WO2011006512A2

WO2011006512A2 - Temperature-reactive control module for nuclear reactors

Info

Publication number: WO2011006512A2
Application number: PCT/EG2009/000018
Authority: WO
Inventors: Mahmoud El-Sayed Dorrah
Original assignee: Mahmoud El-Sayed Dorrah
Priority date: 2009-07-15
Filing date: 2009-07-15
Publication date: 2011-01-20
Also published as: WO2011006512A3

Abstract

A nuclear reactor power control system which consists of multiple modules introduced at different positions within the reactor moderator. Each module contains a gas room (2) whose gas content is at the same temperature and pressure as its surrounding moderator (8) at the desired reactor power, and a small reservoir (3) of a strong neutron absorber liquid. The liquid is separated from the gas in a module by a mobile piston (4). The liquid reservoir is connected to a few small diameter extension pipes (6) that extend into the reactor moderator. The neutron absorber liquid is kept at certain level in these extension pipes (6) by means of some mobile plugs (7) that are regulated by the moderator pressure directly, or by an equal pressure. When the reactor power increases, the moderator temperature will increase and so will the module's gas temperature. The gas will isobarically expand; pushing the mobile piston (4) which will push the neutron absorber liquid far into the extension pipes (6), adding substantial negative reactivity to the reactor; reducing its power.

Description

Title: TEMPERATURE-REACTIVE CONTROL MODULE FOR

NUCLEAR REACTORS

Technical Field This invention relates to nuclear reactors power control, in particular, power stabilization technique by reacting to moderator temperature.

Background Art Nuclear reactors power is conventionally controlled by movement of solid control rods, or by injection, into some pipe system, of a neutron absorber liquid. In all conventional , mechanisms; administrating the control object is accomplished by certain mechanical systems that are provoked by electric signals usually arising in response to neutron flux measurements at certain points in the reactor core, i.e.; electro-mechanical feedback mechanisms.

Such mechanisms are liable to failure, as they involve signaling among different systems; from a neutron flux detector, to a signal analyzer, etc ., ending at a mechanical system that drive the control object into/or out of the reactor core. Such complex mechanism necessitates sophisticated quality assurance and safety measures. In essence; a passive control system is preferable to such complex mechanisms, especially if it relies on some simple physical principle. Disclosure of the Invention This invention introduces a nuclear reactor power control system which consists of multiple identical modules; that relies on the difference between the physical properties of gases and liquids for driving a neutron absorber liquid inside a set of pipes in reactor core. The proposed system is totally independent on neutron flux measurement devices inside the reactor core. It is not activated by electro-mechanical systems. The proposed system reacts passively and inherently to the local reactor moderator temperature around it.

The proposed system consists of multiple cylindrical modules introduced at different positions in reactor core within the moderator; close to the fuel. The number and distribution of the modules depends on the reactor's design. Each module contains a gas room. Its gas content is at the same temperature and pressure as its surrounding moderator at desired reactor power. The gas is chosen to be a low neutron absorbing one. In addition; each module contains a small reservoir of a strong neutron absorber liquid at the same temperature and pressure as the gas and the surrounding reactor moderator. The liquid is separated from the gas in a module by a mobile piston. The liquid reservoir is connected to a few small diameter extension pipes that extend into the reactor moderator. The neutron absorber liquid is kept at certain preset level (i.e.; certain liquid column length) in these extension pipes by means of some mobile plugs that are regulated by the moderator pressure directly or by an equal pressure.

The plugs determining the level of the neutron absorber liquid in the extension pipes can be controlled by being in direct contact with the reactor moderator on their rear faces (i.e.; the faces away from the liquid reservoir). In such case; the extension pipes are made patent on their far ends; being filled, distal to the plugs, by the moderator itself. Thus, a plug position in an extension pipe will be determined by the equilibrium between two pressures; the moderator pressure in the pipe on the plug's rear face, and the pressure of the neutron absorber liquid; on the plug's near face. The latter pressure directly reflects the gas pressure in the module's gas room, since the piston separating the gas room from the liquid reservoir is designed to be mobile.

Alternatively; the plugs position can be indirectly controlled. In such case; the far end of the extension pipes are connected to a gas (also a low neutron absorber gas) tank, whose pressure is kept constant and equal to the moderator pressure at desired steady state reactor power. Thus, a plug position will be determined by the equilibrium between, again, two pressures; the said gas tank pressure in the extension pipe distal to the plug; on the plug's rear face, and the pressure of the neutron absorber liquid on the plug's near face. The latter pressure again reflects the pressure of the gas in the module's gas room.

