Patent Number: 055240330
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to gadolinium for use as a burnable poison for nuclear fuel, comprising: a plurality of isotopes of gadolinium, wherein a content of at least one even mass numbered isotope of said plurality of isotopes is smaller than a content of said at least one even mass numbered isotope in natural gadolinium, and a fuel assembly comprising a plurality of fuel rods comprising the gadolinium. As seen in Table 2, when natural gadolinium is used, Gd-156, Gd-157 and Gd-158 contribute to about the same degree to the loss of reactivity after burnout of the gadolinium. In the case of Gd-156, that produced from Gd-158, neutron absorption and that which is found naturally are present in a ratio of about 15:20. In the case of Gd-158, that produced from Gd-157 by neutron absorption and that which is found naturally are present in a ratio of about 16:25. Therefore, by making the content of Gd-156 and Gd-158 low, the loss of reactivity due to these isotopes can be greatly reduced. Lowering the Gd-156 moreover enables the reactivity loss due to Gd-157 to be reduced. The change with time of the atom density of Gd-157 after the naturally present Gd-157 has all been converted into Gd-158 is expressed by the following equation. EQU dN7/dt=-N7.sigma.7 .phi.+N6.sigma.6 .phi. where N6 and N7 are the atom densities of Gd-156 and Gd-157, respectively .sigma.6 and .sigma.7 are their respective cross-sections, and .phi. is the neutron flux. Since .sigma.6 is very much smaller than .sigma.7, with the lapse of sufficient time dN7/dt=0. At this point, N6 .sigma.6 .phi.=N7.sigma.7 .phi., and an equilibrium condition is produced in which the neutron absorption factors of Gd-156 and Gd-157 are equal and the atom density of Gd-157 is proportional to the atom density of Gd-156. As shown in FIG. 4, the atom density of Gd-156 after the Gd-155 has been fully converted to Gd-156 is practically constant and is determined by the quantities of Gd-155 and Gd-156 that were present initially. The loss in reactivity due to Gd-157 can therefore also be reduced by lowering the Gd-156 content. Also, by maintaining the contents of Gd-155 and Gd-157, which function in essential part as burnable poisons, reactivity control can be achieved for the necessary initial period, so isotopes other than these can be removed and the gadolinia concentration can therefore be lowered by a corresponding amount. As a result, the thermal conductivity of the fuel is raised so the benefits of higher burnup and/or reduction in local power peaking can be obtained and gadolinia concentration can be raised, thereby making it possible to raise the availability factor by long-term operation. Furthermore, since the Gd-155 and Gd-157 are present in the same ratio as in natural gadolinium, the peak value of the infinite multiplication factor will not get too large. It is therefore possible to solve the problems of adverse effect on shutdown margin and increase in channel peaking. In a fuel assembly according to the present invention, Gd-157-enriched gadolinium, having Gd-157 content higher than natural abundance, is employed as a burnable poison, and fuel rods are allocated into a plurality of groups of mutually different burnable poison concentrations. The atom density variation of Gd-157 and Gd-155 shown in FIG. 4 can therefore be obtained by simulation. This makes it possible to reduce the peak value of the infinite multiplication factor, so the problems of adverse effect on the shutdown margin and increased channel peaking can be overcome. In one embodiment of the present invention, gadolinium containing no Gd-156 at all (shown in Table 4) is employed as a burnable poison in a fuel assembly as shown in FIG. 1. The gadolinium is included in the fuel in the form of gadolinia at a gadolinia concentration of 3.2%. The total mass content of Gd-155 and Gd-157 is the same as when natural gadolinium is used with a gadolinia concentration 4.0%. The neutron absorption factor at 25 GWd/st in this embodiment is 0.60%, providing a reactivity gain of 0.21% when compared with the gain of 0.81% which is obtained when natural gadolinium is used, as shown in Table 2. Consequently, compared with a prior art example using natural gadolinium, burnup can be prolonged by about 1% for the same uranium concentration. Alternatively, the uranium concentration to achieve the same burnup as in the prior art example can be lowered by about 0.03%. FIG. 3 shows the burnup variation (13) of the infinite multiplication factor in this embodiment. It can be seen from the Figure that a burnup variation is displayed which is very similar to that of the infinite multiplication factor (11) in the prior art example in which natural gadolinium is employed. Since the peak of the infinite multiplication factor does not get too large, there is no risk of adverse effects on shutdown margin and channel peaking. FIG. 7 is a schematic diagram showing an example of a laser device for manufacturing the gadolinium shown in Table 4. First of all, gadolinium metal is melted and evaporated by a metal vapor generating device (15) in the interior of a