Corresponding author: Georgiy L. Khorasanov (khorasanow@yandex.ru)

Academic editor: Yury Korovin

The paper considers the concept of a fast lead cooled 25MW reactor for a variety of applications, including incineration of minor actinides, production of medical radioisotopes, testing of radiation-damaged nuclear technology materials, etc. A specific feature of the proposed reactor is rather a high neutron flux of 2.6·10^{15} n/(cm^{2}·s) at the core center, high average neutron energy of 0.95 MeV at the core center, and a large fraction (40%) of hard neutrons (_{n} > 0.8 MeV). The extremely high estimated reactor parameters are achieved thanks to the small core dimensions (^{2}), innovative metallic fuel of the Pu-Am-Np-Zr alloy, and the ^{208}Pb enriched lead coolant. A relatively high probability of ^{241}Am fission (about 50%) is achieved in the reactor core’s hard spectrum, this making it possible to incinerate up to 4 kg of ^{241}Am during one reactor campaign of 1000 effective days.

At the present time, along with light sodium coolant, heavy lead-bismuth and lead coolants are considered for advanced fast reactors. The advantages of these include chemical inertness, minor activation, and small neutron absorption (^{208}Pb) with a high threshold value of inelastic neutron scattering (_{thresh} = 2.63 MeV). The unique properties of ^{208}Pb as a promising coolant for fast reactor cores were addressed for the first time in (^{208}Pb enriched lead.

It is shown in the paper that the ^{208}Pb enriched lead coolant, in a combination with low moderating metal fuel, e.g. plutonium-americum-neptunium fuel (^{2}) can provide for an extremely high average energy of neutrons at the core center, which is close to 1 MeV, that is the amount that cannot be reached in the current fast medium sodium-cooled reactors (BN-600 and BN-800) (

A reactor with extremely hard neutrons can be used in a variety of applications, primarily, for incineration of environmentally hazardous minor actinides (^{237}Np, ^{241}Am, and ^{244}Cm) with a high threshold value of nuclei fission (_{thresh} > 0.8 MeV), as well as to study the radiation damage to nuclear technology materials caused largely through the action of fast neutrons. The paper considers the possibility for obtaining a hard spectrum of neutrons in a small reactor of 25 MW(th) but with parameters sufficient for the noticeable (~ 15%) incineration of low fissionable nuclides during one reactor campaign. It is also suggested that the reactor campaign is limited only by the neutron fluence growth to the value which does not lead to a catastrophic damage to the fuel cladding.

The design of the BRUTs fast training reactor was discussed in (

Design parameters of the BRUTs-25 reactor.

Parameter | Value |
---|---|

Thermal power, MW | 25 |

Equivalent core diameter, mm | 500 |

Core height, mm | 418 |

Number of FAs in core | 7 |

Number of pins in FA | 165 |

Core heat density, kW/l | 293 |

Average linear thermal load on pin, kW/m | 50 |

FA flat-to-flat dimension, cm | 20.1 |

FA pitch, cm | 20.2 |

Pin inner diameter, mm | 8.2 |

Cladding thickness, mm | 0.3 |

Fuel pellet diameter, mm | 7.4 |

Pin pitch, mm | 14 |

Fuel, wt. % | Pu_{en}47.6+Am10.5+Np0.3+Zr41.6 |

Fuel density, g/cm^{3} |
10.3 |

Coolant | ^{208}Pb |

Core inlet/outlet coolant temperature, °C | 450 / 530 |

Cladding surface temperature, °C | 610 |

In-core volume fraction of coolant/fuel/structural material, % | 69 / 25 / 6 |

Core loaded fuel weight, kg | 215.8 |

Core loaded weight of power-grade Pu, kg | 102.72 |

In-core Am-241 weight, kg | 28.68 |

In-core Np-237 weight, kg | 0.75 |

_{eff} with fuel loaded into reactor |
1.01616 ± 0.00029 |

Core center neutron flux, 1/(cm^{2}·s) |
2.6∙10^{15} |

Campaign, eff. days | 1000 |

The core center neutron fluxes for the BRUTs-25 reactor were calculated using the MCNP/4B code (_{n} > 0.8 MeV, at the core center; the one group cross sections of the ^{238–242}Pu, ^{241}Am and ^{237}Np fission and nuclei neutron radiation capture, and the probabilities of these nuclei fission.

Fig. ^{nat}Pb и ^{208}Pb).

Spectra of neutrons at the core center for the nat Pb and 208Pb cooled BRUTs-25 reactor. The fractions of hard neutrons of En > 0.8 MeV are respectively 32.97 and 40.28%.

