Denison Announces Independent “Proof of Concept” Achieved FOR Application of Isr Mining Method at Phoenix and Details of 2020 Isr Field Program

Denison Mines Corp. 

1100 – 40 University Ave 

Toronto, ON  M5J 1T1 

www.denisonmines.com 

PRESS RELEASE

DENISON ANNOUNCES INDEPENDENT “PROOF OF CONCEPT”

ACHIEVED FOR APPLICATION OF ISR MINING METHOD AT PHOENIX

AND DETAILS OF 2020 ISR FIELD PROGRAM

Toronto, ON – June 4, 2020. Denison Mines Corp. (“Denison” or the “Company”) (DML: TSX, DNN:

NYSE American) is pleased to announce that the hydrogeologic model developed by Petrotek Corporation

(“Petrotek”), for the high-grade Phoenix uranium deposit (“Phoe nix), produced demonstration of “proof of

concept” for the application of the In-Situ Recovery (“ISR”) mi ning method at Phoenix, with respect to

potential operational extraction and injection rates. The hydrogeologic model was developed based on the

data collected from the ISR field test completed in 2019 (“2019 Field Test”) (see press release dated

December 18, 2019). Based on the positive results from the hydrogeologic model, Denison is also pleased

to announce the details of its plans for the continuation of IS R field testing in 2020, within Phoenix, at the

Company's 90% owned Wheeler River Uranium Project ("Wheeler Riv er"), located in northern

Saskatchewan, Canada.

David Bronkhorst, Denison’s Vice President Operations, commented, “Petrotek applied a rigorous process

of calibration to several numerical models developed based on hydrogeologic data collected from 19 test

wells installed into the Phoenix deposit during the 2019 Field Test. Based on this site-specific data, the

hydrogeologic model allowed for the simulation of an ISR wellfield including a total of 18 extraction wells

and 33 injection wells across Test Area 1 and Test Area 2 of Phoenix. Based on these simulations, Petrotek

concluded that the results demonstrated ‘proof of concept’ for the use of ISR mining at Phoenix. This result

represents a significant milestone in the ongoing de-risking of the use of the ISR mining method at Phoenix.

Our plans for additional field testing in 2020 are expected to build on our success from 2019, further confirm

the hydrogeological model completed by Petrotek, and pr epare for field tests in future years, which are

expected to support a feasibility study – including a potential in-ground lixiviant test or ISR demonstration.”

Hydrogeologic Modeling

Denison engaged Petrotek to facilitate the design and implement ation of ISR field testing at Phoenix in

2019. Petrotek is an independent qualified technical specialist firm with unique expertise in the evaluation

and field operations of subsurface fluid flow and injection projects, with experience ranging from feasibility

studies to facility operation. T he firm has more than 20 years of experience in the ISR uranium mining

industry and has provided consulting services to each of the ISR uranium miners in the United States.

Petrotek and Denison designed the 2019 Field Test for the uniqu e geological characteristics of Phoenix.

The testing program aimed to collect hydrogeological data in Te st Area 1 and Test Area 2 of Zone A at

Phoenix. Data acquired from the 2019 Field Test was collected and analyzed to support the development

of an integrated hydrogeological model to better define areas o f the deposit contributing to fluid flows and

ultimately to facilitate future I SR wellfield designs necessary to support a Feasibility Study (“FS”) and the

Environmental Assessment (“EA”) process.

Highlights of the hydrogeologic modeling completed by Petrotek include the following:

 Numerical groundwater flow models were developed using site-sp ecific data to evaluate the

hydraulic responses of the ore zone aquifer observed during hydrogeologic field testing conducted

at Phoenix in 2019;

 Multiple models were constructed to address the lateral and ve rtical variability in hydraulic

conductivity (a measure of permeability) observed at the site;

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 Modeling was limited to the physical flow of water through the groundwater system and did not

address any geochemical reactions of injected fluids within the aquifer matrix or native

groundwater;

 The hydrogeologic model developed for Phoenix consisted of fiv e layers, including one layer for

the overlying Athabasca sandstone above the ore zone, one layer for the underlying paleo

weathered unmineralized basement below the ore zone, and three subdivided layers within the ore

zone;

 The uppermost layer of the ore zone represents the upper clay unit, the middle layer represents

the more permeable friable unit, and the lower layer represents the lower clay unit. The combined

thickness of the three layers is six metres with the thickness of the middle layer simulated in multiple

models. A two metre interval was assumed to be the most represe ntative of the overall in-situ

conditions of the deposit, with the resultant model being referred to as the “2M” model.;