separation cell (14) maintained at high vacuum. The neutral vapor current (16) that is thus generated is fed into an optical reaction unit (17) where only the Gd-156 is ionized by irradiating the vapor with laser light (19) introduced from a laser system (18). Preferably, rather than carrying out the ionization directly from the ground state, a selective excitation laser beam (20) is used to temporarily selectively excite the Gd-156 to a specific excited condition. The excited Gd-156 is then ionized by irradiating with a further ionizing laser beam (21). Laser devices (22) and (23) are constituted by a pumping laser, variable-wavelength laser, frequency modulating device and pulse laser amplifier. The ionized Gd-156 vapor current (24) which is obtained in this way is adsorbed onto an ion recovery electrode plate (25). Vapor current (26) consisting of the other gadolinium isotopes which have not been ionized is recovered onto a neutral atom recovery plate (27). Gadolinium of lower Gd-156 content than the natural abundance can therefore be obtained by recovering the gadolinium from neutral recovery plate (27). TABLE 4 ______________________________________ Isotope Content (%) Neutron absorption factor (%) ______________________________________ Gd-154 3 0.04 Gd-155 19 0.05 Gd-156 0 0.11 Gd-157 20 0.12 Gd-158 31 0.21 Tb-159 0 0.05 Gd-160 27 0.02 Total 100 0.60 ______________________________________ As a further embodiment, gadolinium containing no Gd-156 or Gd-158 at all, as shown in Table 5, is employed as a burnable poison in a fuel assembly as depicted in FIG. 1. The gadolinia concentration is 2.2%, and the total mass content of Gd-155 and Gd-157 is the same as when natural gadolinium is used with gadolinia concentration of 4.0%. The neutron absorption factor at a burn up of 25 GWd/st in this embodiment is further reduced from that in the first embodiment at 0.45%. In other words, a reactivity gain of 0.36% is obtained compared with the gain of 0.81% which is obtained when natural gadolinium is used, as shown in Table 1. Consequently, compared with a prior art example using natural gadolinium, burnup can be prolonged by about 2 for the same uranium concentration. Alternatively, the uranium concentration to achieve the same burnup as in the prior art example can be lowered by about 0.04%. In this embodiment also, since the peak of the infinite multiplication factor does not get too large, there is no risk of adverse effects on shutdown margin and channel peaking. TABLE 5 ______________________________________ Isotope Content (%) Neutron absorption factor (%) ______________________________________ Gd-154 4 0.04 Gd-155 27 0.05 Gd-156 0 0.11 Gd-157 29 0.12 Gd-158 0 0.09 Tb-159 0 0.02 Gd-160 40 0.02 Total 100 0.45 ______________________________________ Comparing the above described embodiments, it can be seen that the reduction in reactivity loss produced by removing the Gd-158 is 0.15%. In order to reduce reactivity loss, it is therefore of initial importance to remove the Gd-156 and it is next important to remove the Gd-158. The other isotopes of even mass number present in gadolinium are Gd-152, Gd-154 and Gd-160. Since the content of Gd-152 and Gd-154 is in any case small, little reactivity loss improvement results from their removal. Next, Gd-154 has a neutron absorption factor due to Gd-154 itself of 0.04%. However, it acts as a source of Gd-155, which has a neutron absorption factor of 0.05%. Consequently by removing the Gd-154 a reactivity gain of 0.09% is obtained. Removing Gd-154 can therefore come third in order of reference. In contrast, Gd-160 has a large natural abundance of 22%, but, as shown in Table 1, its cross-section is small, so its removal gives little benefit in terms of lowering reactivity loss. However, since there is a large Gd-160 content, removing it is beneficial in raising thermal conductivity, and so is effective in achieving high burnup and/or prolonging reactor operation. FIG. 8 shows a transverse cross-sectional view of a fuel assembly constituting another embodiment of the present invention. This embodiment is an example in which the Gd-157 content is made higher than the natural abundance. In fact, gadolinium consisting solely of Gd-157 is employed as burnable poison. The gadolinia concentration is 1.2% in the case of eight fuel rods G1 and 1.5% in the case of five fuel rods G2. Specifically, the burnup variation of Gd-157 shown in FIG. 4 is simulated by the gadolinia of G1 and the burnup variation of Gd-155, which has a slower burnup rate, is simulated by the gadolinia of G2. Furthermore, since the gadolinia content is substantially greater in G2, the number of gadolinia-containing fuel rods can be reduced by 1 to 13 rods. The uranium concentration in this embodiment is 0.1% lower than 4.0%, i. e., 3.9%. FIG. 3 shows the burnup variation (28) of the infinite multiplication factor of this embodiment. In this embodiment, the infinite multiplication factor (11) of conventional fuel using natural gadolinium is closely simulated. In particular, the peak value of the infinite multiplication factor can be made smaller. The reactivity loss due to the gadolinia can thereby be