MSR Chapter Title 1 Core Design 1 1 Introduction The MSR core design group designed a safe robust and highly reliable reactor core to provide full power of 100kWe for five years on either the Moon or Mars This chapter presents the design process of the MSR core from choosing the neutron energy spectrum all the way through accident analyses In order to meet the goals of the project the design group set two core specific goals for the design The first goal is to attain core temperatures around 1800K in order to keep the mass of the system as low as possible Section P further describes the motivation for this goal The second goal is to create a core that uses only external core control excore as it presumably increases reliability and avoids sudden reactivity addition subtraction events 1 2 Spectrum 1 2 1 Options The first step in designing the core was selecting the neutron energy spectrum Three possible choices exist thermal epithermal and fast A thermal spectrum is one where primarily moderated thermalized neutrons at about 0 025eV induce fission An epithermal spectrum consists of slightly moderated neutrons that are absorbed in fission resonances between about 1 100keV A fast spectrum is mostly comprised of unmoderated neutrons with energy around 0 5MeV which are the primary source of fission 1 Each of these spectra had inherent advantages and disadvantages no one spectrum was obviously superior All of the options are capable of providing a core capable of running for five EFPYs generating sufficient thermal energy to produce 100kWe and operating in a safe environmentally friendly manner on both the Lunar and Martian surfaces Accordingly the team applied formal decision methodology as described in Section X to rank the three options based on the established project goals in Section C The selected spectrum for the core was that which ranked highest in this procedure 1 2 2 Decision Methodology The neutron energy spectrum will drive the rest of the core design and so it is critical at this juncture that we choose the best possible spectrum The decision methodology used spectrum specific design criteria developed for each of the project goals to evaluate each of the choices As seen in Table Spectrum 1 below the numerical rankings of these design criteria indicate a fast spectrum is the best choice Table Spectrum 1 Spectrum Choice Decision 1 2 3 4 Spectrum Thermal Epithermal 1 Fast Criterion MSR Chapter Title Small Mass and Size Cost 1 35 Small Fuel Mass Small Reflector Moderator Mass Small Dimensions Launchable Accident Safe 1 13 No Criticality Accident Fits in Rocket Controllable 1 14 Flat Keff Feasibility of Ex core Control Slow Transients High Reliability and Limited Maintenance 1 00 Few Moving Parts Little Radiation Damage Performance Index 1 1 1 2 2 2 3 3 3 3 1 2 2 1 3 2 1 3 1 2 2 3 3 1 1 3 2 2 19 41 22 32 3 1 28 6 5 1 2 3 Spectrum Comparison by Design Criterion From the decision methodology we not only see that the fast spectrum is the best choice but it also indicates what the shortcomings of this system might be The sections below will address the reason for the rankings as well as a discussing the shortcomings of a fast spectrum Small Mass and Size The spectrum in part determines the mass of the core as it affects the required fuel heavy metal mass The selected spectrum should minimize the fuel mass For a small core with a 5 year life a fast spectrum minimizes the required mass As the goal is not to breed fissile isotopes large blanket materials are not required a fast spectrum therefore allows for the most compact and light design As the spectrum softens to lower energies efficiency of fuel use decreases and mass requirements increase for a given fissile isotope enrichment 2 Holding enrichment constant the reduction in conversion from using a thermal spectrum for the MSR core would have increased mass requirements by as much as half a metric ton Implicit in this analysis is the assumption that low enrichment is desirable to aid in public acceptance of any space reactor design Different spectra require different amounts of reflecting and moderating material Again the chosen spectrum should minimize the mass of these peripheral systems A fast spectrum requires no moderating material and so will best minimize these requirements Epithermal and thermal systems require neutrons to slow thus requiring the addition of bulky moderators 3 The largest component of the reactor mass and volume is the reflector Adding a moderating material between the core and reflector greatly expands the reflector geometry To add even a small 10cm moderator to the final MSR design the 2 MSR Chapter Title required reflector mass would increase by about 760 kg neglecting the mass of the actual moderating material The spectrum should facilitate the construction of a small core This implies high power density and enrichment Again a fast spectrum is best suited for this requirement as it allows the highest power density of the three choices 2 As neutron energy decreases power density decreases as neutrons migrate to slow down Launchable Accident Safe The chosen spectrum should reduce the risk of criticality accidents during launch As a worst case scenario upon reflector removal and water or wet sand ingress in the case of a crash the core must remain subcritical A thermal spectrum core is easiest to design to meet these needs as the system normally operates with moderated neutrons In a fast or epithermal spectrum core the moderation of the neutrons by water increases reactivity beyond normal operations because fission cross sections decrease with increasing neutron energy In fact this issue becomes more significant as the average operating energy of the neutrons increases The spectrum should lend itself to a compact system design that will fit easily inside existing rockets As with the concerns of small mass and size a fast spectrum is best suited to provide a compact design Controllable Spectrum should assist in achieving a flat keff over the life of the core without complicated movement of control assemblies Over life fissile isotope concentrations gradually decrease and the criticality k of the core drops Continuously converting fertile isotopes to fissile isotopes breeding minimizes this effect 2 As it is possible to obtain a higher conversion ratio in a fast spectrum than in either of the other two spectra it will be easiest to meet this goal with a fast spectrum For simplicity in geometry and