US20080056935A1

US20080056935A1 - Apparatus and method for preparing a canister loaded with wet radioactive elements for dry storage

Info

Publication number: US20080056935A1
Application number: US11/691,780
Authority: US
Inventors: Krishna Singh
Original assignee: Holtec International Inc
Current assignee: Holtec International Inc
Priority date: 2002-12-13
Filing date: 2007-03-27
Publication date: 2008-03-06
Also published as: DE60335778D1; ATE496375T1; EP1429344A2; EP1429344B1; KR100840809B1; EP1429344A3; ES2362495T3; US20060288607A1; KR20040052204A; JP2004219408A; JP4800567B2; US20050066541A1; US7210247B2; US7096600B2

Abstract

The present invention provides a method and system of preparing a canister having a cavity loaded with wet radioactive elements for dry storage, the method comprising: (a) providing a gas circulation system comprising means for removing moisture from gas; (b) connecting the gas circulation system to the canister so as to form a hermetically sealed closed-loop path that includes the cavity; and (c) circulating a non-reactive gas through the hermetically sealed closed-loop path until a desired vapor pressure is achieved in the cavity of the canister, the circulating non-reactive gas becoming wetted as the non-reactive gas flows through the canister and dried as the non-reactive gas flows through the moisture removal means. The apparatus comprises a gas circulation system comprising a source of a non-reactive gas and means for removing moisture from gas, the gas circulation system adapted to form a hermetically sealed closed-loop path when operably connected to a cavity of a canister to be prepared for dry storage; and means for circulating the non-reactive gas of the source through the closed-loop path so that the circulating non-reactive gas becomes wetted while flowing through the canister and dried as flowing through the moisture removal means.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation patent application of co-pending U.S. patent application Ser. No. 11/477,189, filed Jun. 27, 2006, now allowed, which is a divisional patent application of U.S. patent application Ser. No. 10/318,657, filed Dec. 13, 2002, now U.S. Pat. No. 7,096,600, the entirety of which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to the field of storing radioactive elements and specifically to systems and methods of preparing canisters having wet radioactive elements, such as spent nuclear fuel rods, for storage in the “dry state.”

