Transuranium element transmuting reactor core

In a transuranium elements transmuting reactor core in which a reactor is charged with a plurality of fuel assemblies at a core and an amount of a transuranium element to be added is controlled so as to prevent a fuel element contained in the fuel assemblies from melting, the amount of the transuranium elements to be added to the fuel element is controlled so as to keep an excess reactivity of the reactor substantially zero through an operation of the reactor. A charging density of minor actinides is set to lessen outwards of a core central portion in a core area where a plutonium content is made even. The charging density of minor actinides is set high accordingly in an area where a plutonium is enriched high at the core of a plutonium enriched area where a plutonium content varies. A transuranium elements transmuting fuel pin is formed by charging a transuranium fuel material in a fuel clad and the transuranium fuel material includes at least one of fuel materials consisting of an enriched uranium and an uranium-plutonium mixed fuel and a fertile material consisting of a natural uranium and a depleted uranium contain transuranium elements. In a transuranium elements transmuting assembly including a wrapper tube and a plurality of fuel pins enclosed in the wrapper tube, each of said fuel pin including a fuel clad. At least one part of the fuel pins are formed by charging a transuranium fuel material in the fuel clad with a transuranium fuel material inside.

BACKGROUND OF THE INVENTION 
The present invention relates to a technology for transmuting transuranium 
elements and more particularly to a transuranium transmuting reactor core 
for transmuting the transuranium elements at a fast reactor and also to a 
transuranium elements transmuting fuel pin and fuel assembly charged into 
a reactor core of a fast reactor. 
A spent fuel discharged from a thermal reactor such as boiling water 
reactor or the like includes transuranium elements (hereinafter called TRU 
elements) such as neptinium-237 (.sup.237 Np), americium-241 (.sup.241 
Am), americium-243 (.sup.243 Am), curium-242 (.sup.242 Cm), curium-244 
(.sup.244 Cm) and others which are high-level radioactive wastes, and in 
minor actinides (hereinafter called MA elements) present after eliminating 
plutonium (Pu) from the TRU elements, there exists elements such as 
.sup.237 Np, .sup.241 Am, .sup.243 Am or the like having an extremely long 
half life such as 2.14 million Years, 432 years, 7,380 years, which cannot 
be quenched within a short period of time. Thus, it is desired that the MA 
elements are transformed into elements with a short half life through a 
nuclear transmutation in a short period of time. 
A prior art includes technique for transmuting the TRU element which 
comprises using a fast reactor extremely high in a neutron energy as 
compared with a thermal reactor and subjecting the TRU elements charged 
into a fuel charged in a core of the fast reactor to a nuclear 
transmutation ((1) "Conceptional Design Study on Actinide burning Fast 
Reactor", T. Osugi et al., JAER1-M 83-217, issued by Japan Atomic Energy 
Research Institute in December 1983; (2) "Transmutation of Transuranics in 
FBR", A. Sasahara, T. Matsumura, F7, Fall Meeting Reports, Atomic Energy 
Society of Japan, 1988). 
The prior art TRU elements transmuting comprises transmuting the 
aforementioned MA elements by causing a transmutation shown in FIGS. 9A to 
9C to the typical MA elements of .sup.237 Np, .sup.241 Am and .sup.243 Am 
which are main objects of transmuting at a fast reactor core. 
In FIGS. 9A to 9C, F.P. denotes fission products, and elements given in a 
square border around indicates that of being easy to cause a fission 
against a neutron energy in the fast reactor, namely, that its energy 
averaged fission cross-sections are about 1 burn or over. 
The prior art TRU elements transmuting process utilizes a feature of the 
fast reactor core effectively, and the feature comes in: 
(1) Since a neutron energy of the fast reactor core is high, a neutron 
capture is hard to occur in .sup.237 Np, .sup.241 Am and .sup.243 Am and 
the like, and thus an evil influence of the fast reactor on a neutron 
economy according to the charging of the TRU elements into the reactor 
core is relatively small (a neutron capture cross-section getting small 
according as the neutron energy becomes high as shown in FIG. 18). 
(2) The fast reactor is generally high by about 1 digit in a neutron flux 
level as compared with the thermal reactor, therefore the TRU elements can 
be subjected to a nuclear transformation even if a fission and neutron 
capture cross section on an energy average is small, and thus a high 
transmuting efficiency of the TRU elements is ensured. 
In the prior art transmuting of the TRU elements, nothing has been taken 
particularly into consideration for charging amount of the TRU elements 
charged into a fast reactor core and its distribution in core when 
carrying out a transmuting of the TRU elements. Still, however, only a 
self-evident technical care on charging the core with the TRU elements as 
much as possible has been considered for enhancing a transmuting 
efficiency of the TRU elements. 
However, if the fast reactor core is charged with the TRU elements as much 
as possible, then the following problems are capable of resulting 
therefrom. 
(1) If the MA elements to be transmuted is added to uranium-plutonium mixed 
fuel, a melting point of the mixed fuel lowers. Then the melting point 
drop is capable of causing a fuel melting, thus a measure such as lowering 
a reactor power or the like will be necessary for avoiding the fuel 
melting, which may deteriorate the transmuting efficiency of the MA 
elements. 
(2) As will be apparent from FIGS. 9A to 9D, the typical MA elements to be 
transmuted is generally hard to bring about a fission, and hence is 
transformed into fissionable elements by a neutron capture. Accordingly, 
if the fast reactor core is charged with the TRU element excessively much, 
then, as shown in FIG. 19, an amount of fissionable elements produced 
newly by the neutron capture of the MA element according to a neutron 
irradiation comes to exceed fissionable elements transmuted by fission, 
thus an excess reactivity of the fast reactor increasing. 
Consequently, if the charging amount of the TRU elements and its 
distribution are not specified properly, an excessive change or distortion 
may arise on a reactor power distribution and a neutron flux distribution, 
thus leading to problems on safety and characteristics of the reactor. 
(3) The TRU elements to be transmuted are easy to cause an alpha-decay in 
most cases, and an alpha ray energy emitted at the time of the alpha-decay 
is relatively high at 4 to 6 MeV generally. Accordingly, if the MA 
elements are added much to a fuel, a calorific value and a source 
intensity of gamma ray, neutron and others become excessive from the state 
of a fresh fuel before loading into the fast reactor core. Further, at the 
time of assembling, storage and transportation of new fuel assemblies in 
which the MA elements are enclosed, a heat removing of the alpha ray 
energy becomes difficult and the fuel overheats to lead to a failure in a 
worst case. 
