Severe Accident Analysis

Containment performance at beyond design basis accident internal pressure and temperature is required as an input for determining the offsite consequences and accident progression of the containment during a severe accident. Extensive research and scale model testing of reinforced and prestressed concrete containments to determine behavior at beyond design basis accident pressure has been performed in the last 25 years at Sandia National Laboratories (SNL) [23] and Central Electricity Generating Board (CEGB), England [24]. Concrete containments start to leak before a complete rupture or failure. It is extremely difficult to accurately predict the location and leakage rate of the concrete containment due to beyond design basis internal pressure and temperature. Hessheimer and Dameron [23], and Dameron, Rashid, and Tang [25] provide guidance for predicting leak area and leak rate in containments. Hessheimer and Dameron [23] recommend a non-linear finite element analysis of the concrete containment to predict containment performance and leakage.

Hessheimer and Dameron [23] have concluded that global, free field strain of 1.5% to 2.0% for reinforced and 0.5% to 1.0% for prestressed concrete can be achieved before failure or rupture. In addition, leakage in concrete containment increases appreciably after the rebars and liner plate yield. Furthermore, under gradual increase in internal pressure, containment leakage continues to grow without failure and rupture. Sheikh

[26] provides the following simplified approach for predicting containment performance during a severe accident. This approach has been used in the NRC’s State-of-the-Art Consequence Analyses Project [27]

^ fail = (Ahoop * Trebar@2% + Aliner * Tliner@2% ) / R Pyield = CAhoop * Yrebar + Aliner * Yliner) / R

where:

Pfaii = Containment failure pressure (containment leakage greater than 100%)

Pyieid = Containment pressure at which hoop rebars and liner plate yield

Ahoop = Area of the hoop rebars

Aliner = Area of the liner plate

Yrebar = Yield stress of the rebar

Yliner = Yield stress of the liner plate

Yrebar@2% = Stress in the rebar at 2% strain

Tliner@2% = Stress in the rebar at 2% strain

R = Radius of the containment

The results using this simple approach are quite consistent with detailed finite element analyses using state of the art computer codes and test data [26].

 
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