BACKGROUND OF THE INVENTION

In the operation of nuclear reactors, hollow zircaloy tubes filled with enriched uranium, known as fuel assemblies, are burned up inside the nuclear reactor core. It is customary to remove these fuel assemblies from the reactor after their energy has been depleted down to a predetermined level. Upon depletion and subsequent removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its subsequent packaging, transporting, and storing. Specifically, the SNF emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times subsequent to removal from the reactor core.
In defueling a nuclear reactor, it is common place to remove the SNF from the reactor and place the SNF under water, in what is generally known as a spent fuel pool or pond storage. The pool water facilitates cooling the SNF and provides adequate radiation shielding. The SNF is stored in the pool for a period long enough to allow the decay of heat and radiation to a sufficiently low level to allow the SNF to be transported with safety. However, because of safety, space, and economic concerns, use of the pool alone is not satisfactory where the SNF needs to be stored for any considerable length of time. Thus, when long-term storage of SNF is required, it is standard practice in the nuclear industry to store the SNF in a dry state subsequent to a brief storage period in the spent fuel pool, i.e., storing the SNF in a dry inert gas atmosphere encased within a structure that provides adequate radiation shielding. One typical structure that is used to store SNF for long periods of time in the dry state is a storage cask.
Storage casks have a cavity adapted to receive a canister of SNF and are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of SNF, size and weight are often secondary considerations (if considered at all). As a result, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh more than 100 tons and have a height greater than 15 ft. A common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problems is that storage casks are generally too large to be placed in spent fuel pools. Thus, in order to store SNF in a storage cask subsequent to being cooled in the pool, the SNF is transferred to a cask, removed from the pool, placed in a staging area, dewatered, dried, and transported to a storage facility. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
As a result of the SNF's need for removal from the spent fuel pool and additional transportation to a storage cask, an open canister is typically submerged in the spent fuel pool. The SNF rods are then placed directly into the open canister while submerged in the water. However, even after sealing, the canister alone does not provide adequate containment of the SNF's radiation. A loaded canister cannot be removed or transported from the spent fuel pool without additional radiation shielding. Thus, apparatus that provide additional radiation shielding during the transport of the SNF is necessary. This additional radiation shielding is achieved by placing the SNF-loaded canisters in large cylindrical containers called transfer casks while still within the pool. Similar to storage casks, transfer casks have a cavity adapted to receive the canister of SNF and are designed to shield the environment from the radiation emitted by the SNF within.
In facilities utilizing transfer casks to transport loaded canisters, an empty canister is first placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the spent fuel pool. Prior to cask storage, the SNF is removed from the reactor and placed in wet storage racks arrayed on the bottom of spent fuel pools. For dry storage, the SNF is transferred in the submerged canister that is flooded with water and within the transfer cask. The loaded canister is then fitted with its lid, enclosing the SNF and the water from the pool within. The loaded canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the SNF-loaded canister for long-term dry storage. In order for an SNF-loaded canister to be properly prepared for dry storage, the United States Nuclear Regulatory Commission (“N.R.C.”) requires that the SNF and interior of the canister be adequately dried before the canister is sealed and transferred to the storage cask. Specifically, N.R.C. regulations mandate that the vapor pressure (“vP”) within the canister be below 3 Torrs (1 Torr=1 mm Hg) before the canister is backfilled with an inert and sealed. Vapor pressure is the pressure of the vapor over a liquid at equilibrium, wherein equilibrium is defined as that condition where an equal number of molecules are transforming from the liquid phase to gas phase as there are molecules transforming from the gas phase to liquid phase. Requiring a low vP of 3 Torrs or less assures that an adequately low amount of moisture exists in the interior of the canister and on the SNF so that the SNF is sufficiently dry for long-term storage.
Currently, nuclear facilities comply with the N.R.C.'s 3 Torr or less vP requirement by performing a vacuum drying process. In performing this process, the bulk water that is within the canister is first drained from the canister. Once the bulk of the liquid water is drained, a vacuum system is coupled to the canister and activated so as to create a sub-atmospheric pressure condition within the canister. The sub-atmospheric condition within the canister facilities evaporation of the remaining liquid water while the vacuum helps remove the water vapor. The vP within the canister is then measured by placing appropriate measuring instruments, such as vacuum gages, into the canister and taking direct measurements of the gaseous contents present therein. If necessary, this vacuum procedure is repeated until a vP of 3 Torrs or less is obtained. Once an acceptable vP is reached, the canister is backfilled with an inert gas and the canister is sealed. The transfer cask (with the canister therein) is then transported to a position above a storage cask and the SNF-loaded canister is lowered into the low storage for long-term storage.
