Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2002-02-06
2003-12-02
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S434000, C376S409000, C376S419000
Reexamination Certificate
active
06658078
ABSTRACT:
This invention relates to an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor employing UO
2
as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing a mixture of PuO
2
and UO
2
(Hereinafter referred to as a MOX nuclear fuel standing for a mixed oxide nuclear fuel) as the nuclear fuel and light water as the moderator/coolant. More specifically, this invention relates to an improvement brought out for the purpose to reduce the production cost of the MOX nuclear fuel assembly and to increase the value of the spent fuel of the MOX nuclear fuel assembly.
BACKGROUND OF THE INVENTION
Nuclear reactors available in the prior art are predominantly thermal neutron reactors wherein the nuclear fuel is a plurality of enriched UO
02
pellets containing fissionable U
235
more than the natural U and the moderator/coolant is light water. In order to prevent chemical reaction from occurring between the UO
02
pellets and light water, disc shaped UO
2
pellets having an approximate diameter of 1 cm and an approximate height of 1 cm are piled in a tube made of a zircaloy sheath, the zircaloy tube confining plural shaped UO
2
pellets being called a nuclear fuel rod. Along the external surface of the nuclear fuel rods, water is allowed to flow for the purpose to act as the moderator/coolant. A light water reactor having a capacity of e.g. 1,100 MW has as many as 50,000 nuclear fuel rods arranged in the reactor core cavity having a diameter of approximately 5 m in parallel one another, remaining space for allowing water to flow between each nuclear fuel rod. Since the quantity of the nuclear fuel rods is so many, it is inconvenient to treat them as a whole. Thus, a concept of grouping some quantity of the nuclear fuel rods, e.g., 60 through 100 nuclear fuel rods, was brought out and the group of nuclear fuel rods was defined as a nuclear fuel assembly. A light water reactor having a capacity of e.g. 1,100 MW has approximately 760 nuclear fuel assemblies. Plural spaces remain between the nuclear fuel rods for allowing water to flow in the nuclear fuel assemblies and for allowing control rods, e.g., boron rods and the like, to be arranged in the nuclear fuel assemblies.
Requirements for the foregoing nuclear fuel assembly are tabulated below.
1. The burn-up of a nuclear fuel assembly is required to be sufficiently large e.g. 40 through 55 GWd/ton during the actual lifetime of 4 through 5 years.
2. The distribution of volumetric power density or actually the heat distribution in the reactor core is required to be uniform for the entire space of each nuclear fuel assembly and resultantly for the entire space of the reactor core.
3. Each nuclear fuel assembly is required to be durable to safely confine the fission products within the nuclear fuel assembly.
Referring to a drawing, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employed for a thermal reactor employing UO
2
as the nuclear fuel and light water as the moderator/coolant, will be described below.
Referring to
FIG. 1
illustrating a horizontal cross-section of a nuclear fuel assembly designed to be employed for a UO
2
/light water reactor available in the prior art, symbols
1
,
2
,
3
,
4
and
5
show 5 independent kinds of UO
2
nuclear fuel rods. Each kind of UO
2
nuclear fuel rods contains UO
2
nuclear fuel of which the enrichment grade is same to each other or the UO
2
nuclear fuel contained in a kind of UO
2
nuclear fuel rods has a single enrichment grade, although the enrichment grade is different for each kind. The enrichment grade is reduced from symbol
1
toward symbol
5
. The quantities of symbols
1
,
2
,
3
,
4
and
5
are 10, 30, 4, 24, and 4 respectively. Each of the UO
2
nuclear fuel rods is approximately 4 m in length and approximately 11 mm in the external diameter. The nuclear fuel assembly has a cross-section which is a square of which the length of each side is approximately 15 cm. Symbol W shows a water rod having sides of approximately 4 cm. As is shown in the drawing, UO
2
nuclear fuel rods having a higher enrichment grade are arranged generally at the center of the reactor core and those having less enrichment grades are arranged surrounding the symbol
1
to turn out the enrichment grade gradually less in the outer space.
The philosophy of the arrangement of nuclear fuel rods in the nuclear fuel assembly designed for a UO
2
/light water reactor is summarized below.
1. Many kinds of UO
2
nuclear fuel rods, each kind being different in enrichment grade, are employed.
2. The enrichment grade of the UO
2
nuclear fuel contained in each kind of the UO
2
nuclear fuel rods is same and the enrichment grade is selected to meet the requirements to make the distribution of the volumetric power density or actually the heat distribution in the reactor, uniform for the entire space of each nuclear fuel assembly, and resultantly for the entire space of the reactor core. Specifically, the enrichment grade of each nuclear fuel rod is selected to be different from one another and is made higher at the center of the reactor core to be made gradually less at the outer space.
It is well known that U
235
contained in the UO
2
nuclear fuel contributes for generation of heat by fission and U
238
contained in the UO
2
nuclear fuel absorbs neutrons and transits itself to isotopes of a higher order or having a higher atomic number e.g. Pu
239
, Pu
240
, Pu
241
or Pu
242
.
Since Pu
239
and Pu
241
alone are fissionable, they are employable for causing fission either in the reactor currently employed or in other reactors after Pu
239
and Pu
241
are recovered from the spent fuel thereof by a spent fuel reprocessing process.
A mixture of oxides of the foregoing isotopes of a higher order than U
235
or having a higher atomic number than U e.g. Pu
239
, Pu
240
, Pu
241
and Pu
242
and of oxides of U is called a MOX nuclear fuel standing for a mixed oxide nuclear fuel.
Following the foregoing background, brought out was a concept of a nuclear reactor which is allowed to employ either an oxide of UO
2
alone or a MOX nuclear fuel. A MOX nuclear fuel assembly produced from the MOX nuclear fuel is the object of this invention.
Referring to a drawing, an exemplary arrangement of nuclear fuel rods in a MOX nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO
2
alone or MOX nuclear fuel will be described below.
Referring to
FIG. 2
illustrating a horizontal cross-section of a nuclear fuel assembly designed to be employable either for a UO
2
/light water reactor available in the prior art or for a MOX nuclear fuel/light water reactor available in the prior art, symbols P
1
, P
2
, P
3
and P
4
show 4 independent kinds of MOX nuclear fuel rods. Each kind of MOX nuclear fuel rods contains fissionable Pu having a single enrichment grade or the same enrichment grade. The fissionable Pu enrichment grade differs from one to another for each kind of nuclear fuel rods and is reduced from symbol P
1
wherein the fissionable Pu enrichment grade is approximately 5% toward symbol P
4
wherein the fissionable Pu enrichment grade is much less. The quantity of MOX nuclear fuel rods represented by symbols P
1
, P
2
, P
3
and P
4
is 20, 17, 8 and 3 respectively. Each of the MOX nuclear fuel rods is approximately 4 m in length and approximately 12 mm in the external diameter. The nuclear fuel assembly has a cross-section which is square of which the length of each side is approximately 15 cm. Symbols G
1
and G
2
show gadolinium fuel rods. The quantity of the gadolinium fuel rods G
1
and G
2
is 4 and 8 respectively. Thus, the MOX nuclear fuel rods accounts for 80% of the total quantity of the nuclear fuel rods employed in the nuclear fuel assembly. The function of the gadolinium fuel rods is to restrict fission from occurring at the beginning of the reactor operation period. In other words, the gadolinium fuel rods are effective to reduce the possi
Anegawa Takafumi
Mizokami Shinya
Takizawa Shin
Oliff & Berridg,e PLC
Palabrica R
Tokyo Electric Power Co.
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