^{238}Pu production in a VVER-1000 power reactor

Corresponding author: Evgeny G. Kulikov (

Academic editor: Snejana Grozev

The paper presents the estimates for the possibility for large-scale production of ^{238}Pu 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 NpO_{2} fuel elements is installed at the reactor core center. The NpO_{2} 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 ^{238}Pu and not more than 2 ppm of ^{236}Pu). 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 ^{238}Pu production in a VVER-type power reactor exceeds considerably the existing scale of its production in research reactors.

^{238}Pu

The ^{238}Pu plutonium isotope has a half-life of _{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 ^{238}Pu isotope a valuable source of thermal and electric energy for use in thermoelectric radioisotope generators (TRG) of spacecraft and in cardiac pacemakers (

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 (

It should be noted that production of ^{238}Pu 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 ^{238}Pu is not less than 85%, and the fraction of ^{236}Pu 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 (_{2} fuel lattice pitch and surrounding the

As preliminarily estimated, the replacement of seven uranium FAs in the VVER-1000 reactor core center for seven ^{238}Pu 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.

It is proposed that the irradiation device be installed in the VVER-1000 core center (Fig.

reactor thermal power – 3200 MW;

number of FAs in the reactor core – 163;

fuel – enriched uranium dioxide (4.4% of
^{235}U), density – 10.7 g/cm
^{3};

fuel cladding – 99%Zr-1%Nb alloy, density – 6.5 g/cm
^{3};

coolant – light water, density – 0.73 g/cm
^{3};

hexagonal FA flat-to-flat dimension – 23.4 cm;

fuel pellet diameter – 7.57 mm;

central hole diameter – 1.40 mm;

fuel-cladding gap thickness – 0.075 mm;

cladding thickness – 0.65 mm;

triangular fuel lattice pitch – 12.75 mm;

fuel column height – 353 cm.

The irradiation device represents a configuration of seven VVER-1000 FAs. A standard VVER-1000 FA, in which enriched uranium dioxide has been replaced for neptunium dioxide (NpO_{2}), is accommodated in the ^{238}Pu production (^{238}Pu in the NpO_{2} FA.

FA with the starting material in the form of NpO_{2} (t is the Np rod triangular lattice pitch).

The preferred spectrum (the ^{237}Np resonance region) is formed by way of the ^{208}Pb) surround the ^{237}Np FA (Fig.

The efficient way to form the preferred spectrum is to use ^{208}Pb that is characterized by an extremely small absorption of neutrons and allows, therefore, increasing the neutron flux in the target material. Using it leads thus to accelerated production of the desired nuclide. Using ^{208}Pb also offers other important advantages, including an enhanced Doppler effect and a longer average prompt neutron lifetime.

It has been shown by preliminary estimates that the heterogeneous configuration of the ^{237}Np irradiation. As a consequence, such method allows efficient and large-scale production of highly conditioned ^{238}Pu.

FA with heavy lead moderator (t is the triangular fuel lattice pitch).

The neutronic calculations were based on the TIME26 computer code (

The geometrical model of the VVER-1000 reactor core represented a system of 8 circular layers of hexagonal FAs isolated from the reactor vessel with a water layer (Fig.

A radial circular model of the VVER-1000 reactor.

In the initial option (No. 1), all 163 FAs were standard VVER-1000 UO_{2} FAs (no _{2} 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 –
^{208}Pb rods in Zr-Nb cladding.

Two important circumstances were the cause for ^{208}Pb rods having been introduced into Option No. 5. The ^{208}Pb 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 ^{238}Pu production in the central NpO_{2} FA.

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

Parameters of plutonium production in the VVER-1000

Option No. | Plutonium production rate, kgPu/yr | Fraction of ^{238}Pu/Pu, % |
Fraction of ^{236}Pu/Pu, ppm |
---|---|---|---|

1 | 3.50 | 1.5 | 0 |

2 | 3.92 | 99.5 | 158 |

3 | 1.11 | 99.6 | 51 |

4 | 3.76 | 99.5 | 21.4 |

5 | 4.24 | 99.3 | 21.3 |

It can be seen that introducing NpO_{2} FAs into the reactor core with no lead surrounding leads to plutonium production with a substantial fraction of unwanted ^{236}Pu. Introducing water into the surrounding FAs (Option 3) leads to an abrupt decrease in the intensity of ^{238}Pu 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 ^{236}Pu in plutonium since their respective fractions proved to be much higher than the 2 ppm permitted.

