A reliable upper bound for the fraction of detonatable mixture[75] is patently an important parameter in reactor safety assessments. In this respect the explosive power of MFCIs restricts experiments to no more than 100 kg of corium stimulants, whereas Severe Accidents could involve tonne-quantities of reactor materials. Such a wide extrapolation
is obviously a moot point. The total heat content of a melt-mass is broadly conserved as

_/

Surface area of melt/thermal capacity « (mass) 3 (5.16)

which implies that with increasing mass its average temperature remains higher for longer. It does not imply that the fraction of coarse mixture asymptotically approaches 1 as the total melt mass increases. Figure 5.1 and volcanic lava flows clearly reveal that a coarse mixture could not form where the melt’s surface is close to or below its freezing point. In fact the formation of a coarse mixture involves many physical processes such as heat diffusion in melt and coolant, heat transfer from it by principally radiation,[76] melt viscosity and contact mode of melt and coolant. The two-dimensional CHYMES code [197] was developed to consolidate the opinion that partial freezing would impose an upper bound on the fraction of coarse mixture if large quantities of corium were to be involved. Though the computed vapor flow rates match experiments [184], thereby suggesting a reasonable simulation of an evolving potentially pre-detonatable mass, the required upper bound did not become available. Nevertheless, present knowledge enables the convincing safety assessment in Section 5.7.

The analysis in Section 5.1 and later in Section 5.7 reveal that debris sizes must be less than about 250 mm for an effective contribution to the explosive energy of an MFCI, and that these fine particles largely stem from the fine fragmentation of a coarse mixture across a shock front. On this basis the CORECT2 and THINA experiments with a sodium coolant provide a 40% upper bound on the detonatable fraction of a coarse mixture [82,206]. However, urania-sodium experiments at AEEW show that hydrogen generated by the prerequisite cleansing of the solidified debris with methanol, and then its vacuum distillation, augments the fine debris fraction.[77] Moreover, unlike a urania-water interaction, a sodium coolant experiment very often produces a number of incoherent weaker detonations [86], which plausibly augment the recovered fine debris. Accordingly, urania-water experimental data
from Rig A and the Molten Fuel Test Facility at AEEW give the arguably more reliable upper bound of 20%. Figure 5.2 schematically illustrates the MFTF in which up to 20 kg of urania from a depleted uranium-molybdenum thermite mix is injected into water so as to replicate in some ways corium slumping into the residual water in a PWR pressure vessel. While the experimental and reactor contact modes are not too dissimilar, the pertinent volumetric ratios of corium to water differ by orders of magnitude. Specifically, experimental ratios

Release mechanism

Coo ant eve

Debris tray

vesse

Figure 5.2 The Molten Fuel Test Facility for the SUS Experiments

are at least 1:1000 but in a reactor situation[78] the ratio can be as low as 1:30. Experimental results show that explosive energies are markedly reduced when “fuel-rich” mixtures are involved because

i. A shortage of coolant restricts the formation of a coarse mixture.

ii. A reduced inertial constraint (a tamp) allows less durable heat transfer between melt particles and coolant.

Expert opinion [59,65] identifies Severe Accidents in PWRs that could probably result in molten corium slumping into residual water in the lower head at pressures in the range 1 to 155 bar. Data on MFCI at higher than atmospheric pressure is sparse, but available evidence [98,198] indicates that with increasing pressure more violent triggers are required and that the energy release is greater. An actual reactor experiment at EG and G-Idaho observed an MFCI at the highest recorded ambient pressure of 64 bar.

Section 4.5 describes the inception of a Severe Accident in a fast reactor subassembly, in which potential MFCI might occur in a radically different geometry from that of a PWR or the MFTF in Figure 5.2. SCARABAEE [200] and TRAN [201] tests establish that the sodium content of an affected subassembly would first vaporize before any melting of the steel-clad fuel pins. At this juncture MFCI are patently impossible. Due to decay heat the steel cladding subsequently melts at ‘1200 °C to be followed by the mixed oxide at ‘3000 °C. Because the boiling point of steel is around 3000 °C, its vapor condenses at the cooler subassembly inlet and outlet to form strong blockages. Molten corium is then considered to perforate the sub­assembly wrapper and thereby allow pressurized injections of molten corium into the inter-wrapper gap or a neighboring subassembly. So far significant quantities of molten fuel would not be involved, and therefore there would be little chance of damaging escalations. How­ever, due to fluid inertias, corium ejection would end by the develop­ment of a negative differential pressure around 2 bar which would encourage re-entry of liquid sodium. As part of a European research program [86] also involving the CORECT2 and THINA experiments,
the MFTF was modified as in Figure 5.3 to investigate this different contact mode.

These independent tests each elicited an erratic series of relatively small MFCIs. Furthermore, with the original unrestricted contact mode in Figure 5.2 just one of these SUS tests had any semblance of coherence with a principal interaction followed by a series of very much smaller ones.

Release mechanism

Upper nozzle

Charge container

To ballast vessel preset I to 10 bar

Charge injection

tube

Flow meter Containment vessel

capacity 1.7 m

Shroud tube

Argon gas

Wrapper

Sodium pool

Punch

Flow meter actuator

Sodium storage tank

Figure 5.3 General Arrangements for B-Series Experiments

Interactions with sodium therefore appear markedly different from and far less damaging than with a water coolant. This can reasonably be attributed to the two orders greater thermal diffusivity of sodium compared to water.[79] As described in Section 5.4, this enables a much higher heat transfer rate from the melt and by promoting localized freezing obstructs an essentially coherent propagation. The most probable outcome for this particular Severe Accident appears to be a less rapid and damaging pressurization of the reactor vessel by an erratic series of small MFCIs or by slower conventional[80] heat transfer from larger corium particles (a so-called Q*-event [202]). Finally, fast — reactor safety is enhanced by the 2000 tonne or so of sodium in its primary circuit. If 50 tonne of molten corium passively equilibrated with this coolant, the predicted temperature rise is only about 42 ° C, which emphasizes that the hazard of an MFCI resides in a localized almost coherent heat transfer to a vaporizable coolant.