NEA/NSC/DOC(2013)9 Accident-tolerant control rod Hirokazu Ohta, Takashi Sawabe, Takanari Ogata Central Research Institute of Electric Power Industry (CRIEPI), Japan Boron carbide (B4C) and hafnium (Hf) metal are used for the neutron absorber materials of control rods in BWRs, and silver-indium-cadmium (Ag-In-Cd) alloy is used in PWRs. These materials are clad with stainless steel. The eutectic point of B4C and iron (Fe) is about 1 150°C and the melting point of Ag-In-Cd alloy is about 800°C, which are lower than the temperature of zircaloy – steam reaction increases rapidly (~1 200°C). Accordingly, it is possible that the control rods melt and collapse before the reactor core is significantly damaged in the case of severe accidents. Since the neutron absorber would be separated from the fuels, there is a risk of re-criticality, when pure water or seawater is injected for emergency cooling. In order to ensure sub-criticality and extend options of emergency cooling in the course of severe accidents, a concept of accident-tolerant control rod (ACT) has been derived. ACT utilises a new absorber material having the following properties: • higher neutron absorption than current control rod; • higher melting or eutectic temperature than 1 200°C where rapid zircaloy oxidation occurs; • high miscibility with molten fuel materials. The candidate of a new absorber material for ATC includes gadolinia (Gd2O3), samaria (Sm2O3), europia (Eu2O3), dysprosia (Dy2O3), hafnia (HfO2). The melting point of these materials and the liquefaction temperature with Fe are higher than the rapid zircaloy oxidation temperature. ACT will not collapse before the core melt-down. After the core melt-down, the absorber material will be mixed with molten fuel material. The current absorber materials, such as B4C, Hf and Ag-In-Cd, are charged at the tip of ATC in which the neutron flux is high, and a new absorber material is charged in the low- flux region, as shown in Figure 1. This design could minimise the degradation of a new absorber material by the neutron absorption and the influence of ATC deployment on reactor control procedure. As a result, the new absorber material may be recycled. Figure 1: Design of ACT, (a) BWR type and (b) PWR type Core End plug B4C End plug (Current control rod) Stainless Spring ball Stainless Stainless cladding cladding Ge2O3 or Ge2O3 or , Ge2O3-HfO 2 , Ge2O3-HfO2 etc... etc... Dimple Ag-In-Cd (Current End plug control rod) End plug Core (a) BWRs (b) PWRs INCREASED ACCIDENT TOLERANCE OF FUELS FOR LIGHT WATER REACTORS, © OECD 2013 29 Accident Tolerant Control Rod CRIEPI (Central Research Institute of Electric Power Industry) H. Ohta, T. Sawabe, T. Ogata International Workshop on Accident Tolerant Fuels of LWRs 10-12 Dec. 2012 1 Current control rod in LWRs BWR: Boron carbide (B4C) powder (with stainless steel cladding) Metallic Hafnium (Hf) (with Stainless sheath) PWR：Silver – Indium – Cadmium (Ag-In-Cd) alloy (with stainless steel cladding) Boric acid (H3BO3) (= chemical shim) Main structure material: Austenitic Steel 金属ハフ二ウム （プレートタイプ） Control rod for BWRs Control rod for PWRs 2 Behavior of current control rod at SA 2850 melting of UO2 2850 melting of UO2 BWR 2810 melting of HfO2 PWR 2810 melting of HfO2 2710 melting of ZrO 2710 melting of ZrO2 2 2600 eutectic of UO -ZrO 2600 eutectic of UO2-ZrO2 2 2 2570 melting of PuO2 2570 melting of PuO2 2400 monotectic of α-Zr(O)-UO2 and U-UO2 2400 monotectic of α-Zr(O)-UO2 and U-UO2 2350 melting of B4C 2350 melting of B4C 2310 2310 melting of Gd O melting of Gd2O3 2 3 2222 melting of Hf 2222 melting of Hf 2180 melting of B 2180 melting of B ) 2054 melting of Al2O3 ) ℃ ℃ 1975 melting of α-Zr(O) 1975 melting of α-Zr(O) 1900 eutectic of Al O -UO and Al