**3. Fuel characteristics calculator (FCCAL)**

The main thrust of FCCAL [2] is to analyze the plenum gas conductivity with fission gas accumulation and its analytical evaluation. With irradiation of the fuel inside the reactor core, fission noble gases Xenon and Krypton accumulate in the plenum gap which changes the gap conductivity from the initial fuel behavior that is for the Helium-filled during manufacturing. The analytical model is a better approximation over the use of correlations to estimate the effect of noble gases in the plenum gap. The change in conductivity is observable in the fuel in CANDU reactors where on-power refueling is practiced and the old fuel bundles move to higher power generating regions of the core. The fresh fuel along with the old bundles leads to a higher fission rate and hence the release of trapped noble gases towards the plenum. This results in a higher temperature of the old bundles even without an appreciable change in the power. We will discuss these phenomena in the sections below.

#### **3.1 Fission gas release**

Fission gases are considered to be released from the fuel when they reach any space that is connected to the free volume within the fuel pin. The released gases accumulate in the fuel-cladding gap, the central void, and porosity within the fuel which communicates directly with the fuel-pin gas space [1]. Cracks or interlinked gas bubbles or pores are an important type of open porosity. The fission gas that has been released from the fuel is responsible for the change in plenum gap conductivity and is assumed to have the following properties. (a) Once the gas is released, the probability of its re-entering the solid from the free volume is negligible (b) the gas pressure in open porosity is equal to that in the free volume of the pin. Because of the insolubility of Xenon and Krypton in solids, there is no effect of plenum fission gas pressure on the rate of gas escape from the fuel (c) while the fission gas contained by the fuel tends to cause swelling, fission gas that has been released promotes shrinkage in the fuel by pressurizing the solid pellets leading to collapse of the internal porosity and bubbles. FCCAL carries out an explicit calculation for changes in gap gas conductivity due to a binary mixture of Helium and Xenon. As the heat generated in the pellet is transferred across the fuel to the coolant, the heat transfer across discontinuities is calculated in the following steps. (i) Heat transfer from the meat of the pellet to the pellet surface. It is estimated by the heat transfer coefficient of the natural uranium oxide pellet (hPÞ. (ii) Heat transfer across the plenum gap and the Zircalloy clad hGg þ hGs þ hs . hGg is the heat transfer coefficient due to plenum gas,hGs is the heat transfer coefficient of the solid-solid contact points between the pellet and the sheath, andhs is the heat transfer of the Zircalloy sheath. (iii) Heat transfer from the clad outer surface to the coolant is estimated by the heat transfer coefficient of the coolant film near the fuel surface hð Þ cf . Total heat transfer coefficient hð Þ<sup>T</sup> is the sum of terms in (i), (ii), and (iii) i.e.

*Plenum Gas Effect on Fuel Temperature DOI: http://dx.doi.org/10.5772/intechopen.101098*

$$\mathbf{h}\_{\rm T} = \mathbf{h}\_{\rm P} + \mathbf{h}\_{\rm Gg} + \mathbf{h}\_{\rm Gs} + \mathbf{h}\_{\rm s} + \mathbf{h}\_{\rm cf} \tag{1}$$
