Nicholas J. Morley

Pellet bed reactor concept for nuclear electric propulsion

For Nuclear Electric Propulsion (NEP) applications, gas cooled nuclear reactors with dynamic energy conversion systems offer high specific power and low total mass. This paper describes the Pellet Bed Reactor (PeBR) concept for potential NEP missions to Mars. The helium cooled, 75–80 MWt PeBR, consists of a single annular fuel region filled with a randomly packed bed of spherical fuel pellets, is designed for multiple starts, and offers unique safety and operation features. Each fuel pellet, about 8–10 mm in diameter, is composed of hundreds of TRISO type fuel microspheres embedded in a graphite matrix for a full retention of fission products. To eliminate the likelihood of a single‐point failure, the annular core of the PeBR is divided into three 120° sectors. Each sector is self contained and separate and capable of operating and being cooled on its own and in cooperation with either one or two other sectors. Each sector is coupled to a separate, 5 MWe Closed Brayton Cycle (CBC) energy conversion unit a...

Passive decay heat removal in the PeBR concept for nuclear thermal propulsion

Passive decay heat removal enhances the safety of a nuclear propelled spacecraft and could increase its Isp by up to 4 percent. The large height‐to‐diameter ratio (≳1.5) of the PeBR provides an effective means for passive removal of decay heat from the reactor core after firing. A multi‐dimensional transient heat conduction/radiation model of the PeBR core and surrounding structures is developed to investigate the potential for passive decay heat removal. The effective conductance of the core region includes both conduction and radiation contributions and the thermophysical properties of the fuel and core structure materials are taken to be temperature dependent. Results indicate that passive decay heat removal, following a short period of active cooling (600–1000 seconds), would maintain the PeBR core safely coolable with the peak fuel temperature well below 3400 K.

Pellet bed reactor concepts for nuclear propulsion applications

Pellet bed reactor (PeBR) concepts have been developed for nuclear thermal and nuclear electric propulsion, and bimodal applications. This annular core, fast spectrum reactor offers many desirable design and safety features. These features include high-power density, small reactor size, full retention of fission products, passive decay heat removal, redundancy in reactor control, negative temperature reactivity feedback, ground testing of the fully assembled reactor using electric heating and nonnuclear fuel elements, and the option of fueling on the launch pad or fueling and refueling in orbit. In addition to these features, the concepts for nuclear electric propulsion and for bimodal power and thermal propulsion have no single point failure. The average power density in the reactor for nuclear thermal propulsion ranges from 2.2 to 3.3 MW/I and for a 15-MWe nuclear electric propulsion system the total power system specific mass is about 3.3 kg/kWe. The bimodal-PeBR system concepts offer specific impulse in excess of 650 s, tens of Newtons of thrust, and total system specific power ranging from 11 to 21.9 We/kg at the 10- and 40-kWe levels, respectively. 35 refs.

Thermal-hydraulic analysis of the pellet bed reactor for nuclear thermal propulsion

Abstract A two-dimensional steady-state thermal-hydraulics analysis of the pellet bed reactor for nuclear thermal propulsion is performed using the NUTHAM-S thermal-hydraulic code. The effects of axial heat and momentum transfers on the temperature and flow fields in the core are investigated. In addition, the porosity profile in the hot frit is optimized to avoid the development of a hot spot in the reactor core. Finally, a sensitivity analysis is performed using the optimized hot frit porosity profile to determine the effects of varying the propellant and core parameters on the peak fuel temperature and pressure drop across the core. These parameters include the inlet temperature and mass flow rate of the hydrogen propellant, average porosity of the core bed, the porosity of the hot frit, and local hot frit blockage. The peak temperature of the fuel is shown not to exceed its melting point as a result of changing any of these parameters from the base case, with the exception of hot frit blockage greater than 60% over a 0.12 m axial segment of the hot frit.