(4) When charging a fast reactor uniformly with the TRU elements to be 
transmuted at the core with a core for which a plutonium enrichment is one 
kind as a base, a radial distribution of the power density, namely a 
radial power distribution during operation of the reactor becomes small 
according as it comes outside, as shown in FIG. 20, therefore a 
transmuting efficiency of the TRU element and a plant power generation 
efficiency being unsatisfying. 
(5) When charging the reactor uniformly with the TRU elements at the core 
with the fast reactor core for which a plutonium enrichment is two or more 
than two kinds as a base, a radial power distribution of the core is 
improved as compared with FIG. 20 by an adjustment of the plutonium 
enrichment, a flatting requirement can thus be satisfied, however, as 
shown in FIG. 21, for example, there arises a portion where the power 
distribution largely fluctuates according to burn-up. 
On the other hand, a flow rate of a coolant flowed for cooling down the 
fast reactor core is constant through the lifetime of a reactor plant. The 
flow rate of the coolant to fuel assemblies is set adaptively to the time 
when the power is maximized. Thus, when the output distribution fluctuates 
largely according to the burn-up of the fuel, a heat removing efficiency 
deteriorates, a heating efficiency gets lowered furthermore, which is not 
preferable from the viewpoint of an economical operation of the reactor 
plant. 
SUMMARY OF THE INVENTION 
An object of the present invention is to substantially eliminate defects or 
drawbacks encountered in the prior art described above and to provide a 
transuranium elements transmuting reactor core capable of transmuting the 
TRU elements efficiently without causing a failure of the fuel assemblies, 
increase of excess reactivity, deterioration of thermal efficiency and 
others. 
Another object of the present invention is to provide a transuranium 
element transmuting fuel and fuel assembly capable of preventing the 
lowering of the power density of a fast reactor and the distortion of the 
power distribution of the fast reactor and effectively transmuting the TRU 
elements. 
These and other objects can be achieved according to the present invention 
by providing, in one aspect, a transuranium element transmuting reactor 
core in which a reactor is charged with a plurality of fuel assemblies at 
a core and an amount of a transuranium element to be added is controlled 
so as to prevent a fuel element contained in the fuel assemblies from 
melting, and in the improvement, the amount of the transuranium elements 
to be added to the fuel assemblies is controlled so as to keep an excess 
reactivity of the reactor substantially zero through an operation of the 
reactor. 
In another aspect, there is provided a transuranium element transmuting 
reactor core in which a reactor is charged with a plurality of fuel 
assemblies at a core and an amount of a transuranium element to be added 
is controlled so as to prevent a fuel element contained in the fuel 
assemblies from melting, and in the improvement, charging amounts of 
.sup.242 Cm, .sup.244 Cm and .sup.241 Am are set so as to satisfy an 
equation 
EQU 1.2.times.10.sup.2 .times.M.sub.242 +2.8.times.M.sub.244 
+1.1.times.10.sup.-1 .times.M.sub.241 &lt;Q.sub.1 
where an upper bound of heating rates of the single fuel assembly outside 
the reactor is.sub.1 Q from the view point of the fuel assembly integrity, 
charging amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am and also 
satisfy an equation 
EQU 1.2.times.10.sup.2 .times.M.sub.242.sup.L +2.8.times.M.sub.244.sup.L 
+1.1.times.10.sup.-1 .times.M.sub.241.sup.L 21 Q.sub.2 
where an upper bound of the heating rates, per unit length of the fuel 
pellet contained in the fuel pins is Q.sub.2 from the view point of the 
fuel element integrity, charging amounts of .sup.242 Cm, .sup.244 Cm and 
.sup.241 Am per the unit length are M.sub.242.sup.L, M.sub.244.sup.L and 
M.sub.241.sup.L. 
In a further aspect, there is provided a transuranium element transmuting 
reactor core in which a reactor is charged with a plurality of fuel 
assemblies at a core and an amount of transuranium elements to be added is 
controlled so as to prevent a fuel element contained in the fuel 
assemblies from melting and in the improvement, a charging density of 
minor actinides is set to lessen outwards of a core central portion in a 
core area where a plutonium content is made even. 
In a still further aspect, there is provided a transuranium element 
transmuting reactor core in which a reactor is charged with a plurality of 
fuel assemblies at a core and an amount of transuranium elements to be 
added is controlled so as to prevent a fuel element contained in the fuel 
assemblies from melting and in the improvement, a charging density of 
minor actinides is set high accordingly in an area where a plutonium is 
enriched high at the core of a plutonium enriched area where a plutonium 
content varies. 
In a still further aspect, there is provided a transuranium element 
transmuting fuel pin wherein a transuranium fuel pin is formed by charging 
a transuranium fuel material in a fuel clad and the transuranium fuel 
material includes at least one of fuel materials consisting of an enriched 
uranium and a uranium-plutonium mixed fuel and a fertile material 
consisting of a degraded uranium, a natural uranium and a depleted uranium 
contain transuranium elements such as Np, Am and Cm. 
In a still further aspect, there is provided a transuranium element 
transmuting fuel assembly including a wrapper tube and a plurality of fuel 
pins enclosed in the wrapper tube, each of the fuel pins including a fuel 
clad, wherein at least one part of the fuel pins are formed by charging a 
transuranium fuel material in the fuel clad with a transuranium fuel 
material inside. 
In a preferred embodiment, the fuel pins enclosed in the wrapper tube 
comprises transuranium fuel pins charged with the transuranium fuel 
material and fuel material pins charged with a fuel material consisting of 
an enriched uranium and a uranium-plutonium mixture fuel, and a 
radioactive fission product such as Sr or alkaline metals is contained in 
the transuranium fuel material. 
In the transuranium element transmuting reactor core according to the 
present invention, since an amount of transuranium elements to be added to 
a fuel pin of the fuel assemblies is controlled so as to keep an excess 
reactivity of the reactor substantially zero through an operation of the 
reactor, a decrease of effective multiplication factor according to the 
lapse of time for operation will be prevented, an excessive deterioration 
or turbulence of the reactor power distribution can be prevented, and as 
looking for improvement of a power plant capacity factor from enhancing a 
reliability of the plant, transuranium elements (TRU elements) can be 
transmuted efficiently. 
