A2-3-7 Decay heat

As far as the transfer of the decay heat of the core to the water-steam mixture is concerned, here too the assumptions are made (usual in this type of calculation) of the total and instantaneous transfer of the available energy from the core to the mixture. These assumptions are not likely to be complied with in an accident, especially when it is assumed that the core always remains dry (i.e. no spray or flooding system operates). In reality the heat released is only partially transmitted to the mixture and, moreover, this phenomenon occurs after a delay. 

The assumption of the total transfer to the mixture of the energy released over time by the core is certainly cautious, while the assumption of an absence of delays in the phenomenon may or may not be cautious according to the aspects of the accident considered. In fact, what can be expected by the assumption of immediate transfer of the heat from the core is a pressure transient characterized at the start by higher values but having a shorter duration. Therefore this assumption is very likely to be conservative for the evaluation of the probability that a second pressure peak higher than the first one in the containment occurs. It will not necessarily be so for the evaluation of prolonged releases of activity from the containment in the absence of pressure abatement systems such as, for example, spray systems. 

The core decay heat is essentially composed of the decay heat of the fission products, the decay heat of the decay chain of uranium-239 and neptunium-239 produced by neutron capture by uranium-238, the decay heat of other actinides, the control rods and the structural materials and the heat generated by the residual fissions and by neutron capture by the fission products. The heat of the residual fissions is generally 

For the time interval 150 / 4xl0 seconds, which generally covers the time span of interest for this transient, Shure suggests the following approximate analytical expression for the decay heat for an infinite irradiation time, valid with a maximum error of five per cent  

The decay heat for a finite irradiation time to, at time t after shutdown, is given by Equation A2.29  

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