O -ZrO 1852 melting of Zr 2 3 2 2 3 2 Start of UO2 dissolution 1852 melting of Zr by molten Zirca loy → Start of UO2 dissolution 1760 melting of Zirca loy Temperature ( 1760 melting of Zirca loy by molten Zirca loy → Temperature ( forma tion of meta llic (U, Zr, O) melt forma tion of meta llic (U, Zr, O) melt 1536 melting of Fe 1536 melting of Fe 1455 melting of Ni 1455 melting of Ni 1450 melting of sta inless steel and Inconel 1450 melting of stainless steel and Inconel 1410 boiling of NaCl, MgCl2 1300 eutectic of Fe-Zr 1300 eutectic of Al(Al2O3)-Zr, Fe-Zr and Ni-Zr Start of rapid Zirca loy Start of rapiｄ 1200 eutectic of Ni-Zr oxidation by H2O → 1200 eutectic of Ag-Zr and Ni-Zr Uncontrolled Zirca loy oxidation by 1150 eutectic of B4C-Fe 1150 eutectic of B4C-Fe H2O → 1133 temperature escalation 1133 melting of U melting of U Uncontrolled temperature 940 eutectic of Fe-Zr and Ni-Zr 940 eutectic of Fe-Zr and Ni-Zr esca la tion ⇒ Control materials would 801 melting of NaCl 800 melting of Ag-In-Cd 714 melting of MgCl2 melt and fall down before 450 eutectic of NaCl-MgCl2 severe core damage. o ⇒ Injecting pure (sea) water Eutectic of Fe-Zr (940 C) Rapid Zircaloy oxidation by H2O o ≦ o into core would lead to Eutectic of Fe-B4C (1150 C) (~1200 C) o o dilution of boric acid Melting of Ag-In-Cd (800 C) Melting of Zircaloy (1760 C) concentration. 3 TMI-2 relocation Boric acid Upper debris bed Molten metal and ceramic Control, structural, and cladding material solidified between fuel rods 4 Accident Tolerant Control rod (ATC) 1. To keep enough shutdown margin at sever accident. Neutron absorber materials should have following properties: 1) Higher neutron absorption than current control rod Gd, Sm, Eu , Dy etc. 2) Higher melting or eutectic point than ~1200oC where rapid Zircaloy oxidation occurs Gd2O3, Sm2O3, Eu2O3, Dy2O3 etc. 3) High miscibility with molten fuel materials Miscible with UO2 = Gd2O3, HfO2 etc. 2. To minimize the influence on the peration conditions of reactors, and To keep control rod worth for a long-term. （ ） 4) High neutron flux regions, loading current control rod B4C or Ag-In-Cd . Mitigate the effect on reactor control in operation 5) Low flux regions, loading ATC materials. Increase of shutdown margin & Long-term use of control rod ※ ATCR materials are reused. 5 Concept of ATC BWRs PWRs B4C or Hf Current Ag-In-Cd Core Core control rod Gd2O3 or Gd2O3-HfO2 etc. - Absorbers: Gd2O3 / HfO2 -To keep burnable poison (Gd2O3) for a long- ATC term Core under operation Core - To minimize the influence on reactor operation. → Vicinity of core: current materials - Higher shutdown margin than current control rod ATC Core Core - Restrictive loss of burnable poison during shutdown → Reusable -Remaining in the core material at SA or Sever accident overheating → Gd2O3 / HfO2 would mix with core debris 6 Structure of ATC BWRs PWRs Core End plug End plug Current control rod B4C (B4C) Stainless ball Spring B4C Stainless Stainless cladding cladding ATC ATC B4C （Ge2O3 or Ge2O3-HfO2） Ag-In-Cd （Ge2O3 or Ge2O3-HfO2） Dimple Ag-In-Cd B C Current control rod 4 (Ag-In-Cd) End plug End plug (a) Current (b) ATC (a) Current (b) ATC control rod control rod Upper part is Core Lower part is same as Current same as Current CR structure CR structure 7 Future works To improve the accident tolerance without significant effect on reactor control in operation, we will confirm (1) Neutron absorption worth of ATC materials, Gd2O3 or Gd2O3 - HfO2 (2) Life time of ATC, Loading position (3) Melting behavior of ATC materials, Compatibility with structural materials, Gd2O3, Gd2O3 - HfO2, Gd2O3 - Fe, Gd2O3 - Zr etc. (4) Miscibility with molten core materials. UO2 - Gd2O3, UO2 - HfO2, ZrO2 - Gd2O3 , Gd2O3 - Fe, etc. 8 .