Moderator Temperature Coefficient – MTC

Moderator Temperature Coefficient

The moderator temperature coefficient – MTC is defined as the change in reactivity per degree change in moderator temperature.

αM = dTM

It is expressed in units of pcm/°C or pcm/°F. The value of moderator temperature coefficient usually ranges from 0 pcm/°C to -80 pcm/°C. The magnitude and sign (+ or -) of the moderator temperature coefficient is primarily a function of the moderator-to-fuel ratio. That means it primarily depends on certain reactor design. It must be noted, according to the design requirements (e.g. NUREG-0800, Chapter 4), reactor design must assure that:

“The MTC should be non-positive over the entire fuel cycle when the reactor is at a significant power level.”

Therefore all light water reactors (LWR) must be designed under moderated, because it ensures, that the reactor may have a negative moderator temperature coefficient. If a reactor is over moderated, it can not reach a positive moderator temperature coefficient. A negative moderator temperature coefficient is desirable because of its self-regulating effect.

The total amount of reactivity, which is inserted to a reactor core by a specific change in the moderator temperature, is usually known as the moderator reactivity defect and is defined as:

dρ = α . dT

 
Example: moderator defect
The moderator temperature coefficient for a reactor is -30 pcm/°C.

Calculate the reactivity defect that results from a temperature increase of 20°C.

Solution:
dρ = α . dT = -30 * 20 = -600 pcm

The reactivity addition due to the moderator temperature increase is negative about -1 $ (for reactor core with βeff = 0.006).

Theory of Moderator Temperature Coefficient

It is very difficult to describe physics of the moderator temperatorure coefficient, because changes in moderator temperature lead to the change of almost all the parameters in a reactor core. For better understanding we describe major physical mechanisms that occur in case of the moderator temperature coefficient in terms of six factor formula.

↑TM ⇒ ↓keff = η.ε.  ↓p  . ↑f .  ↓Pf  .  ↓P (BOC)

↑TM ⇒ ↓keff = η.ε.  ↓p  .f.  ↓Pf  .  ↓P (EOC)

Major impacts on multiplication of the system arise from the change of the resonance escape probability and the change of total neutron leakage (see thermal non-leakage probability and fast non-leakage probability). But as can be seen at the beginning of the cycle (BOC), when the PWR core contains large amount boron disolved in primary coolant (chemical shim), an increase in temperature causes an increase in the thermal utilization factor.

  • moderator-to-fuel ratioChange of the resonance escape probability. It is known, the resonance escape probability is dependent also on the moderator-to-fuel ratio. All PWRs are designed as undermoderated reactors. As the moderator temperature increases the ratio of the moderating atoms (molecules of water) decreases as a result of the thermal expansion of water. Its density simply decreases. This, in turn, causes a hardening of neutron spectrum in the reactor core resulting in higher resonance absorption (lower p). Decreasing density of the moderator causes that neutrons stay at a higher energy for a longer period, which increases the probability of non-fission capture of these neutrons. It must be added moderator density changes are not linear. At high temperatures an increase in the moderator temperature causes a larger reduction in density than an identical increase at low moderator temperatures. This process (the hardening of neutron spectrum) is one of two key processes, which determine the moderator temperature coefficient (MTC). The second process is connected with the leakage probability of the neutrons.
  • Change of the neutron leakage. Since both (Pf and Pt) are affected by a change in moderator temperature in a heterogeneous water-moderated reactor and the directions of the feedbacks is the same, the resulting total non-leakage probability is also sensitive on the change in the moderator temperature. In result, an increase in the moderator temperature causes that the probability of leakage increases. In case of the fast neutron leakagethe moderator temperature influences macroscopic cross-sections for elastic scattering reactionss.NH2O) due to the thermal expansion of water, which results in an increase in the moderation length. This, in turn, causes an increase of the leakage of fast neutrons.
    • For the thermal neutron leakage there are two effects. Both processes have the same direction and together causes the increase in the thermal neutron leakage. This physical process is a part of the moderator temperature coefficient (MTC).