Current methods of satisfying the N.R.C.'s 3 Torrs or less vP requirement are potentially dangerous, operationally time consuming, prone to error, subjects the SNF rods to high temperatures, and costly. First, the intrusive nature of the direct vP measurement is dangerous because the canister contains highly radioactive SNF. Any time the canister must be physically breached, there is the danger of exposing the surrounding an environment and the work personnel to radiation. Moreover, the creation of sub-atmospheric conditions in the canister requires expensive vacuum equipment and can cause complicated equipment problems. Finally, the operational durations for vacuum drying are unacceptably long as vacuum drying times on the order of days is quite common. The vacuum operation is prone to line freeze ups and ice formation inside canister which can give false readings to the instruments. Lowering of the canister pressure causes a progressive loss of the heat transfer medium (gas filling the gaps and open spaces in the canisters) resulting in substantial elevation of temperature of heat producing SNF rods.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and system for drying the interior of an SNF-loaded cavity.
Another object of the present invention is to provide a method and system for drying the interior of an SNF-loaded cavity without the need to intrusively physically measure the vP within the cavity to ensure that an acceptably low vP is within the cavity.
Yet another object of the present invention is to provide a method and system for drying the interior of an SNF-loaded cavity without subjecting the interior of the cavity to sub-atmospheric conditions.
Still another object of the present invention is to provide a method and system for adequately drying the interior of an SNF-loaded cavity without using expensive vacuum equipment.
A further object of the present invention is to provide a method and system for preparing an SNF-loaded cavity for dry storage in a more time efficient manner.
A yet further object of the present invention is to provide a method and system for preparing an SNF-loaded cavity for dry storage in a more cost effective manner.
An additional object of the present invention is to provide a method and system for drying that eliminates excessive SNF rods temperatures associated with prior art.
These objects and other are met by the present invention which in one aspect can be a method of preparing a canister having a cavity loaded with wet radioactive elements for storage, the method comprising: (a) providing a gas circulation system comprising means for removing moisture from gas; (b) connecting the gas circulation system to the canister so as to form a hermetically sealed closed-loop path that includes the cavity; and (c) circulating a non-reactive gas through the hermetically sealed closed-loop path until a desired vapor pressure is achieved in the cavity of the canister, the circulating non-reactive gas becoming wetted as the non-reactive flows through the canister and dried as the non-reactive gas flows through the moisture removal means.
Preferably, the method further comprises controlling and/or measuring at least one psychrometric property of the non-reactive gas along the closed-loop path at a location outside of the cavity to ensure that the desired vapor pressure is achieved in the cavity. The controlled and/or measured psychrometric property can be correlated to an existing vapor pressure within the cavity of the canister via a psychrometric relationship. Thus, the vapor pressure within the cavity can be approximated (and ensured to be below certain mandated levels) without having to obtrusively take measurements within the cavity.
In another aspect, the invention can be a method of drying a canister loaded with wet radioactive elements comprising: (a) providing a gas circulation system comprising a source of a non-reactive gas and a means for removing moisture from gas; (b) connecting the gas circulation system to the canister so as to form a closed-loop fluid path; (c) flowing the non-reactive gas through the canister, the non-reactive gas existing the canister as wet non-reactive gas that flows into the moisture removal means; (d) removing moisture from the wet non-reactive gas via the moisture removal means to create a dried non-reactive gas; (e) flowing the dried non-reactive gas through the canister; and (f) continuing steps (c) through (d) until a desired vapor pressure is achieved in the canister.
In still another aspect, the invention can be an apparatus for preparing a canister having a cavity loaded with radioactive elements for dry storage, the apparatus comprising: a gas circulation system comprising a source of a non-reactive gas and means for removing moisture from gas, the gas circulation system adapted to form a hermetically sealed closed-loop path when operably connected to a cavity of a canister to be prepared for dry storage; and means for circulating the non-reactive gas of the source through the closed-loop path so that the circulating non-reactive gas becomes wetted while flowing through the canister and dried as flowing through the moisture removal means.
In one preferred embodiment, the apparatus can further comprise sensor means operably coupled to the closed-loop path at a location outside of the cavity for measuring a psychrometric property of the non-reactive gas flowing through the closed-loop path, the psychrometric property capable of being correlated to an existing vapor pressure within the cavity of the canister through a psychrometric relationship.
In yet another aspect, the invention can be an apparatus for preparing a canister having a cavity loaded with radioactive elements for dry storage, the apparatus comprising: a gas circulation system comprising a source of a non-reactive gas, means for removing moisture from gas, sensor means for measuring a psychrometric property of gas and means for circulating gas; and wherein the gas circulation system is adapted to (1) form a hermetically sealed closed-loop path when operably connected to a cavity of a canister to be prepared for dry storage, (2) circulate the non-reactive gas through the closed-loop path so that the non-reactive gas becomes wetted flowing through the cavity of the canister and dried flowing through the moisture removal means; and (3) achieve a desired vapor pressure within the cavity of the canister based on measurements of the sensor means and a psychrometric relationship.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an open canister partially in section and empty.
FIG. 2 is a perspective view of a transfer cask partially in section with the canister of FIG. 1 sealed and positioned in the transfer cask.
FIG. 3 is a schematic diagram of a closed-loop system according to the present invention.
FIG. 4 is a graph illustrating the thermodynamic cycle of helium gas flowing through the system of FIG. 3 under specified conditions.
FIG. 5 is a flowchart of a first embodiment of a method according to the present invention.
