# Nuclear Reactor Technology Exam

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```					                   EXAM_NUCI 572 EB - Nuclear Reactor Technology

Prepared by                                     Date                     Page
Randall Lavelot                              15 March 2010               1 of 26

Question 1:a) and b)

For A → B → C , equation 2.33 gives:

Since λAnA is the rate atoms of A decay into atoms of B, the rate atoms of B are produced
is λAnA. The rate of decay of B atoms is λBnB, so the rate of change of dnB/dt is,

dnB/dt = -λBnB +λAnA

Sub ...... n(t)=n0e-λt for nB gives the following equation :

dnB/dt = -λAnA +λBn0e-λt

Where nB0 is the number of atoms of B at t=0. Integrating dnB/dt = -λAnA +λBn0e-λt
equation gives,

nB = nB0 e-λB t + nA0λA /(λB – λA) * (e-λA t - e-λB t)

In terms of activity, the equation may be written as,

αB = αB0 e-λB t + αA0λA /(λB – λA) * (e-λA t - e-λB t)

Therefore

For A → B → C , equation 2.33 gives:

αB = αB0 e-λB t + αA0λB /(λB – λA) * (e-λA t - e-λB t)

For t1/2(B) << t1/2(A) => λB >> λA

With no atoms of B present at t=0, αB0 = 0, therefore:

αB = αA0λB /(λB – λA) * (e-λA t - e-λB t)

and for λB >> λA, λB – λA ≈ λB and e-λA t - e-λB t ≈ e-λA t

Therefore, αB = αA0λB /(λB) * (e-λA t) = αA0 e-λB t, which is equal to αA

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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αB = 0 + 0 (e-λA t - e-λB t)

= - λAe-λB t +λAe-λA t

λB/λA= - e-λB t +e-λA t

λB/λA= et (λB - λA t)

ln(λB/λA)= t (λB - λA)

t= 1/(λB - λA) ln(λB/λA)

Question 2:
BR = Fissile Material Produced / Fissile Material Spent

BR = (558+455) / (789+34)=1.231

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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Question 3: a.)
135
Xe has a very important role in the reactor. It has a very large thermal-neutron
absorption cross section. It is a considerable load on the chain reaction. Its concentration
has an impact on power distribution, but in turn is affected by the power distribution, by
movement of reactivity devices, and significantly by changes in power.

First, define symbols:
Let I and X be the I-135 and Xe-135 concentrations in the fuel.
Let λI and λX be the I-135 and Xe-135 decay constants, and

Let γI and γX be their direct yields in fission

Let σX be the Xe-135 microscopic absorption cross section

Let φ be the neutron flux in the fuel, and
Let Σf be the fuel fission cross section

I-135 has 1 way to be produced, and 1 way to disappear, whereas Xe-135 has 2 ways to
be produced, and 2 ways to disappear. The differential equations for the production and
removal vs. time t can then be written as follows:

dI
= γ I Σ f φ − λI I                                  (1)
dt

dX
= λ I I + γ X Σ f φ − λ X X − σ X Xφ               ( 2)
dt

In steady state the derivatives are zero:

γ I Σ f φss − λI I ss = 0                          (3)

λI I ss + γ X Σ f φss − λ X X ss − σ X X ssφss = 0    (4)

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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Randall Lavelot                         15 March 2010                   4 of 26

135
The Xe        fission-product chain

135
Xe is produced directly in fission, but mostly from the beta decay of its precursor
135
I (half-life 6.585 hours). It is destroyed in two ways: By its own radioactive decay

(half-life 9.169 hours), and By neutron absorption to 136Xe.

The above diagram can be simplified:

The decay of Te-135 is so fast that it can be assumed that I-135 is formed directly from
fission with a yield of 6.4%. The decay of Cs-135 is so slow (2.6 million year half-life)
that it can be taken as stable.

The large absorption cross section of 135Xe plays a significant role in the overall neutron
balance and directly affects system reactivity, both in steady state and in transients. It also
influences the spatial power distribution in the reactor.

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Question 3: b.)
Xenon Transient Following a Shutdown

There is a very large initial increase in 135Xe concentration and decrease in core
reactivity. If the reactor is required to be started up shortly after shutdown, extra positive
reactivity must be supplied, if possible, by the Reactor Regulating System.

The 135Xe growth and decay following a shutdown in a typical CANDU is shown in the
above Figure.