Passive cooling of Pellet Bed Reactor concepts for bimodal applications

A threedimensional transient heat conduction analysis of the Pellet Bed Reactor concepts for bimodal applications is performed to investigate the passive cooling capability of the reactor sectors, following a lossof-coolant accident (LOCA), and after shutdown. Results demonstrated that passive cooling of onc or two sectors of the reactor core following a LOCA is possible, while maintaining the reactor operating at a fraction of its nominal thermal power. It is also concluded that passive removal of decay heat can be achieved, while maintaining the maximum fuel temperature in the reactor core at or below 1600 K.

Pellet Bed Reactor for nuclear thermal propelled vehicles

The Pellet Bed Reactor (PeBR) concept is capable of operating at a high power density of up to 3.0 kWt/cm3 and an exit hydrogen gas temperature of 3000 K. The nominal reactor thermal power is 1500 MW and the reactor core is 0.80 m in diameter and 1.3 m high. The nominal PeBR engine generates a thrust of approximately 315 kN at a Specific Impulse (Isp) of 1000 s for a mission duration to Mars of 250 days requiring a total firing time of 170 minutes. Because of its low diameter‐to‐height ratio, PeBR has enough surface area for passive removal of the decay heat from the reactor core. In addition, the reactor is equipped with two independent shutdown mechanisms; 8‐D4C safety rods and 26 BeO/B4C control drums; each system is capable of operating and scraming the reactor safely. The core k‐effective at Beginning of‐Life (BOL) is about 1.07 and in case of water immersion the reactor core is subcritical (k‐effective of 0.93). Due to the absence of core internal support structures, the PeBR can be fueled and refue...

Neutronics and thermal-hydraulics analyses of the pellet bed reactor for nuclear thermal propulsion

Neutronics and thermal-hydraulics design and analyses of the pellet bed reactor for nuclear thermal propulsion are performed based on consideration of reactor criticality, passive decay heat removal, maximum fuel temperature, and subcriticality during a water flooding accident. Besides calculating the dimensions of the reactor core to satisfy the excess reactivity requirement at the beginning-of-mission of 1.25

Neutronics and safety analysis of pellet bed reactor for nuclear thermal propulsion

(K{sub eff} of 1.01), the TWODANT discrete ordinates code is used to estimate the radial and axial fission power density profiles in the core. These power profiles are used in the nuclear propulsion thermal-hydraulic analysis model (NUTHAM-S) to determine the two-dimensional steady-state temperature, pressure, and flow fields in the core and optimize the orificing in the hot frit to avoid hot spots in the core at full-power operation.

A Comparison of Energy Conversion Systems for Meeting the Power Requirements of Manned Rover for Mars Missions

The Pellet Bed Reactor for Nuclear Thermal Propulsion is modeled using the TWODANT discrete ordinance code to determine a reactor point design based on the selection of a fuel fraction in and a diameter of the pellets, dimensions of the reactor core, maximum fuel temperature, and sub‐criticality during a water flooding accident. A total excess reactivity of approximately

Thermal‐Hydraulics Sensitivity Analysis of the Pellet Bed Reactor for Nuclear Thermal Propulsion

1.25 (or keff of 1.01), an order of magnitude higher than that estimated at BOM for 15 hours of full power, steady‐state operation of the PeBR, is considered. Besides calculating the dimensions of the reactor core to satisfy the excess reactivity at BOM, the results of the neutronics calculations include estimates of the radial and axial fission power density profiles in the PeBR core. These results, in conjunction with a 1‐D, steady‐state thermal hydraulics analysis are used to select the operation and design characteristics of the PeBR point design, namely: (a) core radius and height of 38.4 cm and 120 cm, respectively, (b) pellet matrix fraction of 0.5, (c) total reactor mass of 3500 kg, excluding those of the radiation shield, the propulsion nozzle, external structure for the propellant flow into the core, and the drive mechanisms of the control drums in the radial reflector, (d) power density of 10 and 15 MW/l for a reactor thermal power of 1000 MW and 1500 MW, submersion calculations show that with all safety rods removed from the core, the 16 control drums are insufficient to maintain the reactor sub‐critical. However, when the 8, B4C safety rods are inserted into the reactor, it is possible to maintain the submerged PeBR point design