 Each model was calibrated to the actual test results from the 2019 Field Test, such that the “head”

(defined as a specific measurement of liquid pressure vertically above a reference point) changes

resulting from simulations in the models were similar to the observed changes in the actual tests;

 The calibrated numerical models were then used to estimate hyd raulic responses of the ore zone

aquifer under conditions representative of ISR operations. Single well pattern simulations were run

under a variety of scenarios, inc luding typical 5-spot and 7-sp ot well patterns, variable distances

between wells (5 to 15 metres), and variable pumping and injection rates;

 Results of the single well pattern simulations were used to fu rther investigate the feasibility of ISR

for uranium extraction by developing a wellfield simulation usi ng the 2M model and 5-spot well

patterns placed across Test Area 1 and Test Area 2 of Phoenix at approximate 10 metre spacings

between wells – including a total of 18 extraction / recovery wells and 33 injection wells;

 Test Area 1 extraction wells were simulated at 5 gallons per m inute (“GPM”) or less, and Test Area

2 extraction wells were simulated at 7.5 GPM or less, with total extraction for the simulation at 105.5

GPM and total injection of 105.4 GPM for a nearly balanced operational flow;

 The simulation was carried out within test area boundaries, re flecting the proposed freeze walls

that are planned to encapsulate the ISR wellfield. No attempts were made to optimize operational

rates with respect to sweep efficiency or simulated lixiviant travel time;

 As expected, there wa s large variability in travel times from injection well to extraction well, with the

average flow path travel time es timated at 55 days, with 71% of flow paths being completed in

fewer than 55 days;

 A 180-day simulation was completed with approximately 80% of t he injected fluids estimated to be

captured during the simulation period.

While the results of these simulations (including well patterns, spacing and flow rates) should be viewed as

preliminary, and should not be considered wellfield plans or pr ojections of actual operations, Petrotek’s

Interim Hydrogeologic Report has drawn the following important conclusions:

 The modelling results generally provide a demonstration of “proof of concept” for application of ISR

to the Phoenix ore body, with respect to potential operational extraction and injection rates; and

 With positive results associa ted with the various permeability measures and models reported to

date, the further adjustment of operational parameters (e.g. we ll spacing, injection pressures,

uranium recoveries), along with the engineered enhancement of i n-situ permeability (such as

MaxPERF drilling - see press release dated December 18, 2019), should allow for potentially

significant optimization of the hydrogeologic model in the future.

2020 ISR Field Test Program

During the summer and fall months of 2020, Denison plans to col lect additional hydrogeological data as

part of an ISR field test program (“the 2020 Field Test”) at Ph oenix. The 2020 Field Test is designed to

further evaluate and de-risk the ISR mining conditions present at Phoenix, by supplementing the extensive

dataset acquired as part of the 2019 Field Test. Hydrogeological data collected as part of the 2020 Field

Test is expected to build additional confidence in the Company’s understanding of the fluid pathways within

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Test Area 1 and Test Area 2, to further validate the hydrogeological model completed by Petrotek, and to

prepare for field tests in future years, which are expected to support a FS – including a potential in-ground

lixiviant test or ISR demonstration.

The 2020 Field Test is expected to utilize existing Commercial Scale Wells (“CSWs”) and Small Monitoring

Wells (“SMWs”), installed as part of the 2019 Field Test, as we ll as mineralized and non-mineralized drill

core recovered from the 2019 Field Test.

Key elements of the 2020 Field Test include:

 Additional pump/injection tests within Test Area 1 and Test Area 2 of the Phoenix deposit

o To confirm the long-term st ability of engineered permeability enhancement tunnels; and

o Collection of data from additional injection points not previo usly tested.

 Groundwater sampling

o Collection of data from various horizons within and above the Phoenix ore zone to

characterize the baseline hydrogeochemistry in the deposit area . Understanding of the

hydrogeochemistry is expected to be necessary to support the pl anning and permitting of

a potential field test in future years for an in-ground lixiviant test or ISR demonstration; 

o Groundwater sampling, in conjunction with future column leach test work, is expected to

provide data needed to conduct reactive transport models (i.e. models that involve

kinetically dissolving a mineral in a groundwater system). Thes e models may be used to

determine the overall dissolution rate of the ore (primarily ur aninite) and the flow of the

lixiviant through the formation, including residence time, to estimate overall mill feed rate.

