PBMR - Safety Q & A's

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	Safety Q & A's

safety Q and As

Does the PBMR have a containment structure that will prevent the release of radiation to the environment?

The safety characteristics of the fuel used in HTR’s such as the PBMR design ensure that no large scale fuel damage can occur. In common with similar projects, total containment of radioactivity was thus deemed unnecessary. It is, however, important to protect the building from overpressure. Whilst containment is an appropriate concept for reactors which use water as a coolant, Pebble Bed Modular Reactor (SOC) has determined that it is more important to ensure the building integrity by initially releasing the helium though filters and then revert to a low-pressure, closed containment. This ensures that – for all circumstances – practically all the radioactivity outside the fuel is still retained inside the building.

Is it true the US Nuclear Regulatory Commission (NRC) will not license a PBMR without a containment structure?

PBMR is not aware of any such decision by the NRC. Recent NRC publications indicate, however, that the NRC is intending to set rules relating to building performance that would allow a low-leakage building if the requirements for public dose are met.

What happened with the fuel being tested at the research reactors in Petten in the Netherlands in 2008. Why did it fail?

As part of the European Union (EU) development programme, fuel with a higher enrichment produced earlier for the German programme, is being irradiated in Petten to establish if fuel burn-ups higher than those used in the past can be achieved without additional fuel failures. Tests were performed with five fuel spheres relevant to the PBMR fuel. The first four tests were completed successfully with exceptional fuel performance observed. In one test, fuel was tested outside the PBMR operational conditions (burn-up and temperature), and as could be predicted, a single particle failed near the end of the test. All test results were evaluated in depth with PBMR models and published internationally. All observations could be explained, which confirmed the boundaries of the fuel. The fifth test containing both German and Chinese fuel spheres, is still ongoing with excellent fuel performance observed so far.

What is PBMR doing to ensure the fuel to be manufactured at Pelindaba near Pretoria is the required quality?

Coated particles from the fuel plant will be irradiated in various reactors over the world as part of an international collaboration effort. To this end, the first batch of coated particles containing 9.6 percent enriched uranium was sent to the US early in 2009 for irradiation testing at the Idaho National Laboratory. Once production line fuel is produced, a number of spheres will be irradiated in a suitable material test reactor under PBMR operating conditions and for the maximum indicated burnup. The failure rate must be within the stated limits before the fuel can be loaded in the reactor.

PBMR has instituted a full fuel qualification programme that includes coated particle, fuel material and complete fuel sphere testing. Fuel will be independently tested under PBMR operating conditions in material test reactors. PBMR will only be allowed to load fuel in the reactor once the fuel qualification programme has been completed successfully and the PBMR models have been validated.

The Thorium High Temperature Reactor (THTR) in Germany, which was intended to be the front-runner of the world's first commercial pebble bed machine, operated between 1985 and 1988. Why was it closed down after only 3 years of operation?

The contract of the utility with the German government was that the government would pay for all extra costs of operation compared to the equivalent coal fired plant next door. The calculated cost for decommissioning as well as other items increased every time this was recalculated. The government consequently felt it was being coerced and refused to deposit the finances. The utility threatened to cancel operation and when neither party budged, the reactor was stopped. The then responsible minister, Dr Klaus Töpfer, was quoted as saying at Davos (2003) that he thought he had made a mistake in halting the Germany's High-Temperature Reactor programme.

It is reported that the THTR had an accident that released a large quantity of activity to the environment. Is this true?

There was an event in May 1986 when a manual action to insert absorber spheres into the core was done in the wrong order. A quantity of the primary helium coolant was released to the building and from the stack to the outside. The released activity was below the limit where the authorities needed to be informed. The event was, however, reported in the quarterly review and nuclear opponents subsequently accused the company and the government of hiding the facts.

Some publications and web sites state that the THTR had serious operating problems and that this is why it was discontinued.

The THTR was a demonstration power plant in several respects. As could be expected, some teething problems were experienced, all of which could be rectified. In the last full year of operation, the THTR had an availability of 70 percent, which compared favourably with other type reactors at the time. None of the technical problems can be described as “serious”, nor were they the reason for the discontinuation of the THTR.