The mechanism of work of the proposed module is as follow; When the reactor power increases, e.g. during a power excursion incident, or during a LOFA "Loss Of Flow Accident"; the moderator temperature will increase. However; since a reactor moderator is almost invariably a liquid; moderator pressure will not be significantly changed with small changes in its temperature. Given that the walls of the gas room of the module are fairly thin and being in direct contact with the moderator, the gas temperature will follow the changes in the surrounding moderator temperature (even if at slower rate). For better and faster heat exchange between the walls of the gas room and its gaseous contents; the cavity of the gas room can be penetrated by a number of metal fins.

Since; the gas room is of variable volume (specifically; variable length) due to the mobility of the piston separating it from the neutron absorber liquid reservoir; its gas content will isobarically expand with increase in its temperature. The mobile piston separating the gas room from the neuron absorber liquid reservoir in the module will accordingly move in direction of the liquid side of the module's cylinder. This would push the liquid far into the extension pipes.

Conversely; when the reactor power decreases, the moderator temperature will correspondingly decrease, and the module's gas temperature will also decrease. Accordingly; the gas will isobarically contract. Now; the piston will move in direction of the gas side of the module's cylinder. This would withdraw the liquid from the extension pipes into the liquid reservoir.

Resulting changes in the lengths of columns of the neuron absorber liquid in the extension pipes will change the reactivity of the reactor. Thus; when the liquid is pushed far into the extension pipes; the reactivity of the reactor is decreased. Accordingly; the nuclear fission rate will decrease; reducing the reactor power. Conversely, when the liquid is withdrawn from the extension pipes, the reactivity of the reactor is increased. Now; the nuclear fission rate will increase, increasing the reactor power. Obviously; the proposed system is totally passive, and relies on physical laws rather than on electro-mechanical feedback systems. This guarantees a high degree of reliability of the system, and provides auto-regulation of the nuclear reactor power. Negative reactivity of the neutron absorber liquid when inside its reservoir is less than it is when the liquid is pushed far into the extension pipes in the reactor core. The explanation is that; if the liquid is a strong neutron absorber one; the outer layers of the liquid in its reservoir will shield its inner layers from the neutron flux in reactor core; a nuclear phenomenon known as (self-shielding). Such self-shielding is not significant in the narrow extension pipes. Moreover; the liquid's reservoir can be coated with some neutron reflector material for more shielding of the liquid inside the reservoir.

The proposed module can serve, according to nuclear design accommodation related to each particular reactor, three functions:

First; the proposed modules can serve stabilizing and controlling nuclear reactor power (i.e. as fine adjuster rods); as explained above.

Second; the neutron absorber liquid in the proposed modules can serve as "burnable poison" that compensates for the loss of positive reactivity of the reactor with burnup of the nuclear fuel. That's; as the nuclear fuel is burned-up reducing reactor reactivity, the liquid neutron absorber is also consumed reducing its negative reactivity (i.e. increasing reactor reactivity). Thus the reactor can be kept critical, despite the continuous burnup. of the nuclear fuel. Third; the proposed module can help accident mitigation, i.e.; help nuclear reactor scram in case of nuclear emergencies, e.g.; serious power excursion, or LOFA "Loss Of Flow Accidents". Given in such serious accidental conditions that the temperature and/or the pressure of the moderator in the reactor core were vastly increased; the proposed modules will not physically endure these conditions, and the modules are expected to shatter; voiding their gas and neutron absorber liquid contents into the reactor moderator. This can help reactor scram by two mechanisms; First; the release of the whole amount of the neutron absorber liquid into the reactor moderator, dissolving into it, and adding substantial negative reactivity to the reactor. Second; the release of the modules' gas into the reactor moderator, would produce numerous gas bubbles (voids) that can reduce neutron moderation in core; reducing the nuclear fission reaction rate, especially in case of nuclear reactors with negative void coefficient.

