BN-800 Reactor Simulation Model

The BN-800 is a sodium-cooled fast reactor with a thermal power output of 2100 MWth, corre- sponding to roughly 800 MWe. For the simulations, the reactor is modeled according to the Fast Reactor Database of the IAEA (2006). It can be seen as an example for either plutonium disposition in the core, or more general, for the possibility of breeding nuclear weapons material in a fast reactor.

5.2.1 Description of the Reactor Design

The BN-800 consists of 565 hexagonal fuel elements that are split into three different zones that vary in regard to their plutonium content. Each of the zones is further split into three batches for refueling. In the sketch of the core model, Figure 5.1, the zones are colored in a red, green, or blue color scheme. At the periphery of the core, 90 breeding elements can be inserted. The design further foresees axial blankets placed below the active region, but none above. The core is surrounded by a ring of steel reflector elements and contains 30 control and safety rods. These have been assumed to be withdrawn for the depletion calculations during which the total neutron flux is normalized according to the overall reactor power. Table 5.1 summarizes key core values. More information on the design of the single fuel and blanket elements is given in Table 5.2. For simulation, the fuel elements were treated as homogeneous material mixtures.

2 _{The calculations on the BN-800 presented in this thesis were finished before the suspension of the agreement.}

2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 3 1 2 3 1 2 3 1 2 3 1 2 1 3 1 3 2 1 2 1 3 2 1 3 2 1 3 2 1 2 3 2 1 3 2 1 3 3 2 3 1 3 2 3 1 2 3 1 3 3 1 3 2 2 1 3 2 1 1 3 1 2 3 1 3 2 2 2 2 1 2 1 3 1 3 2 1 2 3 1 3 1 2 3 1 2 1 3 1 2 3 2 1 3 2 1 3 3 1 2 3 1 2 1 2 3 1 2 3 1 3 3 1 3 2 2 1 3 2 1 2 1 2 3 1 2 3 2 3 1 2 3 2 1 2 1 3 1 3 2 1 2 3 1 2 3 2 3 1 2 3 1 3 1 2 1 2 3 2 1 3 2 1 3 3 2 2 3 1 2 3 11 3 2 1 2 3 1 3 1 2 3 1 23 1 2 3 1 21 2 3 1 2 1 2 1 2 11 3 3 1 3 2 3 2 1 2 1 1 2 3 1 21 3 3 3 2 1 2 3 1 2 3 1 2 3 1 2 3 11 2 3 1 2 3 2 3 3 2 1 2 3 23 3 3 3 1 3 2 2 1 2 3 2 31 3 1 2 1 3 1 2 3 12 3 1 2 3 13 1 2 3 1 23 1 2 3 1 1 2 1 3 21 2 2 1 2 2 1 3 2 1 3 3 11 3 2 2 1 3 1 23 1 2 3 1 21 2 3 1 2 31 2 3 1 2 3 2 3 3 1 3 22 3 1 2 2 1 3 2 1 2 1 3 1 2 2 3 1 2 33 2 1 3 3 1 2 3 1 2 3 12 3 1 2 3 13 1 2 3 1 3 3 1 3 23 1 2 2 1 3 1 3 2 2 1 3 12 1 3 1 2 2 3 2 23 1 2 3 1 21 2 3 1 2 1 1 2 2 1 31 3 2 1 3 2 1 3 2 1 3 3 1 3 2 1 2 3 3 1 3 2 3 3 1 3 21 3 2 2 1 2 3 1 2 3 1 2 3 1 2 1 3 2 1 2 1 3 1 3 2 1 3 2 1 3 2 1 3 2 2 1 32 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 3 2 1 32 1 3 2 1 3 2 1 3 - 150 - 100 - 50 0 50 100 150 - 150 - 100 - 50 0 50 100 150 cm cm

Low Enriched Zone

1 2 MediumEnriched Zone 3 High Enriched Zone

Low Enriched Zone

1 2 MediumEnriched Zone 3 High Enriched Zone

Low Enriched Zone

1 2 MediumEnriched Zone 3 High Enriched Zone

Blanket Steel Reflector Control Rods

Figure 5.1.: Cross section of the BN-800 reactor model. The core contains three different fuel zones split into three batches each. The active zone is surrounded by a ring of blanket elements. There are 30 control and safety rods present.

Table 5.1.: Core and element design data for the BN-800 core model. All values from IAEA (2006).

Parameter Unit Value

Total Thermal Power MW 2100

Number of Fuel Elements 211+156+198 Number of Blanket Elements 90 Number of Control Elements 30

Fuel Element Pitch cm 10

Can Thickness cm 0.275

Active Core Height cm 88

Table 5.2.: Geometric dimensions of the fuel and blanket elements in the BN-800 core (IAEA 2006).

Parameter Unit Fuel Blanket

Number of Pins per Element 127 37 Outer Pin Diameter cm 0.66 1.4 Cladding Thickness cm 0.04 0.04

The coolant densityρ_{N A}can be calculated using equation

ρN a= 950.0483 − 0.2298T − 14.6045 · 10−6T2+ 5.6377 · 10−9T3. (5.1)

It results in the density in kg/m3 depending on the temperature T in◦C (Rouault 2010, p. 2354)

An average coolant temperature of 450◦C is assumed. For structure, cladding and reflector material, stainless steel ChS-68 is selected (IAEA 2012c). Material composition was chosen according to Porollo et al. (2009). Table 5.3 lists the original materials and their densities for the derivation of the homogeneous materials.

