IAEA Technical Meeting on on Re evaluation of Maximum Operating Temperatures and Accident Conditions for High Temperature Reactor Fuel and Structural Materials, 10 12 July 2012, Vienna, Austria Naturally Safe HTGR in the response to the Fukushima Daiichi NPP accident Hirofumi OHASHI Nuclear Hydrogen and Heat Application Research Center Japan Atomic Energy Agency (JAEA)
Outline 1. Background 2. Concept of Naturally Safe HTGR 3. R&D and demonstration items 4. Summary 1
Safety Design of Current System (GTHTR300) Applied principle of the defense in depth to assure fundamental safety functions Combination of passive and inherent safety features Control of the reactivity The control rod system The reserved shutdown system (B 4 C/C pellets) Removal of heat from the core The vessel cooling system (passive system) Confinement of radioactive material Multiple barriers to prevent FP release The fuel coatings The fuel matrix and fuel element graphite The reactor pressure boundary The confinement The reactor building Control rod system Air Fuel coatings Graphite Reactor pressure boundary Reserved shutdown system Confinement Vessel cooling system (passive) Reactor building 2
Response to the Fukushima Daiichi NPP Accident Technical features For public acceptance Issues The worst uncontrolled accident must be postulated. Issues PSA approach does not convince average population of safety of the nuclear system. Answer Ensure the reactor safety by natural phenomena Answer Evaluate the reactor safety by deterministic approach Naturally Safe HTGR 3
Defense in depth or Defense by nature LWR Naturally Safe HTGR Levels and objectives of defence in depth 1~3) Plant state 4) Level 1 Level 2 Level 3 Level 4 Prevention of abnormal operation and failures Control of abnormal operation and detection of failures Control of accidents within the design basis Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents Normal operation Anticipated operational occurrence (AOOs) Complex operational occurrence and design basis accidents Design extension conditions (DECs) Essential features Conservative design and high quality in construction and operation Control, limiting and protection systems and other surveillance features Engineered features Accident management Evaluation method (under discussion) Deterministic safety analysis + Probabilistic safety analysis Review of accident management Deterministic safety analysis + Probabilistic safety analysis Essential features Conservative design and high quality in construction and operation Control, limiting and protection systems and other surveillance features Engineered features Physical phenomena Evaluation method Deterministic analysis Deterministic safety analysis (Severe accident free) Level 5 Mitigation of radiological consequences of significant releases of radioactive materials Post DECs situations Off site emergency response Off site emergency response by Revised Special Law on Nuclear Disaster Countermeasures NA Evacuation free 1) INSAG 10, IAEA (1996). 2) INSAG 12, IAEA (1999). 3) NS R 1, IAEA (2000). 4) DS 414, IAEA (2010). 4
Safety Concept of Naturally Safe HTGR Eliminate environmental impact from accidents under any conditions, Only rely on inherent safety features based on physical phenomena, Protect fuel coating against causes of events without reliance on design features, and Retain radionuclides within confinement;intrinsically accomplish safety Confinement function Reactor Phenomena which degrades confinement function Causes of events Physical phenomena Fuel coating Diffusive release Sublimation Fission product Uranium Corrosion Failure Core heat up Oxidation Doppler effect Radiation, Natural convection Oxide layer formation Attain stable state Retain radionuclides within confinement RPV CV Fuel coating Flammable gas explosion Flammable gas oxidation Retain radionuclide within coated particles which are insusceptible to external events * Confinement function is only degraded by well known phenomena * Protect confinement function only by physical phenomena *The occurrence of phenomena is unquestionable Ref: H. Ohashi et al., Concept of an Inherently safe High Temperature Gas cooled Reactor, American Institute of Physics Conference Proceedings, 1448, p. 50 58. (2012). 5
Physical Phenomena to Prevent the Progression of the Core Heat Up The progression of the core heat up is prevented by inherent shutdown by the doppler effect heat removal from the outside of RPV to soil by conduction in the RPV, radiation and natural convection of air in the reactor cavity 6
Physical Phenomena to Prevent the Progression of the Coating Layer Oxidation The progression of SiC oxidation is prevented by SiO 2 layer formed on SiC surface Initial O 2 Pressure (Pa) 10 6 10 4 10 2 10 0 10 2 10 4 2000 1800 1600 Temperature (K) 1400 1200 1000 Passive oxidation SiC + C SiC Active oxidation 5 6 7 8 9 10 1 / T (10 4 /K) O 2 OPyC SiO 2 SiC CFP IPyC Cross section Passive oxidation SiC(s) + 3/2O 2 (g) SiO 2 (s) + CO(g) Active oxidation SiC(s) + O 2 (g) SiO(g) + CO(g) Ref: International Atomic Energy Agency, Response of Fuel, Fuel Elements and Gas Cooled Reactor Cores under Accidental Air or Water Ingress Conditions, IAEA TECDOC 784, IAEA, Vienna (1995). 7
Physical Phenomena to Prevent the Progression of the Flammable Gas Explosion CO concentration is maintained below the explosion limit by CO oxidation reaction. Reflector Fuel block Air ingress Pipe rapture RPV Pipe rapture Fuel block Coolant channel O C O C Air ingress (natural convection) CO oxidation CO + 1/2O 2 CO 2 Graphite oxidation C + 1/2O 2 CO C + O 2 CO 2 Mass transfer, diffusion outlet inlet Design capable Lower limit of explosion CO concentration 8
Safety Demonstration using HTTR 1. Demo. of all the CRs withdrawal Fuel region All the CRs withdrawal CR 3. Demo. of all black out It is to be verified by conducting related tests step by step that even if loss of all black out would occur and rise core temperature due to decrease (loss) of heat removal, reactor would settle to safe state. Horizontal cross sectional view of core Fuel region Vertical cross sectional view of core 4. Composite events It is to be verified by conducting related tests step by step that even if multiple abnormal events would occur, reactor would settle to safe state. 5. Confirmation of radioactive material inventory in primary circuit It is to be confirmed that inventories of radio active materials such as iodine, cesium, tritium, etc. are very low in primary circuit. It is to be verified by conducting related tests step by step that even if abnormal reactivity would be inserted by withdrawal of all the control rods and rise core temperature due to increase of heat generation, reactor would settle to safe state. 2. Demo. of rapid and large coolant flow increase It is to be verified by conducting related tests step by step that even if abnormal reactivity would be inserted by over cooling due to rapid/large increase of coolant flow and rise core temperature due to increase of heat generation, reactor would settle to safe state. Measurement test of deposited iodine in primary circuit 9
R&D Items for Fuel and Graphite Experimental evaluation of oxidation of SiC layer of coated fuel particles measurement of formation of SiO 2 film measurement of O 2 contents and temperature to form SiO 2 film Development of oxidation resistant graphite development of surface structure oxidation resistant performance (incl. thermal expansion and irradiation effects) evaluation of integrity of oxidation layer (detachment, cracks, etc.) Air ingress into core Fuel block Fuel rod Graphite sleeve hold fuel compacts and make coolant flow channels. Fuel compact Oxidation of fuel sleeve May cause mechanical damage on coated fuel particles (CFP) Air flow 10
Water Free System Concept for Naturally Safe HTGR 11
Summary We proposed a Naturally Safe HTGR in the response to the Fukushima Daiichi NPP accident. The safety of the Naturally Safe HTGR is kept by natural phenomena without engineered safety features, AC power or prompt actions by plant personnel even in the worst uncontrolled condition. We proposed deterministic approach instead of probabilistic approach for public acceptance. Several R&D items for the Naturally Safe HTGR is planned in JAEA. 12