is referred to as a Station Blackout . Because many safety systems required for reactor core decay heat removal are dependent on AC power, the consequences of a station blackout could be a severe core damage accident. The technical issue involves the likelihood and duration of the loss of all AC power and the potential for severe core damage after a loss of all AC power. 

Consequence projections during the initial phase of a severe core damage accident provide a basis to establish priorities for the use of limited resources in the implementation of offsite actions such as deployment of field-monitoring teams. In an actual uncontrolled release of radioactive material to the environment, it would be imperative to obtain offsite monitoring team data as quickly as possible. However, for a core-melt sequence, early protective actions in nearby areas (2 to 3 miles) should not await such results. In particular, the evacuation of nearby areas for a severe core damage accident should be initiated on the basis of plant conditions. 

For the initial stages of a severe core damage accident, offsite dose projection has a secondary role, independent of initiating protective actions near the plant. 

In most cases, the specific protective action criteria for severe core damage accidents have been developed after consideration of plant and local conditions. For example, the areas planned to be evacuated may be confined to a valley around the site, or the specific evacuation sector boundaries may be determined by local roads. This delineation is done so that the local population can understand the evacuation instructions. 

When a severe core damage accident is detected or projected, the licensee should recommend and state/local agencies should implement predetermined off-site protective actions. 

If a severe core damage accident occurs, people should immediately evacuate areas near the plant (within a 2- to 3-mile radius) and remain in shelter elsewhere for the immediate future. 

Severe core damage accidents, beyond the design basis (by definition), in which the fuel channels fail and collapse to the bottom of the calandria. 

As noted, PHWR regulators include acceptance criteria for some severe accidents, but there are no formal requirements for severe core damage accidents. However, the CANDU regulatory system is, to a large extent, a... 

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