THE USE OF ENRICHED URANIUM TO MODIFY COOLANT VOID REACTIVITY IN CANDU

BY

A.R. Dastur, P.S.W. Chan and S Girgis

Atomic Energy of Canada Limited CANDU Operations 0 Sheridan Park Research Community Mississauga. Ontario L5K lB2

Studies carried out to evaluate the feasibility of the Slightly Enriched Uranium (SEU) cycle in CANDU indicate potential for improved economics. The improvement is due in part to uprating of the power level (and reduction in capital cost) through Global Differential Enrichment (GDE) and in part to the increase in fuel burnup with the consequent reduction in fuel fabrication and disposal costs.

However~, the power uprating is usually accompanied by changes in the channel thermohydraulics which lead to an increase of the power coefficient of reactivity and thereby of reactor l instability. In this paper we address the question of'increased l reactor instability at uprated power levels by describing a method of using enriched uranium to modify the coolant void and power coefficients of reactivity, the latter being a parameter that has a direct influence on the stability of the neutron flux and power distribution. The feedback reactivity due to changes in power level consists mainly of two components, the coolant void reactivity (a positive effect) and the fuel temperature reactivity (a negative effect). Coolant voiding softens the neutron spectrum in the fuel as thermalization by the coolant is reduced. This reduces the f,lux in the U-238 resonance energy region and also increases the neutron reproduction factor of the fuel at thermal energies. The change in thermal neutron flux shape across ~the bundle is minimal.

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In the proposed method, the power coefficient is modified by reducing the void reactivity coefficient. This is achieved by adjusting the fuel composition such that a negative reactivity component is added to the void coefficient. This negative component is created by adding neutron absorbing material to the fuel (UO2) and simultaneously distributing the fissile material amongst the fuel pins such that voiding of the coolant leads to an increase in the neutron flux level in the added absorber.

The economic penalty of the added absorber is small compared with the savings from the additional uprating of the power level that becomes feasible with a lower power coefficient. The penalty is small as fuel fabrication and disposal costs are high compared with enrichment costs and especially with the incremental enrichment cost required to compensate for the additional absorption. The effectiveness of this methodology is illustrated in the attached figure where the lattice reactivity change per percent increase in power level is compared for the reference (no absorber) and modified fuel bundles. The modification in this case consists of the addition of 6 wt% of gadolinium to the central fuel pin of a 37-element bundle~accompanied with redistribution of 1.4 wt% U-235 across the bundle. An additional power uprating of 5% is possible without affecting the neutronic stability of the system. With 6 wt% gadolinium in the central pin, the lattice void reactivity drops from 13.4 to 7.6 millik. The penalty in incremental enrichment to compensate for the added gadolinium is 0.1 wt% of U-235. This represents a 5% increase in fuelling cost. Further reductions are indicated with increased absorber concentrations.

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EFFECT OF NEUTRON POISON ON LATTICE POWER COEFFICIENT 14290003