It can be seen that, at about 10 hours after shutdown, the (negative) reactivity worth of
135Xe has increased to several times its equilibrium full-power value. At ~35-40 hours

the 135Xe has decayed back to its pre-shutdown level. If it were not possible to add
positive reactivity during this period, every shutdown would necessarily last some 40
hours, when the reactor would again reach criticality.

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To achieve xenon “override” and permit power recovery following a shutdown (or
reduction in reactor power), positive reactivity must be supplied to “override” xenon
growth; e.g., the adjuster rods can be withdrawn to provide positive reactivity.

It is not possible to provide “complete” xenon override capability; this would require >
100 mk of positive reactivity.

The CANDU-6 adjuster rods provide approximately 15 milli-k of reactivity, which is
sufficient for about 30 minutes of xenon override following a shutdown.

Xenon Summary

There are two sources of Xenon: direct from fission and indirect from the decay of
iodine. The latter is the major source.

There are two sinks for Xenon: burnup and decay to Cesium which does not act as a
neutron poison. Burnup is the major sink.

What happens on a reactor shutdown? The major sink for Xenon is removed while the
major source remains active because of the inventory of the fission product iodine. So,
Xenon rises. The rise continues until the supply of iodine is exhausted.

What happens when the reactor is restarted? The major sink (burnup) is resumed. But the
major supply (iodine decay to Xenon) is below its equilibrium value because the reactor
has been off-line. So, Xenon decreases.

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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Randall Lavelot                        15 March 2010                  7 of 26

Question 4:a.)

Four regions are represented in Figure 13. The first and second regions show that as heat
flux increases, the temperature difference (surface to fluid) does not change very much.
Better heat transfer occurs during nucleate boiling than during natural convection. As the
heat flux increases, the bubbles become numerous enough that partial film boiling
(part of the surface being blanketed with bubbles) occurs. This region is
characterized by an increase in temperature difference and a decrease in heat flux. The
increase in temperature difference thus causes total film boiling, in which steam
completely blankets the heat transfer surface.

Question 4:b.)

A pressurized water reactor (PWR) is protected from departure from nucleate boiling and
from hot leg boiling by generating a segmented delta temperature trip function having a

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first line segment which closely follows the departure from nucleate boiling core limit
line and a second segment closely following the hot leg boiling core limit line. Each line
segment is a function of the average core temperature and coolant pressure with the
departure from nucleate boiling segment further adjusted for axial power distribution.
The two set points are continuously compared to actual delta temperature in four
independent channels. If either set point is exceeded in at least two channels, the reactor
is tripped.

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Question 5:a.)

The most significant effect of a variation in temperature upon reactor operation is the
addition of positive or negative reactivity. Reactors are generally designed with negative
temperature coefficients of reactivity (moderator and fuel temperature coefficients) as a
self-limiting safety feature. A rise in reactor temperature results in the addition of
negative reactivity. If the rise in temperature is caused by an increase in reactor power,
the negative reactivity addition slows, and eventually turns the increase in reactor power.
This is a highly desirable effect because it provides a negative feedback in the event of an
undesired power excursion.

Negative temperature coefficients can also be utilized in water cooled and moderated
power reactors to allow reactor power to automatically follow energy demands that are
placed upon the system. For example, consider a reactor operating at a stable power level
with the heat produced being transferred to a heat exchanger for use in an external closed
cycle system. If the energy demand in the external system increases, more energy is
removed from reactor system causing the temperature of the reactor coolant to decrease.
As the reactor temperature decreases, positive reactivity is added and a corresponding
increase in reactor power level results.

As reactor power increases to a level above the level of the new energy demand, the
temperature of the moderator and fuel increases, adding negative reactivity and
decreasing reactor power level to near the new level required to maintain system
temperature. Some slight oscillations above and below the new power level occur before
steady state conditions are achieved. The final result is that the average temperature of
the reactor system is essentially the same as the initial temperature, and the reactor is
operating at the new higher required power level. The same inherent stability can be
observed as the energy demand on the system is decreased.

If the secondary system providing cooling to the reactor heat exchanger is operated as an
open system with once-through cooling, the above discussion is not applicable. In these

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reactors, the temperature of the reactor is proportional to the power level, and it is
impossible for the reactor to be at a higher power level and the same temperature.

Question 5:b.)

