secondary coolant loop

The secondary coolant loop uses thermal energy from the primary loop to generate steam, which is transferred to the power generation subsystem. Exhaust stem from the power generation process is condensed back to liquid form and recycled through the loop.

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steam generator

Secondary coolant loop schematic (click for whole system schematic)A phase-change heat exchanger transfers thermal energy from the primary coolant loop to the secondary coolant loop (a single-phase exchange) which is then used to create steam (a change in phase from liquid to gas).

This system ensures that radiation is contained within the reactor-side primary coolant loop, eliminating the need for radioactive shielding or precautions on the generation-side secondary coolant loop. The generation-side components of the heat exchange are referred to as the steam generator.

The heat exchange uses a series of tubes mounted horizontally. Tubes from the primary and secondary loops run in parallel to allow for thermal energy exchange. As thermal and radiation stresses are significantly less in the heat exchange environment than the reaction chamber, the tubes within the heat exchange are constructed from an austenite nickel-chromium based alloy. This is a less resilient and hard-wearing material than used in the main part of the primary coolant loop but still very well suited to high-temperature environments while being cheaper to construct and easier to maintain.

control bleed

Steam pressure coming out of the steam generator is a function of the temperature being fed in by the primary coolant loop. This cannot be controlled with sufficient granularity to allow effective management of steam pressure levels going to the turbine. While adjustments within the turbine can compensate for different pressure levels, this can only be managed within a relatively narrow range. Steam pressure outside this range is managed by the control bleed, which diverts sufficient steam directly to the RHSA to nomalise turbine feed pressure.

radiant heat sink array

The Radiant Heat Sink Array (RHSA) is the final stage of the coolant loop. In this stage the turbine exhaust is returned to liquid condensate, by radiating the remaining heat into space, prior to being reinjected into the primary coolant loop. The rapid condensation of the coolant fluid to a liquid state in this stage ensures there remains a negative pressure differential between the turbine input nozzle and the turbine exhaust. In this way the radiant heat sink ensures a steady flow of coolant around the system, thereby preventing back flows or reductions in coolant velocity within the turbine due to pressure equalisation. The Radiant Heat Sink Array is comprised of a bank of eight highly efficient isothermal radiators (ISO’s) arranged equidistant around the vessel’s hull. The flow of the turbine’s coolant exhaust into ISOs is controlled by the RHSA Manifold.

The RHSA Manifold is located directly behind the turbine exhaust vent and acts a central hub which distributes the coolant flow evenly across each ISO. During normal operation the RHSA Manifold is able to isolate each ISO from the coolant flow to support operational requirements, or in the event of combat or environmental damage to a radiator. The ability for the RHSA Manifold to rapidly shut-off flow to a damaged ISO is essential to prevent loss of coolant fluid to the environment. Reducing the number of operating ISO’s limits the amount of heat which can be dissipated by the RHSA. Insufficient dissipation may potentially cause coolant backflow, typified by a reduction in steam velocity across the turbine stage and an associated drop in power generation. However, isolating radiators is effective in reducing the vessels overall thermal footprint thereby reducing the chance of detection by enemy combatants.

Each ISO in the array is comprised of a network of aluminium tubes which are shaped to maximise surface area and are shielded by highly Infra-Red (IR) emissive polymer nanocomposite (PNC). As the coolant fluid passes through the manifold into the ISO tubing, heat within the coolant convects through to the PNC layer and then dissipates into space as IR radiation. In the vacuum of a deep space operational environment convection will play a minimal role in ISO heat emission to the environment - all heat loss is achieved by IR Radiation (although convection into the minimal atmosphere of low planetary orbit can increase the rate of ISO heat dissipation).

Having passed through an ISO the coolant emerges in a liquid state and is re-injected into the primary coolant loop before being the cycle again. Due to pressure differential between the coolant as it exits an ISO and the Coolant Loop it is necessary for the coolant to have reached a liquid state before it can be injected into the coolant loop. This is because the liquid acts as a pressure lock between the low pressure RHSA and high-pressure coolant loop. Attempting to inject low pressure steam into the coolant loop would result in steam hammering, the backflow of steam from the coolant loop the RHSA, potentially damaging one or more ISOs. For this reason it is necessary to maintain an appropriate number of operational ISO’s in order to handle the current coolant fluid flow rate. If the capacity of the operational ISO’s to convert the coolant to liquid is insufficient then the reservoir of liquid coolant will not provide an effective pressure lock, nor will it be possible to maintain nominal flow rate through the coolant loop.

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