The fraction of ^{236}Pu in plutonium can be apparently reduced through the neutron spectrum mitigation in the central NpO_{2} FA, that is, at the expense of reducing the ^{237}Np (n, 2n) ^{236}Pu reaction rate. To this end, the central NpO_{2} FA options were calculated, in which the triangular NpO_{2} 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} 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 _{2} 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 ^{236}Pu content decreasing to the required standard value (2 ppm). The fraction of ^{238}Pu decreases slightly as well while remaining, however, in excess of the produced plutonium quality standard value (^{238}Pu fraction of over 85%).

Due to the specifically attractive properties of ^{208}Pb, calculations were conducted for the _{2} FA was surrounded by six ^{208}Pb FAs. The NpO_{2} fuel rod spacing (12.75 mm / 20 mm / 30 mm / 44 mm) was varied in the course of the calculations. No ^{208}Pb rod spacing change was considered because of its insignificant effect. The results obtained are presented in Table

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

Fig. ^{208}Pb 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.

Plutonium production rate and isotopic composition with the Pb rod pitch growth

Pb rod lattice pitch, mm | ||||
---|---|---|---|---|

12.75 | 20 | 30 | 40 | |

Pu mass, kg | 3.76 | 3.83 | 3.87 | 3.89 |

Fraction of ^{238}Pu, % |
99.5 | 99.2 | 99.2 | 99.2 |

Fraction of ^{236}Pu, ppm |
21.4 | 21.1 | 20.9 | 20.8 |

Pu / Np, % | 0.78 | 0.79 | 0.80 | 0.80 |

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

Plutonium production rate and isotopic composition with the NpO_{2} fuel lattice pitch growth (six surrounding Pb FAs)

NpO_{2} fuel lattice pitch, mm |
||||
---|---|---|---|---|

12.75 | 20 | 30 | 47 | |

Quantity of starting material (Np), kg | 484 | 197 | 87.5 | 35.6 |

Pu mass, kg | 3.89 | 3.59 | 3.15 | 2.45 |

Fraction of ^{238}Pu, % |
99.2 | 98.2 | 96.0 | 91.6 |

Fraction of ^{236}Pu, ppm |
20.8 | 7.5 | 3.6 | 1.9 |

Plutonium production rate and isotopic composition as a function of the NpO_{2} fuel lattice pitch growth (six surrounding ^{208}Pb FAs)

NpO_{2} fuel lattice pitch, mm |
||||
---|---|---|---|---|

12.75 | 20 | 30 | 44 | |

Quantity of starting material (Np), kg | 484 | 197 | 87.5 | 40.7 |

Pu mass, kg | 4.47 | 4.16 | 3.70 | 3.06 |

Fraction of ^{238}Pu, % |
99.1 | 97.9 | 95.2 | 91.0 |

Fraction of ^{236}Pu, ppm |
20.8 | 7.0 | 3.3 | 1.9 |

An analysis of the above results from the ^{238}Pu 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 ^{236}Pu isotope (its value is 60 times higher than permitted).

2. Introducing water instead of the surrounding FAs (^{238}Pu production intensity decreasing by four times while reducing greatly, though, the fraction of ^{236}Pu.

3. A layer of ^{208}Pb transmits much more soft neutrons promoting so the accumulation of plutonium in the ^{208}Pb leads to the plutonium accumulation in the ^{236}Pu being the same.

4. Introducing lead surrounding for the NpO_{2} FA maintains intensive production of plutonium while reducing greatly, at the same time, the fraction of the unwanted ^{236}Pu. 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 (

6. A fundamentally essential role is played by the increase in the fraction of water in the NpO_{2} FA. The content of ^{236}Pu drops to the required standard value (2 ppm), and the fraction of ^{238}Pu remains in the limits of the standard value in terms of the produced plutonium quality (the ^{238}Pu 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 NpO_{2} FA. This leads to the plutonium production in the ^{236}Pu fraction is less than 2 ppm). Substituting natural uranium for ^{208}Pb 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 NpO_{2} 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 ^{238}Pu and 1.9 to 2 ppm ^{236}Pu.

9. In the event of a ^{208}Pb layer, the best plutonium parameters are achieved with the NpO_{2} 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 ^{238}Pu and 1.9 to 2 ppm of ^{236}Pu.

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

^{238}Pu production in NpO

_{2}targets irradiated at the High Flux Isotope Reactor.

Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2023, n. 3, pp. 118–126.