Further, from setting loading amounts of .sup.242 Cm, .sup.244 Cm and 
.sup.241 Am so as to realize: 
EQU 1.2.times.10.sup.2 .times.M.sub.242 +2.8.times.M.sub.244 
+1.1.times.10.sup.-1 .times.M.sub.241 &lt;Q.sub.1 
where an upper bound of the single fuel assembly power assembly outside the 
reactor is.sub.1 Q from the view point of the fuel assembly integrity, 
loading amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am which can be 
loaded into the single fuel assembly are M.sub.242, M.sub.244 and 
M.sub.241, and also to realize: 
EQU 1.2.times.10.sup.2 .times.M.sub.242.sup.L +2.8.times.M.sub.244.sup.L 
+1.1.times.10.sup.-1 .times.M.sub.241.sup.L &lt;Q.sub.2 
where an upper bound of the heating per unit length of the fuel element 
contained in the fuel assemblies is Q.sub.2 from the view point of the 
fuel element integrity, charging amounts of .sup.242 Cm, .sup.244 Cm and 
.sup.241 Am per the unit length are M.sub.242.sup.L, M.sub.244.sup.L and 
M.sub.241.sup.L, a melting of the fuel element during operation of the 
reactor and an overheating or failure of the fuel assemblies outside the 
reactor can effectively be prevented, and an accident of a control rod and 
a neutron absorbing material of the control rod can be reduced by a 
neutron absorption effect of .sup.242 Cm, .sup.244 Cm and .sup.241 Am, an 
enhancement of heat removing efficiency of the core can thus be realized, 
an economical operativity is also improved, and a safety and reliability 
of the core and the fuel assemblies are ensured as well, thus transmuting 
the TRU elements efficiently. 
Further, by setting a charging density of minor actinides to lessen 
outwards of a core center in a core area where a plutonium content is 
even, and also by setting a charging density of minor actinides high 
accordingly in an area where Pu is enriched high at the core of a 
Pu-enriched area where a plutonium content varies, a flatting requirement 
of a radial distribution of the reactor power can be satisfied, an 
enhancement of safety and reliability of the core and the fuel assemblies 
will be realized without causing the excessive deterioration and 
turbulence of the reactor power distribution, thus transmuting the TRU 
elements efficiently. 
In a further aspect, according to the transuranium element transmuting fuel 
assembly of the characters described above, even if the transuranium fuel 
material is charged in the transuranium fuel pin, the degradation of the 
core power density and the distortion of the core axial power distribution 
can be effectively prevented, thus improving the core cooling efficiency 
and effectively transmuting the transuranium element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of a transuranium element transmuting reactor core relating 
to the present invention will now be described with reference to the 
accompanying drawings. 
FIG. 1 represents a basic conception of a nuclear reactor such as pool type 
fast reactor enclosing the transuranium elements transmuting reactor core 
of the present invention. The present invention is applicable to a core of 
the nuclear reactor of a type to remove heating from cooling the core by 
flowing a coolant thereto and the coolant then includes liquid Na, liquid 
NaK, He gas and the like, which is intended for the core for which a 
fission is caused chiefly by a fast neutron. 
FIG. 1 represents an example of a pool type fast reactor, however, a loop 
type fast reactor may be exemplified, needless to say, in this case. The 
nuclear reactor includes a reactor vessel 10 which is charged with a 
liquid sodium (Na) 11 as a coolant. A core 12 is provided at a central 
portion in the reactor vessel 10, and a upper core structure 13 is 
provided above the core 12. The upper core structure 13 is supported by a 
roof slab 14 working as a shielding plug to cover a top of the reactor 
vessel 10. A primary coolant pump 15 and an intermediate heat exchanger 16 
and others are suspended on the roof slab 14. A cover gas such as inert 
gas or the like is enclosed between the roof slab 14 and a free liquid 
surface of the liquid Na 11. 
The core 12 enclosed in the reactor comprises, as shown in FIG. 2 for 
example, two region cores 12a, 12b. A fuel assembly 17 relatively high in 
plutonium content is disposed in the outer peripheral area 12a and a fuel 
assembly 18 relatively low in plutonium content is disposed in the central 
area 12b. For charging amount of the MA elements of the TRU elements to be 
transmuted which is charged into the fuel assemblies 17, 18, a charging 
density of the MA elements is controlled at every areas or fuel assemblies 
17, 18. A reference numeral 19 denotes a control rod. 
As exemplified in FIG. 3, the fuel assemblies 17, 18 have a plurality of 
fuel pins 21 enclosed in a bundle within a hexagonal wrapper tube 20 as a 
fuel element in a wide sense. A coolant inlet 22 is formed on a lower 
portion of the wrapper tube 20, and a coolant outlet 23 is formed on an 
upper portion. 
With opposite ends sealed up with an upper end plug 27 and a lower end plug 
28, the fuel pin 21 enclosed within the wrapper tube 20 is that for which 
a plurality of fuel pellets 26 are inserted in a row, as shown in FIG. 4, 
within a fuel clad 25 as a fuel element. The fuel pellets 26 are retained 
elastically by a spring 29 within the fuel clad 25, a fuel stack portion 
30 being constructed as an effective fuel length. The fuel pellet 26 is 
that for which as oxide fuel matter as fuel element is sintered through 
ceramics. 
A so-called metallic fuel is available for the fuel element instead of the 
oxide fuel matter, and the fuel element is not necessarily a thoroughly 
sealed type covered by the fuel clad 25. For example, a so-called vented 
fuel element (fuel pin) which discharges fission products gas (hereinafter 
called FP gas) such as He gas or the like outside the fuel element by 
fission is acceptable otherwise. 
On the other hand, the fuel assemblies 17, 18 are charged with the TRU 
elements (transuranium elements) to be transmuted, and a concrete case for 
controlling a charging amount of a main MA elements of the TRU elements to 
be charged is exemplified as follows: 
(1) An amount of the elements is controlled so that the MA elements to be 
transmuted which is added into the fuel pellets 26 will be distributed 
uniformly. 
(2) The fuel pellets with the MA elements to be transmuted relatively high 
in density and the fuel pellets with the elements relatively low in 
density or almost not added are prepared and a charging number of each 
fuel pellet to a fuel pin is controlled, thereby controlling the charging 
amount. An array order of the fuel pellets is arbitrary. 
(3) The fuel pin 21a with the MA elements to be transmuted relatively much 
in charging amount and the fuel pin 21b with the MA elements relatively 
little in charging amount or almost not charged are prepared, and a number 
of the fuel pins 21a, 21b enclosed within the wrapper tube 20 is 
controlled as shown in FIG. 5 and FIG. 6, thereby controlling a charging 
amount of the MA elements per the fuel assemblies 17 and 18. 