There is also another effect that is a consideration only on reactors that use dissolved
boron in the moderator (chemical shim). As the fuel is burned up, the dissolved boron in
the moderator is slowly removed (concentration diluted) to compensate for the negative
reactivity effects of fuel burnup. This action results in a larger (more negative) moderator
temperature coefficient of reactivity in a reactor using chemical shim. This is due to the
fact that when water density is decreased by rising moderator temperature in a reactor
with a negative temperature coefficient, it results in a negative reactivity addition because
some moderator is forced out of the core. With a coolant containing dissolved poison, this
density decrease also results in some poison being forced out of the core, which is a
positive reactivity addition, thereby reducing the magnitude of the negative reactivity
added by the temperature increase. Because as fuel burnup increases the concentration of
boron is slowly lowered, the positive reactivity added by the above poison removal
process is lessened, and this results in a larger negative temperature coefficient of
reactivity.

Question 5:c.)

The void coefficient is always negative, since the moderator is the same as the coolant.
The reactor temperature rises and the water either decreases in density or boils, reducing
the amount of moderator available for fast neutrons from fissions and thereby reducing
the number of thermal (slow) neutrons and making the number of fissions decrease. This
makes the reactor slow, lowering the temperature. There is an increase in reactions of fast
neutrons with 238U nuclei also as the temperature increases, decreasing the number of

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neutrons sleeting around the reactor, so reducing the number of neutrons available for
fission. Both these effects tend to turn the reactor down.

In PWRs the water’s thermal expansion reduces the reaction rate, because of the high
pressure, however, the water in the core does not boil. The increase of temperature in
PWR reactor is self-regulating. When voids (bubbles) form or the water itself expands to
become less dense, the reactor decreases thermal energy production. It is said to have a
negative void coefficient.

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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Randall Lavelot                      15 March 2010                   12 of 26

Question 6:a.)

1 Uranium fuel        9 Turbine
2 Pressure tube       10 Generator
3 Graphite            11 Condenser
moderator
4 Control rod         12 Cooling water
pump
5 Protective gas      13 Heat transport
6 Water/steam         14 Feedwater pump
7 Moisture            15 Preheater
separator
8 Steam to turbines

The RBMK reactor has a huge graphite block structure as the Moderator that slows down
the neutrons produced by fission. Graphite is a good moderator with low mass nucleus,
slows neutrons down to thermal speeds and is a low absorber with small cross-section.
The RBMK is a unique reactor type: its moderator is graphite (in this respect it resembles
the AGRs), the coolant is boiling light water (as in the case of BWRs), more over it has
pressure tubes (like the CANDUs).

Question 6:b.)

The combination of graphite moderator and water coolant is found in no other power
reactors othe than RBMK’s. RBMK employs long (7 metre) vertical pressure tubes
running through graphite moderator, and is cooled by water, which is allowed to boil in
the core at 290 °C, much as in a BWR. Fuel is low-enriched uranium oxide made up into
fuel assemblies 3.5 metres long. With moderation largely due to the fixed graphite,
excess boiling simply reduces the cooling and neutron absorption without inhibiting the

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fission reaction, and a positive feedback problem can arise (such as at Chernobyl, which
was a RBMK reactor).

Question 6:c.)

Advantage – Ability to be refueled while operating, permitting a high level of
availability. The low core power density of RBMKs provides a unique ability to
withstand station blackout and loss of power events of up to an hour with no expected
core damage. The graphite moderator design allows the use of fuel that is not suitable for
use in conventional water-moderated reactors.

Disadvantage – Lack of a massive steel and concrete containment structure as the final
barrier against large releases of radiation in an accident. Accident mitigation systems are
limited. Power increases when cooling water is lost, i.e. positive void coefficient. Limited
capability for steam suppression in the graphite stack. Flawed separation and redundancy
of electrical and safety systems.

RBMK                                              Other Thermal reactors

Pressure tubes eliminate the need for large       LWRs use large pressure vessels
pressure vessels

Can be enlarge to very large reactor system by    Larger pressure vessels get very expensive
adding pressure tubes                             (There is a practical limit due to transport
considerations)

The light water as a coolant has a positive void LWRs void coefficient is negative because the
coefficient                                       water is the moderator as well as the coolant.