reduced and the same burnup as in the conventional example can be achieved with a lower uranium concentration, without adverse effect on the shutdown margin or channel peaking. It is desirable to set the ratio of gadolinia concentration of the rods G1 to the rods G2 at about 1.2 to 1.3 as in this embodiment. In this way, the burnup variation in atom number density of Gd-155 and Gd-157 shown in FIG. 4 can be roughly simulated. Although, in this embodiment, two levels of gadolinia concentration were employed, three or more levels of gadolinia concentration could be used. In a fuel assembly according to an additional embodiment of the present invention, the transverse cross-sectional plane is the same as in the prior art example of FIG. 1, but the gadolinia in gadolinia-containing fuel rods (10) is different in the vertical axial direction. Specifically, gadolinium consisting solely of Gd-157 is employed in the bottom while in the top portion natural gadolinium is employed. The gadolinium concentrations are 1.2% in the bottom and 4.0% in the top portion. The neutron importance on reactor shutdown is a maximum at a location 1/4 to 1/3 of the total length below the top of the core. The infinite multiplication factor at the bottom of the core has scarcely any effect on the effective multiplication factor of the core at shutdown. With this embodiment, gadolinium of high Gd-157 content is only used in the bottom of the fuel assembly, so the possibility of excessive peaking of the infinite multiplication factor is confined to the bottom of the fuel assembly. Reactivity loss in the bottom of the fuel assembly can therefore be decreased with no risk of adverse effect on the shutdown margin. In general in a boiling water nuclear reactor, power peaking in the axial direction is liable to occur in the bottom of the core. Consequently, in this embodiment, if the uranium concentrations are the same in the top and bottom portions of the fuel assembly, the infinite multiplication factor is largest in the bottom, where the reactivity loss produced by the gadolinia is smallest. As a result, there is a possibility of increased axial power peaking in the bottom of the core. In such a case, means may be adopted such as making the uranium concentration at the bottom of the fuel assembly lower than at the top, using more gadolinia-containing fuel rods in the bottom than in the top, or making the gadolinia concentration at the bottom of the fuel assembly richer than in this embodiment. Early Japanese Patent Publication Sho. 54-13899 discloses a technique of making the gadolinia concentration used in the bottom of the fuel assembly richer than at the top, in order to reduce axial power peaking occurring at the bottom of the core in a boiling water reactor. In a fuel assembly making use of this technique shown in FIGS. 9(A) and (B), in all the fuel rods, natural uranium is employed in portions at the top and bottom ends while enriched uranium is employed in the middle portion. The gadolinia-containing fuel rods (10) are of two kinds: in fuel rods G3, the gadolinia concentration is 4.0% at both top and bottom; in fuel rods G4, it is 4.0% at the top and 5.0% at the bottom. In this example, since the gadolinia concentration is higher at the bottom of fuel rods G4, the thermal conductivity here is lower, so the degree of uranium enrichment of the entire middle portion of fuel rods G4 is lower than the degree of uranium enrichment of the middle portion of fuel rods G3. As a result, local power peaking in the cross-sectional plane of the fuel assembly is adversely affected. In a further embodiment of the present invention, the gadolinium which does not contain Gd-156, as indicated in Table 4, is used in the bottom of fuel rods G4, and natural gadolinium is used for the top. The gadolinia concentration is 4.0% at both the top and the bottom. With such a gadolinia distribution, the same reactivity control as in the prior art example can be achieved. In this embodiment, the uranium concentration of fuel rods G3 and fuel rods G4 is the same. As a result, lower power peaking in the cross-sectional plane of the fuel assembly is improved in comparison with the prior art example. Of course, gadolinium which does not contain Gd-156 could be used in both the top and bottom of G3 and G4, but this is more expensive to manufacture than natural gadolinium, so in order to keep costs down, it may be used, as in this embodiment, only in the bottom portion of the fuel assembly. Obviously, instead of using gadolinium which does not contain Gd-156, the same effect can be obtained by using gadolinium in which the Gd-157 content is raised above its natural abundance. The initially loaded fuel loaded in the first cycle of reactor operation has a gadolinia concentration higher than that of the replacement fuel loaded in the second and subsequent cycles. Gadolinia of concentration 7 to 8% is used in this initially loaded fuel. This is because the operating period of the first cycle for the start-up test is longer than the operating period of the second and subsequent cycles. If the initially loaded fuel is made of high enrichment in order to raise fuel