The characteristic feature of the coated particle fuels is retention of FPs by the ceramic coating layers. The principle of ensuring fuel integrity of LWRs is avoiding failure of the fuel clad, which contains FPs, by applying the criterion of the minimum critical heat flux ratio. On the other hand, since the character­istic of FP retention in the coated particle fuels is different from that in the LWR fuel, a new principle for ensuring fuel integrity is applied.

The mechanisms of FP gas release from the coated particle fuel can be categorized as below.

(i) FP gas release from the coated particle fuels where the coating layers have failed at fabrication

(ii) FP gas release from the intact coated particle fuels due to diffusion enhanced by heat up

(iii) FP gas release due to failure of the coating layers during operation (additional failure)

The allowable design limit of the coated particle fuels is determined to keep failure of the coating layers associated with (iii) within an allowable range. The failure fraction associated with (i) is limited below 0.2 %. The diffusion of fission fragments associated with (ii) is much smaller than the release by (iii).

Various tests on failure of the coating layers have been carried out in several countries. Failure fractions of the coated particle fuels in some heating tests are shown in Fig. 4.19. Heating the coated particle fuels causes failure of the coating layers. Then fission gas (85Kr) is abruptly released. The failure fraction is estimated by the release of radioactivity per coated particle fuel. From the figure, it can be seen that the failure fraction is almost zero below 1,800 °C, it gradually increases above 1,800 °C, and sharply increases around 2,200 °C. It is supposed that rapid thermal decomposition (thermal degradation) of the SiC layer above 2,200 °C leads to abrupt failure of the coating. The allowable design limit is set as 1,600 °C, taking a margin from 1,800 °C. In order to keep the fuel temperature below 1,600 °C at anticipated operational occurrences

Control rod

Support ring

Neutron absorber iB. C)

Connecting rod

Refractory metal sleeve

Guide ring

Shock absorber

Element

such as “Uncontrolled withdrawn of control rod at operation”, the limit of the fuel temperature at normal operation is set as 1,495 °C.