The paper presents the estimates for the possibility for large-scale production of ²³⁸Pu in the core of a VVER-1000 power reactor. The Np-fraction of minor actinides extracted from transuranic radioactive waste is proposed to be used as the starting material. The irradiation device with NpO₂ fuel elements is installed at the reactor core center. The NpO₂ fuel lattice pitch is varied and the irradiation device is surrounded by a heavy moderator layer to create the best possible spectral conditions for large-scale production (~ 3 kg/year) of conditioned plutonium with the required isotopic composition (not less than 85% of ²³⁸Pu and not more than 2 ppm of ²³⁶Pu). Plutonium with such isotopic composition can be used as the thermal source in thermoelectric radioisotope generators and in cardiac pacemakers. It has been demonstrated that the estimated scale of the ²³⁸Pu production in a VVER-type power reactor exceeds considerably the existing scale of its production in research reactors.

²³⁸Pu, thermoelectric radioisotope generators, irradiation device, VVER-1000 reactors

The ²³⁸Pu plutonium isotope has a half-life of Т_1/2 = 87.7 years. This value, on the one hand, is comparatively small for one to be able to say that the specific heat generation is intense (~ 570 W/kg), and, on the other hand, is high enough to be able to state that heat will be generated for a long time.

These properties make the ²³⁸Pu isotope a valuable source of thermal and electric energy for use in thermoelectric radioisotope generators (TRG) of spacecraft and in cardiac pacemakers (Pustovalov et al. 2005).

At the same time, the capacities currently available in Russia and worldwide for its production are insufficient and fail to cover the growing annual demand (Start-up Plan for Plutonium-238 Production for Radioisotope Power Systems 2010, Austin et al. 2020). This isotope is produced largely in research reactors.

It should be noted that production of ²³⁸Pu involves the need for satisfying a number of requirements (e.g., the NASA’s) as regards its suitability for space-borne TRGs (the content of ²³⁸Pu is not less than 85%, and the fraction of ²³⁶Pu is not more than 2 ppm (Daily, McDuffee 2020) which makes it more difficult to solve the original problem. The purpose of this study is to find out if it is possible to create the optimal spectral conditions in the irradiation device (ID) for large-scale production of plutonium suitable for being used in TRGs by selecting the NpO₂ fuel lattice pitch and surrounding the ID with a heavy moderator layer.

As preliminarily estimated, the replacement of seven uranium FAs in the VVER-1000 reactor core center for seven ID assemblies will lead to the reactor power drop at a level of 5%. This loss is expected to be however made up in part at least via production of ²³⁸Pu in kg quantities, the cost of which is estimated at $4000 per gram (The VVER Today: Evolution, Design, Safety). Nevertheless, the authors apprehend the need for justifying further the reactor safety during operation in a dual-purpose mode.

In the initial option (No. 1), all 163 FAs were standard VVER-1000 UO₂ FAs (no ID). Further options took into account the availability of the ID, that is, the central NpO₂ FA was surrounded by a layer of six neighboring FAs, which included the following materials:

• Option No. 2 – a layer of standard UO ₂ FAs.
• Option No. 3 – a layer of light water.
• Option No. 4 – natural lead rods in Zr-Nb cladding.
• Option No. 5 – ²⁰⁸Pb rods in Zr-Nb cladding.

Two important circumstances were the cause for ²⁰⁸Pb rods having been introduced into Option No. 5. The ²⁰⁸Pb lead isotope is characterized by a very small neutron absorption and by the capability for shifting the delayed neutron spectrum into the resonance region, this expected to accelerate the ²³⁸Pu production in the central NpO₂ FA.

For options Nos. 2 through 5, the parameters were estimated which characterize the plutonium production rate and isotopic composition in the central NpO₂ FA. The calculations were conducted in conditions of an invariable FA triangular lattice pitch (12.75 mm). The results are presented in Table 1.

It can be seen that introducing NpO₂ FAs into the reactor core with no lead surrounding leads to plutonium production with a substantial fraction of unwanted ²³⁶Pu. Introducing water into the surrounding FAs (Option 3) leads to an abrupt decrease in the intensity of ²³⁸Pu production. At the same time, introducing lead moderator into the surrounding FAs has improved greatly the plutonium production rate and isotopic composition. However, none of the options considered satisfied the limit for the fraction of ²³⁶Pu in plutonium since their respective fractions proved to be much higher than the 2 ppm permitted.

The fraction of ²³⁶Pu in plutonium can be apparently reduced through the neutron spectrum mitigation in the central NpO₂ FA, that is, at the expense of reducing the ²³⁷Np (n, 2n) ²³⁶Pu reaction rate. To this end, the central NpO₂ FA options were calculated, in which the triangular NpO₂ rod lattice pitch was increased with increasing, respectively, the FA water fraction. This is expected to lead to a mitigated spectrum of the starting material irradiation.