Table 5.3.: Selected material properties used in the BN-800 core model. Material Density in g/cm Reference

Fuel MOX 8.60 (IAEA 2006, p. 57)

Blanket Uranium Dioxide 9.70 (IAEA 2006, p. 59) Cladding, Structure ChS-68 Steel 7.75 (IAEA 2012a, p. 525)

Coolant Sodium 0.84 (IAEA 2006, p. 15)

For the MOX fuel and the blanket material, the "smeared density of fuel with fuel assumed to occupy

whole space inside the cladding tube" is tabled (IAEA 2006, p. 57, p. 59). In the original reactor

design, MOX fuel using reactor-grade plutonium is planned. Table 5.4 shows the exemplary isotopic compositions for weapon- and reactor-grade plutonium used for the model. The reactor- grade composition originates from fuel with low burn-up of about 30 MWd/kgHM. During the design phase of the BN-800 this was a common burn-up in commercial light water reactors. The exact composition of Russian excess weapon-grade plutonium is not known but it is assumed that the content of plutonium-239 is higher than in the given vector. According to the PMDA, Russia is allowed to blend down her original weapon-grade plutonium to reduce the fraction of plutonium-239.

Table 5.4.: Isotopic composition of weapon-grade and reactor-grade plutonium. Reactor-grade plutonium as given in NEA (1995, p. 77) and weapon-grade plutonium as given in Holdren et al. (1995, p. 45).

Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Reactor-grade Plutonium 1.80 59.00 23.00 12.20 4.00 Weapon-grade Plutonium 0.01 93.80 5.80 0.13 0.02

There are three different zones in the reactor: the inner zone consisting of 211 fuel elements with a plutonium fraction of 19.3 %, the intermediate zone consisting of 156 fuel elements with a plutonium fraction of 21.9 % and the outer zone consisting of 198 elements and containing 24.5 % plutonium. In the following, the zones are labeled low, medium, and high enriched zone (LEZ, MEZ, HEZ). The plutonium content is not published explicitly in the IAEA database, but rather an

enrichment defined as the mass of the fissile isotopes divided by the mass of the fertile and fissile isotopes. The published values of 19.5 %, 22.1 %, and 24.7 % enrichment include uranium-235 in the fissile mass. Using the figures given for the total uranium-235 and uranium-238 inventory, it was derived that depleted uranium with 0.3 % uranium-235 is used for the design and thus the above values for the plutonium enrichment could be calculated.

For the analysis concerning the PMDA, the reactor-grade plutonium in the fuel is replaced by weapon-grade plutonium. To keep core characteristics as similar as possible to the already developed model, the plutonium fraction in the fuel is decreased. The resulting average fission cross section of the weapon-grade plutonium is set to be roughly the same as for the reactor-grade plutonium. The plutonium fraction in the different zones is thereby calculated to be 17.8 %, 20.2 % and 22.7 % respectively. This adjustment in the plutonium enrichment also leads to changes in the neutron economy in the core, but they are considered to have only small effects on reactor dynamics. Further, several other options to adjust for the different plutonium vectors can be imagined. This includes the final number of MOX elements, the possible introduction of uranium fuel elements or other plutonium fractions in the fuel.

The axial blankets follow the same geometry as the fuel elements, whereas for the radial elements a special structure applies (cf. Table 5.2). As blanket material, depleted uranium dioxide is used. If no blankets are present, the elements are filled with sodium. This replacement leads to only minor changes in reactivity and the average burn-up of the fuel elements. The temperature is set to 600 Kelvin, except for the fuel and breeding elements with an assumed temperature of 1200 Kelvin.

5.2.2 Simulation Parameters

For the depletion calculation, a reactor power of 2100 MWth and a cycle length of 420 full power days (FPD) for every fuel element is modeled. Every 140 FPD, one third of the fuel elements from each zone is replaced. The different batches3are labeled from one to three in Figure 5.1, according to the period after which they are replaced by a fresh fuel element. The axial blankets are replaced every 420 FPD, together with the fuel elements. The radial blanket elements have a longer irradiation time and remain in the periphery of the core for 840 FPD.

Since the simulation starts with a core completely filled with fresh fuel, more than three cycles are simulated for the core to reach equilibrium operation mode4. For the evaluation, fuel elements are taken into consideration from the cycle when each batch has been refueled at least twice. To shorten the simulation time, the first radial blanket batch was already replaced after 420 days. The IAEA fast reactor database lists an average burn-up of 66 MWd/kgHM (ibid., p. 41). Since one of the requirements set by the PMDA is the fulfilling of certain radiation barrier by the spent fuel elements, in the agreement different minimal burn-ups are defined. Those must be reached before the elements can be removed from the core. During the commissioning period, lower values are allowed. Additional to the values for one single element, also average batch values are stated. In the agreement, the burn-ups are given in percent of heavy metal atoms that are fissioned. Table 5.5 gives the published values and those translated into MWd/kgHM. Depletion calculations were performed using MCMATH.

3 _{One batch are all elements discharged at the same time. It contains elements from all fuel zones.}

4 _{At BOL, the core is only fueled with fresh fuel elements, resulting in a higher criticality. Consecutively, the batches}

are replaced by fresh elements until the equilibrium operation mode is reached. The first batch reaches only one third of the targeted burn-up, the second batch already two thirds. Not even the first batch reaching full burn-up shows equilibrium behavior since it was exposed to higher neutrons fluxes at BOL. Only when elements from the same position show the same burn-up and composition after removal from the core, equilibrium mode is reached.

Table 5.5.: Minimum burn-up values as agreed-upon in the Plutonium Management and Disposition Agreement.

HM atoms fissioned % HM MWd/kg HM Fuel Element, Commissioning Period 3.9 36.4 Fuel Element, Main Operation 4.5 42.1 Batch, Commissioning Period 5.0 46.7

Batch, Main Operation 6.0 56.1