economy, the number of rods to be replaced on first replacement is reduced so the excess reactivity of the second cycle must be borne by gadolinia present in the initially loaded fuel. Consequently, with higher enrichment of the initially loaded fuel, if one seeks to raise the gadolinia concentration, and if in the initially loaded fuel the gadolinia concentration in the bottom portion is richer than in the top portion, the gadolinia concentration will in fact be restricted by the gadolinia concentration present in the bottom portion of the fuel assembly. In such cases too, it is appropriate to use more in the bottom than in the top of the fuel assembly of either (a) gadolinium whose Gd-156 content is lowered below the natural abundance or (b) gadolinium whose Gd-157 content is raised above the natural abundance as in this embodiment. A fuel assembly according to an additional embodiment of this invention is the same as the prior art example of FIG. 1 in the transverse cross-sectional plane but the gadolinia concentration in gadolinia-containing fuel rods (10) is different between top and bottom in the axial direction. Specifically, for the top portion, gadolinium containing no Gd-156 and no Gd-158 at all, as shown in Table 5 and used in an earlier embodiment, is employed, while natural gadolinium is employed for the bottom portion. The gadolinium concentration is 2.2% in the top portion and 4.0% in the bottom portion. The infinite multiplication factor of the fuel assembly according to this embodiment is greater, at 0.49%, in the top than in the bottom, so the occurrence of axial power peaking in the bottom of the core in the boiling water reactor is diminished and the thermal margin is therefore raised. Obviously, instead of using gadolinium which does not contain Gd-156 and Gd-158, the same effect can be obtained by using gadolinium in which the Gd-157 content is raised above its natural abundance. As shown in FIG. 1, in a fuel assembly employed in a boiling water reactor, normally gadolinia is used in the fuel rods other than those at the outer periphery. In contrast, Early Japanese Patent Publication Sho. 58-216989 discloses an invention in which the shutdown margin is improved and higher fuel burnup achieved by using gadolinia for the fuel rods in the outer periphery. However, owing to the more rapid elimination of gadolinium resulting from the large neutron flux in the fuel rods at the outer periphery, the gadolinia concentration has to be raised, by reducing the number of gadolinium containing fuel rods. However, this led to the problem that it was difficult to raise the gadolinia concentration sufficiently, owing to the drop in thermal conductivity which this caused. FIG. 10 is a transverse cross-sectional view of a fuel assembly according another embodiment of this invention. In this embodiment, apart from two fuel rods G5 in the central region, gadolinia is used in eight outer peripheral fuel rods G6. The central fuel rods G5 contain natural gadolinium in a gadolinia concentration of 4.0%; the peripheral fuel rods G6 contain gadolinium which does not contain Gd-156, Gd-158 or Gd-160, in a gadolinia concentration of 2.0%. By using such a gadolinia concentration, an infinite multiplication factor equivalent to infinite multiplication factor (11) of the prior art example shown in FIG. 1 can be achieved. In this embodiment, if natural gadolinium is employed for the outer peripheral fuel rods G6, a gadolinium concentration of 6.0% must be used. This presents an obstacle to obtaining higher burnup since it results in lowered thermal conductivity or lower thermal output of the fuel rods due to lower uranium concentration. In this embodiment the same effect can be obtained by using for the peripheral fuel rods G6, gadolinium in which the Gd-157 content is raised. Also for central fuel rods G5, gadolinium which does not contain Gd-156, Gd-158 or Gd-160, or gadolinium in which the content of Gd-157 is raised, can be employed. Moreover this embodiment can clearly also be applied to many of the above described embodiments. The embodiments obtained by such combinations are preferred in that their respective benefits can be achieved concurrently. In the above embodiments the cases were described in which gadolinium from which specific gadolinium isotopes of even mass number had been completely removed, or gadolinium consisting solely of Gd-157 was employed. However, if Gd-156 removal is performed using a separation device as shown in FIG. 7, complete ionization is in fact impossible to achieve. The gadolinium recovered from neutral atom absorption plate (27) therefore still contains some Gd-156 which was not ionized. Also in the case of ionizing Gd-157, a proportion of neutral atoms which have not been ionized adheres to ion recovery electrode plate (25) so the gadolinium element recovered from ion recovery electrode plate 25 still contains some isotopes other than Gd-157. Clearly, however, the present invention can still be applied and corresponding benefits achieved even using gadolinium element of this type as actually obtained in practice. Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein.