Reactor
Reactor Safety
Reactor safety can mean different things to different people. At STL we believe that a safe reactor must have the following attributes:
- It must not be able to meltdown – ever! (even when subjected to the severity of a Fukushima type event).
- This means that fission products will always be retained within a safe containment structure.
- The containment structure will have multiple independent barriers to ensure fission product retention.
Meltdown Proof:
The pebble bed reactor is designed to have a low power density of approximately 4-5MW/m³ as opposed to more than 100MW/m³ in most water cooled reactors. This low power density has the following advantages:
Firstly it means that it is not critical to cool the core at all times as is the case in a water cooled reactor. In the event that the helium coolant flow is interrupted the reactor core temperature will take in the order of 20 hours to increase slowly to a maximum temperature of approximately 1550°C during which time it will automatically shut itself down (i.e. become subcritical) due to the effect of the negative temperature coefficient.
Secondly the low power density means that the reactor geometry can be designed to make use of natural heat removal mechanisms to remove heat from the reactor and disperse the heat into the environment.
One of the main reasons that a pebble bed reactor can be designed to be meltdown proof is the fact that the core power density is approximately 30 times lower than in most water cooled reactors. The power density is the amount of heat (from nuclear fission) typically generated in one cubic meter in the reactor core. The figure below illustrates the size and core volume of a pebble bed reactor producing 100MWt compared to a typical water cooled reactor which produces 3000MWt. It can be seen that the reactor pressure vessels are of similar size (height and diameter) and that the cores (i.e. the volume where the nuclear fuel is placed to produce heat from nuclear fission) are of similar physical size. In both cases a coolant is used to cool the core during normal operation; however the pebble bed reactor has a number of inherent safety features which ensure that the core cannot melt down when the coolant flow stops due to an accident or some unforeseen event.
The strong negative temperature coefficient together with the low power density of a pebble bed reactor means that if the active coolant flow ceases, the reactor will automatically become sub-critical (i.e. shut itself down). LWRs also have a negative temperature coefficient, however they have a high power density and require active cooling to keep the core cooled.
Proliferation resistance
STL's HTMR100 reactor operates on a much longer burn-up fuel cycle of pebble fuel when compared to conventional nuclear reactors. No thorium fuel cycle involving the separation and recycle of 233U would approach the proliferation resistance of unprocessed spent fuel.
HTMR100 has been designed to circulate pebbles through the core just once, therefore eliminating all the complexities of recirculating pebbles. This fuelling concept is called Once-Through-Then-Out, or OTTO cycle. An important non-proliferation characteristic of the OTTO cycle is the long residence time of pebbles inside the core, making it impossible to divert partially burnt fuel for recycling of weapon grade uranium/plutonium.
By the time the pebbles are extracted from the core, the remaining fissile material (mostly 233U in the case of thorium based fuel) is contaminated with a considerable amount of absorbent material (namely 234U). Another strong deterrent with233U as a fissile material for proliferation purposes is that the spent fuel contains an admixture of 232U, whose decay products produce penetrating gamma rays. This spent fuel containing 232U is undesirable as weapons material by virtue due to the fact that their gamma emissions bring with them the potential for significant radiation doses and shielding requirements during weapons proliferation.
STL's commitment of using a proliferation resistant technology naturally extends to a philosophy not supportive of recycling. The HTR technology, characterized by relatively high burn ups, allows using the converted fissile material in situ, at the reactor itself, before extracting the fuel as nuclear waste. In addition, the silicon carbide coating of the fuel kernels can be considered a first step in immobilization (vitrification) of nuclear waste, making HTMR100 fuel less suitable for reprocessing.
Modularization
A fundamental requirement to make HTMR100 plants viable especially for remote areas, due to its transportability. Centralized manufacturing facilities with Quality Assurance (QA) procedures in place will be responsible of building the Reactor Pressure Vessels (RPV), which later will be transported to the reactor sites. This transportability requirement imposes a limit in the RPV outer diameter, which in turn limits the maximum diameter of the pebble bed as well as the height.
The steam turbine and helium blowers along with various other components will be purchased straight off the shelf from a number of different suppliers; this modularity allows the HTMR100 Nuclear Power Plant to be built in a short period of time. The construction of the buildings can continue independently from the supply of the critical components. Most of the sub-systems will be constructed in skid mounted units, ready for coupling when delivered to site.