 Permeameter analysis

o Collection of additional matrix permeability data from drill c ore previously recovered from

within Test Area 1 and Test Area 2, to support further refinement of hydrogeological models

with an enhanced understanding of both large- and small-scale fluid flow pathways.

 Rock mechanics tests

o Collection of data to aid in evaluating the potential utility of certain permeability

enhancement techniques.

The Company previously announced a decision to temporarily susp end the environmental assessment

process for the Wheeler River project and other discretionary a ctivities due to the significant social and

economic disruption that has emerged as a result of the COVID-1 9 pandemic and the Company's

commitment to ensure employee safety, support public health eff orts to limit transmission of COVID-19,

and exercise prudent financial discipline. The work related to the 2020 Field Test is not part of the

suspended activities, and is included in the Company’s evaluati on budget contained within the current

outlook and operating plan for 2020 (see the Company’s Manageme nt Discussion and Analysis for the

period ended March 31, 2020).

The Company previously indicated that field activities in 2020 could include the expansion of ISR field test

work into Test Area 3 and Test Area 4 of Phoenix Zone A; however, since then, field testing of the MaxPERF

Drilling Tool validated the potent ial to normalize the impact o f geologic variations throughout the orebody

by mechanically increasing acce ss to existing permeability thro ugh the installation of lateral penetration

tunnels from a CSW. Based on this success, the Company has decided to focus its testing efforts on Test

Area 1 and Test Area 2 with the potential to carry out, in futu re years, an in-ground lixiviant test or ISR

demonstration within the existing test areas (rather than expanding testing efforts to Test Area 3 and Test

Area 4). This approach is expected to substantially de-risk th e application of the ISR mining method at

Phoenix and to support a future FS.

Operational planning for the 2020 Fi eld Test is currently in pr ogress, with significant consideration being

given to public health guidelines and industry best practices a ssociated with operating a remote mining

camp site in northern Saskatch ewan amidst the COVID-19 pandemic . In addition to camp operating

procedures and physical distancing protocols, transportation an d travel protocols are being developed in

consultation with various Indigenous and non-Indigenous communities situated between the Wheeler River

project site and Saskatoon. The Company is committed to ensuri ng that the site is a safe operating

environment for its staff and contractors and that the Company’ s field activities do not compromise the

health and safety of the residents of northern Saskatchewan. Despite the Company’s current intentions, it

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is possible that the program may not advance as planned, or as described above, owing to the social and

economic disruptions associated with the COVID-19 pandemic, whi ch are outside of the control of the

Company – for example, the ava ilability of Company or contracto r staff to attend to the site, Provincial or

local travel restrictions, and changing public health guidelines.

In-Situ Recovery Mining Method

Denison selected the ISR mining method for the future mining of Phoenix in the Pre-Feasibility Study

(“PFS”) completed for Wheeler River in 2018 (see press release dated October 30, 2018). In an ISR mining

operation, a mining solution is injected into the ore zone thro ugh a series of injection wells, and then

dissolves the uranium as it travels through the ore zone, befor e the uranium bearing solution (“UBS”) is

then pumped back to surface via a series of recovery wells. Once on surface, the UBS is sent to a surface

processing plant for the chemical separation of the uranium. Fo llowing the uranium removal, the mining

solution is reconditioned and returned to the wellfield for fur ther production. The ISR mining method

accounts for a significant portion of uranium mine production globally and is generally considered the lowest

cost uranium mining method in the world – owing to the fact tha t the method eliminates the surface

disturbances and costs associated with physically removing ore and waste from the ground, as well as the

tailings treatment and storage, that are normally associated with underground or open pit mining operations.

The geologic setting of the uranium ore body and the ability of the mining solution to travel through the

orebody (permeability) is an important element of an ISR mining operation. Accordingly, much of Denison’s

work in 2019 was focused on enhancing its assessment of the per meability of Phoenix through the

completion of a series of ISR field tests – the positive prelim inary results of which have been previously

released (see press releases dated December 18, 2019 and February 24, 2020), and the data from which

was incorporated into the hydrogeologic modelling described above.

About Wheeler River

Wheeler River is the largest undeveloped uranium projec t in the infrastructure rich eastern portion of the

Athabasca Basin region, in northern Saskatchewan – including combined Indicated Mineral Resources of

132.1 million pounds U3O8 (1,809,000 tonnes at an average grade of 3.3% U 3O8), plus combined Inferred

Mineral Resources of 3.0 million pounds U 3O8 (82,000 tonnes at an average grade of 1.7% U 3O8). The

project is host to the high-grade Phoenix and Gryphon uranium deposits, discovered by Denison in 2008

and 2014, respectively, and is a joint venture between Denison (90% and operator) and JCU (Canada)

Exploration Company Limited (10%).