Meanwhile, if a percentage by weight of minor actinides (element with the 
atomic number 92 or over excluding uranium and plutonium; hereinafter 
called MA element) to total actinides with the atomic number 92 or over is 
called minor actinides content (hereinafter called MA content), a relation 
in the melting point between the MA content and the fuel pellet 26 is as 
indicated in FIG. 7, and a melting point of the fuel pellet 26 drops 
according as the MA content increases. Thus, a melting point of the fuel 
pellet to which the MA elements are added for transmuting the MA elements 
drops as compared with the fuel pellet to which the MA elements are not 
added, thereby facilitating a fuel melting. 
It is then necessary that a reactor power be lowered in a nuclear reactor 
so as to keep the fuel elements from being melted during operation of the 
nuclear reactor regardless of the melting point drop, and a neutron flux 
level of the nuclear reactor lowers due to a decrease of the reactor 
power. 
Meanwhile, a transmuting rates of the TRU elements is proportional to the 
MA content and the neutron flux level. When taking an adjustment of the 
reactor power further into consideration for prevention of a melting of 
the fuel element, the transmuting rates of the TRU elements is not always 
to rise monotonously along with an increase in the MA content, and, as 
shown in FIG. 8, there exists a peak portion whereat the TRU elements 
transmuting rates is maximized correlatively to the MA content. Then, an 
MA content P.sub.MA when the TRU elements transmuting rates is maximized 
is effective and hence is capable of realizing a transmuting of the TRU 
element most efficiently without causing a melting on the fuel element. 
On the other hand, in the case of ordinary fast reactor core, fissionable 
elements contained in the fuel are lost in accordance with burnup and thus 
fission products are accumulated, and therefore, an effective 
multiplication factor indicating the degree of a criticality of the 
nuclear reactor (fast reactor) decreases according to the lapse of time 
for the reactor operation. 
However, in the case of fast reactor core operation for transmuting of the 
TRU elements, main MA elements .sup.237 Np, .sup.241 Am, .sup.243 Am, 
.sup.242 Cm of the TRU elements to be transmuted are transformed, as shown 
in FIGS. 9A to 9D, into a fissionable elements easy to cause a fission by 
fast neutrons. Consequently, the effective multiplication factor comes to 
decrease moderately in accordance with the lapse of time for reactor 
operation. 
As shown in FIG. 10, when the MA content becomes excessive, the TRU 
elements are transformed into the fissionable elements too much, and the 
effective multiplication factor is capable of increasing according to the 
lapse of time for the reactor operation. Accordingly, a transition of the 
effective multiplication factor according to the lapse of time for the 
reactor operation may be controlled by the MA content. Now, therefore, 
such reactor core as will suppress a change in the effective 
multiplication factor in accordance with the reactor operation and keep an 
excess reactivity of the reactor substantially zero through the reactor 
operation may be designed by controlling the MA content. 
Further, a heating rates according to a decay of the MA elements of the TRU 
element can be calculated from a decay constant of each MA element and an 
energy emitted per decay. A decay constant of the MA elements to be 
transmuted and an energy emitted per decay are shown in FIG. 11A. 
A heating rates per gram of each MA element may be calculated by means of 
data given in FIG. 11A as shown in FIG. 11B. The MA elements of those of 
the TRU elements which contribute influentially to heating are .sup.242 
Cm, .sup.244 Cm and .sup.241 Am, and hence, it is understood that a 
heating of the TRU elements will be calculated by taking these MA elements 
into consideration. 
On the other hand, if an upper bound of the heating rates per fuel assembly 
capable of removing heat is Q.sub.1 (w), then amounts of .sup.242 Cm, 
.sup.244 Cm and .sup.241 Am capable of charging into the single fuel 
assembly must satisfy the following equation: 
##EQU1## 
where M.sub.242 : amount (g) of .sup.242 Cm charged into a single fuel 
assembly 
M.sub.244 : amount (g) of .sup.244 Cm charged into a single fuel assembly 
M.sub.241 : amount (g) of .sup.241 Am charged into a single fuel 
assembly 
Then, granted that the heating rates per a fuel assembly comes within the 
range capable of removing heat, if the heating is one-sided at a position, 
the fuel element is capable of being damaged and therefore, there is a 
limit to the charging of the MA elements to be transmuted from the 
viewpoint of preventing a fuel rupture. That is, if an upper bound of the 
heating rates per cm in an axial length of a local one fuel pellet .sub.2 
is Q (w), a charging amount of the MA elements to be transmuted at the 
position must satisfy the following equation: 
##EQU2## 
where, M.sub.242.sup.L : amount (g) of .sup.242 Cm charged per cm in axial 
length of local one fuel pellet 
M.sub.244.sup.L : amount (g) of .sup.244 Cm charged per cm in axial length 
of local one fuel pellet 
M.sub.241.sup.L : amount (g) of .sup.241 Am charged per cm in axial length 
of local one fuel pellet 
Accordingly, to prevent a failure of the fuel assembly due to overheating, 
it is necessary that a charging amount of the MA elements of the TRU 
elements to be transmuted is controlled so as to satisfy the above Eqs. 
(1) and (2) at the same time. 
For example, an upper bound of the heating rates per a fuel assembly, where 
a fuel stack general as the fuel assembly is 100 cm long, and fuel pins 
charged into the single fuel assembly are 271 pieces in number, is about 5 
kw/assembly. In this case, an upper bound of addable amount of main MA 
elements .sup.242 Cm, .sup.244 Cm and .sup.241 Am which are contributive 
to heating to the fuel pellets is: 
##EQU3## 
Accordingly, in order that the fuel assembly with the TRU elements added 
thereto may not cause overheating and failure of the fuel element for 
transmuting of the TRU element at the time of assembling, storage and 
transportation, if percentages by weight of the MA elements .sup.242 Cm, 
.sup.244 Cm and .sup.241 Am to be transmuted to a total weight of the 
heavy metal elements of a fresh fuel pellet are f.sub.242, f.sub.244 and 
f.sub.241, then it is necessary that: 
##EQU4## 
be realized. So far as an amount of the main MA elements of the TRU 
elements added to the fresh fuel pellet satisfies Eq. (3), the fresh fuel 
assembly will never be overheated or damaged. 
Further, as will be understood from FIG. 9, the MA elements of the TRU 
elements to be transmuted functions generally as a neutron absorber, and 
its degree is stronger than uranium 238 (.sup.238 U), as shown in FIG. 12, 
in the fast reactor core. 
