Nuclear Thermal Propulsion at UNIPI

Main characteristics of the UNIPI NTP design

Topics for Master’s Thesis work

• Conceptual design of the nuclear reactor system
  The proposed Thesis is in the framework of the BANTER EIC project and foresees the following two subtasks:
  
  
  • Particle bed fuel optimization:
    According to mission scenarios identified by a dedicated work package of the BANTER project, the present conceptual design of the nuclear reactor needs to be optimized. In particular, attention has to be devoted to the fuel burnup management. The very high power density of the TRISO fuel constituting the particle bed core affects the maximum burning time. A parametric study is necessary to find the proper size of the fuel particles which allows fulfilling both the mission requirements and the needed performance of the reactor.
  
  
  • Propellant management system:
    With the bimodal design of the nuclear propulsion system, the same propellant is used for both thermonuclear and electric propulsion. Ammonia with its high hydrogen content and potential for non-cryogenic storage, offers a viable alternative to classic propellant like LH₂, especially after its discovery on the Moon and Mars. Two different paths are expected for the ammonia propellant. In the propulsive open loop, ammonia is heated up to 3000 K while it flows through the particle bed fuel channels, until reaching the nozzles. In a Brayton closed loop, the ammonia crosses the reactor core through dedicated fueled or unfueled channels before entering the turbine for electric power production. These channels’ size, location, and composition must be studied and optimized.

• Design of an experimental loop for ammonia radiolysis studies
  The proposed thesis is in the framework of the BANTER EIC project. Using ammonia as a propellant simplifies the system design but has the drawback of a reduced specific impulse, halving the performance compared to hydrogen due to its higher molecular weight. A fundamental aspect of this project is to induce the NH₃ decomposition into nitrogen and hydrogen within the nuclear reactor core before being accelerated in the nozzles. The decomposition will halve the molecular weight of the exhaust gases thus increasing the specific impulse to approximately 500s. This thesis concerns the design of an out-core experimental loop for the LVR-15 reactor of CVR (Rez, Prague) to perform ammonia radiolysis studies. The tests will be done at one of the horizontal channels where the irradiation conditions can be changed by varying the fraction between the neutron and photon components. The combination of radiolysis and catalysis (obtained by adding a suitable catalyst in the experimental loop) can also be studied.

• Inadvertent reentry
  The launch approval process for an NTP system is complex and requires the analysis of many scenarios among which there is the inadvertent reentry of the spacecraft. Among the different types of inadvertent reentries, we can mention the reentry during ascent to orbit, the reentry from low-, mid-, or high-earth orbits, as well as, the reentry during a wrong flyby and so on. The consequences of the reentry can be divided into three categories: i) burn up in the atmosphere, ii) intact impact, and iii) scattered impact. Thus, for the UNIPI NTP design, different scenarios for inadvertent reentry must be analyzed to implement technical solutions or suitable strategies to mitigate the possible consequences on the population and the environment of such events.

• Nuclear Safe Orbit (Thesis assigned): An important aspect of any nuclear-based space system is the level of radiation produced by the reactor both during and after its operation. The determination of a nuclear-safe orbit identifies the orbit at which a nuclear-powered spacecraft should be placed so that its orbital lifetime, in case of full loss of power, is long enough thus allowing its radioactive content to be reduced by decay to acceptable levels. This study should be performed for different reactor core configurations and operating times.

• Radiation shielding system (Thesis assigned): The radiation shield is a necessary component of an NTP system to protect the payload and the crew, in case of a manned mission, from the radiation produced by the nuclear reactor core. The shield constitutes a deadweight for the spacecraft, which increases the launching costs and might dominate the mass of the entire engine, dramatically reducing the propulsion system thrust-to-weight ratio. Moreover, in classical design the radiation shields are generally sized to protect the hydrogen propellant tanks from the radiation emitted by the reactor (to prevent its warming), thus the enormous envelope of the tanks governs the size of the shield necessary to achieve adequate protection. In the UNIPI NTP design, part of the radiation from the core is used to heat the ammonia tank which pressurizes the whole system. A trade-off on materials and their combination in a multilayered configuration and on the shield position relative to the reactor, the ammonia tanks, and the payload is needed.