6.2.1 Core Damage

Essentially, the first barrier, that of the fuel matrix and its cladding, can be chal­lenged in one of two ways: loss of cooling or increase of power. First, loss of ef­fective cooling of the fuel can lead to overheating as happened at TMI-2. Alternatively, a significant increase of neutron population (or reactor power) can result in excess energy deposition within the fuel, leading to fuel expansion and melting and consequent failure of the cladding. This can occur in spite of apparently adequate cooling. The accident at Chernobyl is an extreme example of this class of fuel failure.

Let us concentrate initially on the consequences of a loss-of-cooling situation. There are many ways this could develop with the primary circuit at either high pressure or depressurized, and on a time scale of a few seconds to a few hours. The progressive failure of the fuel can be summarized as follows:

1. As the fuel canning material increases in temperature, it will either burst or under some circumstances swell because of the gas pressure inside it. This may lead to a restriction of the coolant flow between and around the fuel el­ements and make more difficult the problem of cooling them. This factor is, in fact, taken account of in the design of fuel for water-cooled reactors, and it has been shown that blockages of up to 90% can be coped with.

2. As the fuel temperatures rise, so the volatile fission products are released and a temperature is reached (1200-1400°C) at which the first signs of molten material in the core begin to be observed. The melting process is ve1y com­plex with the formation of eutectics and occurs most rapidly in the regions of the core that have had the highest neutron flux (and therefore the highest concentration of fission products whose decay is causing the heating). The
grids that hold the fuel together also melt around 1400°C, followed by the control rods passing through the fuel.

3. At core temperatures above 1100°C the steam reacts with the zirconium can, destroying the can. The reaction is exothermic, that is, the chemical reaction itself releases additional heat. As the temperature increases, so the reaction rate increases and at high temperatures the chemical reaction can contribute as much or even more heat than the fission decay process. The chemical re­action produces hydrogen, which we shall see is a potential threat to con­tainment integrity.

4. Zircaloy itself starts to melt around 1700°C; the sequence of melting may he as illustrated in Figure 6.1. Molten droplets and rivulets of eutectic are formed (rather like wax running down a candle; Figure 6.1a). They solidify in the lower, cooler regions of the core, causing further blockage, which ex­acerbates the lack of cooling (Figure 6.1 b). The solidified material forms a crncible. With the cladding around them gone, the fuel pellet stacks are un­stable. Any transient (like the starting of the primary pumps in the TMI-2 ac­cident) can cause a redistribution of this material with the pellets falling into the crncible to form a debris bed. This material is still generating heat and there will he a tendency for it to melt and move down through the core, growing in volume as it does so (Figure 6.1 d)

5.

The mass of molten material will eventually reach the bottom core support plate and will he held there for a period of time until that core plate also fails

Figure 6.1: Sequence of core melting. Initial stages: (a) Molten droplets and rivulets beginning to flow clown intact fuel rods: ( /J) formation of local blockage in colder re­gions of fuel rods and formation and growth of a molten pool; (c) formation of a small molten pool: (d) radial and axial growth of the pool.

and the core debris then has access to the lower plenum of the reactor pres­sure vessel.