1. A.A. Bochvar High-technology Research Institute of Inorganic Materials
The authors: Krivov M.P., Kireev G.A., Davydov A.V., Tenishev A.V.
Title of work
«THE METHOD OF THE URANIUM SESQUINITRIDE RESIDUAL AMOUNT FOR DETERMINING IN MIXED NITRIDE NUCLEAR FUEL»
2019

2. Nitride dissociation Oxide fuel : UO2. Nitride fuel
Tmelt(UO2)= 2805 °C
Nitride fuel
UN(s) → U(l) + 1/2N Tdiss≈1800 оС
Tdiss (U) = 1132 oC;
PuN(s) → PuN(g) → Pu(l) + 1/2N2 Тdiss – no data
Tmelt (Pu) = 640 oC;
(UPu)N → ?

3. Equipment and research methods

4. Equipment and research methods
Thermal analysis: thermogravimetry (TG);
Mass spectrometry;
Additionally:
chemical analysis.

5. Investigated samples
Nitrides that was obtained by carbothermic synthesis:
UN;
PuN;
(U1-хPuх)N.
ω(Pu)=5,9; 9,8; 12; 14; 20 и 50 масс.%
msample ≈ mg.
Nitrogen content 5,2-5,6 wt.%.

6. Uranium mononitride Composition: N – 5,368 wt.%, С – 0,208 wt.%,
U2N3 – 2,39 wt.%
Results of a thermogravimetric study of uranium nitride at the temperature of 1900 and 2100 оС

7. MNIT fuel (U0,902Pu0,098)N Composition : О – 0,104 wt.%,
U2N3 – 4,55 wt.%
Results of a thermogravimetric study of MNIT fuel (U0,902Pu0,098)N
at the temperature of 1900 оС

8. Thermogravimetric data PuN 1800 оС

9. Uranium sesquinitride
The existance of four phases of uranium nitrides was revealed in the uranium-nitrogen system: UN, cubic α-U2N3, hexagonal β-U2N3, UN2-х
Cubic α-U2N3 has composition UN1,54 – UN1,75
Hexagonal β-U2N3 has composition UN1,45 – UN1,49
The α-β phase transition occurs above 800 ºС. The decomposition of sesquinitride at a nitrogen pressure of 1 atm occurs at the temperature of 1300 ºС [1].
The decomposition of higher uranium nitrides in a vacuum occurs as follows [2]:
Uranium mononitride (UN) content 5,56 wt.% nitrogen
[1]. Kurosaki K. et al. A molecular dynamics study on plutonium mononitride //Journal of alloys and compounds. – – Т – № – С
[2] Wang X. et al. Study of the Decomposition and Phase Transition of Uranium Nitride under UHV Conditions via TDS, XRD, SEM, and XPS //Inorganic chemistry. – – V. 55. – №. 21. – P

10. Determination of the composition of MNIT fuel by thermogravimetry
The level of nitrogen and carbon, oxygen impurity was determined by the infrared absorption method using the МЕТАВАК for sample nitride fuel (U0,941Pu0,059)N:
О – 0,034 wt.%;
N – 5,51 wt.%;
С – 0,145 wt.%.
Comparison of calculated and experimental data on the mass loss of mixed nitride in the low-temperature area №
The estimated content of the phase U2N3 in the sample, wt.%
Weight loss from TG experiments, mg
Expected weight loss, mg 1
1,84
0,05±0,01
0,08±0,03 2
0,11±0,01
0,2±0,08 3
0,06±0,01
(UPu)N – 96,49 wt.%;
(UPu)O2 – 0,29 wt. %;
(UPu)C2 – 1,37 wt.%;
U2N3 – 1,84 wt.%.

11. MNIT fuel (U0,88Pu0,12)N Composition : N – 5,36 wt.%; С – 0,07 wt.%;
According to chemical analysis, sesquitnitride (U2N3) must be absent.
Results of a thermogravimetric study of MNIT fuel (U0.88Pu0.12) N, at the temperature of 1900 °C

12. MNIT fuel (U0,86Pu0,14)N Composition : О – 0,101 wt.%, N – 5,45 wt.%,
U2N3 – 0,95 wt.%
Results of a thermogravimetric study of MNIT fuel (U0,86Pu0,14)N, at the temperature of 1900 оС

13. Sesquinitride According to the specification of the tablet MNIT fuel:
N – 5,2-5,6 wt.%;
С and О ≤ 0,15 wt.% each.
According to calculations of the phase composition at the maximum allowable concentration of oxygen and carbon impurities and nitrogen content at the upper boundary, the nitride fuel can contain up to 8.06 wt.% of the phase U2N3

14. The distribution of maximum fuel temperature at the height of the reactor core

15. Findings
It has been shown that mixed nitride and uranium nitride have started to lose mass at much lower temperatures, than the melting point. The loss of mass occurs in several stages. At the same time, the mass loss at the first stage ( ºС) is small. At the second stage, above the temperature of 1700 ºС, fuel decomposition occurs.
Mass loss in the low-temperature area ( ºС) is associated with the decomposition of residual sesquinitride, which can be formed in nitride fuel during its fabrication. In this case, the thermogravimetry method established that the decomposition of sesquinitride takes place in 2 stages for MNIT fuel and in the 1 stage for uranium nitride.
Based on the results of thermogravimetric analysis and mass spectrometry, a method was offered for determining the amount of U2N3 in the nitride fuel.

16. Thanks for attention!