Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-03-17
2001-10-09
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S327000, C376S412000, C376S421000, C376S428000, C376S419000, C376S447000
Reexamination Certificate
active
06301320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with improvements in and relating to fuel rods and assemblies for nuclear reactors, principally of the mixed oxide fuel (MOX) type and in particular, but not exclusively, for pressurised water reactors (PWR).
2. Present State of the Art
Operators of nuclear reactors throughout the world increasingly wish to run their reactors on a longer fuel cycle than that previously entertained. Eighteen or even twenty four month cycles are now preferred between reloads. Whilst such runs call for more expensive fuel to be provided in the first place, cost savings are made in terms of the overall power generation from the fuel cycle as the shut down period for re-fuelling occurs less frequently. A re-fuelling cycle can take up to six weeks during which time the reactor is off line. The procedure also involves an experienced and consequently expensive team to be involved on site. In addition, re-fuelling is normally followed by having to obtain regulatory approval before the reactor can be brought on line once more.
Where longer fuel cycles are employed, to achieve the desired level of activity and hence power output from the reactor at the end of the cycle, the fuel must provide a higher reactivity level to start with. To provide even output over the cycle despite this higher initial reactivity a form of neutron poison, such as boron-10, must be provided at start up to inhibit the reactivity at this stage. The poisons involved, which may be of a variety of types, commonly act as neutron absorbers and consequently depress the reactivity whilst present. Whilst these poisons are essential during the initial part of the cycle, towards its end they must no longer be present as this would defeat the object by lowering the reactivity at the end of the cycle too.
Control of reactivity has previously been addressed in a number of ways in conjunction with solely uranium fuel reactors.
Control has been provided in the past by providing poison materials in the reactor at start up and during the initial part of the cycle and subsequently physically removing them as the cycle progresses. Clearly such an arrangement is mechanically and methodically complex.
Other control techniques have involved the provision of discrete poison rods which are inserted into the guide thimbles of a fuel assembly. Thus the fuel material is provided in one or more grades in rods and the poison is provided in a separate set of rods. Unfortunately, the provision of such discrete rods in the right amount to provide the desired level of reactivity control at the beginning of the cycle and yet not interfere with the end of the cycle is very difficult indeed. In addition, even if the poisons burn out during the subsequent part of the cycle the overall assembly is effectively unbalanced where the poison rod used to be. This lack of balance can present itself as hot spots which are highly undesirable and can lead to stresses which deform the assembly in that region. Additionally, these “empty” spaces formerly occupied by the poison rods are not available for water passage and consequently the transfer rate is diminished.
In a further prior art system UO
2
fuel rods only have been provided in conjunction with lower grade UO
2
which contains the poison; 6 wt % poison in 1.8 wt % U
235
. The poison is provided as a part of the fuel or alternatively as a coating around the circumference of the pellets. Construction of such assemblies is complex and time consuming as different levels and contents are frequently employed for different parts of the reactor fuel cell. Even longitudinally, within a given rod the enrichment grade and poison presence may vary.
The prior art problems are particularly acute in MOX reactor cores as MOX itself is a bigger absorber of neutrons than UO
2
. As a consequence poison provision using discrete rods is even harder to control than in other fuel types as the burn out rates for poisons in such MOX reactors is lower. If combined UO
2
poison rods are employed in such reactor systems a further problem occurs where the poison does eventually burn out due to the differential absorption properties of MOX and UO
2
. As a consequence power peaking around the formerly poison containing UO
2
rod occurs at a later date. Once again, this can lead to undesirable hot spots within the core leading to undesirable stresses and potential melting of the fuel rods.
Similar problems occur where discrete poisoning is provided within fuel rods or pellets, for instance as a central core. Neutron access for the poison is poor in such cases and once burnt out gives significant variations with location.
Poison systems have not successfully been employed in MOX fuelled reactors to date. At present at least 3 plutonium grades are provided in the fuel rods.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
Amongst the aims of the present invention it attempts to provide an improved fuel assembly, fuel rods, a method of reactor control and improved fuel regime.
According to a first aspect of the invention we provide a fuel rod, said fuel rod containing mixed oxide fuel and a neutron poison.
The provision of the neutron poison in the fuel rod with the mixed oxide fuel has advantages in simplifying the production technique.
Preferably the neutron poison is gadolinia (Gd
2
O
3
) or substantially consists of gadolinia. The gadolinia may be present at between 0.25 and 3 weight per cent, or 0.5 and 2.5 weight per cent compared with the total fuel rod contents. The provision of between 0.75 and 1.75, and most preferably 1 and 1.5 weight per cent is particularly preferred. The reduced level of gadolinia employed, whilst effectively providing the desired degree of neutron absorbtion, does not significantly detract from the thermal conductivity of the fuel rod.
Preferably the mixed oxide fuel accounts for the remainder of the fuel contents. The plutonium content of the fuel rod may be between 3% and 12% and most preferably 6% and 10%. The balance of the fuel rod contents preferably comprise depleted UO
2
.
Preferably the neutron poison is mixed with the mixed oxide fuel, most preferably intimately. Homogenous dispersion of the poison in this way in the MOX is highly advantageous. The possibility also exists to coat the mixed oxide fuels around its circumference with the neutron poisons. The fuel rod may contain a middle segment containing mixed oxide fuel and the neutron poison with a mixed oxide only portion being provided at one or either end of the fuel rod. The portion provided with the neutron poison, and in particular gadolinia, may comprise between 40% and 95% of the fuel rod length measured from end cap to end cap. In a particularly preferred form the portion containing the gadolinia is between 75 and 85% of the length.
The fuel rod may be for a pressurised water or boiling water reactor.
According to a second aspect of the invention we provide a fuel assembly comprising a multiplicity of fuel rods wherein the assembly comprises a first fuel rod type provided with mixed oxide fuel and a second fuel rod type provided with mixed oxide fuel in conjunction with a neutron poison.
A MOX fuel assembly provided in this way offers controlled reactivity during the initial phase of the cycle and yet offers longer cycles than previously possible.
Preferably the neutron poison is gadolinia. The gadolinia is preferably present as between 0.5 and 2.5 weight per cent compared with the total fuel content of a second type rod.
Preferably the first type rod contains mixed oxide fuel only. Preferably the plutonium content of such first type rods is between 3% and 12%, most preferably 6 to 10%, by weight compared with the total fuel content of the first type rod. The remaining material in the first type rod is preferably depleted UO
2
.
Preferably the mixed oxide of the second type rod contains between 1.5% and 8%, preferably 3 to 6% plutonium.
The plutonium content of the second type is preferably between 20% and 75% that of the first type, for instance 25 to 50%.
The plutonium conten
Hesketh Kevin Wynn
Thomas Gwilym Michael
British Nuclear Fuel plc
Jordan Charles T.
Richardson John
Workman & Nydegger & Seeley
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