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JAEA-Data/Code 2007-004 - Welcome to Research Group for ...

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One of the physical problems associated with this type of fuel is a double heterogeneity<br />

through the self-shielding of the grain and also of the lattice configuration of fuel rods on the<br />

resonance absorption.<br />

Here, the “Accretion” method by Leslie & Jonsson 70) <strong>to</strong> calculate collision probabilities in a<br />

cluster-type fuel element is applied <strong>to</strong> evaluate the resonance absorption in the doubly heterogeneous<br />

system in the fuel block of the HTTR. The details will be found in the reference 26) <strong>to</strong>gether with some<br />

typical numerical examples.<br />

We assume that the fuel grains in a fuel compact are uni<strong>for</strong>mly distributed so that each coated<br />

particle, <strong>to</strong>gether with graphite diluent, <strong>for</strong>ms a two-region spherical cell (microscopic cell) containing<br />

only a fuel grain (region f) and the associated amount of graphite diluent (region m).<br />

The neutron slowing-down equations can be written by using the collision probabilities under<br />

the assumption that the neutron flux is flat in each spatial region and the neutron scattering is isotropic<br />

and elastic:<br />

V<br />

jΣ<br />

j<br />

( u)<br />

ϕ<br />

j<br />

( u)<br />

= ∑ Pij<br />

( u)<br />

Vi<br />

Si<br />

( u)<br />

,<br />

i<br />

(7.3.5-1)<br />

S ( u)<br />

=<br />

i<br />

∫<br />

u<br />

0<br />

Σ ( u'<br />

→ u)<br />

ϕ ( u'<br />

) du'<br />

i<br />

i<br />

, (7.3.5-2)<br />

where V i , Σ i , ϕ i , Σ i (u’→u), and S i are the volume, the <strong>to</strong>tal cross-section, the neutron flux, the<br />

differential scattering cross-section, and the slowing down source of the i-th region, respectively. The<br />

quantity P ij is the collision probability that a neutron emitted in the region i has the next collision in<br />

the region j, evaluated by assuming the flat flux in each region.<br />

BURNABLE POISON ROD (BPR)<br />

FUEL PELLET<br />

COATED PARTICLE<br />

HANDLING<br />

HOLE<br />

GRAPHITE BINDER<br />

DETAILED<br />

FUEL PELLET<br />

GRAPHITE SLEEVE<br />

GRAPHITE BLOCK<br />

SPARE HOLE FOR BPR<br />

Fig.7.3.5-1 Horizontal cross-section of the standard fuel block of the VHTRC<br />

VHTRC: Very High Temperature Reac<strong>to</strong>r Critical Assembly, decommissioned<br />

265

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