Table

Neutronic parameters of the BRUTs-25 reactor core and several transuranics. OCNRCN – One-group cross-section for neutron radiation capture by nucleus.

Parameter | Value |
---|---|

Core center average neutron energy [_{n}], MeV |
0.955 |

Fraction of fast neutrons _{n} > 0.1 MeV, % |
89.34 |

Fraction of hard neutrons _{n} > 0.8 MeV, % |
40.28 |

One-group cross-section for ^{238}Pu fission, barn |
1.516 |

OCNRCN for ^{238}Pu, barn |
0.298 |

Probability of ^{238}Pu fission, % |
83.56 |

One-group cross-section for ^{239}Pu fission, barn |
1.684 |

OCNRCN for ^{239}Pu, barn |
0.128 |

Probability of ^{239}Pu fission, % |
92.95 |

One-group cross-section for ^{240}Pu fission, barn |
0.836 |

OCNRCN for ^{240}Pu, barn |
0.178 |

Probability of ^{240}Pu fission, % |
82.45 |

One-group cross-section for ^{241}Pu fission, barn |
1.754 |

OCNRCN for ^{241}Pu, barn |
0.174 |

Probability of ^{241}Pu fission, % |
90.98 |

One-group cross-section for ^{242}Pu fission, barn |
0.670 |

OCNRCN for ^{242}Pu, barn |
0.155 |

Probability of ^{242}Pu fission, % |
81.23 |

One-group cross-section for ^{241}Am fission, barn |
0.731 |

OCNRCN for ^{241}Am, barn |
0.708 |

Probability of ^{241}Am fission, % |
50.81 |

One-group cross-section for ^{237}Np fission, barn |
0.821 |

OCNRCN for ^{237}Np, barn |
0.540 |

Probability of ^{237}Np fission, % |
60.30 |

The table uses the following calculation pattern:

– the probability of the _{fis} nuclei fission was calculated based on the relation _{fis} = [σ_{fis}] / ([σ_{fis}] + [σ_{cap}]), where [σ_{fis}] and [σ_{cap}] are one-group cross-sections of nucleus fission and cross-sections of the neutron radiation capture by nucleus respectively;

– composition of the power-grade plutonium after 20 years of decay in wt. %: ^{237}Np – 0.10, ^{238}Pu – 1.19, ^{239}Pu – 63.05, ^{240}Pu – 21.50, ^{241}Pu – 4.07, ^{242}Pu – 4.12, ^{241}Am – 5.87.

It follows from the presented data that the values of the one-group nuclear fission cross-sections for ^{240, 242}Pu, ^{241}Am, and ^{237}Np in the hard neutron spectrum at the BRUTs-25 reactor core center differ by not more than twice from the values of the one-group cross-sections for the highly fissionable plutonium isotopes (^{238, 239, 241}Pu).

We shall use the following relation to estimate the value of the isotope incineration in the BRUTs-25 reactor fuel:

Δ_{n}·[σ_{fis}]·

where Δ_{n} is the neutron flux at the core center, 1/(cm^{2}·s); [σ_{fis}]·is the one-group cross-section of the isotope nucleus fission, 10^{–24} cm^{2}; and

Table ^{238–242}Pu and ^{241}Am isotope mass for the BRUTs-25 reactor core calculated based on relation (1) with Φ_{n} = 2.6∙10^{15} 1/(cm^{2}·s) and ^{7} s. It is assumed that pins with the cladding of EP 823 steel will remain serviceable during one reactor campaign, equal to 1000 eff. days, with the neutron fluence consistent with this time (Φ_{n}·^{23} 1/cm^{2}) and the damaging dose less than 75 dpa (displacements per atom).

Estimated fuel isotope mass incinerated during a life of 1000 eff. days in the BRUTs-25 reactor core.

Fissionable isotope and its percent content in loaded fuel | Mass of fissionable isotope with the load М, kg | Mass of incinerated isotope, ∆М, kg | Fraction of incinerated isotope, ∆М/М, % | Mass of non-incinerated isotope and its percent content in fuel at the end of campaign |
---|---|---|---|---|

