{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/263068"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/263068","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"Structural and thermodynamic properties of sodium actinide ternary oxides","abstract":"In the potential event of a clad breach in a Sodium-cooled Fast Reactor (SFR), the sodium metallic coolant could come into contact with the (U,Pu,Np)$O_2$ nuclear fuel. The reaction products are numerous, but there is little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent $Na_3AnO_4$ (An=U,Pu,Np) is expected to form, but its structure was the subject of controversy until now. We showed that $\\alpha-Na_3UO_4$ adopts a monoclinic symmetry in space group $\\textit{P2/c}$. Neutron diffraction combined with X-ray Absorption Near Edge Structure (XANES) spectroscopy at the U-M$_4$ edge also revealed that this phase could accommodate excess sodium on the uranium site, with subsequent charge compensation of the uranium cation from U(V) to U(VI), which was not previously foreseen. The corresponding mixed valence state composition is written $Na_3(U_{1-x},Na_x)O_4$ with 0<$\\textit{x}$<0.16(2). To complete the data on the Na-U-O system, the thermodynamic functions of $Na_2U_2O_7$ and $Na_4UO_5$ were evaluated using Knudsen effusion mass spectrometry (KEMS) and thermal-relaxation calorimetry. In addition, the oxygen content required at 900 K within liquid sodium to form pentavalent $Na_3UO_4$ and hexavalent $Na_4UO_5$ were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs. A thermodynamic model for the Np-O system was then developed using the CALPHAD method. This is particularly relevant since it is envisaged to incorporate minor actinides into the fuel to minimize the nuclear waste inventory. The poorly known structures of the Na- Np-O and Na-Pu-O phases diagrams, i.e., tetravalent $Na_2AnO_3$ (An=Np,Pu), pentavalent $Na_3AnO_4$, hexavalent $Na_4AnO_5$ and $\\alpha-Na_2NpO_4$, and heptavalent $Na_5AnO_6$, were also re-fined by the Rietveld method. The structures of $Na_3NpO_4$ and $Na_3PuO_4$ were determined ab initio from powder X-ray diffraction data, and found to be orthorhombic in the space group $\\textit{Fmmm}$. The valence states of the neptunium cations were confirmed from the isomer shift values of their Mössbauer spectra. Having established the charge states without ambiguity, XANES spectra were collected at the $Np-L_3$ and $Pu-L_3$ edges to serve as reference data for An(V), An(VI), and An(VII) oxide phases in the solid state. Finally, KEMS studies of $\\alpha-Na_2NpO_4$ showed very promising results for the determination of the enthalpies of formation of the sodium neptunates and plutonates, for which there is almost no data available. The heat capacities and entropies at 298.15 K of $\\alpha-Na_2NpO_4$, $Na_4NpO_5$, $Na_5NpO_6$, and $Na_5PuO_6$ were also determined. Comparing their Gibbs energy values, the sodium neptunates were found to be slightly more stable than their isostructural uranium analogues.","abstract_html":"In the potential event of a clad breach in a Sodium-cooled Fast Reactor (SFR), the sodium metallic coolant could come into contact with the (U,Pu,Np)<span class=\"etd-inline-math\">O<sub>2</sub></span> nuclear fuel. The reaction products are numerous, but there is little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent <span class=\"etd-inline-math\">Na<sub>3</sub>AnO<sub>4</sub></span> (An=U,Pu,Np) is expected to form, but its structure was the subject of controversy until now. We showed that <span class=\"etd-inline-math\">&alpha;-Na<sub>3</sub>UO<sub>4</sub></span> adopts a monoclinic symmetry in space group <span class=\"etd-inline-math\"><em>P2/c</em></span>. Neutron diffraction combined with X-ray Absorption Near Edge Structure (XANES) spectroscopy at the U-M<span class=\"etd-inline-math\"><sub>4</sub></span> edge also revealed that this phase could accommodate excess sodium on the uranium site, with subsequent charge compensation of the uranium cation