A PFS was completed for Wheeler River in late 2018, considering the potential economic merit of

developing the Phoenix deposit as an ISR operati on and the Gryphon deposit as a conventional

underground mining operation. Taken together, the project is estimated to have mine production of 109.4

million pounds U3O8 over a 14-year mine life, with a base ca se pre-tax NPV of $1.31 billion (8% discount

rate), Internal Rate of Return ("IRR") of 38.7%, and initial pre-production capital expenditures of $322.5

million. The Phoenix ISR operation is estimated to have a stand-alone base case pre-tax NPV of $930.4

million (8% discount rate), IRR of 43.3%, initial pr e-production capital expenditures of $322.5 million, and

industry leading average operating costs of US$3.33/lb U 3O8. The PFS is prepared on a project (100%

ownership) and pre-tax basis, as each of the partner s to the Wheeler River Joint Venture are subject to

different tax and other obligations.

Further details regarding the PFS, including additional scientific and technical information, as well as after-

tax results attributable to Denison's ownership intere st, are described in greater detail in the NI 43-101

Technical Report titled "Pre-feasibility Study for the Wheeler River Uranium Project, Saskatchewan,

Canada" dated October 30, 2018 with an effective date of September 24, 2018. A copy of this report is

available on Denison's website and under its prof ile on SEDAR at www.sedar.com and on EDGAR at

www.sec.gov/edgar.shtml.

About Denison

Denison is a uranium exploration and development company with interests focused in the Athabasca Basin

region of northern Saskatchewan, Canada. In addition to the Wheeler River project, Denison's Athabasca

Basin exploration portfolio consists of numerous projects covering over 250,000 hectares. Denison's

interests in the Athabasca Basin also include a 22.5% ownership interest in the McClean Lake joint venture

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("MLJV"), which includes several uranium deposits and the McClean Lake uranium mill, which is currently

processing ore from the Cigar Lake mine under a to ll milling agreement, plus a 25.17% interest in the

Midwest and Midwest A deposits, and a 66.57% interest in the J Zone and Huskie deposits on the

Waterbury Lake property. Each of Midwest, Midwest A, J Zone and Huskie are located within 20 kilometres

of the McClean Lake mill.

Denison is also engaged in mine decommissioning and environmental services through its Closed Mines

group (formerly Denison Environmental Services), which mana ges Denison's Elliot Lake reclamation

projects and provides post-closure mine care and ma intenance services to a variety of industry and

government clients.

Denison is also the manager of Uranium Participat ion Corp., a publicly traded company which invests in

uranium oxide and uranium hexafluoride.

For more information, please contact

David Cates (416) 979-1991 ext 362

President and Chief Executive Officer

Sophia Shane (604) 689-7842

Investor Relations

Follow Denison on Twitter @DenisonMinesCo

Qualified Persons

The results and interpretations contained in this release related to the hydrogeological model for Phoenix

were prepared by Mr. Errol Lawrence, PG (Senior Hydrogeologist), and Mr. Aaron Payne, PG (Senior

Hydrogeologist), at Petrotek, each of whom is an i ndependent Qualified Person in accordance with the

requirements of NI 43-101.

The other technical information contained in this release has been reviewed and approved by Mr. David

Bronkhorst, P.Eng, Denison's Vice President, Operati ons, or Mr. Dale Verran, MS c, P.Geo, Pr.Sci.Nat.,

Denison's Vice President, Exploration, each of whom is a Qualified Person in accordance with the

requirements of NI 43-101.

Cautionary Statement Regarding Forward-Looking Statements

Certain information contained in this news release constitutes ‘forward-looking information’, within the meaning of the applicable United

States and Canadian legislation concerning the business, operations and financial performance and condition of Denison.

Generally, these forward-looking statements c an be identified by the use of forward-look ing terminology such as ‘plans’, ‘expects’,

‘budget’, ‘scheduled’, ‘estimates’, ‘forecasts’, ‘intends’, ‘anticipates’, or ‘believes’, or the negatives and/or variations of such words and

phrases, or state that certain actions, events or results ‘may’, ‘could’, ‘would’, ‘might’ or ‘will be taken’, ‘occur’, ‘be achieved’ or ‘has

the potential to’.