FIG. 6 is a flowchart of a second embodiment of a method according to the present invention.
FIG. 7 is a data graph plotting simulated data for resulting vapor pressure in a cavity of a canister after dry heated helium gas is flowed through the cavity for various times.
FIG. 8 is a data graph plotting the number of turnovers of free volume of a cavity versus resulting vapor in the cavity.
FIG. 9 is an enlarged view of area IX-IX in FIG. 8.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a canister 20 that is suitable for use in the present invention. Canister 20 is illustrated for exemplary purposes only. The present invention is not limited to specific geometries, structures, or dimensions but is applicable to any type of enclosure vessel used to transport, store, or hold radioactive elements. Thus, the scope of the present invention also encompasses cask embodiments where spent nuclear fuel (“SNF”) is loaded directly into the cask's cavity without the use of a canister.
Canister 20 comprises bottom plate 22 and cylindrical wall 24 which forms cavity 21. As used herein, that end 25 of canister 20 that is closest to bottom plate 22 will be referred to as the bottom of the canister 20 while that end 26 of canister 20 that is furthest from bottom plate 22 will be referred to as the top of the canister 20. The cavity 21 of canister 20 has honeycomb grid 23 positioned therein. Honeycomb grid 23 comprises a plurality of rectangular boxes adapted to receive spent nuclear fuel (“SNF”) rods. Canister 20 further comprises a drainage hole (not illustrated) located at or near the bottom of canister 20 that provides a sealable passageway from outside canister 20 to cavity 21. The drainage hole can be located on bottom plate 22. Furthermore, the drainage hole can be opened or hermetically sealed using conventional plugs, drain valves, or welding procedures.
As illustrated in FIG. 1, canister 20 is empty (i.e. cavity 21 does not have SNF rods placed in honeycomb grid 23) and the top 26 of canister 20 is open. In utilizing canister 20 to transport and store SNF rods, canister 20 is placed inside transfer cask 10 (FIG. 2) while canister 20 is open and empty. The drainage hole located at or near the bottom of canister 20 is closed and hermetically sealed at this time. Open transfer cask 10 and open canister 20 are then submerged into a spent fuel pool which causes the remaining volume of cavity 21 to become filled with water. SNF rods that are removed from the nuclear reactor are then submerged into the spent fuel pool and placed inside cavity 21 of canister 20. Preferably, a single bundle of SNF rods is placed in each rectangular box of honeycomb grid 23. Once cavity 21 is loaded with the SNF rods, canister lid 27 (FIG. 2) is secured to the top 26 of canister 20 so as to seal the SNF rods and pool water in cavity 21. Canister lid 27 has a plurality of sealable lid holes 28 that form a passageway into cavity 21 from outside of the canister 20 when open. Transfer cask 10 (having loaded and sealed canister 20 therein) is then lifted from the spent fuel pool by a crane and placed uprightly in a staging area (as shown in FIG. 2) so that canister 20 can be properly prepared for dry-storage.
Referring to FIG. 2, when in the staging area, canister 20 (containing the SNF rods and pool water) is within transfer cask 10. Both canister 20 and transfer cask 10 are in an upright position. Once properly positioned in the staging area, the drainage hole (not illustrated) at or near the bottom 25 of canister 20 is opened and the bulk water that is trapped in cavity 21 of canister 20 is drained. Despite draining the bulk water from cavity 21, water moisture remains in cavity 21 and on the SNF rods due to trace amounts of liquid water and water vapor remaining inside cavity 21. However, before the canister 20 can be permanently sealed and transported to a storage cask for long-term dry storage, it must be assured that cavity 21 and the SNF rods contained therein are adequately dried. Because a low vapor pressure (“vP”) within a container indicates that a low level of moisture is present, the United States Nuclear Regulatory Commission (“N.R.C”) has decided that adequate drying is achieved when a vapor pressure (“vP”8) of 3 Torrs or less is obtained within cavity 21.
FIG. 3 schematically illustrates an embodiment of a closed-loop system 300 capable of drying cavity 21 to acceptable N.R.C. levels without the need to intrusively measure the resulting vP within cavity 21. FIG. 5 is a flowchart of one embodiment of a method of operating system 300 according to the present invention. FIG. 6 is a flowchart of a second embodiment of a method of operating system 300 according to the present invention.
System 300 is connected to canister 20 so as to form a closed-loop system. However, the present invention can be practiced without having a closed-loop system. System 300 comprises non-reactive gas reservoir 340, gas cooling element 310, temperature sensor 320, gas circulator 330, auxiliary heater 350, gas flow controller 360, and gas distributor 370. As illustrated, gas cooling element 310 comprises condenser 370 and demoisturizer module 380. All of these elements are fluidly connected to that a non-reactive gas can flow through system 300 without escaping into the exterior environment. All fluid connections discussed herein can be accomplished through the use of suitable tubing or piping which fluidly connects the elements of system 300 through the use of threaded connections, seals, ring clamps, and/or gaskets. The piping and tubing can be constructed of flexible or non-flexible metal conduits. The design to fluidly connect the piping and tubing to the various elements of system 300 will be mandated by the specific design structure of the equipment chosen and the material composition of the piping or tubing chosen.
It is preferred that non-reactive gas reservoir 340 to be used to store helium gas. While helium gas is the preferred non-reactive gas, any non-reactive gas can be used in system 300 and still be within the scope of the present invention. For example, the non-reactive can be nitrogen, carbon-dioxide, light hydrocarbon gases such as methane, or any inert gas, including but not limited to helium argon, krypton and xenon.
Flow of gaseous helium through system 300 is effectuated by gas circulator 330. Gas circulator 330 circulates the helium gas though system 300 and canister 20 by increasing the pressure of the helium gas as it exits gas circulator 330. As such the helium gas that exits gas circulator 330 is forced counter-clockwise through system 300. The direction of gas flow through system 300 is delineated by the arrows present on the fluid connections.