from U(V) to U(VI), which was not previously foreseen. The corresponding mixed valence state composition is written <span class=\"etd-inline-math\">Na<sub>3</sub>(U<sub>1-x</sub>,Na<sub>x</sub>)O<sub>4</sub></span> with 0&lt;<span class=\"etd-inline-math\"><em>x</em></span>&lt;0.16(2). To complete the data on the Na-U-O system, the thermodynamic functions of <span class=\"etd-inline-math\">Na<sub>2</sub>U<sub>2</sub>O<sub>7</sub></span> and <span class=\"etd-inline-math\">Na<sub>4</sub>UO<sub>5</sub></span> were evaluated using Knudsen effusion mass spectrometry (KEMS) and thermal-relaxation calorimetry. In addition, the oxygen content required at 900 K within liquid sodium to form pentavalent <span class=\"etd-inline-math\">Na<sub>3</sub>UO<sub>4</sub></span> and hexavalent <span class=\"etd-inline-math\">Na<sub>4</sub>UO<sub>5</sub></span> were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs. A thermodynamic model for the Np-O system was then developed using the CALPHAD method. This is particularly relevant since it is envisaged to incorporate minor actinides into the fuel to minimize the nuclear waste inventory. The poorly known structures of the Na- Np-O and Na-Pu-O phases diagrams, i.e., tetravalent <span class=\"etd-inline-math\">Na<sub>2</sub>AnO<sub>3</sub></span> (An=Np,Pu), pentavalent <span class=\"etd-inline-math\">Na<sub>3</sub>AnO<sub>4</sub></span>, hexavalent <span class=\"etd-inline-math\">Na<sub>4</sub>AnO<sub>5</sub></span> and <span class=\"etd-inline-math\">&alpha;-Na<sub>2</sub>NpO<sub>4</sub></span>, and heptavalent <span class=\"etd-inline-math\">Na<sub>5</sub>AnO<sub>6</sub></span>, were also re-fined by the Rietveld method. The structures of <span class=\"etd-inline-math\">Na<sub>3</sub>NpO<sub>4</sub></span> and <span class=\"etd-inline-math\">Na<sub>3</sub>PuO<sub>4</sub></span> were determined ab initio from powder X-ray diffraction data, and found to be orthorhombic in the space group <span class=\"etd-inline-math\"><em>Fmmm</em></span>. The valence states of the neptunium cations were confirmed from the isomer shift values of their Mössbauer spectra. Having established the charge states without ambiguity, XANES spectra were collected at the <span class=\"etd-inline-math\">Np-L<sub>3</sub></span> and <span class=\"etd-inline-math\">Pu-L<sub>3</sub></span> edges to serve as reference data for An(V), An(VI), and An(VII) oxide phases in the solid state. Finally, KEMS studies of <span class=\"etd-inline-math\">&alpha;-Na<sub>2</sub>NpO<sub>4</sub></span> showed very promising results for the determination of the enthalpies of formation of the sodium neptunates and plutonates, for which there is almost no data available. The heat capacities and entropies at 298.15 K of <span class=\"etd-inline-math\">&alpha;-Na<sub>2</sub>NpO<sub>4</sub></span>, <span class=\"etd-inline-math\">Na<sub>4</sub>NpO<sub>5</sub></span>, <span class=\"etd-inline-math\">Na<sub>5</sub>NpO<sub>6</sub></span>, and <span class=\"etd-inline-math\">Na<sub>5</sub>PuO<sub>6</sub></span> were also determined. Comparing their Gibbs energy values, the sodium neptunates were found to be slightly more stable than their isostructural uranium analogues.","abstract_has_math":true,"creators":["Smith, Anna Louise"],"institution":"University of Cambridge","degree_name":"Doctor of Philosophy (PhD)","degree_level":"Doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Cheetham, Anthony"],"committee_chairs":[],"committee_members":[],"year":2015,"date_issued":"2015-10-06","date_published":"2015-10-06","updated_at":"2026-07-22T22:24:06Z","subjects":[],"languages":["en"],"rights":[],"rights_urls":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/b97119d9-8a25-402a-952f-144dc6a1d60b/download","https://www.rioxx.net/licenses/all-rights-reserved/","https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f9c1c255-4359-4ea7-bb08-991e6f063883/download"],"identifier_entries":[]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.8361","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Cheetham, Anthony"]},{"key":"dc:creator","label":"Author","values":["Smith, Anna