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System and safety studies of accelerator driven transmutation ... - SKB

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3 ADS SOURCE EFFICIENCY STUDIES<br />

3.1 INTRODUCTION<br />

Efficient incineration <strong>of</strong> americium, curium <strong>and</strong> even higher acitindes require fast<br />

spectrum cores to yield a reduction <strong>of</strong> the overall radiotoxicity, avoiding production <strong>of</strong><br />

even heavier nuclides. However loading a fast reactor with large fractions <strong>of</strong> minor<br />

actinide fuels is lead to cumbersome <strong>safety</strong> issues. As an example, the Doppler effect<br />

vanishes with the removal <strong>of</strong> 238 U, <strong>and</strong> the situation is further worsened by<br />

introduction <strong>of</strong> americium. Further the effective delayed neutron fraction is<br />

significantly reduced.<br />

Subcritical core are commonly discussed as a means <strong>of</strong> overcoming the <strong>safety</strong> issues.<br />

Such a core would be operated with a large enough margin to criticallity to allow for<br />

sudden accidental reactivity insertions. External spallation neutron sources are<br />

envisioned to supply the extra neutrons needed to sustain a chain reaction. Typically,<br />

1 GeV neutrons impinging on a liquid metal target are foreseen. At impact, the<br />

protons produce neutrons via spallation reactions. These multiply in the target through<br />

further spallation reactions <strong>and</strong> through (n, xn)-reactions.<br />

The efficiency <strong>of</strong> the spallation source is an important parameter for the <strong>transmutation</strong><br />

system as a whole. The multiplication production <strong>of</strong> neutrons in the target <strong>and</strong> the<br />

subsequent multiplication <strong>of</strong> these neutrons in target <strong>and</strong> fuel region depends on a set<br />

<strong>of</strong> parameters such as target size <strong>and</strong> geometry, target material, proton beam energy et<br />

cetera. In our work designing subcritical system, the impact <strong>of</strong> the core coolant <strong>and</strong><br />

fuel on source efficiency have been recurrent questions.<br />

To study the impact on source efficiency <strong>of</strong> changes in fuel <strong>and</strong> coolant, we have<br />

modeled the two core designs suggested by the PDS-XADS project. Helium <strong>and</strong> leadbismuth<br />

eutectic (LBE) have been tested as coolants. MOX <strong>and</strong> a transuranium fuel<br />

have been tested as fuels.<br />

3.1.1 Source efficiency<br />

Seltborg suggested proton source efficiency to be defined as [13]:<br />

⎛ ⎞<br />

⎜<br />

1<br />

⎟<br />

< Fφs<br />

><br />

ψ * = − 1 ⋅ . (1)<br />

⎜ ⎟<br />

⎝<br />

keff<br />

⎠<br />

< S p ><br />

For a given keff, is approximately proportional to the total power produced in<br />

the core, <strong>and</strong> thus to the beam power amplification.<br />

The neutron source efficiency on the other h<strong>and</strong> relates the multiplication <strong>of</strong> “source<br />

neutrons” to fission multiplication. However, the definition <strong>of</strong> what is considered as<br />

source neutrons may vary inbetween different systems. This is why we choose to<br />

discuss effects on the proton source efficiency, which is uniqely defined.<br />

32

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