Description Of Drawings In order that the invention may be more readily understood and put into practical effect, a preferred example illustrating one method of using the invention in a model reactor will now be described, with reference to the accompanying drawings, in which : FIG.1 illustrates a simple model nuclear reactor. Six of the modules "Temperature-Reactive Control Modules" proposed in this invention are introduced in the reactor core (only two modules can be seen in the FIG.l). Note that, the modules can be placed on alternating sides (one up and one down; on alternating pattern) of the reactor core. This is especially important in case such arrangement was found, by calculations or a by a Monte Carlo simulation, to provide more neutron flux and power flattening in core; FIG.2 illustrates the detailed structure of a single "Temperature- Reactive Control Module", as proposed in the present invention;

FIG.3 illustrates a single module's extension pipe and its plug, with pipe's content of neutron absorber liquid, and moderator;

FIG.4 illustrates a proposed structure of the module's mobile piston separating the gas room from the neutron absorber liquid reservoir;

FIG.5 illustrates a cross section of the model reactor used in the illustrative example, and in the verification study (presented later);

FIG.6 illustrates an indirect mechanism for controlling the positions of the extension pipes' plugs. FIG.1 shows two identical "Temperature-Reactive Control Modules", as described in the present invention, on the sides of a model reactor core. The main compartment 1 of a module is a cylinder, preferably made of stainless steel. This cylinder is divided into two compartments, viz.; the gas room 2 and the neutron absorber liquid reservoir 3. The above two compartments are separated by a mobile piston 4. The mobile piston 4 is preferably made of stainless steel with layer(s) of a neutron reflector material (described in more details in FIG.4). The neutron absorber liquid reservoir 3 is coated with a neutron reflector material 5. A number of small diameter extension pipes 6 extend out of the neutron absorber liquid reservoir 3. These extension pipes 6 extend into the reactor moderator 8 along the nuclear fuel "reactor core" 9. The neutron absorber liquid which is basically present inside the liquid reservoir 3 ; fills the extension pipes 6 up to the position of the mobile plugs 7. The modules are introduced at the upper and/or the lower plenums of the reactor. The modules as a whole are placed within the reactor moderator fluid 8. The reactor walls 10 (whatever) are represented by an outer frame. FIG.2 shows the detailed structure of a single module. It shows: the main module's cylinder 1, the gas room 2, the neutron absorber liquid reservoir 3, the mobile piston 4, the neutron reflector coat of the liquid reservoir 5, the extension pipes 6, and their mobile plugs 7. The gas room cavity is shown to be penetrated by a number of rod shaped "metal" fins 11 to allow better and faster heat exchange between the gas room walls and its gaseous content. FIG.3 shows the mobile plug 7 in one extension pipe 6. The pipe 6 is filled with the neutron absorber liquid 12 proximal to the plug 7 (the upper part of the pipe 6). The pipe 6 is filled with the reactor moderator fluid 13 distal to the plug 7 (the lower part of the pipe 6). FIG.4 shows a proposed structure of the module's mobile piston 4. The piston 4 is made of stainless steel 14 with a layer of a neutron reflector 15 (e.g.; Beryllium). A good design is the sandwich design; with a meat of Beryllium 15 between two slabs of stainless steel 14. FIG.5 shows a cross section of the model reactor used in the illustrative example. The reactor is cylindrical in shape. The reactor core 9 is a hexagonal matrix of [169] fuel pins 16.

FIG.6 shows the model reactor with the modules, described in this invention, inside its moderator 8. All the extension pipes 6 are connected at their far ends to a Helium-4 tank 17 located outside the reactor moderator 8. Thus the pressure inside the tank is transmitted to and affects the rear face of the mobile plugs 7 in extension pipes 6. A Monte Carlo study was carried out using the fore-mentioned model reactor and six of the "Temperature-Reactive Control Modules"; as described in the present invention. The study aimed at verifying the practicability of the proposed module for controlling and stabilizing the reactor power. The study also calculated the quantity of negative reactivity that can be inserted into the model reactor due to increase in the neutron absorber liquid column length in the extension pipes with increase in reactor moderator temperature. The nuclear design of the model reactor was as follows;

Fuel Type UO₂

Fuel Enrichment 8.5 ^w/₀

Fuel Density 10.5 g/cm³

Number of Fuel Pins 169

Fuel Pins Matrix Hexagonal

Fuel Pin Length 200 cm

Fuel Pin Diameter 1.0 cm

Fuel Pin Pitch in Matrix 2.5 cm

Total Fuel (UO₂) Weight 1,114.95 kg

Reactor Moderator D₂O

Moderator Temperature 573 ⁰K

Moderator Pressure 15 MPa The modules used in the study had the following design;

Number of Modules Used 6 (at core matrix sides)

