The first boundary for fission products is the fuel and the cladding (Fig. 9.1). The fuel normally holds up about 90% of the fission products during the normal operating cycle of an LWR. The main exceptions are elements that are gaseous at normal fuel operating conditions (~400°C to ~1200°C) such as I2 , Kr, and Xe which are held up by the cladding. Even under normal

I

Demineralizer

9.2

Graphic of the typical pressurized water reactor. (Published in NUREG -1350, Volume 23, August 2011.)

operation the cladding and other fuel assembly components operate under very extreme conditions. These components are constantly bombarded by neutrons (~1012 neutrons/cm2/s), at moderate temperatures (~250-340°C), pressures (~15.5 MPa), mechanical conditions (boiling surfaces, high veloc­ity two-phase flow with large amounts of vibration), and chemical condi­tions (up to ~2000 ppm of boron and up to 10 ppm Li for PWRs).

9.2.1 Understanding the fuel

A stable UO2 fuel structure holds up most of the fission products and even a fair amount of the fission gases (up to ~90%) in the pores of the UO2 pellet. Even though UO2 pellets undergo a significant amount of cracking due to

Emergency water supply systems

9.3 Graphic of the typical boiling water reactor. (Published in NUREG -1350, Volume 23, August 2011.)

the low thermal conductivity and the resulting stresses from the very steep temperature gradients experienced under normal operating conditions, the resulting pieces are relatively large and most of the pores of the UO2 fuel act as reservoirs for fission gas products and the fission products themselves. Under circumstances where the cladding leaks and coolant enters the rod, the UO2 fuel can further disintegrate releasing much of the soluble fission product (primarily Cs and Sr) and most of the gaseous fission products. This effect becomes more pronounced as the burnup of the fuel increases sig­nificantly above the current levels of about 50 MWtd/kgU. Therefore basic research work on fuel that is exposed to burnups >~60 MWtd/kgU is needed and includes: [20]

Steam dryer and shroud head alignment and guide rods

Steam separator and standpipe assembly

Feedwater inlet Feedwater sparger

Core spray sparger Fuel assembly Control rod Fuel support

Core shroud Core plate

9.4 BWR/3 or BWR/4 reactor vessel (G. E. Technology Advanced Manual Differences/Introduction, USNRC Technical Training Center Rev 1195). [21]

• Extension of the understanding of these effects to new fuel pellet mate­rials (e. g. uranium nitride (UN)).

• Effects of long-term wet and dry storage, as well as environmental con­ditions in any potential disposal site, on the integrity of the fuel in terms of its holdup of long-lived radioactive components (mainly U, Pu, Am, Cm, and Np).

• Interactive effects of non-homogeneous portions of the fuel (such as the rim after high burnups) on the performance of the fuel and its interac­tion with the cladding during upset events such as reactivity insertion accidents (RIAs).

Understanding of these effects in a mechanistic way is at the frontier of nuclear fuel research. Since the current level of burnup between 50 and 60 MWd/kgU is just about the practical and economic upper boundary of 5% enriched U-235 fuel today, this is the limit of our empirical knowledge. There is a need to extend this boundary if enrichments above 5% become accepted or if higher density fuels (such as UN) come into use. Phenomenological models, not correlation of empirical data, will be needed to allow predic­tions to be made without the huge cost associated with totally empirical approaches.