[1] Design of fuel element

(1) Operating environment features for fast reactor fuel

Fast reactor fuel has a different operating environment from LWR fuel has. That difference needs to be considered in fuel design. As an example, general features and special points to note for consideration are summa­rized below for MOX fuel, which has acquired much operating experience as fast reactor fuel.

(a) High fast neutron flux and fast neutron exposure enhance swelling and creep deformation of the fuel clad and wrapper tube. Measures should be taken for them.

(b) High operating temperatures of coolant and materials lead to changes of the materials, and care should be taken for them, including especially corrosion-caused changes.

(c) The high pellet temperature of MOX fuel leads to significant restructuring of the pellet and increases the FP gas release rate.

(d) The temperature gradients in the radial and axial directions of the pellet are large causing cracking and relocation (slipping of the cracked pellet fragment), and also causing redistribution of the O/M (oxygen-metal ratio) and migration (movement inside the pellet) of plutonium and FPs such as cesium. Those pellet behaviors during irradiation should be taken into account in the design.

(e) High burnup enhances pellet swelling and FP gas release.

(f) Due to the high burnup and low coolant pressure compared to LWRs (especially PWRs), the fuel cladding internal pressure is higher than its external pressure. Thus, creep rupture of the fuel cladding by the high internal pressure is selected as the major failure mode of fast reactor fuel to be considered.

(2) Major design principles of fast reactor fuel element

Based on the above operating environment features, the major design

principles of the fast reactor fuel element are summarized as the following.

(a) The fuel element must not fail at normal operation and anticipated operational occurrences. Concretely, creep rupture by internal pressure is avoided and pellet cladding mechanical interaction (PCMI) is miti­gated. To avoid creep rupture by internal pressure, an adequate length of the gas plenum, optimization of the cladding thickness and limitation of the cladding temperature must be provided. For mitigating PCMI, optimization of the fuel density and adequate initial pellet-cladding gap must be provided.

(b) Cladding deformation must not be excessive. Since excess deformation of the cladding blocks the coolant flow path, deteriorating the cooling performance, the cladding deformation by swelling and creep needs to be kept small. For that purpose, the fuel internal pressure must be reduced as much as possible, materials with small swelling and creep must be used, and interaction between the fuel bundle and wrapper tube must be mitigated.

(c) Fuel pellets must not melt at anticipated operational occurrences. (It should be noted that most fast reactor fuel designs do not allow fuel melting even at design basis accidents.) A melting limit of linear heat rate based on power to melting experiments and an adequate design criterion must be set up.

(3) Structure of fast reactor fuel element

According to the design principles above, the structure of fast reactor core fuel element is outlined here. Many MOX pellets along with the depleted UO2 pellets as the axial blanket are inserted into a stainless steel fuel cladding. The plenum spring is inserted into the upper part of the fuel element to prevent the pellets from moving. The fuel element is sealed by welding the stainless steel end plugs into the top and bottom of the cladding. An adequate pellet-cladding initial gap is arranged and the gas plenum is provided, so that excess stress is avoided on the cladding or welds of the end plugs; this stress is due to the internal pressure of released FP gas and the cladding deformation caused by the difference of thermal expansion and swelling between the cladding and the pellet. Figure 4.3 showed the concept of the core fuel element.

In the blanket fuel element, many depleted UO2 pellets are inserted into the stainless steel fuel cladding. In order to increase the fuel volume fraction for high breeding ratio, the diameter of the blanket fuel element is a little bit larger than that of the core fuel element. The shape and structure of the blanket fuel element are almost the same as those of the core fuel element.

(4) Major evaluation items in fuel element design

(a) Evaluation of cumulative damage fraction (CDF)

FP gas accumulates with burnup of fuel and that increases the cladding internal pressure. CDF is the evaluation item of cladding integrity associated with creep rupture and is defined below.

(4-8)

t : Time

tR: Creep rupture time (time to creep rupture under constant tempera­ture and stress)

T: Cladding temperature a : Stress.

In the evaluation of CDF, cumulative fatigue fraction by thermal transients is added.

According to the definition of CDF, the fuel element ruptures when CDF reaches 1.0. In the design, the CDF limit is set taking an adequate margin to 1.0. Fuel integrity is assessed by ensuring that the evaluated CDF, considering various uncertainties, is below the limit.

(b) Evaluation of melting limit of linear heat rate and fuel temperature In order to avoid fuel melting, the melting limit of the linear heat rate is identified by power to melting experiments. Based on the melting limit, the allowable design limit of the linear heat rate is set by considering various uncertainties and the operational overpower coefficient (ratio of the scram setpoint and rated power). In the core neutronics design,

By integrating this equation,

dT r2 rkTr+c> T=c‘

(4.10)

is obtained. By providing the boundary conditions as

dT at r = 0 (pellet center), T dr

Ts at r = Rs (pellet surface)

C1 = 0 is obtained. By integrating Eq. (4.10),

(4.11) is obtained. By integrating from the pellet surface to the pellet center and assuming T = T0 at r = 0,

/Тт;к(т)сіт=^-я

(4.12)

is obtained. By using, x = QnRs2 the relation between the power density and the linear heat rate is written as Eq. (4.13).

X=4xf£k(.T)dT

(4.13)

X : Linear heat rate k : Thermal conductivity of fuel pellet T : Temperature T0: Pellet centerline temperature Ts: Pellet surface temperature

The maximum fuel centerline temperature is estimated for normal operation and overpower conditions based on the cladding inner sur­face temperature and using the heat transfer coefficient between the pellet and cladding (gap conductance) and the thermal conductivity of the fuel pellet. In the case of overpower condition, the overpower coefficient is applied.

The gap conductance used for fuel design is based on irradiation test results from experimental reactors in Japan and overseas. The empirical correlations of the pellet thermal conductivity have been prepared by researchers in Japan and overseas as well. The thermal conductivity of MOX pellet, which has extensive operating experience as fast reactor fuel, is arranged as a function of the pellet density, O/M ratio and temperature.

It is known from the irradiation data that the MOX pellet structure is changed by irradiation under high temperature and high heat flux conditions. After being irradiated at high temperature, the internally fabricated pores in the pellet move towards the fuel center which has the peak temperature, so that the structure inside the pellet changes from that at the time of fabrication. In the post-irradiation examinations of fuel pellets which had been irradiated at high temperature, the following features have been observed: an unchanged region remained at the external layer of the pellet; an equiaxial crystal region was present inside the unchanged layer; a columnar crystal region existed inside the equiaxial crystal region; and a void was formed at the pellet center. The temperatures at the boundaries between the two regions and the densities of those structures have also been evaluated by post­irradiation examinations.

In the fuel element design, those data and correlations are installed into the fuel temperature evaluation model. The fuel temperature is calculated by solving the thermal conduction equation in consideration of the fuel restructuring.