Consideration was also given to the produced plutonium parameters as a function of the lead rod dense lattice pitch in the Pb FAs. Table 2 shows the change in the plutonium production rate and isotopic composition with the Pb rod pitch growth for the NpO₂ rod lattice pitch equal to 12.75 mm. The results obtained have shown the lead rod lattice pitch to have minor effect on the produced plutonium parameters. A decision was made therefore for further calculations to be undertaken with an invariable Pb rod lattice pitch (40 mm) to reduce the excessive calculated data.

Table 3 shows the change in the plutonium production rate and isotopic composition with the NpO₂ fuel rod pitch growth with six surrounding Pb FAs.

It can be seen that an increase in the neptunium fuel rod spacing (that is, an increased water fraction) leads to the ²³⁶Pu content decreasing to the required standard value (2 ppm). The fraction of ²³⁸Pu decreases slightly as well while remaining, however, in excess of the produced plutonium quality standard value (²³⁸Pu fraction of over 85%).

Due to the specifically attractive properties of ²⁰⁸Pb, calculations were conducted for the ID design options, in which the central NpO₂ FA was surrounded by six ²⁰⁸Pb FAs. The NpO₂ fuel rod spacing (12.75 mm / 20 mm / 30 mm / 44 mm) was varied in the course of the calculations. No ²⁰⁸Pb rod spacing change was considered because of its insignificant effect. The results obtained are presented in Table 4.

For the NpO₂ fuel wide lattice option, as it can be seen in the event of the transition to ²⁰⁸Pb, the produced plutonium mass increases by 25% and the specific production grows by 9%. The quality of ²³⁸Pu, both for natural lead and ²⁰⁸Pb, meets the criterion that the fraction of ²³⁶Pu ≤ 2 ppm.

Fig. 5 shows specific plutonium production (per unit of the loaded neptunium mass) as a function of the increase in the neptunium fuel rod spacing for options with natural lead and ²⁰⁸Pb used in the surrounding FAs. There is an evident growth in specific production of plutonium (by a factor of eight and more) with the Np lattice pitch growth.

Figure 5. 

Specific production of plutonium with increased neptunium rod sparing in options with natural uranium and ²⁰⁸Pb used in the surrounding FAs.

An analysis of the above results from the ²³⁸Pu production calculation allows for the following conclusions.

1. No moderator around the Np target leads to plutonium being produced with a very large fraction of the unwanted ²³⁶Pu isotope (its value is 60 times higher than permitted).

2. Introducing water instead of the surrounding FAs (ID design option 3) leads to the ²³⁸Pu production intensity decreasing by four times while reducing greatly, though, the fraction of ²³⁶Pu.

3. A layer of ²⁰⁸Pb transmits much more soft neutrons promoting so the accumulation of plutonium in the ID. As the result, the substitution of Pb for ²⁰⁸Pb leads to the plutonium accumulation in the ID increasing to 4.24 kg/yr, the fraction of ²³⁶Pu being the same.

4. Introducing lead surrounding for the NpO₂ FA maintains intensive production of plutonium while reducing greatly, at the same time, the fraction of the unwanted ²³⁶Pu. The fraction of this isotope remains however ~ 10 times higher than permitted.

5. An increase in the lead rod dense lattice pitch (layer 2 of the radial model), as compared with the standard pitch (h = 12.75 mm), has minor effect on the performance of produced plutonium. This circumstance allowed further calculations to be conducted with an invariable lead rod spacing.

6. A fundamentally essential role is played by the increase in the fraction of water in the NpO₂ FA. The content of ²³⁶Pu drops to the required standard value (2 ppm), and the fraction of ²³⁸Pu remains in the limits of the standard value in terms of the produced plutonium quality (the ²³⁸Pu fraction is over 85%).

7. A major growth in the specific production of plutonium (by a factor of eight and more) is observed as the fraction of water increases in the NpO₂ FA. This leads to the plutonium production in the ID decreasing by a factor of just 1.6 even if the starting material quantity is reduced from 484 kg to 35.6 kg. The plutonium becomes so conditioned (the ²³⁶Pu fraction is less than 2 ppm). Substituting natural uranium for ²⁰⁸Pb leads to a further 9% increase in its specific production.

8. In the event of a natural lead layer, the best plutonium parameters are achieved with the NpO₂ fuel lattice pitch being 47 mm. Plutonium is accumulated in the amount of 2.35 to 2.45 kg and contains 91.6 to 91.9% of ²³⁸Pu and 1.9 to 2 ppm ²³⁶Pu.

9. In the event of a ²⁰⁸Pb layer, the best plutonium parameters are achieved with the NpO₂ fuel lattice pitch being 44 mm. Plutonium is accumulated in the amount of 2.88 to 3.06 kg and contains 91.0 to 91.3% of ²³⁸Pu and 1.9 to 2 ppm of ²³⁶Pu.

The research was supported by a grant from the Russian Science Foundation, project No. 22-22-00287.