Louise"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.issued","label":"Date","values":["2015-10-06"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Cambridge"]},{"key":"dc:relation.isreferencedby.uri","label":"Dc Relation Isreferencedby URI","values":["https://www.repository.cam.ac.uk/handle/1810/263068"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Doctoral"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["Doctor of Philosophy (PhD)"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/b97119d9-8a25-402a-952f-144dc6a1d60b/download","https://www.rioxx.net/licenses/all-rights-reserved/","https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f9c1c255-4359-4ea7-bb08-991e6f063883/download"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.doi","label":"DOI","values":["10.17863/CAM.8361"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/868f0a80-b683-4adf-b379-9fb7dc57dcd8/download"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["In the potential event of a clad breach in a Sodium-cooled Fast Reactor (SFR), the sodium metallic coolant could come into contact with the (U,Pu,Np)$O_2$ nuclear fuel. The reaction products are numerous, but there is little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent $Na_3AnO_4$ (An=U,Pu,Np) is expected to form, but its structure was the subject of controversy until now. We showed that $\\alpha-Na_3UO_4$ adopts a monoclinic symmetry in space group $\\textit{P2/c}$. Neutron diffraction combined with X-ray Absorption Near Edge Structure (XANES) spectroscopy at the U-M$_4$ edge also revealed that this phase could accommodate excess sodium on the uranium site, with subsequent charge compensation of the uranium cation from U(V) to U(VI), which was not previously foreseen. The corresponding mixed valence state composition is written $Na_3(U_{1-x},Na_x)O_4$ with 0<$\\textit{x}$<0.16(2). To complete the data on the Na-U-O system, the thermodynamic functions of $Na_2U_2O_7$ and $Na_4UO_5$ were evaluated using Knudsen effusion mass spectrometry (KEMS) and thermal-relaxation calorimetry. In addition, the oxygen content required at 900 K within liquid sodium to form pentavalent $Na_3UO_4$ and hexavalent $Na_4UO_5$ were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs. A thermodynamic model for the Np-O system was then developed using the CALPHAD method. This is particularly relevant since it is envisaged to incorporate minor actinides into the fuel to minimize the nuclear waste inventory. The poorly known structures of the Na- Np-O and Na-Pu-O phases diagrams, i.e., tetravalent $Na_2AnO_3$ (An=Np,Pu), pentavalent $Na_3AnO_4$, hexavalent $Na_4AnO_5$ and $\\alpha-Na_2NpO_4$, and heptavalent $Na_5AnO_6$, were also re-fined by the Rietveld method. The structures of $Na_3NpO_4$ and $Na_3PuO_4$ were determined ab initio from powder X-ray diffraction data, and found to be orthorhombic in the space group $\\textit{Fmmm}$. The valence states of the neptunium cations were confirmed from the isomer shift values of their Mössbauer spectra. Having established the charge states without ambiguity, XANES spectra were collected at the $Np-L_3$ and $Pu-L_3$ edges to serve as reference data for An(V), An(VI), and An(VII) oxide phases in the solid state. Finally, KEMS studies of $\\alpha-Na_2NpO_4$ showed very promising results for the determination of the enthalpies of formation of the sodium neptunates and plutonates, for which there is almost no data available. The heat capacities and entropies at 298.15 K of $\\alpha-Na_2NpO_4$, $Na_4NpO_5$, $Na_5NpO_6$, and $Na_5PuO_6$ were also determined. Comparing their Gibbs energy values, the sodium neptunates were found to be slightly more stable than their isostructural uranium analogues."]