Like several others, it seems, I was intrigued by the design of the SSR, and being a long term advocate of MSRs, I contacted Moltex with a few questions of my own. So later, I also followed this thread with interest, and as a number of questions were left unanswered, it seemed appropriate to draw the thread to their attention. Rory O’Sullivan of Moltex has now asked me to play the role of messenger, and to pass on some feedback to the points that have been raised. So what follows is from Rory.
“Please find some comments about the Moltex Energy SSR following a dialogue with members of the team:
- There is no steam near the reactor core, there is a secondary coolant loop through the heat exchanger which is the same salt as the primary salt within the tank ZrF4/NaF/KF. A secondary heat exchanger (boiler) creates steam in the turbine hall which is an adjacent building. The pdf on the learn more page of the Moltex site on "reactor design" does specify that there is a secondary loop, though it does not go into detail as to the coolant.
- Gaseous Fission Products. This is dealt with in their paper on line http://moltexenergy.com/learnmore/Moltex_Gaseous.pdf
. The summary is that sufficient time has been allowed for the gases to decay within the top of the fuel tube before they release to the argon containment zone. Kr-89 escapes which is radioactive. But harmless is an accurate description, Kr-89 is routinely discharged to atmosphere during conventional fuel reprocessing and that is seen as acceptable by regulators because of the low biological effect inherent with it being a noble gas. The key point is that Xe-137 which is dangerous because it decays to Ce-137 is virtually all contained in the fuel tubes. All this is set out in the relevant pdf.
- The sacrificial zirconium within the fuel tube scavenges any sulphur, prevents chromium dissolution and tellurium embrittlement as it reacts with the Cl salt first.
- The air cooling system is indeed close to the core, this is possible because the coolant salt is a highly effective neutron shield so neutron activation of argon and nitrogen in air is a non issue.
- The point about cladding failure being "inevitable" is just wrong. Modern clad PWR tubes have failure rates approaching zero. That is with high internal pressure, fuel/clad interactions and the water/zirconium corrosion problem. We have no corrosion, no pressure, no solid/tube interaction. Provided the fuel tubes were properly pressure tested and QC'd, it would be an extremely rare event for a tube to leak. A rupture of a fuel tube will contaminate the coolant and will involve an expensive clean up process. This is the same for any reactor. With the SSR there is no safety risk to the public with this accident.
- High Temperatures – the fuel salt peaks at 1200C in isolated areas at the centre of the tube. The fuel salt near the boundaries are similar to the coolant salt temperature of 650C. This means that no material is exposed to temperatures higher than 650C. This is because the coolant keeps the fuel tubes cool at all times. This is the exact same effect in solid fuel elements. Extensive modelling has been to validate this and experiments are ongoing.
- The moderator in the thermal spectrum version is part of the fuel assembly and will be replaced with the fuel assembly.
- Coolant activation. The coolant in the fast spectrum will be activated and can be recycled or disposed of at the end of the plant life. Coolant activation of an Na K Zr F salt was addressed in ORNL/TM-2006/12. In short activation of the coolant does occur but after 10 years of storage, a reasonable time in decommissioning terms, radioactivity falls to levels allowing straightforward disposal or (preferable) recycling. There is a 10^4 neutron screening effect of the coolant for the fast version. This means that the only materials within the high flux zone are the fuel tubes which are obviously replaced once sufficient burnup has been achieved. Design is such that no humans will ever need to go anywhere near the core. All reactors should be designed this way but this is not possible given the complexity of today’s reactors. Activation of the thorium coolant is troublesome as you will have a wide range of fission products in the fuel salt along with fission in the heat exchangers – circa 1% of fission from thorium including some delayed neutrons. This is a far bigger problem for all other MSRs as they have fuel salt going through the heat exchangers."