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CANDU MTC tends to be slightly positive
(requires automatic control system

The large core is neutronically loosely coupled The smaller thermal reactors are more tightly
(many separate critical regions with positive     coupled neutronically and have negative MTC
void coefficient leading to potential stability and (More stable)
control problems)

RBMK uses automatic radial/azimuthal power        Not required for other thermal reactors since
density control system                            they are stable

RBMK design flaw results in positive reactivity LWR and CANDU control rods always insert
insertion for some conditions due to graphite     negative reactivity
followers

On-Line refueling                                 LWRs need a refueling outage. CANDU
reactors also use on-line refueling

No containment building to contain radioactivity Containment designed to remain intact
following a LOCA (design basis event)

Graphite burns. Chernobyl burned for 9-10         LWR reactor internal materials (steel,
days.                                             Zirconium, water, ceramic UO2) do not burn
in any postulated accident.

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EXAM_NUCI 572 EB - Nuclear Reactor Technology

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Question 6:d.)

The world's first nuclear power plant reactor was an RBMK (Soviet Union Obninsk,
1954). To this type belongs the Chernobyl reactor unit that suffered an accident on 26
April 1986.

The Chernobyl reactor suffered from a positive void coefficient. Control rods were
removed far out of the core. Control rods were dropped too fast into the core adding
positive reactivity. Then the coolant came in contact with the hot graphite causing the
steam explosion that destroyed the reactor building.

The RBMK reactors are moderated by graphite, but cooled by water that flows over fuel
rods. There is not enough water to significantly moderate the reactor. The temperature
rises, the water in the core boils. This reduces poisoning, so more neutrons are available
to make more fissions. In addition, the graphite expands, and this effectively increases the
cross section for thermalizing neutrons, also causing more fissions.

Changes – In-core feedback sensors monitor the amount of reactivity during operation.
RBMK reactors also have a radiation monitoring station that monitors radiation from the
plant and the nearby environment. The RBMK reactor also has an Accident Localization
System which serves as a containment but this system can only handle minor pipe breaks.
An increase in fuel enrichment from 2% to 2.4% to compensate for control rod
modifications and the introduction of additional absorbers. Manual control rod count
increased from 30 to 45 (Increase operational reactivity margin). SCRAM (rapid shut
down) sequence reduced from 18 to 12 seconds. Precautions against unauthorized access
to emergency safety systems. Installation of 80 additional absorbers (inhibit operation at
low power. Redesign of control rods with no graphite tips.

Replacement of the fuel channels at all units. Replacement of the group distribution
headers and addition of check valves. Improvements to the emergency core cooling
systems. Improvements of the reactor cavity over-pressure protection systems.
Replacement of the SKALA process computer.

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Question 7:

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Question 7:1.) PHYSICAL CHARACTERISCTICS

PWR                                                                   X-Reactor

Large reactor with onsite fueling                                        Small reactor without onsite fueling

Non Modular design                                                       Modular design

Suspended fixed core (with core limiter is of the piston type used to
Core in Reactor vessel                                                   adjusts the core height and controls the amount of fuel elements that
are permitted to enter the core)

Fine control rods – five control rods (one at the center and four
Control rods
located at 90deg) are meant to be used for fine control of the reactor

Pressurizer - Pressure in the primary circuit is maintained by a         Pressurizer - Pressure in the primary circuit is maintained by a
pressurizer                                                              pressurizer

Steam Generator - PWR turbine cycle loop is separate from the
Integrated Steam Generator - A steam generator is integrated in the
primary loop, so the water in the secondary loop is not contaminated
upper part of the module and is contaminated by radioactive materials.

Turbine-PWR turbine cycle loop is separate from the primary loop, so Turbine - is contaminated by radioactive materials.

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the water in the secondary loop is not contaminated by radioactive
materials.

Light water coolant - coolant water is used as a moderator            Light water coolant

HTGR type fuel elements - 15mm diameter spherical fuel elements
(has a fuel chamber and reserve fuel chamber) - Using a coated
Ceramic pellets in Fuel bundle - Zircaloy is chosen because of its
particle fuel form tailored to a water reactor environment can
mechanical properties and its low absorption cross section
eliminate the constraints of the present pressurized water reactor
(PWR) fuel system.

No Flange seal                                                        Two Flange sealed by international authorities

Feedwater system                                                      Feedwater system

No Exhaust steam                                                      Exhaust steam

No Grid                                                               Grid is to hold the fuel elements in place

Remote controlled accumulator valves - The water flowing from an
accumulator that is controlled by a multi redundancy valve system
None Remote controlled accumulator valves
cools the fuel chamber as a measure of emergency core cooling
system.