Neutron Moderators - Parameters

  • Change of the thermal utilization factor. The impact on the thermal utilization factor depends strongly on the amount of boron which is diluted in the primary coolant (chemical shim). As the moderator temperature increases the density of water decreases due to the thermal expansion of water. But along with the moderator also boric acid is expanded out of the core. Since boric acid is a neutron poison, and it is expanding out of the core, positive reactivity is added. The positive reactivity addition due to the expansion of boron out of the core offsets the negative reactivity addition due to the expansion of the moderator out of the core. The resulting MTC is therefore strongly dependent on the concentration of boron in primary coolant. It is obvious this effect is significant at the beginning of the cycle (BOC) and gradually loses its significance as the boron concentration decreases. In case of PWRs, burnable poisons are usually used to reduce the amount of dissolved boron. Without burnable absorbers it would be impossible to reach non-positive MTC, especially today, when utilities are demanding the vendors to design cores that can operate from 18 to 24 months at full power. In PWRs, samarium concentration can also influence the MTC at BOC. Note that, the samarium peak concentration after shutdown, is flux dependent! Sudden shutdown of a reactor at the end of the cycle lowers MTC (lowers critical boron concentration) at the beginning of the next cycle.
 
Example: Core inlet temperature increase
The negative MTC and the negative fuel temperature coefficient (DTC) ensure stable power operation of the reactor core. If the temperature of the moderator is increased, negative reactivity is added to the core. This negative reactivity causes reactor power to decrease, which acts against any further increase in temperature or power. As the thermal power decreases, the power coefficient acts against this decrease and the reactor returns to the critical condition. The reactor power stabilize itself. This effect is shown on the picture below. Let assume all the changes are initiated by the changes in the core inlet temperature.
moderator temperature coefficient - MTC
Example: Increase in the core inlet temperature. If the temperature of the moderator is increased, negative reactivity is added to the core. This negative reactivity causes reactor power to decrease. As the thermal power decreases, the power coefficient acts against this decrease and the reactor returns to the critical condition. The reactor power stabilize itself. Since the reactivity insertion is slow, the reactor seems to be in semi-critical state along entire transient. If the temperature of the moderator is further decreased, the positive reactivity is offset by control rods motion.
Example: Self-regulating properties of nuclear reactor
Operating with a negative MTC is favorable operational characteristics also during power changes. At normal operation there is an exact energy balance between the primary circuit and secondary circuit. Therefore when the operator decreases the load on the turbine (e.g. due to a grid requirement), the steam demand decreases (see the initial electrical output decrease at the picture). At this moment, the reactor will produce more heat than the steam turbine can consume. This disbalance causes the steam pressure, the saturation temperature in the steam generators to increase (see II. pressure at the picture). As a result of increasing saturation temperature in the steam generators the moderator temperature will simply increase (see inlet temperature). Increasing the temperature of the moderator adds negative reactivity, which reduces reactor power (without any operator intervention). As can be seen, to a certain extent the reactor is self-regulating and the reactor power may be controlled  via the steam turbine and via grid requirements. This feature is limited, because also the range of allowable inlet temperatures is limited. It is power plant specific, but in general, power changes of the order of units of % are common.

self-regulating reactor-min

Range of values of MTC

Upper limit

It was written:

“The MTC should be non-positive over the entire fuel cycle when the reactor is at a significant power level.”

This requirement is common for most of PWRs and with the negative fuel temperature coefficient (DTC) ensures stable power operation of the reactor core. It was written magnitude and sign (+ or -) of the moderator temperature coefficient is dependent on a boron concentration in the moderator. But during all shutdown states (e.g. Hot Standby mode or durigg refueling), there are requirements on subcriticality of the reactor core, that require significantly higher concentrations. The increase in boron concentration is large enough in magnitude that the MTC will be positive. But during these conditions the reactor is safe shutdown and the shutdown margin (SDM) must be ensured.

Lower limit

On the other hand, there is also a limit on the minimal value of MTC (most negative). It is due to the fact the negative temperature feedback acts also againts decrease in the moderator temperature. Consider what happens when moderator temperature is decreased quickly, as in the case of the main steamline break (MSLB – standard initiating event for PWRs). The steamline break causes the steam pressure, the saturation temperature in the steam generators to fall rapidly. As a result of falling saturation temperature in the steam generators the moderator temperature will rapidly decrease. The rapid moderator temperature drop causes a positive reactivity insertion. The amount of reactivity inserted depends also on a magnitude of the MTC and therefore it must be limited. Typical values for lower limit is MTC = -80 pcm/°C, but it is a plant specific value limited in technical specifications.