On the other hand, if the ratio of weight of plutonium element, or the 
plutonium content, to the weight of total heavy metal elements in the fuel 
pellet is called a plutonium (Pu) enrichment, in the fast reactor core, 
since the neutron flux level normally decreases according as it goes 
outside from the core center, the power density lowers according as it 
goes outside in the same Pu enrichment area. Accordingly, in the same Pu 
enrichment area at the fast reactor core operating for transmuting the TRU 
elements, the charging density of the MA elements to be transmuted which 
functions as a neutron absorber will be lessened gradually outside from 
the reactor core, thereby satisfying a flatting requirement. Thus, 
fluctuation and change of the reactor power are suppressed, and 
reliability and safety of the nuclear reactor can be enhanced. 
Further, in the fast reactor core having the Pu enrichment in two kinds or 
more, since the area higher in the Pu enrichment has a high content of 
fissionable elements, and the fissionable elements are transmuted quick by 
burnup in accordance with the reactor operation. Thus in the area high in 
the Pu enrichment, a power density according to the operation lowers more. 
On the other hand, the MA elements to be transmuted are transformed, as 
shown also in FIGS. 9A to 9D, into fissionable elements by a neutron 
capture. Accordingly, from increasing the charging density of the MA 
elements to be transmuted in an area higher in the Pu enrichment, a 
decrease of the power density according to the operation of the nuclear 
reactor will be compensated for with the transformation into a fissionable 
elements of the MA elements to be transmuted, thus moderating the decrease 
of the power density according to the reactor operation. 
FIG. 13 exemplifies a main specification characteristic of a fast reactor 
core to which the TRU elements transmuting reactor core according to the 
present invention is applied, and FIG. 14 shows a charging density radial 
distribution of the MA elements charged into the transmuting reactor core. 
The mean MA content of the fast reactor core in the example shown in FIG. 
13 is, for example, 5 percent by weight. As shown in FIG. 8, the MA 
content is set so as to minimize an excess reactivity of the reactor to 
substantially zero, for example, in the range where a fuel fusion does not 
occur during operation of the reactor. 
When the MA content is 5 percent by weight, an amount of the fissionable 
elements produced by a neutron capture of the charged MA elements is 
moderate to be balanced just sufficiently with the amount transmuted by 
fission, therefore an excess reactivity of the reactor during operation of 
the reactor being checked to be about 0.5% .DELTA. K/K maximumly. In the 
case of an ordinary fast reactor core equivalent to the present embodiment 
in a reactor thermal power and the operation cycle length, the maximu 
excess reactivity during the operation is about 3% .DELTA. K/K. 
In the example, since the MA elements has a neutron absorbing effect as a 
control rod, a required reactivity worth of the control rod may be 
minimized, thus the number of control rods and the required amount of 
neutron absorber such as boron, hafnium or the like which is charged into 
the control rod being reduced, and an economical efficiency will be 
enhanced. 
If an excess reactivity of the reactor is low, a reactivity insertion into 
the reactor at the time when the control rod is drawn erroneously can be 
minimized, thereby ensuring a safety. Further, the excess reactivity of 
the reactor can be kept low through the reactor operation, and thus a 
reactivity loss of the reactor due to the reactor operation, namely the 
decrease of the effective multiplication factor according to burnup can be 
minimized. 
Thus, the period of continuous operation of the reactor or the operation 
cycle length can be prolonged, and a plant capacity factor or besides the 
transmuting efficiency of the TRU elements may be enhanced. 
Further, in the present embodiment, the charging density of the MA elements 
varies, as shown in FIG. 14, according to a position where the fuel 
assembly is charged into the reactor core, however, when a fresh fuel 
assembly outside the reactor is taken particularly into consideration, 
even in the case of the fresh fuel assembly whereby a heating rates is 
maximized, the MA elements are set within the range satisfied by the Eqs. 
(1) and (2) or (3) so as not to increase the heating rates excessively by 
a decay of the MA elements. Accordingly, the maximum heating rates of the 
fresh fuel assembly outside the reactor is about 5 kw/assembly, as shown 
in FIG. 13, and hence, a trouble such as failure due to the overheating of 
the fuel assembly or the like will not result at the time when assembling, 
storing and transporting the fresh fuel assembly. 
Still further, in the present embodiment, a radial distribution of the 
charging density of the MA elements at the reactor core is made less, as 
shown in FIG. 14, according as it goes outside of the reactor core in the 
same Pu-enriched area. Besides, since the MA elements functions as a 
neutron absorber, the neutron flux level getting large toward the core 
central portion is suppressed normally by the MA elements, and in result, 
the radial distribution of the neutron flux becomes flatter. Accordingly, 
as shown in FIG. 15, the radial distribution of the reactor power density 
is flattened during the period of the reactor operation cycle as compared 
with a prior art exemplified in FIG. 21. Thus, a local power peak will be 
kept from arising, and hence a thermal tolerance of the fuel assembly can 
be ensured thoroughly, thus enhancing an economical efficiency such as 
compacting and lightening the reactor core in structure. 
Further, in the present embodiment, the area higher in Pu enrichment is 
kept high in the MA elements charging density as compared with the area 
lower in Pu enrichment, as shown in FIG. 14, the area higher in Pu 
enrichment has fissionable elements resulting from the neutron capture of 
the MA elements more than that. 
On the other hand, a consumption due to the burnup of the fissionable 
plutonium is more with the area high in Pu enrichment where the 
fissionable plutonium is present much. Accordingly, the distribution of 
the MA elements charging density shown in FIG. 14 is set at every area of 
Pu enrichment so that the area with much consumption of the fissionable 
plutonium has much fissionable elements produced by the neutron capture of 
the MA elements, therefore a net decrease of the fissionable elements 
according to the reactor operation being suppressed, and, as shown in FIG. 
15, a fluctuation of the power density according to operation is 
suppressed. Thus, if a fluctuation of the power density is minimized 
according to the operation of the fast reactor (nuclear reactor) or 
burnup, the heat removing efficiency is improved, the enhancement of 
economical efficiency is realized, and thus fuel temperature is decreased, 
and safety and reliability of the reactor core can be enhanced 
accordingly. 
FIG. 16 is a graph representing a second embodiment of the TRU elements 
transmuting reactor core according to the present invention. 
Even in the case of a reactor core where the Pu enrichment is one kind, the 
fast reactor core represented in this embodiment has a radial distribution 
of the MA element charging density flattened preferably as compared with 
the prior art as shown in FIG. 6 and also has a radial power distribution 
of the core flattened as shown in FIG. 17. Accordingly, from employing the 
present invention, the reactor power distribution is flattened despite Pu 
enrichment being one kind and therefore, the plant efficiency will be 
enhanced, and the economical efficiency such as decrease in fuel 
fabrication cost or the like may also be enhanced. Needless to say, a 
similar characteristic to the first embodiment applies to those other than 
the reactor power distribution. 