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Pump - The pump circulates the coolant inside the reactor moving it     Pump - The pump circulates the coolant inside the reactor moving it
up through the fuel chamber, the core, and the steam generator          up through the fuel chamber, the core, and the steam generator

Question 7: 2.a.) ECONOMICS

PWR                                                                 X-Reactor

Life cycle cost / Risk to capital                                       Life cycle cost / Risk to capital

Advantage Life cycle cost - Assuming an output of 1600MW, this          Advantage Life cycle cost - The modular design also allows a small
represents a cost of about £1250/kW                                     reactor to be mass-produced, reducing the life-cycle costs of safety-
certification and design qualification.

Risk to capital - Comparable level of financial risk to other reactors in Risk to capital – Comparable Comparable level of financial risk to
it’s class.                                                             other reactors in it’s class.

Operating cost of running the plant and generating energy.              Operating costs. There would be a significant risk that performance
Westinghouse claims its Advanced PWR reactor will cost USD \$1400 would be poorer than forecast. Reliability is largely under the control
per KW                                                                  of the owner and it is not clear whether developers would be

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sufficiently confident in their abilities to take the risk of poorer than
expected reliability;
Non-fuel operations & maintenance cost. PWR’s nuclear power
plant in 2008 was 1.37 cents / kWh                                   Non-fuel operations & maintenance cost. Similarly, this is largely
under the control of the owner and they may be willing to bear this
risk;
Nuclear fuel cost. fuel cost is 0.71 c/kWh.
Nuclear fuel cost. Purchasing fuel has not generally been seen as a
risky activity. Uranium can easily be stockpiled and the risk of
increasing fuel purchase cost can be dealt with. The cost of spent fuel
disposal (assuming reprocessing is not chosen) is however much more
contentious and nuclear owners might press for some form of cap on
disposal cost.

Decommissioning cost. Nuclear energy averages 0.4 euro cents/kWh Decommissioning cost. The cost of decommissioning is very hard to
forecast, but the costs will arise far into the future. Contributions to a
well-designed segregated decommissioning fund appear relatively
manageable, although if experience with decommissioning and waste
disposal does reveal that current estimates are significantly too low, or
if returns on investment of the fund are lower than expected,
contributions might have to be increased significantly. Private
developers might therefore seek some ‘cap’ on their contributions

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Question 7: 2.b.) SAFETY

PWR                                                                      X-Reactor

Water cooled reactors are characterised by the three ‘basic safety Water cooled reactors are characterised by the three ‘basic safety
functions’ in case of a malfunction / accident                     functions’ in case of a malfunction / accident

1. Control of the nuclear fission process (nuclear power). Is the             1. Control of the nuclear fission process (nuclear power). The control
avoidance of an uncontrolled power excursion. In PWR’s this is                system is based on the inherent safety philosophy. Any initiating event
avoided by an inherently stable core configuration with negative              will cut-off power to the pump, causing the fuel elements to leave the
feedback upon increasing power. Control in PWR’s also means to                core and fall back into the fuel chamber, where they remain in a
reduce the fission power to lower levels and even to zero                     highly sub critical and passively cooled condition.
(subcriticality) if it is needed, e.g. after a loss of the normal heat sink
(turbine and turbine bypass).

2. Cooling of the fuel (includes removal of the fission product decay         2. Cooling of the fuel (includes removal of the fission product decay

heat). It is the guarantee of PWR’s reliable fission product decay heat heat). The fuel chamber is cooled by natural convection transferring
removal from the core and also from the spent fuel pool.                heat to the water in the tank housing the fuel chamber. In case of

any abnormity the circulating pump stops and due to gravity

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the fuel pellets fall automatically out of the core region into the fuel
chamber underneath the core where their decay heat is transferred
passively by natural circulation to a water tank housing the fuel
chamber.

3. Confinement of radioactive material. Special emphasis is put on the 3. Confinement of radioactive material. TRISO type fuel element acts
minimisation of radioactive releases in PWR’s to values far below the as the first barrier. Particle fuel reduces fuel temperatures, lowers
prescribed intervention levels for the public by using the ALARA (as stored energy, and has better fission product retention.
low as reasonably achievable) principle.

Question 7: 2.c.) WASTE

PWR                                                                   X-Reactor

PWR Low-level waste                                                     Low-level waste and High-level waste disposal

Disposal occurs at commercially operated low-level waste disposal       Refueling is done by changing fuel chamber. It is controlled by
facilities.                                                             international authorities. Fueling is assumed to be fuelled in the
factory thus eliminating Low-level waste and High-level waste risks.

