Burnable poison

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. 

Chemical shim control is effected by adjusting the concentration of boric acid dissolved ia the coolant water to compensate for slowly changing reactivity caused by slow temperature changes and fuel depletion. Eixed burnable poison rods are placed ia the core to compensate for fuel depletion. 

These are made of boron carbide ia a matrix of aluminum oxide clad with Zircaloy. As the uranium is depleted, ie, burned up, the boron is also burned up to maintain the chain reaction. This is another intrinsic control feature. The chemical shim and burnable poison controls reduce the number of control rods needed and provide more uniform power distributions. 

The low-power-density, low enrichment reactor core uses soluble boron and burnable poisons for shutdown and fuel bumup reactivity control. Low worth grey rods provide load following. A heavy uranium flywheel extends the pump coastdown to allow for emergency action during loss-of-flow transients. 

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. 

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

The Pressurized Water Reactor (PWR) reload core optimization problem, though easily stated, is far from easily solved. The designer s task is to identify the arrangement of fresh and partially burnt fuel (fissile material) and burnable poisons (BPs) (control material) within the core which optimizes the performance of the reactor over that operating cycle (until it again requires refueling), while ensuring that various operational (safety) constraints are always satisfied. 

The first row of symbols in each square is the serial number of the assembly. The first symbol is the fuel lot number lot 1 contains 2.25 w/o U lot 2 contains 2.8 w/o and boron burnable poison and lot 3 contains 3.3 w/o and burnable poison. The second... 

At the end of cycle 1, 64 of the 65 assemblies of lot 1 (called lot lA) are removed. One of the IGC assemblies (called lot IB) that had the lowest burnup of the lot 1 group is moved to the central AA position. Residual burnable poison is removed from the remaining lot 2 and lot 3 assemblies, which are shifted to the new positions shown in Fig. 3.21. Sixty-four new assemblies (called lot 4), enriched to 3.2 w/o U and containing no burnable poison, are placed in the positions with heavy borders in this figure. This placement of assemblies was... 

Fuel element UOj pellets, 1.43 cm diameter 49 rods per assembly 444 assemblies in core enrichment 2.6% Gd203 burnable poison zircaloy cladding 0.8 mm. 

Because of such effects, spent uranium fuel elements from PWR, BWR, HWR, GCR and FBR differ in composition both from each other and between fuel batches from the same reactor. Furthermore, the composition differs betwe pins in the same fuel elemrat and for each pin also along its Iragth, especially when initial burnable poison concentration and enrichm t is graded along pins. The differraice is not so large that very different fuel... 

Boron is a useful control material for thermal (and other) reactors. The very high thermal-absorption cross section of (boron-10) and the low cost of boron has led to wide use of boron-containing materials in control rods and burnable poisons for thermal reactors. The absorption cross section of boron is large over a considerable range of neutron energies, making it suitable for not only control materials but also for neutron shielding. 

Boron may also be used as a burnable poison to compensate for the change in reactivity with lifetime. In this scheme, a small amount of boron is incorporated into the fuel or special burnable poison rods to reduce the beginning-of-life reactivity. Bumup of the poison causes a reactivity increase that partially compensates for the decrease in reactivity due to fuel burnup and accumulation of fission products. Difficulties have generally been encountered when boron is incorporated directly with the fuel, and most applications have used separate burnable poison rods. 

Advantages Very high thermal-absorption cross-section and low cost. Commonly used in thermal reactors for control rods and burnable poison. 

Burnable poison is adopted in the reactor, so boron concentration control and regidation systenr is need no longer. 

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 core reactivity is controlled by a combination of fixed lumped burnable poison (LBP) and movable poison. The LBP consists of boronated graphite rods located in the active core. The movable poison consists of two diverse, independent control devices of different design principles, each with the... 

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