Further, as another embodiment, the axial distribution is applied to the MA 
elements charging density as in the case where a radial reactor power 
distribution is flattened according to the present invention as described 
above, thereby flattening the axial reactor power distribution. 
If the power distribution need not be flattened, an MA content of the whole 
core will be set according to the present invention even in case a 
distribution is not applied particularly to the MA elements charging 
density, thereby realizing effects such as decrease in excess reactivity 
of the reactor through the operation, prevention of overheating and 
failure of the fresh fuel assembly outside the reactor and the like. 
As described above, in the transuranium element transmuting reactor core 
relating to the present invention, since an amount of transuranic elements 
added to the fuel element of fuel assemblies is controlled so as to keep 
an excess reactivity of the reactor substantially zero through the 
operation of the reactor, the decrease of the effective multiplication 
factor according to the lapse of time for the operation is prevented, the 
excessive deterioration and turbulence of the reactor power distribution 
can be prevented, the reliability of the power plant is thus enhanced, and 
as seeking an improvement of the plant operating efficiency, transuranium 
elements (TRU elements) can be transmuted efficiently. 
Then, from setting charging amounts of.sup.242 Cm, .sup.244 Cm and .sup.241 
Am into the fuel assembly so as to realize: 
EQU 1.2.times.10.sup.2 .times.M.sub.242 +2.8.times.M.sub.244 
+1.1.times.10.sup.31 1 .times.M.sub.241 &lt;Q.sub.1 
where an upper bound of heating rates of the single fuel assembly outside 
the reactor is Q.sub.1, charging amounts of.sup.242 Cm, .sup.244 Cm and 
.sup.241 Am which can be charged into the single fuel assembly are 
M.sub.242, M.sub.244 and M.sub.241, and also to realize: 
EQU 1.2.times.10.sup.2 .times.M.sub.242.sup.L +2.8.times.M.sub.244.sup.L 
+1.1.times.10.sup.-1 .times.M.sub.241.sup.L Q.sub.2 
where an upper bound of heating rates of per unit length of the fuel pellet 
contained in the fuel pins is Q.sub.2, charging amounts of.sup.244 Cm and 
.sup.244 Cm and .sup.241 Am per the unit length are M.sub.242.sup.L, 
M.sub.244.sup.L and M.sub.241.sup.L, a melting of the fuel pellet during 
the reactor operation of the reactor and an overheating or failure of the 
fuel assemblies outside the reactor can be prevented and a neutron 
absorption effect of .sup.242 Cm, .sup.244 Cm and .sup.241 Am is available 
for eliminating accidents of a control rod and a neutron absorbing 
material of the control rod and also for realizing the improvement of the 
heat removing efficiency of the core, the economical efficiency will be 
enhanced, the safety and the reliability of the core and the fuel 
assemblies will be enhanced as well, and the TRU elements can be 
transmuted efficiently. 
Further, by setting a charging density of minor actinides to lessen 
outwards of a core center in a core area where a plutonium content is even 
and also by setting a charging density of minor actinides high accordingly 
in an area where Pu is enriched high at the core of a Pu-enriched area, 
the radial distribution of the reactor power can be flattened, the 
excessive deterioration or turbulence of the reactor power distribution 
will never result, the enhancement of safety and reliabiilty of the core 
and the fuel assemblies may be realized, and thus the TRU elements can be 
quenched efficiently. 
Further embodiments of the present invention will be described hereunder in 
conjunction with FIGS. 22 to 30, with reference to transuranium elements 
transmuting fuel pins and fuel assemblies. 
FIG. 22 exemplifies a transuranium elements transmuting fuel assembly 
according to another embodiment of the present invention which is charged 
into the fast reactor core. The fuel assembly 110 has a coolant inlet 111 
at the lower portion thereof for letting in the coolant such as liquid 
sodium (Na), liquid NaK, helium (He) gas or the like and a coolant outlet 
112 at the upper portion thereof, and a plurality of fuel pins 114 are 
enclosed in a bundle within a wrapper tube 113 square tubular or, for 
example, hexagonal in section which works as a fuel channel. The fuel pin 
114 enclosed within the wrapper tube 113 may be constructed only of a TRU 
(transuranium elements) fuel pin 115, or of the TRU fuel pin 115 and an 
ordinary fuel material pin 116 otherwise as shown in FIG. 23. 
In the case of TRU fuel assembly having constructed the fuel assembly 110 
only of the TRU fuel pins 115, nothing will be taken into consideration 
for arrangement of the TRU fuel pins 115, an administration on manufacture 
and assembling of the TRU fuel pins 115 is facilitated, the number of fuel 
assemblies for which a special measure such as heat removing, shielding or 
the like is required may be minimized, thus enhancing an economical 
efficiency on a core administration and a fuel handling. 
Then, in case the TRU fuel assembly 110 is constructed of the TRU fuel pin 
115 and the ordinary fuel material pin 116, the change in reactor power 
according to the reactor operation is minimized for presence of the TRU 
elements, and the TRU elements can be transmuted efficiently without 
lowering a cooling efficiency of the core as, in addition, keeping the 
integrity of the reactor internal structure. 
FIG. 23 exemplifies the case where the TRU fuel pins 115 are dispersed and 
disposed almost uniformly within the wrapper tube 113 of the TRU fuel 
assembly 110. By dispersing and disposing the TRU fuel pins 115 uniformly, 
a relatively low-temperature coolant around the TRU fuel pins 115 and a 
relatively high-temperature coolant around the usual fuel material pins 
116 are mixed acceleratedly in the presence of the TRU element, a fuel 
clad temperature of the ordinary fuel material pin 116 is decreased, the 
cooling efficiency of the core can be enhanced, the ordinary fuel material 
pin 116 can be made to last so long, and the economical efficiency and 
safety of the fast reactor can thus be enhanced. 
Meanwhile, as shown in FIG. 24, the TRU fuel pin 115 has a fuel clad 118 
charged with a TRU fuel material 119, and its upper and lower portions 
closed by an upper end plug 120 and a lower end plug 121. The TRU fuel 
material 119 has at least one of an ordinary fuel material and a fertile 
material consisting of degraded uranium, natural uranium and depleted 
uranium contain TRU elements (trans- uranium elements) of minor actinides 
elements such as Np (neptinium), Am (americium), Cm (curium) and the like. 
Here, the ordinary fuel material refers to a fuel material consisting of 
an enriched uranium and an uranium-plutonium mixed fuel (uranium fuel 
having enriched plutonium). An oxide fuel such as uranium oxide or the 
like is used for the fuel material, however, a metallic fuel may be 
employed instead of the oxide fuel. 
