05.08.2013 Views

ORNL-4191 - the Molten Salt Energy Technologies Web Site

ORNL-4191 - the Molten Salt Energy Technologies Web Site

ORNL-4191 - the Molten Salt Energy Technologies Web Site

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

shown as solid points in Fig. 4.1 (points shown<br />

only for <strong>the</strong> MSRE 3U spectra). The calculated<br />

points are connected by straight line segments. As<br />

might be expected from <strong>the</strong> gross similarities between<br />

<strong>the</strong> two reactor systems (temperatures, moderating<br />

materials, total salt volume fractions),<br />

<strong>the</strong>re is a marked similarity between <strong>the</strong> two<br />

spectra. Above energies of about 1 kev, <strong>the</strong> NISBK<br />

flux spectra lie above that of <strong>the</strong> MSIIE, due to <strong>the</strong><br />

smaller neutron leakage from <strong>the</strong> MSBR core. Below<br />

about I ltev <strong>the</strong> spectrum in <strong>the</strong> MSRR becomes progressively<br />

reduced by resonance neutron absorption<br />

in <strong>the</strong> fertile material, and <strong>the</strong> two flux spectra<br />

tend to approach one ano<strong>the</strong>r in magnitude.<br />

Result.:; of TIIEriMOS <strong>the</strong>rmal spectra calculations<br />

are shown in Figs. 4.2 and 4.3. Figure 4.2 conipares<br />

<strong>the</strong> MSRE spectrum for <strong>the</strong> 233U fuel salt<br />

with that for <strong>the</strong> current 235U fueI loading. The<br />

spectra at <strong>the</strong> center of <strong>the</strong> fuel channel are chosen<br />

as a basis of comparison. In Fig. 4.3, <strong>the</strong> MSRE<br />

<strong>the</strong>rmal spectrum with <strong>the</strong> 2 3 3 fuel ~ salt is compared<br />

with <strong>the</strong> corresponding spectra at several<br />

points in <strong>the</strong> MSBR lattice, Note that. curve A has<br />

<strong>the</strong> same relative position in <strong>the</strong> MSBR lattice<br />

(center of <strong>the</strong> larger fuel salt channel) as that of<br />

<strong>the</strong> MSKE. For bo1.h <strong>the</strong>se reactors <strong>the</strong> flux spectra<br />

of Figs. 4.2 and 4.3 are nonnalized to give <strong>the</strong><br />

same total integrated neutron flux below 1.86 ev,<br />

averaged over <strong>the</strong> total volume of salt and graphite.<br />

On this basis, relative comparisons can be made of<br />

<strong>the</strong> energy (or lethargy) dist1ibutions and position<br />

dependence of <strong>the</strong> spectra in each reactor. One<br />

should note, however, that <strong>the</strong> actual magnitudes<br />

53<br />

’ 2 5 10‘ 2 5 io2 2<br />

ENERGY (evl<br />

of <strong>the</strong> total <strong>the</strong>rmal flux wi.11 be quite different in<br />

<strong>the</strong> systems, depending on <strong>the</strong> relative fission<br />

densities and <strong>the</strong> fissile material concentrations.<br />

In Fig. 4.2, <strong>the</strong> slight “energy hardening,” or<br />

preferential removal of low-energy neutrons for <strong>the</strong><br />

235U fuel loading, reflects <strong>the</strong> larger uranium concentration<br />

for this case (-0.9 mole % to1.A UF,,<br />

33% enriched in 235U). Again, in Fig. 4.3 <strong>the</strong><br />

higher concentration and absorptions in fissile<br />

uranium in <strong>the</strong> MSBR and also <strong>the</strong> absorptions in<br />

<strong>the</strong> fertile salt produce a hardening of <strong>the</strong> energy<br />

spectrum as compared with <strong>the</strong> NISRI.:. Just us in<br />

<strong>the</strong> case of <strong>the</strong> epi<strong>the</strong>rmal spectra, however, <strong>the</strong><br />

similar temperatures and gross material compositions<br />

result in a marked similarity in <strong>the</strong> spectra.<br />

An alternate means of comparing <strong>the</strong> neutron<br />

spectra in <strong>the</strong> two systems is shown in Figs. 4.4<br />

and 4.5. Here <strong>the</strong> calculated group fluxes have<br />

been multiplied by <strong>the</strong> corresponding group microscopic<br />

fission cross section for 2331J, summed<br />

over <strong>the</strong> energy groups above each energy point,<br />

and normalized to one fission event in 2 3 3 ~<br />

occurring in each of <strong>the</strong> <strong>the</strong>rmal (E< 1.86 ev) and<br />

epi<strong>the</strong>rmal (E > 1.86 ev) energy regions. In <strong>the</strong><br />

latter case, most of <strong>the</strong> fission reactions occur<br />

between <strong>the</strong> lower cutoff energy and about 30 ev,<br />

where <strong>the</strong> flux spectra are quite close to one<br />

ano<strong>the</strong>r (Fig. 4.1). As a result, <strong>the</strong>re is a close<br />

correspondence in <strong>the</strong> normalized distribution of<br />

epi<strong>the</strong>rmal fission events in <strong>the</strong> two systems.<br />

Larger differences are encountered in <strong>the</strong> <strong>the</strong>rmal<br />

fissions, as shown in Fig. 4.5. Mote, however,<br />

that <strong>the</strong> spectra at <strong>the</strong> center of <strong>the</strong> fuel channels<br />

5<br />

ORUl.--DWG 67-11796<br />

-7- -m<br />

1G3 2<br />

Fig. 4.4. Normalized Distributions of Epi<strong>the</strong>rmal Fission Events in <strong>the</strong> 233U-Fueled MSRE and in <strong>the</strong> MSBR.<br />

5

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!