Helium reservoir 340 provides an adequate source of helium gas to system 300. Helium reservoir 340 is fluidly connected to condenser 370. As helium gas is forced through system 300, helium gas that is within helium reservoir 340 flows into condenser 370. As the helium gas flows through condenser 370, the helium gas is cooled. This cooling results in some of the moisture (i.e. water vapor) that is trapped in the helium gas to condense out of the helium gas and gather in condenser 370 in liquid form. The liquid water is removed from condenser 370 via a fluid drain (not illustrated) while the partially dried helium gas flows into demoisturizer module 380 (also known as a freeze dryer) via a separate fluid connection. The partially dried helium gas then flows through demoisturizer module 380 where the helium gas is further cooled by freeze-drying the helium gas with a refrigerant fluid that flows through a tubular heat exchanger. This further reduction in temperature of the partially dried helium gas results in additional moisture being removed from the helium gas. As such, the helium gas that exits demoisturizer module 380 has a very low moisture content (i.e. a low vapor pressure) and is very dry. The moisture that is removed from the helium gas in demoisturizer module 380 is frozen directly on the refrigerated fluid cooled heat exchanger tubes.
By monitoring the temperature of the helium gas flowing out of demoisturizer module 380 and assuring that the temperature is adequately low, it can be ensured that the helium gas is very dry. Demoisturizer module 380 is adapted to ensure that the exiting helium gas is cooled to a desired low temperature T_C. This is done by monitoring the temperature of the helium gas exiting demoisturizer module 380 with temperature sensor 320. Temperature sensor 320 is operably coupled to demoisturizer module 380 through electrical connection 325. Through the use of a user interface and properly programmed processors (not illustrated), the desired temperature T_Cis inputted by a user. Temperature sensor 320 repetitively measures the temperature of the helium gas exiting demoisturizer module 380. If the temperature of the exiting helium gas does not match the inputted desired temperature T_C, an appropriate signal is sent to demoisturizer module 380. Demoisturizer module 380 will appropriately respond to the signal by either increasing or decreasing the cooling of the helium gas, depending on the signal sent. This adjustment process is repeated until the helium gas exiting demoisturizer module achieves (and maintains) the desired a temperature at or below T_C, thus completing steps 500 and 600 of FIGS. 5 and 6 respectively. As will be discussed in detail below, T_Cis chosen by the user to ensure that the exiting helium gas is sufficiently dry so as to be capable of removing adequate amounts of moisture from cavity 21 (FIG. 1) to achieve a desired vapor pressure (vP_D) in cavity 21.
The dry helium gas (which is at the desired temperature T_C) flows into gas circulator 330, which as discussed above, increases the pressure of the dry helium gas so as to force the dry helium gas to circulate into auxiliary heater 350 and throughout system 300. Auxiliary heater 350 is fluidly connected downstream of gas circulator 330. Upon entering auxiliary heater 350, the dry helium gas is super-heated to a desired heated temperature (T_H), thus completing steps 510 and 610 of FIGS. 5 and 6 respectively. Increasing the temperature of the helium gas to T_Hhelps facilitate the removal of moisture from cavity 21 (including moisture on the SNF rods) when the helium gas enters cavity 21. However, because SNF rods often generate great amounts of heat, the use of auxiliary heater 350 may not be necessary in such instances. Auxiliary heater 350 ensures that the dry helium gas exiting auxiliary heater 350 is maintained at temperature T_Hthrough the use of an operably coupled temperature sensor (not illustrated). In order to avoid redundancy, discussion of this operable connection will be omitted with the understanding that the connection and controls are similar to those used for demoisturizer module 380 and temperature sensor 320.
Upon leaving auxiliary heater 350, the dry helium gas (now at temperature T_H) flows into gas flow controller 3660. Gas flow controller element 360 is operably coupled to gas circulator 330 by electrical connection 390. Depending on which method of the present invention is being used, gas flow controller 360 can be either a volumetric flow rate controller or a mass flow rate controller.
If gas flow controller 360 is a volumetric flow rate controller, a valve will be adapted to flow helium gas therethrough at a constant desired volumetric flow rate (R). The desired volumetric flow rate R is inputted by a user via a coupled user interface having properly programmed processors (not illustrated). The properly programmed processors will send a signal to the volumetric flow rate valve to adjust the valve so that the desired volumetric flow rate R is achieved. In this embodiment, the user will also enter a desired time (t) into the user interface. Upon time t passing, the properly programmed processor will send a cease operation signal to gas circulator 330, stopping the flow of helium gas through system 300. It is in this way that gas flow controller 360 facilitates step 520 of FIG. 5.
Another way in which step 520 of FIG. 5 can be facilitated is through the use of a mass flow controller. If gas flow controller 360 is a mass flow rate controller, a valve will be adapted to measure the mass of the heated dry helium gas flowing through the mass flow controller. The user will input a desired mass flow rate (R) and a desired time t rather than a volumetric flow rate value. Discussion of how the user determines what the desired flow rate R, whether a mass flow rate or a volumetric flow rate, and time t are will be had in detail below.