High-level waste disposal

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To be disposed of underground, in a deep geologic repository.

Storage of Spent Nuclear Fuel                                               Storage of Spent Nuclear Fuel and Transportation of Spent Nuclear

Has two acceptable storage methods for spent fuel after it is removed Fuel
from the reactor core:                                                      Fuel module in a sealed form and transported to and from site. It has
a long fuel cycle time thus reducing Low-Level Waste Disposal ,
1. Spent Fuel Pools - Currently, most spent nuclear fuel is safely
High-Level Waste Disposal and storage on site.
stored in specially designed pools at individual reactor sites around the
country.

2. Dry Cask Storage - If pool capacity is reached, licensees may move
toward use of above-ground dry storage casks.

Transportation of Spent Nuclear Fuel of PWR’s are shipped in
containers or casks that shield and contain the radioactivity and
dissipate the heat.

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Question 7: 2.d.) PROLIFERATION RESISTANCE

PWR                                                                    X-Reactor

Concept of the NPT (Non-Proliferation Technology) fuel, which has        This "Generation IV nuclear energy systems will increase the
proliferation-resistant nature and is retrofittable to current PWR was   assurance that they are a very unattractive and the least desirable route
proposed by minimizing plutonium generation, increasing decay heat for diversion or theft of weapons-usable materials, and provide
and degrading plutonium composition throughout burnup.                   increased physical protection against acts of terrorism by controlling
the refuelling by International authorities.

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Question 7: 3.) and 4.)

The preliminary comparison between PWR’s and the X reactor shows similarity. The existing
problems of conventional nuclear reactors are man-made waste product of nuclear fission (nuclear
waste), which can be used either for fuel in nuclear power plants or for bombs (proliferation risk).
Other concerns are environmental degradation and risk of accidents. X reactor will minimize and
manage nuclear waste and notably reduce the long-term stewardship burden, thereby improving
protection for the public health and the environment as the fuel module is in a sealed form
transported to, from site and refueling is done by changing fuel chamber. It has a clear life-cycle
cost advantage over other energy systems and a level of financial risk comparable to other energy
projects.

The X reactor has many advantages like provision of longterm energy generation resources, cost
and environmental pollution point of view to sustain growth in the form of long-term availability
of energy resources. X reactor operations will excel in safety and reliability and will have a very
low likelihood and degree of reactor core damage. X reactor increases the assurance that they are a
very unattractive and the leave desirable route for diversion or theft of weapons-usable materials,
and provide increased physical protection against acts of terrorism. The refuelling involves the
connecting and disconnecting of a fuel chamber to the reactor by a flange that is sealed by the
safeguard / international authorities.

Reactivity reveals that the X reactor would have very high excess reactivity at start up. PWR
reactors compensate this with a greater number of control rods in its design. The temperature
effects and xenon effect can be manipulated by these fine control rods. At start up, the reactor
reactivity is manipulated by the adjusting the level limiter. The core level limiter can be used to
add reactivity during reactor operation by introducing fresh fuel to the core and compensate for
long-term reactivity change caused by fuel depletion.

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Randall Lavelot                      15 March 2010                    26 of 26

A grid at the lower part of the tube is to hold the fuel elements in place. A steam generator is
integrated in the upper part of the module. The reactor is provided with a pressurizer system to
keep the coolant at a constant pressure. The pump circulates the coolant inside the reactor moving
it up through the fuel chamber, the core, and the steam generator. In a shut down condition, the
suspended core breaks down and the fuel elements leave the core and fall back into the fuel
chamber.

The water flowing from an accumulator that is controlled by a multi redundancy valve system
cools the fuel chamber as a measure of emergency core cooling system. Using a coated particle
fuel form tailored to a water reactor environment can eliminate the constraints of the present
pressurized water reactor (PWR) fuel system.

The "modular" concept of the X reactor utilizes several small reactors in a large power plant and
therefore new investment can be gradual, and tuned to the actual demand for electric power. Sites
that require larger generation capacity can simply install more reactors. Depending on the design,
there also can be economies of scale and better reliability when several reactors share equipment,
and can switch sets of equipment when some part fails. The modular design also allows a small
reactor to be mass-produced, reducing the life-cycle costs of safety-certification and design
qualification.

In conclusion the X reactor is a completely new pressurized water reactor, based on a robust
TRISO type microspheres fuel and a purely passively working decay heat removal system, which
is recommended for future power generation.

END

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