Further, the ordinary fuel material pin 116 has the fuel clad charged with 
a plurality of so-called fuel pellets (or metallic fuel otherwise) having 
sintered the ordinary oxide fuel material, and the upper and lower ends 
closed by the upper end plug and the lower end plug, a gas plenum part 
being formed on at least one portion, or upper and lower portions, for 
example, in the fuel clad. 
Then, the TRU fuel pin 115 and the ordinary fuel material pin 116 have been 
described with reference particularly to a closed type one closed by the 
upper and lower end plugs, however, this need not always be a full-closed 
type, and hence a so-called vented type fuel pin which is capable of 
discharging fission products gas (hereinafter called FP gas) generated by 
the fission outside the fuel pins 115 and 116 may be employed. 
FIG. 24 exemplifies the TRU fuel pin 115 working as a transuranic elements 
transmuting fuel. The fuel pin 115 is that of having a multiplicity of TRU 
fuel areas provided axially, a TRU fuel 119a high in content of Np of the 
minor actinides elements is disposed in a core upper portion area of the 
fuel clad 18, a TRU fuel 119c high in content of Am and Cm is disposed in 
a core lower portion area, and a TRU fuel 119b is also disposed in a core 
height center area high in a neutron flux level, and an ordinary fuel 122 
is disposed among the TRU fuels 119a, 119b, 119c to constitute the TRU 
fuel material 119. A gas plenum part 123 is formed on an upper portion of 
the fuel clad 118, the upper and lower ends being closed by the upper end 
plug 120 and the lower end plug 121. In the gas plenum part 123, a spring 
is installed, as occasion demands, so as to stably hold the TRU fuel 
material 119. 
By disposing the TRU fuels 119a, 119b, 119c with which the fuel clad 118 of 
the TRU fuel pin 115 is charged inside as indicated in FIG. 25A, a core 
axial power distribution curve A of the fast reactor is as indicated by a 
full line and a peak power is decreased more than a core axial output 
distribution curve B indicated by a broken line in the case of the 
ordinary fuel material pin 116, thus the reactor characteristic and the 
transmuting efficiency of the TRU elements being improved. Typical minor 
actinides elements .sup.237 Np, .sup.241 Am, .sup.243 Am, .sup.242 Cm, 
.sup.244 Cm of the TRU fuels 119a to 119c with which the TRU fuel pin 115 
is charged inside the capture neutrons to fissionable elements as shown in 
FIGS. 9A to 9D when charged into the fast reactor core to the reactor 
operation, thus transmuting the TRU elements. 
Further, due to the TRU fuel pin 116 having the fuel clad 118 charged with 
the TRU fuel pellets 119 inside, there arises a problem that the melting 
of the fuel drops due to the presence of the minor actinides elements. 
The fuel melting will not result even at the transient conditions of the 
fast reactor according to a normal core design, and in the TRU fuel pin 
115 shown in FIG. 24, the TRU fuel 119a high in content of low melting 
point MA elements is disposed in the core area with high fuel temperature, 
namely the core upper portion area in the case of the metallic fuel, and 
the core height center area in the case of the oxide fuel, and the TRU 
fuel 19c high in content of high melting point MA elements is disposed in 
the core lower portion area where the fuel temperature is low. By 
disposing the TRU fuel 119c containing Am and Cm with the melting point 
dropping comparatively large for the oxide fuel in the area where the fuel 
temperature is low, the drop of the fuel melting point due to the content 
of the TRU element such as Am and Cm or the like does not exert an 
influence directly on determination of the core power density, and the TRU 
elements can be transmuted efficiently without lowering the core power 
density. 
On the contrary, the melting point of Np is lower than that of Am and Cm 
for the metallic fuel. 
Meanwhile, typical minor actinides elements .sup.237 Np, .sup.241 Am, 
.sup.243 Am, .sup.242 Cm, .sup.241 Cm of the TRU fuel pellets 119 with 
which the fuel clad 18 of the TRU fuel pin 115 are charged inside are 
transformed into fissionable elements, as shown in FIGS. 9A to 9D, by a 
neutron absorption, and therefore, a power of the TRU fuel assembly 
constructed only of the TRU fuel pin 115 sharply increases as indicated by 
a symbol I in FIG. 27 according to the number of days for operation. 
Accordingly, the power of the TRU fuel assembly constructed only of the 
TRU fuel pin 115 is low at the point in time of start for operation but 
increases largely according to the operation. 
Then, as shown in FIG. 24, in the TRU fuel assembly 110 for which the TRU 
fuel pin 115 and the ordinary fuel material pin 116 are disposed mixedly, 
since the ordinary fuel material pin 116 generates power, the power of the 
TRU fuel assembly 110 is also high relatively at the point in time of 
start for the reactor operation as indicated by a symbol II in FIG. 26, 
and since the decrease in the fissionable material of the ordinary fuel 
material pin 116 and the transformation of the TRU fuel element 119 into a 
fissionable material are offset, the degree of power flunctuation of the 
TRU fuel assembly 110 due to the operation is smaller and smoother than 
the fuel assembly constructed only of the TRU fuel pin 115. 
In a core design, a coolant flow rate of the fuel assembly charged into the 
core is specified so as to remove heat at the time of maximum power of the 
fuel assembly. The TRU fuel assembly constructed only of the TRU fuel pin 
115 does not generate a power corresponding to the coolant flow rate at 
the time of start for the operation or for a several time after the start, 
an outlet temperature is low consequently, and therefore, the coolant 
outlet temperature does not rise, thus leaving a problem on the cooling 
efficiency of the core and the integrity of the reactor internal 
structure. 
However, in the case of the TRU fuel assembly for which the TRU fuel pin 
115 and the ordinary fuel material pin 116 are disposed mixedly as shown 
in FIG. 23, the power coming up substantially to the maximum power is 
generated at the time of the start for the operation or for a several time 
after the start, and therefore, an outlet temperature of the TRU fuel 
assembly 110 will not drop extremely, and thus the problem on the cooling 
efficiency of the core and the integrity of the reactor internal structure 
can be avoided. 
Then, as shown in FIG. 24, by mixing a radioactive fission product (F. P) 
such as strontium (Sr), alkaline metals (Cs or the like), technetium (Tc) 
or the like into the TRU fuel pellets 119 with which the fuel clad 118 of 
the TRU fuel pin 115 is charged inside, the transmuting of the TRU 
elements and also the transmuting of a long-lived radioactive fission 
products in the reactor and the internal administration of the reactor are 
realizable. The disposal and administration of the radioactive waste 
products will be facilitated as compared with the case where these are 
carried out outside the reactor. 