It is stated in some reports that the THTR suffered from compaction of the fuel which could lead to high fuel temperatures. Your comment?

The initial loading of the fuel in THTR was done manually with people walking on the pebble bed. This compacted the fuel, but it was corrected when fuel circulation was started and the pebble bed attained the expected density of about 61%. No high fuel temperatures were experienced.

Some scientists claim the AVR research reactor in Jülich, on which the South African pebble bed concept is based, cannot be used as an example for high temperature reactor design because it is so contaminated due to bad fuel performance. Your response?

The AVR contamination was mainly due to the use of experimental fuel during its operating life. The unexpected high fuel temperatures were only discovered shortly before shutdown in 1987. Due to the shutdown, the reason for the high temperatures was never explained. PBMR is consequently in the process of re-analysing the design and operating history. The following issues have arisen and are being investigated:

* Why was the high fuel temperature not noticed?
* Were there hot spots?
* Why should the fuel manufactured by PBMR be better?

Read more...

Some scientists believe pebble bed reactors produce a lot of graphite dust that trap radioactivity which can be released in an accident. Is this true?

The modular pebble bed reactors are designed with fuel that will not melt even at very high temperatures. Like the fuel in all nuclear reactors, however, there is a phenomenon known as diffusion where fission products can migrate from the fuel to the outside. In Light Water Reactors (LWR’s) this is noticed when fuel rods start to leak. In pebble bed reactors the silicon carbide (SiC) layer that protects the uranium kernels provides a strong barrier to most fission products, but at high temperatures (> 1000 °C) some fission products start to diffuse through the layer into the matrix graphite. These fission products may end up in the coolant and deposit on cool parts of the system.

Due to the movement of the pebbles, some graphite is rubbed off from the surface of the spheres. This “dust”, which contains some of the released fission products like Caesium 137, settles in stagnant or low flow areas of the main coolant loop. Should there be a sudden break of a medium or large pipe, part of the dust will go into suspension and may be carried out with the escaping gas. For this reason the building has a dust filter in the stack to catch the escaping dust, despite the fact that only a small proportion would escape in such an event. The actual activity is low and only becomes a factor when a 50-year dose is calculated.

Read more on the following:

1. What is diffusion and does it limit operating temperatures?
2. How does PBMR expect to measure the fuel temperature if it cannot put sensors in the core?

Is it not better to have a containment building and not vent to the outside?

LWR’s have a containment that can withstand high pressure for a short time. This pressure is due to high temperature steam escaping from a break in the pressure boundary. In case of a serious accident, the loss of coolant may lead to part or total fuel damage (e.g. Three Mile Island). This will release very high quantities of radioactive material which must not escape into the environment. Studies over the years for the PBMR and other HTR projects have shown that keeping the gas at pressure over a long time, creates a bigger potential danger to the public, as even small leaks or part containment failure will lead to a higher public dose than would be the case if the first gas volume is vented and filtered in a controlled way. Therefore all designs so far have selected a vented, but closable containment.

Block-fuel type High Temperature Reactors do not produce graphite dust. Are they not better than pebble bed designs?

While it is true that block-fuel reactors do not produce graphite dust, this is only one of the considerations when assessing the two fuel formats. While the spherical sphere concept allows for online refuelling, block-fuel reactors must regularly replace used fuel with new blocks in a complicated change-out. After any change, some of the fuel will, for the same gas temperatures, experience temperatures well above those seen in pebble reactors. This will also lead to enhanced Caesium release to the matrix graphite and eventually deposits in the system.

From an economic viewpoint, the availability of on-line fuelled reactors like the PBMR, is advantageous for applications where continuous operation is needed, such as the petrochemical industry.

Why does PBMR aim at such high gas temperatures if this may produce added contamination?

For the direct cycle selected by PBMR for electricity production, the efficiency is directly dependent on the outlet gas temperature. For process heat applications, however, such high temperatures are not needed, except for the chemical production of hydrogen. PBMR aims to demonstrate that high temperatures can be achieved without serious contamination. Hydrogen production can therefore be a goal for high temperature reactors with some development of high temperature materials that are needed in the heat transport cycle. Note that very high temperatures were inadvertently achieved in the AVR over the years, without noticeable fuel damage.