Alternatively, gas flow controller 360 can be adapted so that the user inputs a numerical value X into the user interface, wherein the value X represents the number of times the user wishes the free volume (V_F) of cavity 21 to be turned-over by the helium gas flow. As used herein, the free volume (V_F) of cavity is defined as that space of the cavity that is not occupied by the loaded SNF nor the honeycomb grid 23. While the value of V_Fwill be different for various canisters and will depend on whether the canister is fully loaded with SNF rods, the V_Ffor any given canister can be approximately by knowing the number and size of the SNF rods to be loaded into the cavity and the volume of the cavity when empty. As used throughout this patent, the V_Fis turned-over when enough gas is flowed into the cavity to fill the free volume V_Fand replace the gas that formerly occupied the volume. For example, if V_Fis equal to 300 cubic feet and a volume of 600 cubic feet of dry helium gas is flowed into V_F, the free volume V_Fis “turned-over” twice. Thus, X would be 2.0. In this embodiment of system 300, the user chooses X so that the V_Fis turned-over a specific number of times resulting in the vapor pressure in the cavity achieving a desired vapor pressure vP_D. How a user calculates the desired X will be discussed in detail below. Assuming that the desired X is know, the user inputs this value into the user interface instead of an R and t. A properly programmed processor will communicate with control gas flow controller 360, be it a mass flow controller or a volumetric flow rate controller, to supply a volume of dry helium gas that will turn-over V_Fof the cavity the desired X times, thus completing step 620 of FIG. 6. Once a volume of dry helium gas resulting in the desired X is supplied to cavity 21, the properly programmed processor wills end a cease operation signal to gas circulator 330, stopping the flow of helium gas through system 300.
Upon exiting gas flow controller 360, the dry helium gas flows into gas distributor 370. Gas distributor 370 is fluidly coupled to canister 20 and is adapted to introduce the heated dry helium gas into cavity 21 (FIG. 1) at or near the bottom of canister 20 possibly through the same drain valve (not illustrated) used to drain the bulk water. Gas distributor 370 introduces the heated dry helium gas into the V_Fof cavity 21, thus completing steps 520 and 620 of FIGS. 5 and 6 respectively. Once inside cavity 21, the dry helium gas becomes wet by absorbing moisture. This adsorption of moisture by the dry helium gas occurs both by mixing with the water vapor already in cavity 21 and by evaporating liquid water that may remain in cavity 21 and on the SNF-rods. If the dry helium gas is superheated to temperature T_H, the added energy in the dry helium gas will help initiate evaporation of the liquid water. As the dry helium gas is continually introduced into cavity 21, the V_Fof cavity 21 becomes filled. Once the V_Fis filled, the continual supply of dry helium gas pushes the wet helium gas out of cavity 21 from an outlet orifice at or near its top, thus completing steps 530 and 630 of FIGS. 5 and 6 respectively. The outlet orifice may be sealable canister lid holes 28 (FIG. 2). This removal of wet helium gas from the cavity 21 lowers the resulting vapor pressure in cavity 21. As the wet helium gas flows out of cavity 21, the wet helium gas is carried through a fluid connection to helium reservoir 340. As such, the wetted helium gas can be re-circulated through system 300 to sustain a continuous canister drying process.
As mentioned above, once the heated dry helium gas is circulated through cavity 21 at rate R for period of time t, the introduction of the heated dry helium gas into cavity 21 is ceased, thus completing step 540 of FIG. 5. In the alternative embodiment, the introduction of the heated dry helium gas into cavity 21 is ceased when the heated dry helium gas is circulated through cavity 21 so that V_F“turns-over” X times, thus completing step 640 of FIG. 6. At this point, system 300 is disconnected from canister 20 and all opening into cavity 21, including the drain valve and orifices, are sealed, thereby trapping the dry helium gas in cavity 21 and creating a non-reactive atmosphere within cavity 21, thus completing steps 550 and 650 of FIGS. 5 and 6 respectively.
Referring to FIG. 4, a thermodynamic cycle of the helium gas circulating through system 300 is illustrated on a Vapor Pressure-Temperature diagram. Plotted in this diagram is the vapor pressure curve for water. Above the vapor pressure curve, water is in the liquid state and below the curve water is in the vapor state. Vapor and liquid co-exist in equilibrium at any point on the curve. For illustrative purposes, the thermodynamic graph assumes that system 300 is being operated so that T_Cis 21 degrees Fahrenheit, T_His 300 degrees Fahrenheit, and the starting vapor pressure (vP_S) in cavity 21 is 87 Torrs. As used herein the starting vapor pressure vP_Sin a cavity is the vapor pressure in that cavity after draining the bulk water therefrom but before any dry helium gas is circulated therethrough. Starting at point 1 on the diagram, superheated helium gas laden with water vapor exits cavity 21 of canister 20 and enters condenser 370. Once inside condenser 370, the superheated helium gas is cooled along line 1-2 until the dewpoint is reached at the intersection with the vapor pressure curve, i.e., point 2. Cooling of the helium gas continues in condenser 370 below the dewpoint, progressively condensing water vapor out of the helium gas down the vapor pressure curve along line 2-3. The liquid that is condensed from 2-3 is separated and removed from the condenser while the partially dried helium gas flows into demoisturizer module 380 where it is freeze-dried. While in demoisturizer module 380, the helium gas is further cooled below dewpoint, condensing even more water vapor out of the helium gas along 3-4 until T_C(which is 21 degrees Fahrenheit) is reached. The dried helium gas leaves condenser module 380 and is superheated to T_Hin auxiliary heater 350 along line 4-5 to help effectuate the evaporation of liquid water in cavity 21. If the SNF rods are hot enough, this superheating will occur inside cavity 21, thus eliminating the need for auxiliary heater 350. Once inside cavity 21, the dry helium gas is heated even further due to the heat given off by the SNF rods and the heated dry helium gas absorbs water vapor from the evaporating liquid water, thus wetting and increasing the vapor pressure of the dry helium gas along line 5-1. So long as the dry helium gas flows through cavity 21, the vapor pressure within cavity 21 will continue to decrease. This will also result in the vapor pressure of the exiting wet helium gas (at point 5) to decrease as time passes. The vapor pressure within cavity 21 will continue to decrease monotonically to approach the vapor pressure of the heated dry non-reactive gas entering cavity 21. Thus, by ensuring that the temperature (T_C) of the helium gas exiting demoisturizer module 380 is low enough so that it corresponds to a vapor pressure equal to or lower than the desired vapor pressure (vP_D), in this case 3 Torrs, the desired vapor pressure (vP_D) can be achieved in cavity 21 so long as a sufficient amount of dry helium gas is flowed through cavity 21. Thus, so long as the desired dryness is known in terms of vapor pressure, a user of system 300 can calculate T_Cusing FIG. 4 or a similar graph to achieve a level of dryness. Because current N.R.C. regulations require a vapor pressure of 3 Torrs or less to be achieved in the cavity 21 as a measure of dryness, vP_Dwill be equal to 3 Torrs or less, thus completing steps 560 and 660 of FIGS. 5 and 6 respectively. As such, it can be seen from FIG. 4 that this corresponds to a T_Cof 21 degrees of Fahrenheit or below.