For example .sup.99 Tc is transformed into a stable elements which is not 
radioactive in the fast reactor by neutron capture (neutron absorption) 
and others, and .sup.90 Sr and .sup.137 Cs become: 
EQU .sup.90 Sr.fwdarw..sup.90 Y.fwdarw..sup.90 Zr (stable) 
EQU .sup.137 Cs.fwdarw..sup.137m Ba.fwdarw..sup.137 Ba (stable) 
by natural decay while residing in the reactor, and .sup.90 Zr and .sup.137 
Ba are stable materials, which are removed from a fuel through the spent 
fuel reprocessing. 
In the TRU fuel pin 115 shown in FIG. 24, the case where TRU fuels 119a, 
119b, 119c are dispersed and arranged axially is exemplified, however, a 
TRU fuel pin 115A may be constructed as shown in FIG. 27 so as to minimize 
the influence to be exerted on the reactor power distribution by the TRU 
elements. Like reference characters are applied to the like portions in 
FIG. 24, and a further description will be omitted here. 
The TRU fuel pin 115A exemplifies the case where the TRU fuels 119b 
containing the TRU element which constitute the TRU fuel pellets 119 are 
disposed even axially at a relatively low content (several percent by 
weight or below). The TRU fuel pin 115 for which the TRU fuels are 
disposed uniformly has an advantage that an administration on fabrication 
and transportation is facilitated, a fabrication cost can be reduced, and 
an influence to be exerted on an axial power distribution is uniform and 
minimized. Further, the TRU fuel pin 115A is much in the TRU elements 
loading amount per pin, and thus is available for enhancing the 
transmuting efficiency of the TRU elements. 
A TRU fuel pin 115B shown in FIG. 28 exemplifies the case where the TRU 
fuel pellets 119 with which the fuel clad 118 is charged inside has the 
TRU fuel 119b disposed only in the core height center area, and such 
disposition is effective in decreasing a peak power and flattening the 
core axial power distribution. By disposing the TRU fuel 119b in the core 
height center area where a neutron flux density is high, the power peak is 
reduced preferably by the distortion of the core axial power distribution, 
the power distribution is flattened, and a reactor characteristic is 
improved to enhance the transmuting efficiency of the TRU elements. 
Then, in this case, the content (percent by weight) of the TRU elements 
contained in the TRU fuel 119b is relatively high to stand, for example, 
at 10% or over as in the case of the TRU fuel pin 115 of FIG. 24. 
Further, in the first embodiment of the transuranic elements transmuting 
fuel assembly 110, the case where the TRU fuel pins 115 are dispersed and 
disposed uniformly in the TRU fuel assembly 110 as shown in FIG. 23, 
however, the fuel pins 115 (115A, 115B) will be disposed intensively in 
the center area of the TRU fuel assembly 110, as shown in FIG. 29, and the 
ordinary fuel material pins 116 will be disposed around the TRU fuel pins 
115 otherwise. 
In case the TRU fuel pins 115 are disposed intensively in the center area, 
since the TRU fuel assembly 110A has the neutron flux density maximized at 
the center area, the transmuting efficiency of the TRU elements can be 
enhanced. 
In the TRU fuel assembly 110A, the fuel pins disposed around the TRU fuel 
pins 115 (115A, 115B) may comprise a fertile material pin consisting of 
natural uranium and depleted uranium, and by disposing such fertile 
material pins, alpha rays and neutrons emitted from the TRU fuel pins 115 
are shielded, and thus measures on transportation and shielding of the TRU 
fuel assembly 110A may be relieved. 
A TRU fuel assembly 110B shown in FIG. 30 exemplifies the case where the 
TRU fuel pins 115 (115A, 115B) are disposed at an outer peripheral 
position abutting on a inside wall of the wrapper tube 113. The ordinary 
fuel material pins 116 are disposed inside of the TRU fuel pins 116 
arrayed as above. 
Generally, the coolant flow rate per one fuel pin is larger on the wall 
side of the wrapper tube 113 than in the center area, and temperature of 
the fuel clad 118 on the wall side becomes lower than the center area. 
When the metallic fuel is employed as the fuel of the TRU fuel assembly 
110B, it is necessary that the fuel clad temperature be adjusted as low as 
possible for the prevention of an eutectic reaction of the fuel clad 118, 
with the metallic fule slug. 
The TRU fuel assembly 10B shown in FIG. 30 is that for which the TRU fuel 
pins 115 are disposed along the inside wall of the wrapper tube 113 with a 
coolant flowing much therethrough, the fuel clad temperature is reduced 
thereby, the economical efficiency of the TRU fuel assembly 110B is thus 
secured to prolong the lifetime. 
Then, the TRU fuel pins 115 (115A, 115B) have typical minor actinides 
elements easy to cause alpha-decay contained much in the TRU fuel material 
119, and thus a relatively high alpha ray energy is emitted at the time of 
alpha-decay, and therefore, special measures on fabrication, heat removing 
and shielding which are different from the ordinary fuel will be necessary 
for the TRU fuel pins 115 (115A, 115B) and the TRU fuel assemblies 110 
(110A, 110B). Thus, for the TRU fuel pin 115, the TRU fuel pellets 119 is 
constructed by containing the TRU elements or specific minor actinides 
elements at a predetermined content. 
For fuel temperature and others to keep in order, it is preferable that the 
TRU fuel pin 115 be provided with a TRU fuel material area in the area 
lower than the core height center more than the area upper than the core 
height center. Further, from inserting a control rod into the core, the 
transmuting efficiency of the TRU elements may be enhanced by providing 
the TRU fuel area in the core height center area where a neutron flux 
level gets high, thereby flattening the core axial power distribution. 
Further, by disposing the TRU fuel in the area lower than a core height 
center level whereat the neutron flux level becomes high, the melting of 
the TRU fuel can be avoided effectively and securely. 
As a described above, in the transuranium element transmuting fuel pin and 
fuel assembly relating to the present invention, an improvement of core 
characteristics and an enhancement of the core cooling efficiency may be 
achieved without causing deterioration of the core characteristics 
according to various restrictions such as fuel melting during the 
operation of the reactor, excessive change of the core axial power 
distribution, distortion of the distribution and the like. Thus the safety 
and reliability of the fuel assemblies and fuel pins to be charged into 
the core may thus be achieved and the transmuting efficiency of the TRU 
elements can be improved and enhanced.