For integral burnable poisons (gadolinia, erbia, and dysprosia), the poison material is intimately mixed with the UO2 fuel, and it is important that the mate­rials properties of the fuel/poison matrix are not too far removed from those of pure UO2. Intermixing UO2 with another ceramic oxide usually has the effect of decreasing the thermal conductivity. For a given fuel rating, this leads to somewhat elevated fuel pellet center temperatures, with consequent implica­tions for fuel melting, fission gas release, and other fuel behavior parameters.

For this reason, the fuel properties of urania/ gadolinia and urania/erbia fuel have been investi­gated very carefully.5-9 The main parameters of interest are the thermal expansion coefficient, heat capacity, fuel melting point, and thermal conductiv­ity, which have been extensively measured to ensure that the thermal performance of the fuel/poison matrices remains acceptable. In this respect, the per­formance of urania/gadolinia and urania/erbia as fuels is very similar, which might be expected, since
gadolinium and erbium are closely related rare earth elements and form oxides with the same structure. Other properties of the fuel that may be affected by the gadolinia or erbia phase include the UO2 grain size (which decreases with gadolinia or erbia fraction) and the fuel diffusivity (which affects fission gas release). However, this is a small effect that does not have a significant impact on fuel behavior.

The thermal expansion coefficients of gadolinia and erbia are both compatible with that of UO2 and there are no significant implications for fuel behavior. The concentrations of gadolinia used today range up to a maximum of about 8.0 wt%. At these concentra­tions, the melting point is decreased by just a few tens of degrees, which is relatively insignificant compared with the ^2600° C melting point of UO2. Erbia has a similar effect, but because the design concentrations of typically 2-3 wt% are lower, the overall impact is a reduction of the order of 10° C.

Similarly, the thermal properties of UO2 are affected when gadolinia or erbia is intermixed. Figures 12-14 show the variation in linear expansion coefficient, heat capacity, and thermal conductivity in urania/gadolinia as a function of temperature and gadolinia content. These data are recommendations made by IAEA5 and are intended to be used here only for illustration. For details of the experimental data underlying these recommendations, the reader is referred to IAEA-TECDOC-1496 or the original sources cited therein.

Recommended materials properties data are not as readily available for urania/erbia fuel, but Figure 15 illustrates one evaluation of the thermal conductivity that has been published in the open literature (see Kim et a/.8). This illustrates that the thermal properties of urania/erbia are similar to those of urania/gadolinia, weight for weight. However, since lower concentrations of erbia are required for practi­cal applications (because of the slower depletion of

erbia compared with gadolinia), the depression of the thermal conductivity is less significant.

Much of the data for gadolinia and erbia fuels used by industry are regarded as being proprietary information and are therefore not made available in open publications.

While the absorbing nuclides are still present, the change in thermal properties caused by the poi­son material is not a concern, because the fission

power produced in the fuel rods containing the poi­son material is depressed to the point where these rods are far from being limiting. However, a problem may occur when the poison material is fully depleted because the poisoned fuel rods will then increase in power and possibly become limiting. Indeed, because the poison material initially holds down the fission power in the poisoned rods, the fissile material is initially depleted more slowly than in the rest of the assembly and this can cause the poisoned rods to become the highest power rods in the entire assembly. This is undesirable, since the thermal con­ductivity is depressed and the fuel center tempera­ture elevated. Furthermore, the materials properties of two-phase ceramics such as urania/gadolinia or urania/erbia may not be known with the same degree of precision as for the urania phase on its own, and it may be necessary to apply larger uncertainties in the fuel behavior assessments that feed into the reactor safety case.

The remedy is simple and involves reducing the 235U enrichment in the poisoned rods relative to the remaining rods in the assembly. When the poison material is fully depleted, the power in the poison rods is reduced because of the reduction in fissile content. Since the two rod types are invariably fabri­cated in different facilities, this approach does not cause any difficulties in the fabrication logistics. There is a slight additional enrichment cost for the entire assembly, but this is not very significant. Fuel
vendors have been able to demonstrate that this approach ensures that the poisoned rods are never the limiting ones and that the materials properties uncertainties for the two-phase poison fuels do not affect the safety case.