Once vP_D(3 Torrs) is known and T_Cis calculated (21 degrees Fahrenheit), the next step is determine the free volume V_Fthat exists within cavity 21. This is done by reducing the total volume of cavity 21 by the approximate volume of the SNF rods contained therein and the volume of any other solids that may be present in cavity 21, such as honeycomb grid 23. Upon the free volume V_Fbeing determined, the user of the system will then pick a temperature T_Hthat the dry helium gas will be heated to by auxiliary heater 350 and a flow rate R at which this heated dry helium gas will be supplied to cavity 21. Depending on the value of V_Ffor the cavity to be dried, and the values chosen for T_Hand R, the heated dry helium gas will have to be flowed through the cavity for a period of time t before the cavity achieves the desired vapor pressure vP_Dof 3 Torrs. By simulating conditions within cavity 21 and setting hypothetical values for the variables of system 300, time t can be determined for any given set of variables by reading an appropriate graph of the data.
Referring to FIG. 7, a data graph is illustrated graphing time of gas flow through a cavity versus the resulting vapor in that cavity for a given set of simulated conditions. In generating this time-vapor pressure graph, the following conditions were assumed or chosen: (1) a vP_Dof 3 Torrs (which, from FIG. 4, relates to a T_Cof 21 degrees Fahrenheit); (2) a V_Fof 300 cubic feet; (3) a T_Hof 300 degrees Fahrenheit (which relates to a helium density ρ of about 0.032 pounds per cubic feet); (4) an R of 78.125 cubic feet per minute; (5) a cavity pressure (P_C) of 50 pounds per square inch; and (6) a starting vapor pressure vP_Sof 87 Torrs inside the cavity. As can be seen from the data plot, as the dry helium gas is allowed to flow through the cavity over time, the resulting vapor pressure within the cavity decreases. By plotting this data, it can be seen that at a time t of about 22.5 minutes, the resulting vapor pressure within cavity will be equal to the vP_Dof 3 Torrs. In determining time t, the clock is started after the temperature T_Cis reached for the helium gas exiting the demoisturizer module. The conditions of FIG. 7 were chosen for exemplary purposes only. Similar graphs can be simulated for any given set of starting conditions within the cavity and for any variables chosen by the user. Thus, the time necessary to achieve any desired vP_Dcan be approximated for any given set of conditions without the need to intrusively measure the vapor pressure within the cavity. Proper graphs can be created by either properly programmed computer simulations or by graphing actual experimental data.
Alternatively, R can be chosen to be mass flow rate. Mass flow rates are easily interchangeable with volumetric flow rates by simply knowing the density of the gas flow. Mass flow rate of gas is equal to the volumetric flow rate of that gas multiplied by the gas density. For example, the volumetric flow rate R chosen above, 78.125 cubic feet per minute, corresponds to a mass flow rate of 2.5 pounds per minute, where the density of the helium is approximate to be 0.032 pounds per cubic feet. Thus, data plots similar to FIG. 7 can be simulated for given mass flow rates.
Moreover, any single variable can be calculated by setting the remaining variables. For example if a certain drying time t is desired, a graph plotting resulting vapor pressure versus gas flow rate R can be created, wherein time t is held constant and the flow rate R is varied. Thus, the present invention is not limited to a situation where any specific variable is held constant.
Referring to FIG. 8, a data graph is illustrated graphing the number of turnovers (X) of the free volume (V_C) time with a gas versus the resulting vapor in the cavity, for a given set of simulated conditions. In generating this turnover number-vapor pressure graph, the following conditions were assumed or chosen: (1) a vP_Dof 3 Torrs (which, from FIG. 4, relates to a T_Cof 21 degrees Fahrenheit); (2) a V_Fof 300 cubic feet; (3) a T_Hof 300 degrees Fahrenheit (which relates to a helium density ρ of about 0.032 pounds per cubic feet); (4) a cavity pressure (P_C) of 50 pounds per square inch; and (5) a starting vapor pressure vP_Sof 87 Torrs inside the cavity. As system 300 is continually operated for any given canister, the total volume of gas (V_tot) flowed into the free volume V_Fof the cavity increases. Because X is equal to V_totdivided by V_F, X also increases as the helium gas flow continues. It can be seen from FIG. 8 that as X increases, the resulting vapor pressure decreases along the Vapor Pressure Plot line. From this data plot line, one can determine the X at which the resulting vapor pressure inside the cavity will be equal to or less than the desired vapor pressure vP_Dof 3 Torrs.
Turning to FIG. 9, it can be seen that when enough volume of heated dry helium gas is flowed through the cavity so that an X greater to or greater than 5.86 results, the desired vapor pressure (vP_D) of 3 Torrs is achieved within the cavity. Thus, by programming system 300 to achieve an X equal to 5.86 or greater, it can be ensured that the desired vapor pressure vP_Dof 3 Torrs is achieved in the cavity without the need to intrusively measure the interior of the cavity. The conditions of FIGS. 8 and 9 were chosen for exemplary purposes only. Similar graphs can be simulated (or plotted using actual experimental data) for any given set of starting conditions within the cavity and for any variables desired or chosen by the user, without departing from the spirit of the invention.

