There are a number of reasons to doubt the safety of the PFBR design.21 As with other breeder reactors, the PFBR design is susceptible to catastrophic accidents involving large and explosive energy releases and dispersal of radioactivity following a core meltdown. The potential for such Core Disruptive Accidents (CDA) comes from the reactor core not being in its most reactive configuration. If conditions during an accident cause the fuel bundles to melt and rearrange, the reactivity could increase leading to further core rearrangement and a potential positive feedback loop. Another unsafe feedback effect that is present in the PFBR design is its positive sodium void coefficient. This means that if the coolant heats up and becomes less dense, forms bubbles, or is expelled from the core, the reactivity increases. The magnitude of the void coefficient is a measure of the feedback and tends to increase with core size.22 For the core design that has been adopted for the PFBR, it has a value of $4.3.23

Compounding the safety risks that come with this large and positive sodium void coefficient, the PFBR design also has a relatively weak containment, which is designed to withstand only 25 kilopascals (kPa) or one quarter of an atmosphere of overpressure.24 This maximum overpressure that the PFBR containment is designed for is lower than some other demonstration reactors (table 3.1). If one considers the ratio of the containment volume times its design overpressure divided by the reactor power, V*P/E, the PFBR containment is weaker than those of all other demonstration breeder reactors except the Prototype Fast Reactor (PFR).25 The difference appears more acute when the higher positive sodium void coefficient of the PFBR in comparison to other breeder reactors is taken into account

It is of course possible to design containments to withstand much higher pressures. Containments for light-water reactors routinely have design pressures above 200 kPa.26 The DAE justifies this choice of containment design by arguing that its safety studies demonstrate that the maximum overpressure expected in a CDA involving the PFBR is smaller than this overpressure. But these results are based on favourable assumptions, in particular, that only limited parts of the reactor core would participate in the CDA and that approximately 1 percent of the thermal energy released would be converted into mechanical energy. Based on such assumptions, the DAE estimates that the maximum credible energy release in a CDA is 100 megajoules (MJ).27 It then calculates that such a CDA leading to sodium leakage into the containment would result in a containment overpressure of 20 kPa.

There are, however, good reasons to consider much larger energy releases from a worst-case CDA to the extent of several hundreds of megajoules in the evaluation of the safety of a reactor design, especially one as large as the PFBR. Table 3.2 shows that the calculated CDA energy releases for a number of breeder reactors are much higher than that of the PFBR, both absolutely and when scaled by reactor power.

The energy releases from core collapse depend sensitively on the reactivity insertion rate, which is the rate at which the fuel rearrangement increases (inserts) reactivity.28 The DAE’s calculation of the maximum CDA energy release is based on a reactivity insertion rate of $65/s, which itself is the result of assuming only limited core disassembly.29 There is ample reason and precedent to use an insertion rate of $100/s as a benchmark for disassembly calculations, with the caveat that it still is not quite an upper bound.30 Likewise, the efficiency of conversion could be much larger than the 1 percent assumed by the DAE. Tests at the UK’s Winfrith facility with core melt amounts of up to 25 kg suggest energy-conversion efficiencies of approximately 4 percent.31 For a reactivity insertion rate of 100 $/s, and an energy conversion efficiency of 1 percent, the energy release from a CDA is 650 MJ.32 It has been estimated that a 650 MJ CDA could lead to an overpressure of approximately 40 kPa on the containment, clearly much higher than the design limit of the containment building.33 Higher conversion factors would imply higher mechanical energy releases and thus higher overpressures and higher likelihoods of containment failure.

To summarize, there are good reasons to believe that the containment of the PFBR does not offer adequate protection against a severe CDA, especially given the many uncertainties inherent in calculations of CDA release energies.