Passive Reactivity Control of Nuclear Thermal Propulsion Reactors

Venneri, Paolo F.,Eades, Michael,Kim, Yonghee American Nuclear Society 2017 Nuclear technology Vol. No.

INNOVATIVE CONCEPT FOR AN ULTRA-SMALL NUCLEAR THERMAL ROCKET UTILIZING A NEW MODERATED REACTOR

남승현,PAOLO VENNERI,김용희,이정익,장순흥,정용훈 한국원자력학회 2015 Nuclear Engineering and Technology Vol.47 No.6

Although the harsh space environment imposes many severe challenges to space pioneers,space exploration is a realistic and profitable goal for long-term humanity survival. One ofthe viable and promising options to overcome the harsh environment of space is nuclearpropulsion. Particularly, the Nuclear Thermal Rocket (NTR) is a leading candidate for neartermhuman missions to Mars and beyond due to its relatively high thrust and efficiency. Traditional NTR designs use typically high power reactors with fast or epithermal neutronspectrums to simplify core design and to maximize thrust. In parallel there are a series ofnew NTR designs with lower thrust and higher efficiency, designed to enhance missionversatility and safety through the use of redundant engines (when used in a clusteredengine arrangement) for future commercialization. This paper proposes a new NTR designof the second design philosophy, Korea Advanced NUclear Thermal Engine Rocket(KANUTER), for future space applications. The KANUTER consists of an Extremely HighTemperature Gas cooled Reactor (EHTGR) utilizing hydrogen propellant, a propulsion system,and an optional electricity generation system to provide propulsion as well as electricitygeneration. The innovatively small engine has the characteristics of high efficiency,being compact and lightweight, and bimodal capability. The notable characteristics resultfrom the moderated EHTGR design, uniquely utilizing the integrated fuel element with anultra heat-resistant carbide fuel, an efficient metal hydride moderator, protectively coolingchannels and an individual pressure tube in an all-in-one package. The EHTGR can bebimodally operated in a propulsion mode of 100 MWth and an electricity generation modeof 100 kWth, equipped with a dynamic energy conversion system. To investigate the designfeatures of the new reactor and to estimate referential engine performance, a preliminarydesign study in terms of neutronics and thermohydraulics was carried out. The resultindicates that the innovative design has great potential for high propellant efficiency andthrust-to-weight of engine ratio, compared with the existing NTR designs. However, thebuild-up of fission products in fuel has a significant impact on the bimodal operation of the moderated reactor such as xenon-induced dead time. This issue can be overcome bybuilding in excess reactivity and control margin for the reactor design.

TRU self-recycling in a high temperature modular helium reactor

Jo, C.K.,Kim, Y.,Noh, J.M.,Venneri, F. North-Holland Pub. Co 2012 Nuclear engineering and design Vol.242 No.-

The Deep Burn Project is developing high burnup fuel based on Ceramically Coated (TRISO) particles, for use in the management of spent fuel Transuranics. This paper evaluates the TRU deep-burn in a High Temperature Reactor (HTR) that recycles its own transuranic production. The DB-HTR is loaded with standard LEU fresh fuel and the self-generated TRUs are recycled into the same core (after reprocessing of the original spent fuel). This mode of operation is called self-recycling (SR-HTR). The final spent fuel of the SR-HTR can be disposed of in a final repository, or recycled again. In this study, a single recycling of the self-generated TRUs is considered. The UO<SUB>2</SUB> fuel kernel is 12% uranium enrichment and the diameter of the kernel is 500μm. TRISO packing fraction of UO<SUB>2</SUB> fuel compact is 26%. In the SR-HTR fuel cycle, it is assumed that the spent UO<SUB>2</SUB> fuel is reprocessed with conventional technology and the recovered TRUs are fabricated into Deep Burn TRISO fuel. The diameter of 200μm is used for the TRU fuel kernel. A typical coating thickness is used. The core performance is evaluated for an equilibrium cycle, which is obtained by cycle-wise depletion calculations. From the analysis results, the equilibrium cycle lengths of Case 1 (5-ring fuel block SR-HTR) and Case 2 (4-ring fuel block SR-HTR) are 487 and 450 EFPDs (effective full power days), respectively. And the UO<SUB>2</SUB> fuel discharge burnups of Case 1 and Case 2 are 10.3% and 10.1%, respectively. Also, the TRU discharge burnups of Case 1 and Case 2 are 64.7% and 63.5%, respectively, which is considered extremely high. The fissile (Pu-239 and Pu-241) content of the self-generated TRU vector is about 52%. The deep-burning of TRU in SR-HTR is partly due to the efficient conversion of Pu-240 to Pu-241, which is boosted by the uranium fuel in SR-HTR. It is also observed that the power distribution is quite flat within the uranium fuel zone. The lower power density in TRU fuel is because the TRU burnup is very high. Also, it is found that transmutation of Pu-239 is near complete in SR-HTR and that of Pu-241 is extremely high in all cases. The decay heat of the SR-HTR core is very similar to the UO<SUB>2</SUB>-only core. However, accumulation of the minor actinides is not avoidable in the SR-HTR core. The extreme high burnup of the Deep Burn fuel greatly reduces the amount of heat producing isotopes that could be problematic in spent fuel repositories (like Pu-238).