Why would PBMR not experience high fuel temperatures without being aware of it, such as was the case with the AVR research reactor?

In the AVR, the highest fuel temperatures were experienced where the fresh fuel entered the core. The gas flow was from bottom to top, thereby causing the highest power levels where the gas was hottest. The gas temperature, however, was not measured at the core exit, but at a position where returning bypasses had reduced the gas temperature. Inadequate analytical tools and lack of incentive caused the problem to be unnoticed for a long time. For the PBMR, there will always be more instrumentation as well as advanced analytical tools to predict actual fuel temperatures much more accurate than was possible for the AVR. Furthermore, for fresh fuel, the difference between the gas temperature and the actual higher fuel kernel temperature is large because of the high power per kernel for fresh fuel. In the PBMR, however, this high power is at low gas temperatures. In planning the AVR, gas bypasses were ignored. Analysis being performed by PBMR, show that the bypasses and the flow direction were mainly responsible for the high fuel temperatures.

In the PBMR, where the gas flow is from top to bottom, bypass gas is diverted back to the core at the bottom. As there is no fresh fuel at the bottom of the core, the power peak is well above the region where gas temperatures are high. The result is that there is only a small difference between the gas temperature (which can be measured) and the fuel kernel temperature.

Why was the fuel burn-up not measured accurately at the AVR? Why should we believe PBMR has a better method?

In simple terms, a reactor ceases to produce power when there is insufficient fissile material in the core (usually U-235). For continuously fuelled reactors like AVR and the PBMR this means that old fuel must be removed and replaced with fresh fuel to keep the chain reaction going. If the burn-up is not measured well enough, the core will shut down due to lack of fuel. A serious consistent underestimate of the fuel burn-up is therefore impossible. The AVR method, however, was not very accurate so that a small percentage of the fuel could possibly have stayed in the core longer than planned. The method selected for the PBMR is much more accurate, but in the fuel qualification it will be confirmed that longer stays in the core will not lead to additional fuel failure.

Would inaccurate fuel burn-up contribute to fission product release?

If some of the fuel remains in the core well beyond its planned life, additional particle failures may occur. This is unlikely to contribute much to the source term due to the large number of particles (>6-billion). Any additional failures, should it be more than a few tenths of a percent, will be noticed by the monitored level of fission products in the coolant gas.

It is claimed that the fuel sphere flow was only measured with small glass spheres in a liquid. Can this be a good prediction for actual pebble flow in the core?

This is erroneous information. Extensive tests were done in experimental facilities to test graphite pebble flows in a helium environment. With newer analytical software, these experimental results are well reproduced and there is confidence that actual flows will not deviate significantly from the predicted values. Even should it happen, it would not significantly contribute to changes in fuel behaviour. Read more...

Reports state that the packing density of the core may be much higher that predicted by PBMR and that this caused AVR high temperatures and may do so in PBMR. Is that correct?

For the PBMR spheres, there is a theoretical maximum packing density of 0.74. This, however, applies only to hand-packed spheres in an infinite array. There is plenty of experimental evidence that the average value of 0.61 used in PBMR calculations is a very good value to use and that deviations from this are predictable (near the walls) and vary little. The same is true for the AVR. It is very unlikely to have played a role in the high fuel temperatures found experimentally at the end of life.

Is it not true that in the AVR the concentration of Cs in the outer layer of so-called modern fuel elements was very high?

Yes, but the concentration decreases towards the centre of the sphere. This indicates that the contamination is from the outside by deposition of Cs released from bad fuel still in the core.

Does the PBMR design include emergency cooling capabilities in case of a loss of gas coolant?

Yes, there is a double redundant system to remove decay heat following a cessation of active cooling for any scenario. This is to prevent fuel temperatures rising to near the licensed limit which could lead to long delays in restarting the reactor. The system is seen as mainly for investment protection.