Claims

1. A method of preparing a canister having a cavity loaded with wet radioactive elements for dry storage, the method comprising:

(a) providing a gas circulation system comprising means for removing moisture from gas;

(b) connecting the gas circulation system to the canister so as to form a hermetically sealed closed-loop path that includes the cavity; and

(c) circulating a non-reactive gas through the hermetically sealed closed-loop path until a desired vapor pressure is achieved in the cavity of the canister, the circulating non-reactive gas becoming wetted as the non-reactive gas flows through the canister and dried as the non-reactive gas flows through the moisture removal means.

2. The method of claim 1 further comprising controlling and/or measuring at least one psychrometric property of the non-reactive gas along the closed-loop path at a location outside of the cavity to ensure that the desired vapor pressure is achieved in the cavity.

3. The method of claim 2 wherein the controlled and/or measured psychrometric property can be correlated to an existing vapor pressure within the cavity of the canister via a psychrometric relationship.

4. The method of claim 3 further comprising, upon the controlled and/or measured thermodynamic property being indicative that the existing vapor pressure within the cavity of the canister is at or below the desired vapor pressure, ceasing the circulation of the non-reactive gas through the hermetically sealed closed-loop path.

5. The method of claim 1 further comprising:

upon the desired vapor pressure being achieved in the cavity, ceasing the circulation of the non-reactive gas through the hermetically sealed closed-loop path;

disconnecting the gas circulation system from the canister; and

sealing the cavity thereby forming an atmosphere of the non-reactive gas within the cavity.

6. The method of claim 1 wherein the gas circulation system further comprises a source of the non-reactive gas.

7. The method of claim 1 wherein the gas is nitrogen, carbon dioxide, light hydrocarbon gases, or an inert gas selected from the group consisting of helium, argon, krypton and xenon.

8. The method of claim 1 wherein the canister is vertically oriented and located within an overpack module during steps (a) through (c).

9. The method of claim 1 further comprising:

controlling and/or measuring at least one psychrometric property of the non-reactive gas along the closed-loop path at a location outside of the cavity, the controlled and/or measured psychrometric property capable of being correlated to an existing vapor pressure within the cavity of the canister;

upon the controlled and/or measured psychrometric property being indicative that the existing vapor pressure within the cavity of the canister is at or below a desired vapor pressure, ceasing the circulation of the non-reactive gas through the hermetically sealed closed-loop path; and

10. The method of claim 9 wherein the desired vapor pressure is at or below 3 Torrs and the gas is nitrogen, carbon dioxide, light hydrocarbon gases, or an inert gas selected from the group consisting of helium, argon, krypton and xenon.

11. The method of claim 9 wherein the controlled and/or measured psychrometric property is temperature.

12. The method of claim 9 further comprising measuring at least one psychrometric property of the non-reactive gas along the closed-loop path at a location outside of the cavity, the measured psychrometric property capable of being correlated to an existing vapor pressure within the cavity of the canister through the use of a psychrometric relationship.

13. A method of drying a canister loaded with wet radioactive elements comprising:

(a) providing a gas circulation system comprising a source of a non-reactive gas and a means for removing moisture from gas;

(b) connecting the gas circulation system to the canister so as to form a closed-loop fluid path;

(c) flowing the non-reactive gas through the canister, the non-reactive gas exiting the canister as wet non-reactive gas that flows into the moisture removal means;

(d) removing moisture from the wet non-reactive gas via the moisture removal means to create a dried non-reactive gas;

(e) flowing the dried non-reactive gas through the canister; and

(f) continuing step (c) through (d) until a desired vapor pressure is achieved in the canister.

14. The method of claim 13 further comprising:

controlling and/or measuring at least one psychrometric property of the non-reactive gas along the closed-loop path at a location outside of the canister, the controlled and/or measured psychrometric property capable of being correlated to an existing vapor pressure within the canister;

sealing the canister thereby forming an atmosphere of the non-reactive gas within the cavity.

15. An apparatus for preparing a canister having a cavity loaded with radioactive elements for dry storage, the apparatus comprising:

a gas circulation system comprising a source of a non-reactive gas and means for removing moisture from gas, the gas circulation system adapted to form a hermetically sealed closed-loop path when operably connected to a cavity of a canister to be prepared for dry storage; and

means for circulating the non-reactive gas of the source through the closed-loop path so that the circulating non-reactive gas becomes wetted while flowing through the canister and dried as flowing through the moisture removal means.

16. The apparatus of claim 15 further comprising sensor means operably coupled to the closed-loop path at a location outside of the cavity for measuring a psychrometric property of the non-reactive gas flowing through the closed-loop path, the psychrometric property capable of being correlated to an existing vapor pressure within the cavity of the canister through a psychrometric relationship.

17. The apparatus of claim 15 wherein the gas circulation system further comprises means to heat the non-reactive gas flowing through the closed-loop path, the heating means loaded downstream of the moisture removal means.

18. The apparatus of claim 15 wherein the moisture removal means comprises a condenser module fluidly coupled upstream to a demoisturizer module, the demoisturizer module adapted to freeze the non-reactive gas flowing through the closed-loop path.

19. The apparatus of claim 15 wherein the source of the non-reactive gas is a source of nitrogen, carbon dioxide, light hydrocarbon gases, or an inert gas selected from the group consisting of helium, argon, krypton, and xenon.

20. The apparatus of claim 15 wherein the circulating means comprises a gas circulator.

21. An apparatus for preparing a canister having a cavity loaded with radioactive elements for dry storage, the apparatus comprising:

a gas circulation system comprising a source of a non-reactive gas, means for removing moisture from gas, sensor means for measuring a psychrometric property of gas and means for circulating gas; and

wherein the gas circulation system is adapted to (1) form a hermetically sealed closed-loop path when operably connected to a cavity of a canister to be prepared for dry storage, (2) circulate the non-reactive gas through the closed-loop path so that the non-reactive gas becomes wetted flowing through the cavity of the canister and dried flowing through the moisture removal means; and (3) achieve a desired vapor pressure within the cavity of the canister based on measurements of the sensor means and a psychrometric relationship.