Modules Arrangement (Alternating) 3 Up, 3 Down

Module's Main Cylinder Diameter 10.0 cm

Gas Room Length 55.0 cm

Gas Used Helium-4

Initial Gas Temperature 573 ⁰K

Gas Pressure 15 MPa

Liquid Reservoir Length 5.0 cm

Liquid Reservoir Coat 1.0 cm Beryllium

Neutron Absorber Liquid Used 20,000 ppm Gadolinium

Nitrate Solution in D₂O

Mobile Piston Neutron Reflector 1.0 cm Beryllium

Number of Extension Pipes per Module 5

Extension Pipe's Diameter 1.0 cm

Preset Liquid Column Length in Pipes 0.0 cm

With such design and dimensions; it was calculated that an increase of moderator temperature (and accordingly of modules' gas temperature) by about [+10.42] ⁰K (i.e.; ΔT = +10.42 ⁰K); will result in an expansion of the gas in the gas room (that has a fixed diameter) by a length (ΔL) = +1.0 cm. Consequently; the mobile pistons 4 between the gas rooms 2 and the neutron absorber liquid reservoirs 3, in all modules, will move towards the liquid sides of the modules by 1.0 cm. This would push the Gadolinium Nitrate solution 12 into each of the extension pipes 6 for a length of 20.0 cm. Using a Monte Carlo simulation study; this was found to add considerable negative reactivity to the reactor; reducing the reactor power.

The above calculations assume constant moderator pressure (despite the increase in its temperature; which is valid with small increases in the "liquid" moderator temperature), and isobaric volume expansion of the gas in the gas room 2 (due to the free mobility of the piston 4).

The above mentioned conditions replicate the core condition in a pressurized water reactor in cases of mild power excursions, or during early stages of a LOFA (Loss Of Flow Accidents).

The results of the fore-mentioned Monte Carlo simulation and criticality "K_eff" (multiplication factor) calculations for the model reactor described above; at different increments of the modules' mobile pistons 4, and their corresponding neutron absorber liquid columns lengths in the extension pipes 6; these results are presented in the following table;

The above results verify the claim that the modules described in the present invention ("Temperature-Reactive Control Modules"), do work efficiently for controlling and stabilizing a nuclear reactor power, especially for pressurized water reactors PWRs.

The proposed modules ("Temperature-Reactive Control Modules"), work completely passively and inherently; reacting automatically to reactor moderator's temperature changes. Similar modules can be employed for power control and stabilization in different types of nuclear reactors other than the pressurized water reactors PWRs. The "Temperature-Reactive Control Modules" described in the present invention are expected to be very useful and very efficient for power stabilization in especially small power reactors such as propulsion reactors for submarines, naval ships, or space reactors, where a constant power output is very essential both for vehicle control and for nuclear reactor economy.

Claims

1. A nuclear reactor power control system which consists of multiple identical modules; that relies on the difference between the physical properties of gases and liquids for driving a strong neutron absorber liquid into a set of pipes in reactor core, where the said modules are introduced at different positions in the reactor core within the moderator close to the nuclear fuel, with the number and distribution of the modules varies according to a reactor's nuclear design; each of the said modules contains a gas room (2); the gas content of which is at the same temperature and pressure as its surrounding moderator (8) at the desired reactor power, and the said gas is chosen to be a low neutron absorbing one, in addition; each of the said modules contains a small reservoir (3) of a strong neutron absorber liquid at the same temperature and pressure as the gas in the said gas room and its surrounding reactor moderator (8); where the neutron absorber liquid is separated from the gas in the gas room by a mobile piston (4), and the said neutron absorber liquid reservoir (3) is connected to a few small diameter extension pipes (6) that extend into the reactor moderator (8), where the neutron absorber liquid is kept at certain preset level (i.e.; certain liquid column length) in these extension pipes (6) by means of some mobile plugs (7) that are regulated by the moderator pressure directly or by an equal pressure indirectly.

2. The said system in claim 1 characterized in that the said mobile plugs determining the level of the neutron absorber liquid in the extension pipes can be controlled by being in direct contact with the reactor moderator (13) on their rear faces (i.e.; the faces away from the liquid reservoir), where in such case the extension pipes are made patent on their far ends; being filled distal to the plugs by the moderator (13) itself; thus a plug position in an extension pipe will be determined by the equilibrium between two pressures; the moderator (13) pressure in the pipe on the plug's rear face, and the pressure of the neutron absorber liquid (12) on the plug's near face; where the latter pressure directly reflects the gas pressure in the module's gas room, since the said piston (4) separating the gas room from the neutron absorber liquid reservoir is designed to be mobile.