2.16.2 Effect of Burnable Poisons on Fuel and Core Operating Parameters

Burnable poisons affect the operating characteristics of the core by reducing the excess reactivity control requirements, reducing power peaking in fresh fuel assemblies, and by modifying the reactivity feedback coefficients. BWRs have the most stringent require­ments for reactivity control, since in the absence of burnable poison, an excessive amount of control rod insertion would be required, especially if the reactor is operating with long fuel cycles (18 months or more). Control rod insertion in power operation is undesirable as it distorts the radial and axial neutron flux distributions, increasing the core power peaking factor (the ratio of peak power to average power in the core). Burnable poisons allow a much more uniform flux distribution and a lower peaking factor. A further deleterious effect of control rod insertion is that fuel rods near the control rods operate at reduced powers and can be subject to a rapid power ramp when the control rods are removed. This can lead to fuel failures, which burnable poisons help to avoid.

PWRs have a smaller reactivity control require­ment than BWRs because of the use of boron dis­solved in the coolant. PWRs typically operate with 1000-1500 ppm of boric acid in the coolant at the start of a fuel cycle, ramping gradually to zero by the end of the cycle. This can control as much as 15 000pcm of excess reactivity. However, even this is insufficient for long fuel cycles, and more boric acid cannot be added indefinitely due to the need to maintain a negative moderator temperature coefficient and also for other reasons such as the need to carefully control the acidity of the coolant.

An increase in water temperature in a PWR nor­mally reduces reactivity because the water is also the moderator and the fuel assembly design is such that reducing moderation reduces the density of thermal neutrons available to propagate the chain reaction. The presence of boron has the opposite effect, how­ever, because when the temperature of the water increases, the density decreases, and with it the density of 10B atoms decreases. At a high-enough concentra­tion of boric acid (usually 1500 ppm), the effect of changing absorption outweighs that of moderation and the moderator temperature coefficient becomes positive. This sets an upper limit to the reactivity hold-down that is achievable with soluble boron and burnable poisons are now routinely used to supple­ment soluble boron.

The presence ofburnable poisons in a fuel assembly needs to be accounted for in the thermal-hydraulic and fuel thermomechanical performance design assess­ments. For discrete burnable poisons, it is important to ensure that there is sufficient cooling to remove the heat production that accompanies neutron captures in the poison material. As noted in Section 37.3, the fuel thermal conductivity is the material property that is the most affected and has to be carefully taken into account in any thermomechanical simulations of the individual fuel rods. It is dependent on the total con­centration of gadolinia or erbia and has the same effect irrespective ofwhether the neutron-absorbing isotopes are still present or have been burned up.

Some additional physical phenomena are relevant in the first phase of irradiation. One important phys­ical effect results from the strong resonance absorp­tion of neutrons within a fuel rod. Resonance absorption effects distort the radial power profile across the fuel pellets and lead to a different radial temperature gradient at low burnups. Another effect is that the low fuel temperatures lead to reduced in­pile densification and delayed cracking of the fuel at beginning-of-life.

As these issues are very difficult to measure experimentally, thermomechanical simulations usu­ally rely on radial power profiles that are precalcu­lated using neutron transport codes. Densification effects are difficult to model and usually neglected in the thermomechanical models, or allowed for by applying a reduced swelling rate. Existing fuel relocation models may need to be refined in order to simulate delayed cracking. The overall effect of these physical mechanisms, however, dis­appears after the burnable poison is consumed (which typically occurs before the end of the first cycle of irradiation cf. Figure 1). For example, the time of closure of the fuel-to-cladding gap is hardly affected at all. Moreover, fission gas release in the later phases of irradiation is similar to that of nonpoisoned fuel and is consistent with experi­ments where the fission gas diffusion coefficient in gadolinia fuels was judged to be independent of the Gd2O3 content. Also, no differences have been found between the temperature dependence of the fission gas diffusion coefficient in gadolinia fuels and nonpoisoned UO2 fuels.