Burnable poison gadolinium

The core has heterogeneous arrangement and uses dispersion-type nuclear fuel. Core consists of a set of FA and sets of reactivity control and safety rods. FAs incorporate burnable poison (gadolinium) rods to compensate the core excessive reactivity. The core uses smooth-pin type fuel element with a clad ng made of zirconium alloy. 

The third control is by use of a fixed burnable poison. This consists of rods containing a mixture of aluminum oxide and boron carbide, included in the initial fuel loading using the vacant spaces in some of the fuel assembhes that do not have control clusters. The burnable poison is consumed during operation, causing a reactivity increase that helps counteract the drop owing to fuel consumption. It also reduces the need for excessive initial soluble boron. Other reactors use gadolinium as burnable poison, sometimes mixed with the fuel. 

As one might expect, the nuclear industry has not been slow to put this property to good use and today gadolinium, in the form of its oxide, is an essential component of certain fuel systems where it is employed as a burnable poison, providing rapid core control under emergency conditions. 

Burnable poison material and form Rods on gadolinium basis... 

Different U-235 enrichments are used in fuel bundles to reduce local power peaking. Low enrichment uranium rods are used in corner rods and in the rods nearer the water gaps higher enrichment uranium is used in the central part of the fuel bundle. Selected rods in each bundle are blended with gadolinium burnable poison. The fuel rods are designed with the characteristics described below. 

Supplementary solid burnable poisons are used to assist in providing reactivity compensation for fuel burnup. For all operating cycles, the supplementary control is provided by gadolinium mixed into a portion of the UO2 reload fuel rods. 

The radial distribution of gadolinium, which is added to both PWR and BWR fuels as a burnable poison (see Section 1.1.2.), is also altered during fuel operation... 

Compensation of reactivity margin for fuel bum-up by gadolinium based burnable poisons ... 

Compensation of initial reactivity margin Fuel elements with burnable poison based on gadolinium... 

Burnable poison rods are based on the gadolinium like in icebreaker reactors, they provide a near-complete compensation of the burn-up reactivity swing. 

An alternative to poisoning the moderator is the inclusion of a burnable poison in or adjacent to the fuel elements themselves. The poison is chosen to have an absorption cross section and a concentration which cause it to burn out at a rate which will match the rate of loss of reactivity of the fuel. The elements which have been used as burnable poisons include boron and gadolinium (in the form of gadolinia, Gd203). 

Some fuel elements within the core contain gadolinium in the uranium dioxide fuel pellets and are used as burnable poisons. The geometric characteristics of such gadolinium fuel elements are similar to those of the core fuel elements. The gadolinium content in the fuel elements is identical with that in the VVER reactors. 

To suppress initial reactivity of the core, gadolinium (Gd203) is used in the fuel pellets as a burnable poison. The enrichment distribution in the CCR bundle is optimized to flatten the local power. The CCR core has several control cells where the control rods are inserted during operation. To minimize neutron leakage from the core, fuel assemblies with high bum-up fuel are shuffled to the periphery of the core. These design aspects are similar to the approach used in current BWRs. 

The addition of burnable poison is also needed to hold down the core reactivity at beginning of life and, thus, allow more plutonium in the fresh fuel. Other poisons such as boron or gadolinium could also be used in the fuel or cladding or as separate rods. Because the poisons and the plutonium are consumed over the cycle, it is not clear how the temperature coefficients change with exposure. This still needs to be examined. 

In addition to their intrinsic theoretical interest and as models for other systems, the oxides of the rare earths have many practical uses. They are receiving attention in industry because of their potential use as control rods for nuclear reactors where samarium, gadolinium and europium oxides are incorporated into cermets or are used in fuel elements as burnable poisons. Radioactive europium and thulium oxides are used as heat sources and promethium oxide as a jS source. Lanthanide oxide catalysts may be... 

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