Immersion in Wet Sand with Water Flooding

Increasing the thickness of both the rhenium liner and the uranium nitride pin were required in the reactor design to meet the safety conditions. During normal operations the rhenium had a negative effect on the k-effective but was countered by the extra fuel in the core. In two of the three accident scenarios, the neutron spectrum is more thermal than during normal operation due to the addition of water to the core. For the dry sand accident case, the spectrum is faster than the normal operation case. For the accident scenarios, the extra thermal absorption of neutrons from the additional rhenium dominated the effects from the additional fissionable fuel. 

In this scenario the reactor is immersed in water, and the gas flow region is flooded. It is assumed that the radial reflectors are all removed by any splashdown into water. This scenario results in the moderation of the fast neutrons and normally the increase in cross section with decreased neutron energy would result in an increase in reactivity. The inclusion of rhenium in the core between the fuel and the NblZr cladding negates this effect, as it is a Spectral Shift Absorber. SSA s are materials that are relatively transparent to neutrons in the fast spectrum but a massive absorber at the lower end of the energy spectrum [King, 2005]. 

The neutron multiplication factor (k-effective) was 0.964 0.001, well below the desired value of 0.985. 

In this scenario the core was immersed in wet sand (70% sand by volume, 1.924 g/cc) with the core flooded with water. This accident scenario includes a variety of negative aspects the water is moderating the neutrons to a significant degree, and the sand is an 

For this case the k-effective was 0.975 0.001 much closer to the margin of 0.985 

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