LWR behavior under severe core damage conditions

The accident in the Three Mile Island-2 (TMI-2) Reactor has clearly demonstrated that severe accidents can be terminated successfully also far beyond the regulatory guidelines. It also has shown, that one needs to be prepared to analyze them on a non-hypothetical basis. As a result, various experimental and theoretical programs have been initiated to analyze the fuel rod and core behavior during degraded core cooling conditions.During the course of severe accidents in light water reactors, the heat transfer conditions between the fuel rods or steam generator tubes and the fluid are mainly affected by the uncovery of the fuel rods, tubes or the dryout of the flow channels. For the case of the uncovery transients, a relevant parameter is the level of the two-phase mixture. The height of the so-called mixture level is generally marked as a sharp increase of the void fraction and the surface temperature.After uncovery, the fuel rods will heat-up because of the relatively low heat transfer rates from the rods to the steam.At temperatures above approximately 1500 K, the metal-water reaction predominates the fuel rod behavior. It also reduces the steam to hydrogen ratio, which possibly may limit the chemical reaction.The processes described are typical for the situations analyzed in the frame of risk studies. Especially in small break accidents and transients, the system response on core behavior is substantial. The analyses contribute to the following questions:

• &;#x02022;- Find the minimum availability of safety components to avoid uncoolable conditions. Calculations with the severe accident code MELSIM3 have bound the degree of core uncovery which leads to coolable core conditions [1,2]. The core temperatures of the TMI-2 reactor were much higher than the regulatory limits.
• &;#x02022;- Determine the amount of time available for operators to take actions to bring water into a reactor during a severe accident.
• &;#x02022;- Provide a better understanding of the plant behavior leading to severe core damage conditions.
• &;#x02022;- Improve judgement and reduce conservatism in the risk assessment.