Moderator Temperature Coefficient and Reactor Design

moderator-to-fuel ratioAs was written, the moderator temperature coefficient is primarily a function of the moderator-to-fuel ratio (NH2O/NFuel ratio). The moderator-to-fuel ratio is the ratio of the number of moderator nuclei within the volume of a reactor core to the number of fuel nuclei. As the core temperature increases, fuel volume and number density remain essentially constant. The volume of moderator also remains constant, but the number density of moderator decreases with thermal expansion. As the moderator temperature increases the ratio of the moderating atoms (molecules of water) decreases as a result of the thermal expansion of water (especially at 300°C; see: Density of Water). Its density simply and significantly decreases. This, in turn, causes a hardening of neutron spectrum in the reactor core resulting in higher resonance absorption (lower p). Decreasing density of the moderator causes that neutrons stay at a higher energy for a longer period, which increases the probability of non-fission capture of these neutrons. This process is one of three processes, which determine the moderator temperature coefficient (MTC). The second process is associated with the leakage probability of the neutrons and the third with the thermal utilization factor.

The moderator-to-fuel ratio strongly influences especially:

  • Resonance escape probability. An increase in moderator-to-fuel ratio causes an increase in resonance escape probability. As more moderator molecules are added relative to the amount of fuel molecules, than it becomes easy for neutrons to slow down to thermal energies without encountering a resonance absorption at the resonance energies.
  • Thermal utilization factor. An increase in moderator-to-fuel ratio causes a decrease in thermal utilization factor. The value of the thermal utilization factor is given by the ratio of the number of thermal neutrons absorbed in the fuel (all nuclides) to the number of thermal neutrons absorbed in all the material that makes up the core.
  • Thermal and fast non-leakage probability. An increase in moderator-to-fuel ratio causes a decrease in migration length, which in turn causes an increase in non-leakage probability.

As can be seen from the figure, at low moderator-to-fuel ratios the product of all the six factors (keff) is small because the resonance escape probability is small. At optimal value of moderator-to-fuel ratio, keff reaches its maximum value. This is the case of so called “optimal moderation”. At large ratios, keff is again small because  the thermal utilization factor is small.

Under-moderated vs. Over-moderated Reactor

From the moderator-to-fuel ratio point of view, any multiplying system can be designed as:

  • Under-moderated. Under-moderation means that there is less than optimum amount of moderator between fuel plates or fuel rods. An increase in moderator temperature and voids decreases keff of the system and inserts negative reactivity. An under-moderated core would create a negative temperature and void feedback required for a stable system.
  • Over-moderated. Over-moderation means that there is higher than optimum amount of moderator between fuel plates or fuel rods. An increase in moderator temperature and voids increases keff of the system and inserts positive reactivity. An over-moderated core would create a positive temperature and void feedback. It will result in an unstable system, unless another negative feedback mechanism (e.g. the Doppler broadening) overrides the positive effect.

Reactor engineers must balance the composite effects of moderator density, fuel temperature, and other phenomena to ensure system stability under all operating conditions. Most of light water reactors are therefore designed as so called under-moderated and the neutron flux spectrum is slightly harder (the moderation is slightly insufficient) than in an optimum case. But this design provides important safety feature. An increase in the moderator temperature results in negative reactivity which tends to make the reactor self-regulating. It must be added, the overall feedback must be negative, but local positive coefficients exists in areas with large water gaps that are over-moderated such as near control rods guide tubes.

Another phenomenon associated with under-moderated core is called the neutron flux trap effect. This effect causes an increase in local power generation due better thermalisation of neutrons in areas with large water gaps (between fuel assemblies or when fuel assembly bow phenomenon is present). Note that “flux traps” are a standard feature of most modern test reactors because of the desire to obtain high thermal neutron fluxes for the irradiation of materials, but basically it can occur also in PWRs.

On the other hand, also under-moderation has its limits. In general, it causes a decrease in overall keff, therefore more fissile material is needed to ensure criticality of the core. Moreover, there is also a limit on the minimal value of MTC (most negative). It is due to the fact the negative temperature feedback acts also against decrease in the moderator temperature. Consider what happens when moderator temperature is decreased quickly, as in the case of the main steamline break (MSLB – standard initiating event for PWRs). The steamline break causes the steam pressure, the saturation temperature in the steam generators to fall rapidly. As a result of falling saturation temperature in the steam generators the moderator temperature will rapidly decrease. The rapid moderator temperature drop causes a positive reactivity insertion. The amount of reactivity inserted depends also on a magnitude of the MTC and therefore it must be limited. Typical values for lower limit is MTC = -80 pcm/°C, but it is a plant specific value limited in technical specifications.

 
References:
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See above:

Reactivity Feedbacks