3. The said system in claim 1 characterized in that the said mobile plugs position can alternatively be indirectly controlled, where in such case the far end of the extension pipes are connected to a gas "a low neutron absorber gas" tank (17) situated outside the reactor core whose pressure is kept constant and equal to the moderator pressure at desired reactor power, thus; a plug position in an extension pipe will be determined by the equilibrium between the pressure in the said gas tank (17) on the plug's rear face, and the pressure of the neutron absorber liquid (12) on the plug's near, face.

4. The said system in claim 1 characterized in that when the reactor power increases, the moderator temperature will also increase, yet, being a liquid, the moderator pressure is not significantly changed (specially with small changes in its temperature); and as the walls of the gas room of the said module are in direct contact with the moderator; the gas temperature will also increase, and as the gas room is of variable volume (specifically; variable length) due to the mobility of the piston (4) separating the gas room (2) from the neutron absorber liquid reservoir (3); the gas will isobarically expand and the said mobile piston (4) separating the gas room from the neuron absorber liquid reservoir in the module will move towards the liquid side of the module; pushing the neutron absorber liquid far into the extension pipes (6), and conversely; when the reactor power decreases, the moderator temperature will decrease and the module's gas temperature will also decrease, resulting in that the gas will isobarically contract; thus the said mobile piston (4) will move towards the gas side of the module; withdrawing the neutron absorber liquid from the extension pipes.

5. The said system in claim 1 characterized in that the resulting change in the lengths of columns of the neuron absorber liquid (12) in the extension pipes (6) would change the reactivity of the reactor; that's if the neutron absorber liquid is pushed far into the extension pipes (6); the reactivity of the reactor is decreased reducing the reactor power, conversely, if the neutron absorber liquid is withdrawn from the extension pipes (6); the reactivity of the reactor is increased increasing the reactor power; thus the said modules react passively and inherently to the local moderator temperature around it.

6. The said system in claim 1 characterized in that the negative reactivity of the neutron absorber liquid when inside its reservoir (3); is less than it is when the neutron absorber liquid is pushed far into the extension pipes, this is due to the fact that since the liquid is a strong neutron absorber one; its outer layers in its reservoir will shield its inner layers from the neutron flux in reactor core (self-shielding).

7. The said system in claim 1 characterized in that for better and faster heat exchange between the walls (1) of the gas room and its gaseous contents; the cavity of the gas room can be penetrated by a number of metal fins (11) extending from its walls inwards.

8. The said system in claim 1 characterized in that the said neutron absorber liquid reservoir (3) and the said mobile piston (4) can both be coated with layers of a neutron reflector material, e.g.; Beryllium (5,15) for more shielding of the neutron absorber liquid inside its reservoir from the neutron flux in reactor core.

9. The said system in claim 1 characterized in that the gas used in the module's gas room (2) and in the constant pressure gas tank (17) [if used] is preferably Helium-4, since it keeps in gaseous state even at very high pressure and temperature conditions, plus it has a fairly low neutron absorption cross section.

10. The said system in claim 1 characterized in that the neutron absorber liquid used in the module's liquid reservoir is preferably an aqueous solution of Gadolinium Nitrate [Gd(NO₃)₃] at high concentration; approximately 20,000 ppm or more.

11. The said system in claim 1 characterized in that it can serve stabilizing and controlling nuclear reactors power, i.e.; the said modules can serve as fine adjuster rods.

12. The said system in claim 1 characterized in that the neutron absorber liquid in the said modules' liquid reservoirs can serve as a "burnable poison" compensating for the loss of positive reactivity of the reactor with burnup of the nuclear fuel, thus the reactor can be kept critical, despite the continuous burnup of the nuclear fuel.

13. The said system in claim 1 characterized in that it can help reactor scram in cases of nuclear emergencies e.g.; serious power excursion, or LOFA "Loss Of Flow Accidents" ; were in such accidental conditions that the temperature and/or pressure in the reactor core were vastly increased; the said module may shatter voiding its gas and neutron absorber liquid contents into the moderator, helping reactor scram by two mechanisms: firstly; the release of the whole amount of the neutron absorber liquid into the moderator would add substantial negative reactivity to the reactor scramming the reactor, and secondly; the released modules' gas will produce numerous gas bubbles "voids" in the moderator that can reduce neutron moderation; reducing nuclear fission reaction rate and consequently reducing reactor power, especially if. the reactor has negative void coefficient.