A fuel performance analysis for a 450MW_th deep burn-high temperature reactor

Kim, Y.M.,Jo, C.K.,Jun, J.S.,Cho, M.S.,Venneri, F. North-Holland Pub. Co 2011 Nuclear engineering and design Vol.241 No.9

A performance analysis for a 450MW<SUB>th</SUB> deep burn-high temperature reactor (DB-HTR) fuel was performed using COPA, a fuel performance analysis code of Korea Atomic Energy Research Institute (KAERI). The code computes gas pressure buildup in the void volume of a tri-isotropic coated fuel particle (TRISO), temperature distribution in a DB-HTR fuel, thermo-mechanical stress in a coated fuel particle (CFP), failure fractions of a batch of CFPs, and fission product (FP) releases into the coolant. The 350μm DB-HTR kernel is composed of 30% UO<SUB>2</SUB>+70% (5% NpO<SUB>2</SUB>+95% PuO<SUB>1.8</SUB>) mixed with 0.6moles of silicon carbide (SiC) per mole of heavy metal. The DB-HTR is operated at the constant temperature and power of 858<SUP>o</SUP>C and 39.02mW per CFP for 1395 effective full power days (EFPD) and is subjected to a core heat-up event for 250h during which the maximum coolant temperature reaches 1548.70<SUP>o</SUP>C. Within the normal operating temperature, the fuel showed good thermal and mechanical integrity. At elevated temperatures of the accident event, the failure fraction of CFPs resulted from the mechanical failure (MF) and the thermal decomposition (TD) of the SiC barrier is 3.30x10<SUP>-3</SUP>3.

Thermal-fluid characteristics of the transuranics fuel in a deep-burn HTR core

Jun, J.S.,Lim, H.S.,Jo, C.K.,Noh, J.M.,Venneri, F. North-Holland Pub. Co 2011 Nuclear engineering and design Vol.241 No.9

The Deep Burn Project is evaluating the feasibility of the DB-HTR (Deep Burn High Temperature Reactor) to achieve a very high utilization of transuranics (TRU) derived from the recycle of LWR spent fuel. This study intends to evaluate the thermal-fluid and safety characteristics of TRU fuel in a DB-HTR core using GAMMA+ code. The key design characteristics of the DB-HTR core are more fuel rings (five fuel-rings), less central reflectors (three rings) and decay power curves due to the TRU fuel compositions that are different from the UO<SUB>2</SUB> fuel. This study considered three types of TRU kernel compositions such as 100%(PuO<SUB>2</SUB>+NpO<SUB>2</SUB>+Am), 99.8%(PuO<SUB>1.8</SUB>, NpO<SUB>2</SUB>)+0.2%UO<SUB>2</SUB>+0.6mole SiC getter, and 70%(PuO<SUB>1.8</SUB>, NpO<SUB>2</SUB>)+30%UO<SUB>2</SUB>+0.6mole SiC getter. The first fuel type of TRU kernel produces higher decay power than the UO<SUB>2</SUB> kernel. For the second and the third fuel types, removing the initial Am isotopes and reducing the volumetric packing fraction of TRISO particles will reduce the decay power. The flow distribution, core temperature and TRISO temperature profiles at the steady state were examined. As a safety performance, this study mainly evaluated the peak fuel temperature during LPCC (low pressure conduction cooling) event with considering the impact of decay power, the annealing effect of the irradiated thermal conductivity of graphite, and the impact of the FB (fuel block) end-flux-peaking. For the 600MW<SUB>th</SUB> DB-HTR core, the peak fuel temperature of 100%(PuO<SUB>2</SUB>+NpO<SUB>2</SUB>+Am) TRU was found to be much higher than the transient fuel design limit of 1600<SUP>o</SUP>C due to the lack of heat absorber volume in the central reflector as well as to the increased decay power of the TRU fuel compositions. For a 0.2%UO<SUB>2</SUB> mixed or a 30%UO<SUB>2</SUB> mixed TRU, the peak fuel temperature was decreased due to the reduced decay power, however, it was still higher than 1600<SUP>o</SUP>C due to the lack of heat absorber volume in the central reflector.

Physics study of deep-burning of spent fuel transuranics using commercial LWR cores

Hong, S.G.,Park, S.Y.,Lee, K.H.,Cho, J.Y.,Jo, C.K.,Lee, W.J.,Venneri, F. North-Holland Pub. Co 2013 Nuclear engineering and design Vol.259 No.-

This paper describes a physics study of the utilization and destruction of the transuranic (TRU) nuclides contained in spent fuel through a high burnup irradiation (Deep Burn) in commercial light water reactors (LWR). The TRU nuclides are contained in the Fully Ceramic Micro-encapsulated (FCM) fuel, comprising of TRISO particles dispersed in a matrix of silicon carbide and manufactured in fuel pellets and fuel pins. This study is performed by using the traditional two-step core design procedure, which consists of fuel assembly calculations and core analysis. The main focus is on the core physics characteristics including safety-related parameters. The target reference core is the YongGwang unit 3, cycle 6 reload core. New fuel assemblies consisting of the conventional UO<SUB>2</SUB> pins and new FCM TRU fuel pins are charged into the cores from cycle 6 to an equilibrium cycle. The results show that the reload cores can be designed to achieve the deep-burning (~60% burnup) of TRU without adverse changes to the safety-related parameters and the core performance parameters. The equilibrium core has TRU self-recycling capabilities, being able to ''deep burn'' as much TRU nuclides as it generates.