In particular, this news release contains forward-looking information pertaining to the following: the results of the 2019 Field Test; the

hydrogeologic model and its underlying assu mptions; the 2020 Field Test, including it s intended scope and timing, objectives an d

evaluation interpretations; the duration and scope of impacts of the COVID-19 pandemic and affiliated operational adjustments; the

current and continued use and availability of third party technologies for testing; the results of the PFS and expectations with respect

thereto; development and expansion plans and objectives, including plans for a feasibility study; and expectations regarding it s joint

venture ownership interests and the continuity of its agreements with its partners.

Forward looking statements are based on the opinions and estimates of management as of the date such statements are made, and

they are subject to known and unknown risks, uncertainties and other factors that may c ause the actual results, level of activi ty,

performance or achievements of Denison to be materially different from those expre ssed or implied by such forward-looking

statements. For example, the results of the 2019 Field Test, hydrogeologic model and/or 2020 Field Test discussed herein may no t

be maintained after further testing or be representative of ac tual conditions within the Phoeni x deposit. In addition, Denison may

decide or otherwise be required to discont inue the 2020 Field Test or other testing, evaluation and developm ent work at Wheeler

River if it is unable to maintain or otherwise secure the nece ssary resources (such as testing facilities, capital funding, reg ulatory

approvals, etc.) or operations are otherwise affected by COVID-19 and its potentially far-reaching impacts. Denison believes that the

expectations reflected in this forward-looking information are reasonable but no assurance can be given that these expectations will

prove to be accurate and results may differ materially from t hose anticipated in this forward-l ooking information. For a discus sion in

respect of risks and other factors that could influence forward-looking events, please refer to the factors discussed in Denison’s Annual

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Information Form dated March 13, 2020 or subsequent quarterly financial reports under the heading ‘Risk Factors’. These factors are

not, and should not be construed as being exhaustive.

Accordingly, readers should not place undue re liance on forward-looking statements. The forward-looking information contained i n

this news release is expressly qualified by this cautionary statement. Any forward-looking information and the assumptions made with

respect thereto speaks only as of the date of this news release. Denison does not undertake any obligation to publicly update or revise

any forward-looking information after the date of this news rel ease to conform such information to actual results or to changes in

Denison's expectations except as otherwise required by applicable legislation.

Cautionary Note to United States Investors Concerning Estimates of Measured, Indicated and Inferred Mineral Resources

and Probable Mineral Reserves: This press release may use the terms 'measured', 'indicated' and 'inferred' mineral resources.

United States investors are advised that while such terms have been prepared in accordance with the definition standards on mineral

reserves of the Canadian Institute of Mining, Metallurgy and Pe troleum referred to in Canadian National Instrument 43-101 Miner al

Disclosure Standards ('NI 43-101' ) and are recognized and required by Canadian regul ations, these terms are not defined under

Industry Guide 7 under the United States Securities Act and, unt il recently, have not been permitted to be used in reports and

registration statements filed with the United States Securities and Exchange Commission ('SEC'). 'Inferred mineral resources' have a

great amount of uncertainty as to their existence, and as to their economic and legal feasibility. It cannot be assumed that al l or any

part of an inferred mineral resource will ever be upgraded to a higher category. Under Canadian rules, estimates of inferred mi neral

resources may not form the basis of feasibility or other economic studies. United States investors are cautioned not to assume that

all or any part of measured or indicated mineral resources will ever be converted into mineral reserves. United States investor s are

also cautioned not to assume that all or any part of an inferr ed mineral resource exists, or is economically or legally mineabl e. In

addition, the terms "mineral reserve", "pro ven mineral reserve" and "probable mineral reserve" for the purposes of NI 43-101 di ffer

from the definitions and allowable usage in Industry Guide 7. Effective February 2019, the SEC adopted amendments to its disclosure

rules to modernize the mineral property di sclosure requirements for issuers whose secu rities are registered with the SEC under the

Exchange Act and as a result, the SEC now re cognizes estimates of "measured mineral re sources", "indicated mineral resources"

and "inferred mineral resources". In addition, the SEC has amended its definitions of "proven mineral reserves" and "probable mineral

reserves" to be "substantially similar" to the corresponding definitions under the CI M Standards, as required under NI 43-101.

However, information regarding mineral res ources or mineral reserves in Denison's disclosure may not be comparable to similar

information made public by United States companies.