},{"key":"dc:format.checksum.md5","label":"Dc Format Checksum Md5","values":["87eda9de84448d1f82354d60eee3eb5f","adbea2dabb2b15da364d0ea5184f8f71","059c1e8af7b1cdd1779c584017cd2180"]},{"key":"dc:title","label":"Title","values":["Structural and thermodynamic properties of sodium actinide ternary oxides"]}]}],"canonical_facts":{"dc:contributor.advisor":["Cheetham, Anthony"],"dc:creator":["Smith, Anna Louise"],"dc:date.issued":["2015-10-06"],"dc:description.abstract":["In the potential event of a clad breach in a Sodium-cooled Fast Reactor (SFR), the sodium metallic coolant could come into contact with the (U,Pu,Np)$O_2$ nuclear fuel. The reaction products are numerous, but there is little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent $Na_3AnO_4$ (An=U,Pu,Np) is expected to form, but its structure was the subject of controversy until now. We showed that $\\alpha-Na_3UO_4$ adopts a monoclinic symmetry in space group $\\textit{P2/c}$. Neutron diffraction combined with X-ray Absorption Near Edge Structure (XANES) spectroscopy at the U-M$_4$ edge also revealed that this phase could accommodate excess sodium on the uranium site, with subsequent charge compensation of the uranium cation from U(V) to U(VI), which was not previously foreseen. The corresponding mixed valence state composition is written $Na_3(U_{1-x},Na_x)O_4$ with 0<$\\textit{x}$<0.16(2). To complete the data on the Na-U-O system, the thermodynamic functions of $Na_2U_2O_7$ and $Na_4UO_5$ were evaluated using Knudsen effusion mass spectrometry (KEMS) and thermal-relaxation calorimetry. In addition, the oxygen content required at 900 K within liquid sodium to form pentavalent $Na_3UO_4$ and hexavalent $Na_4UO_5$ were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs. A thermodynamic model for the Np-O system was then developed using the CALPHAD method. This is particularly relevant since it is envisaged to incorporate minor actinides into the fuel to minimize the nuclear waste inventory. The poorly known structures of the Na- Np-O and Na-Pu-O phases diagrams, i.e., tetravalent $Na_2AnO_3$ (An=Np,Pu), pentavalent $Na_3AnO_4$, hexavalent $Na_4AnO_5$ and $\\alpha-Na_2NpO_4$, and heptavalent $Na_5AnO_6$, were also re-fined by the Rietveld method. The structures of $Na_3NpO_4$ and $Na_3PuO_4$ were determined ab initio from powder X-ray diffraction data, and found to be orthorhombic in the space group $\\textit{Fmmm}$. The valence states of the neptunium cations were confirmed from the isomer shift values of their Mössbauer spectra. Having established the charge states without ambiguity, XANES spectra were collected at the $Np-L_3$ and $Pu-L_3$ edges to serve as reference data for An(V), An(VI), and An(VII) oxide phases in the solid state. Finally, KEMS studies of $\\alpha-Na_2NpO_4$ showed very promising results for the determination of the enthalpies of formation of the sodium neptunates and plutonates, for which there is almost no data available. The heat capacities and entropies at 298.15 K of $\\alpha-Na_2NpO_4$, $Na_4NpO_5$, $Na_5NpO_6$, and $Na_5PuO_6$ were also determined. Comparing their Gibbs energy values, the sodium neptunates were found to be slightly more stable than their isostructural uranium analogues."],"dc:format.checksum.md5":["87eda9de84448d1f82354d60eee3eb5f","adbea2dabb2b15da364d0ea5184f8f71","059c1e8af7b1cdd1779c584017cd2180"],"dc:identifier.doi":["10.17863/CAM.8361"],"dc:identifier.uri":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/868f0a80-b683-4adf-b379-9fb7dc57dcd8/download"],"dc:language":["en"],"dc:publisher.institution":["University of Cambridge"],"dc:relation.isreferencedby.uri":["https://www.repository.cam.ac.uk/handle/1810/263068"],"dc:rights":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/b97119d9-8a25-402a-952f-144dc6a1d60b/download","https://www.rioxx.net/licenses/all-rights-reserved/","https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f9c1c255-4359-4ea7-bb08-991e6f063883/download"],"dc:title":["Structural and thermodynamic properties of sodium actinide ternary oxides"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["Doctoral"],"dc:type.qualificationname":["Doctor of Philosophy (PhD)"]},"updated_at":"2026-07-22T22:24:06Z"}