^{238}Pu, 0.57 wt. % |
1.22 | 0.35 | 28.78 | 0.87 kg, 0.47 wt. % |

^{239}Pu, 30.01 wt. % |
64.77 | 20.47 | 31.60 | 44.30 kg, 23.99 wt. % |

^{240}Pu, 10.23 wt. % |
22.09 | 3.82 | 17.30 | 18.26 kg, 9.89 wt. % |

^{241}Pu, 1,94 wt. % |
4.18 | 1.35 | 32.30 | 2.83 kg, 1.53 wt. % |

^{242}Pu, 1,96 wt. % |
4.23 | 0.59 | 13.90 | 3.644 kg, 1.97 wt. % |

^{241}Am, 13,29 wt. % |
28.68 | 4.30 | 15.00 | 24.38 kg, 13.20 wt. % |

^{237}Np, 0,40 wt. % |
0.75 | 0.12 | 16.50 | 0.66 kg, 0.34 wt. % |

Total: 58.40 wt. % in fuel of 215.8 kg | Total: 125.92 kg | Total: 31.00 kg | Total: 94.91 kg, 51.39 wt. % in fuel of 184.69 kg |

The results of the actinide incineration in fuel during one BRUTs-25 reactor life can be summed up as follows:

– out of 125.92 kg of loaded fissionable isotopes, 31.00 kg or 24.62 wt. % are incinerated;

– the most highly incinerated isotopes are fissionable isotopes of plutonium (^{238, 239, 241}Pu) in the amount of 28 to 32 wt. % of the initial mass of fissionable isotopes;

– ^{241}Am and ^{237}Np are incinerated in the amount of 15 to 16 wt. % of their initial mass;

– the percent content of the ^{238–242}Pu, ^{241}Am and ^{237}Np isotopes in non-incinerated Pu changes insignificantly or by 1 to 5 %, as compared with their initial percent content in Pu_{power-grade}.

As to ^{241}Am, this is contained in the BRUTs-25 fuel in the amount of 28.68 kg, including 6.03 kg contained in power-grade plutonium and 22.65 kg contained in the Pu_{energ}-Am-Np-Zr alloy. In accordance with the above ^{241}Am incineration percentage (15%), the mass of the americium incinerated during one reactor campaign will be Δ_{Am241} = 4.3 kg. And another 4.3 kg of ^{241}Am are transmuted into ^{242}Am decaying further rapidly to ^{242}Cm and ^{242}Pu.

In this case, transmutation (conversion to fission products) of the americium generated by one VVER-1000 reactor during one year (25.75 kg (

The proposed actinide incineration pattern in an extremely hard spectrum of small reactors can be considered along with other currently suggested scenarios (

A concept has been proposed of a lead-cooled 25MW(th) reactor with innovative plutonium-americium-neptunium fuel (Pu-Am-Np-Zr) currently under development. This fuel, combined with the small core dimensions and ^{208}Pb lead enriched coolant, provides for an extremely high average energy of neutrons (about 0.95 MeV at the core center) and a high fraction (~ 40%) of neutrons with the energy of over 0.8 MeV to be achieved. It has been shown that, in this extremely hard spectrum of neutrons, the values of the one-group cross-sections of the ^{241}Am and ^{237}Np isotope fission are in a range of 0.7 to 0.8 barn with the one-group cross-sections of these low fissionable isotopes differing from the one-group cross-sections of highly fissionable isotopes of ^{238, 239, 241}Pu by not more than twice. This circumstance makes it possible to incinerate low fissionable isotopes of americium and neptunium by about 15 to 16 % of the initial mass during one reactor campaign. The presence of 28.68 kg of ^{241}Am in the loaded innovative fuel allows 4.3 kg of its mass to be transmuted (converted to fission products) during three years of one 25 MW(th) reactor operation. Incineration of the americium generated by one VVER-1000 reactor during one year (25.75 kg) will require power operation of six BRUTs-25 reactors for about three years.

The proposed method for the conversion of ^{241}Am and ^{237}Np to fission products can be considered along with other currently proposed scenarios of actinide transmutation for reducing the long-lived RW hazard.

^{th}Information Exchange Meeting on Actinide and Fission Products Partitioning and Transmutation (14IEMPT). OECD NEA.

^{th}Conference “Heavy Liquid Metal Coolants in Nuclear Technologies”. Obninsk, SSC RF-IPPE Publ.

^{208}Pb as such a FR coolant.

^{208}Pb isotope. Perspektivnye materialy (special issue). Moscow: 444–448. [in Russian]

^{208}Pb. Proc. of the 2

^{nd}Int. Workshop “Technology and Components of Accelerator-driven Systems”. NEA/NSC/DOC (2015)7: 65–69.

^{nd}Int. Conf. of Young Scientists, Specialists, Postgraduates and Students, “Innovative Small and Ultra-small Nuclear Reactors”, May 15–17, Obninsk. Obninsk. IATE NIYaU MIFI Publ., 19–21. [in Russian]

^{241}Am incineration probability in lead fast reactors.

^{th}Russian Conf. on Reactor Material Science. NIIAR Publ., Dimitrovgrad, 2. [in Russian]

* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2019, n. 4, pp. 118–129.